37 Bid? N Mo - UNT Digital Library/67531/metadc330827/...mechanism involving an initial fission of...
Transcript of 37 Bid? N Mo - UNT Digital Library/67531/metadc330827/...mechanism involving an initial fission of...
37? N Bid
Mo.asif
KINETICS AND MECHANISMS OF LIGAND EXCHANGE
REACTIONS OF CHELATE COMPLEXES
DISSERTATION
Presented to the Graduate Council of the
University of North Texas in Partial
Fulfillment of the Requirements
For the Degree of
DOCTOR OF PHILOSOPHY
By
Jose E. Cortes, B.S.
Denton, Texas
May, 1989
Cortes, Jose E., Kinetics and Mechanisms fif Lj.gafld
Exchange Reactions of Chelate Complexes. Doctor of
Philosophy (Physical Chemistry), May 1989, 150 pp., 17
tables, 50 figures, bibliography, 125 titles.
The ligand substitution reactions of (fts-DTHp)W(COU,
(fP-DTD)W(COU, and (fF-DTU)W(COU, (DTHp = 2,2,6,6-tetra-
methy1-3,5-dithiaheptane, DTD = 2,2,9,9-tetramethyl-3,8-
dithiadecane, DTU - 2,2,10,10-tetramethyl-3,9-dithia-
undecane), with L, (L = phosphites and phosphines), proceed
to a complete displacement of the chelate ligand. During the
course of the reactions there is appreciable formation of
cis-(n*-DTA) (L)W(CO)*, (DTA - DTHp, DTD, DTU).
The reactions of <n«-DTA)W(COU, (DTA = DTD, DTU),
with L to produce cis-(n*-DTA)(L)W(COU proceed through a
mechanism involving an initial fission of the tungsten-sulfur
bond to afford a coordinatively-unsaturated intermediate,
cis-[(n*-DTA)W(COUJ, which is rapidly attacked by
chlorobenzene. The resulting solvated intermediate
establishes an equilibrium which involves desolvation-
solvation. Then, cis-C(fl1 -DTA)W(C0U3 undergoes ring-closure
and attack by L.
The reactions of (fl®-DTHp)W(CO)^ with phosphites and
tri(n-butyl) phosphine proceed through a mechanism involving
an initial interaction between the incoming ligand, L, and
the substrate. Activation parameters for the reactions
leading to the ring-opened 1igand-substituted intermediate,
cis-(ft1-DTHp)(L)W(CO)^, are consistent with this mechanism.
The thermally-generated intermediate cis- {ft* -DTA) -
(L)W(COU, (DTA = DTHp, DTD, DTU), will undergo further
reaction with L to produce cis-(L)aW(CO)<•. The rate law for
these reactions, first order with respect to the
concentration of cis- (fl* -DTA) (L)W(CO)^. and zero order with
respect to the concentration of the incoming 1igand, and the
activation parameters, indicate rate-determining dissociation
of the anchored end of the chelate 1igand. Cis-(L)aW(COU
complexes, (L = tri(isopropyl) phosphite, trimethyl
phosphite, and tri(n-butyl) phosphine) undergo cis-trans
isomerization to produce a mixture of trans- and cis-
(L)eW(COU.
The kinetics evidence for the cis-trans isomerization
of the final product, {(L)eW(CO)<», L = tri(n-butyl)
phosphine) suggests that it is non-dissociative.
TABLE OF CONTENTS
Page
v LIST OF TABLES
LIST OF ILLUSTRATIONS v 1 1
Chapter
I. INTRODUCTION 1
History and General Aspects Metal Carbonyls Reactions The Problem Chapter Bibliography
II. EXPERIMENTAL 2 8
General Purification of Solvents Purification of Ligands Syntheses of Bidentate Ligands Syntheses of Metal Complexes Identification of Intermediates Identification of Reaction Products Recrystalization of Metal Complexes Kinetics Runs Flash Photolysis Chapter Bibliography
III. REACTIONS OF (fla-DTA)W(COU 54
General Reactions of (fle-DTHp)W(COU Formation of Cis-{fl*-DTHp) (L)W(CO)** Ring-Closure Cis- [ (fl1-DTHp) (S)W(C0)«*3 Mechanism for the Formation of Cis-(ni-DTHp) (L)W(COU Reactions of Cis-(H*-DTA) (L)W(CO)*-Chapter Bibliography
xxx
Chapter
TABLE OF CONTENTS-Continued
Page
f **• IV. REACTIONS OF CIS- (fl'-DTA) (L)W(CO)-DTA = DTD, DTU 1 1 1
Formation of Cis-(ni_DTA) (L)W(CO)*. Ring-Closure of Cis-DTA) (CB)W(CO)«» Summary Chapter Bibliography
V. CIS-TRANS ISOMERIZATION OF (L)sW(COU 132
Exper imenta1 Cis-trans Isomerization 3 1P NMR Studies Chapter Bibliography
VI. CONCLUSIONS 1 4 7
VII. BIBLIOGRAPHY 1 5 2
IV
LIST OF TABLES
Table Page
I.
II.
Ill
IV.
V.
VI.
VII.
VIII
IX.
Carbonyl Stretching Frequencies of Cia-(n*-DTHp) (L)W(COU in Chlorobenzene 41
First-Order Rate Constants for the Reactions of (D®-DTHp)W( CO)*. with Phosphites in Chlorobenzene at Various Temperatures 57
Rate Constants and Activation Parameters for the Reactions of (fle-DTHp)W(CO)<=» with Phosphites in Chlorobenzene at Various Temperatures 64
Rate Constants for the Ring-Closure of (fl1-DTHp) (Solvent)W(COU in
Bromobenzene and Chlorobenzene at Various Temperatures 68
Rate Constants for the Reactions of £i£-(n*-DTA)(L)W(COU with L in Chlorobenzene at Various Temperatures 76
Rate Constants for DTHp-Dissociation from £i£-(ni-DTHp) (L)W(COU in Chlorobenzene at 44.5 °C 79
Activation Parameters for the Dissociation of DTA from £i£-(fl 4-DTA)(L)W(C0U 87
Rate Constants for the Displacement of Chlorobenzene from QiSr [ (P(O-i-Pr)a) (CB)W(CO)*.] by Tri(isopropyl) Phosphite at 35.2 °C 91
Pseudo-First Order Rate Constants for the Reactions of (n*-DTA)W(CO)«. with Phosphites in Chlorobenzene at Various Temperatures 96
Table
LIST OF TABLES-Continued
Page
X.
XI
XII.
XIII
XIV.
XV.
XVI,
XVII.
Rate Constants for the Ring-Opening of (rF-DTA)W(CO)^ in Chlorobenzene at Various Temperatures 105
Rate Constants for the Ring-Closure of Sia~[(Hl-DTA)(Chlorobenzene )W( CO)*.] in Chlorobenzene at Various Temperatures
Rate Constants and Activation Parameters for the Displacement of DTA from (rF-DTA)W(COU at 35.2 °C
110
116
Rate Constants for the Isomerization of (P(n-Bu)a)aW(C0U in Chlorobenzene at Various Temperatures 128
CIS : Trans Ratios of (P(n-Bu)s ) aW(C0U in Chlorobenzene at Various Temperatures 134
Rate Constants for CIS-Trans Isomerization of (P(n-Bu)a)aW(CO)^ in Chlorobenzene at Various Temperatures 136
Rate Constants Involved in Mechanism Described in Figure 49 141
Rate Constants for the Overall Mechanism Described in Figure 50 142
vi
Figure
LIST OF ILLUSTRATIONS
Page
1. Fenske Direct Donation Model 5
2. Substitution Reaction Mechanism Involving Rate-Determining Dissociation of CO; Interchange or Associative Interaction with L 7
3. Generation of Coordinatively-Unsaturated Cis- [LW(C0U1 and Attack by CH, CB, and Pip. CH = Cyclohexane, CB = Chlorobenzene, Pip = Piperidine
4. Displacement of Bidentate Ligand, Bonded Through Nitrogen to the Central Metal, Involving a Competitive Me chanism
5. Displacement of Bidentate Ligand, Bonded Through Sulfur to the Central Metal, Involving a Competitive Mechanism 1?
6. Carbonyl Stretching Spectrum of Cis-(na-DTHp)W(CO)^ in Chlorobenzene 3 3
7. Carbonyl Stretching Spectrum of Cis - (PI1 -DTHp) (CP)W(CO )<» in Chlorobenzene 3 5
8. Carbonyl Stretching Spectrum of £i£-(fF-DTD)W(COU in Chlorobenzene 3 6
9. Carbonyl Stretching Spectrum of Cis-(fl1-DTD) (CP)W(COU in Chi orobenzene 3 ?
10. Carbonyl Stretching Spectrum of Cis-(ne-DTU)W(COin Chlorobenzene 3 9
v n
LIST OF ILLUSTRATIONS-Continued
Figure Page
11. Carbonyl Stretching Spectrum of £is- {P(n-Bu)a)eW( CO)«. in Chlorobenzene 40
12. Carbonyl Stretching Spectrum of Cis-and Trans-(P(n-Bu)a)EW(C0U in Chi or obenz ene 42
13. Carbonyl Stretching Spectrum of Cis-(CP)BW(COU in Chi orobenz ene 43
14. Carbonyl Stretching Spectrum of Cis- and Trans-(P(Q-i-Pr)a)aW(C0)*» in Chlorobenzene 44
15. Carbonyl Stretching Spectrum of Cis- and Trans- (P(0Me)A)EW(C0)«» in Chlorobenzene 45
16. Carbonyl Stretching Spectrum of Cis- (P(QCaHb )A )E»W(C0)*. in Chlorobenzene 46
17. Schematic of Flash-Laser Photolysis Equipment: SS, Slow Shutter; FS, Programmable Fast Shutter; Ft Filter; L, Lenses; S, Sample; A, Attenuator; MC, Monochromator; P, Photomultiplier Tube; D, Photodiode Energy Monitor; Mf Mirror; G, Quartz.... 50
18. Plot of (A*-A.) vs. Time for the Reactions of (fP-DTHp )W( CO )*. with Tri( isopropyl) Phosphite at 21.1 °C. [L] = 0.2008 M. Ordinate = (At - A-), Abscissa = Time X 10-** Sec. 55
19. Plots of kob«d 3£s. [L] for the Reactions of (na-DTHp)W(CO)«. with Tri (isopropyl) Phosphite in Chlorobenzene at Various Temperatures. Ordinate = kob.d X 10® Sec"1, Abscissa = [L] X 10 M 59
Vlll
Figure
LIST OF ILLUSTRATIONS-Continued
Page
20. Plots of kobmd vs. [L] for the Reactions of (ne-DTHp)W(CO) ** with L, L — Tri (isopropyl) Phosphite, Trimethyl Phosphite, in Chlorobenzene at 21.1 °C. Ordinate = kcf.ci X 10®, Abscissa = [L] X 10 M 60
21. Competitive Mechanism for the Displacement of One End of DTHp from ( n ® - D T H p ) W ( C O U by L Involving Initial Rate-Determining Ring-Opening; Bimolecular Attack by L 61
22. Eyring Plot of l n ( W T ) yg. 1/T for the Reactions of (fF-DTHp)W(COU with Tri(isopropyl) Phosphite in Chlorobenzene at Various Temperatures. Ordinate = ln(ke/T), Abscissa = 1/T X 10s K"1 6 3
23. Ring-Closure of £is-[(n*-DTHp)(CB)W(CO)*] Involving a Bimolecular Displacement of CB. (CB = Chlorobenzene) 67
24. Eyring Plots of the Infk-^/T) vs. l/T for the Ring-Closure of Cis- Ufl1 -DTHp)(solvent )W( CO U 3 at Various Temperatures. BB = Bromobenzene, DCE = 1,2-Dichloroethane. Ordinate = ln(k-i/T), Abscissa * 1/T X 10s K-1
6 9
25. Plausible Mechanism for the Reactions of (rF-DTHp)W(COU with L to afford £i£-(n*-DTHp) (L)W<CO)* 73
26. Eyring Plots of ln(k*/T) vs. 1/T for the Reactions of fiiS-m^-DTHp) (L)W(COU with L, L = Tri(isopropyl) Phosphite, Trimethyl Phosphite, and CP in Chlorobenzene at Various Temperatures. Ordinate = ln(k*/T), Abscissa = 1/T X 10s K-1 qj
IX
Figure
LIST OF ILLUSTRATIONS-Continued
Page
27. Eyring Plot of ln(k*»/T) vs. 1/T for the Reactions of Cis-tn^-DTD)(L)W(COU with L, L = Tri(isopropyl) Phosphite, in Chiorobenzene at Various Temperatures. Ordinate = ln(k*/T), Abscissa = 1/T X 103 82
28. Eyring Plot of ln(k>»/T) vg,. 1/T for the Reactions of Cis-(n*-DTU) (L)W(COU with L, L = Tri < isopropyl) Phosphite, in Chlorobenzene at Various Temperatures. Ordinate = ln(k*./T), Abscissa = 1/T X 10s 83
29. Mechanism for the Displacement of DTA from Cis-<n1-DTA) (L)W(COU by L, L = Phosphites, Phosphines 84
30. Plot of Ink* vs. Tolman Cone Angles for the Reactions of Cis- (ni~DTHp) (L)W(C0)«. with L, L = 1, Tri(n-butyl) phosphine; 2, Triphenyl Phosphite; 3, Tri(isopropyl) Phosphite; 4, Trimethyl Phosphite; 5, CP 85
31. Photochemical Generation of Cis-[LW(COU1 in Chlorobenzene and Attack by L 89
32. Plot of kob.d va- CL3 f°r the Reactions of Cis- [ (L) (CB)W(COU] with L at 35.2 °C in chlorobenzene. L = Tri(isopropyl) Phosphite. Ordinate = k X 103 sec-1, Abscissa = [L] X 10 M 92
33. Plots of kob.« vs. CL] for the Reactions of (fF-DTD)W(COU with Tri(isopropyl) Phosphite in Chlorobenzene at Various ^ Temperatures. Ordinate = kob>d X 10® Sec-1, Abscissa = [L] X 10 M 98
Figure
LIST OF ILLUSTRATIONS-Continued
Page
34. Plots of keb«d vg. [L] for the Reactions of (rF-DTTJ)W(COU with Tri( isopropyl) Phosphite in Chlorobenzene at Various Temperatures. Ordinate = ]<«**,.* x 103 Sec"1
Abscissa = [L] X 10 M [ 99
35. Plots of 1/kob.ci vs. 1/[L3 for the Reactions of (fF-DTD)W(COU with Tri(isopropyl) Phosphite in Chlorobenzene at Various Temperatures. Ordinate = 1/koe-c X 10~B
Sec, Abscissa = 1/[L] M_1 10i
36. Plots of 1/kofa.d vs. 1/[L] for the Reactions of (n®-DTU)W(COU with Tri(isopropyl) Phosphite in Chlorobenzene at Various Temperatures. Ordinate = l/k=>b_d X 10~« Sec, Abscissa = 1/[L] M-1
102
37. Mechanisms for the Displacement of One End of DTA from (fla-DTA)W(COU by L 103
38. Plots of l/kot.««j vs. 1/[L] for the Reactions of (f^-DTD)W(CO)<» with L in Chlorobenzene at 21.1 °C. Ordinate = l/kob«d X 103 Sec, Abscissa = 1/[L] M-1
104
39. Eyring Plots of ln(k/T) v§. 1/T for the Reactions of (rF-DTD)W(COU with Tri(isopropy1) Phosphite in Chlorobenzene at Various Temperatures. Ordinate = ln(k/T) Abscissa = 1/T X 10s It-1 ' 106
40. Eyring Plot of ln(k/T) l/T for the Reactions of (rF-DTU)W(COU with Tri(isopropyl) Phosphite in Chlorobenzene at Various Temperatures. A = k*, B = ka/k-a Ordinate = In{k/T), Abscissa = 1/T X 10s K _ 1
107
41. Eyring Plot of lndc-^/T) vs. 1/T for the Ring-Closure of Cis-[(n*-DTD)(CB)W(CO)^] in Chlorobenzene at Various Temperatures. Ordinate = ln(k~VT), Abscissa = 1/T X 10s kr1 1 1 3
XI
LIST OF ILLUSTRATIONS-Continued
Figure Page
42. Eyring Plot of ln(k-i/T) v&. 1/T for the Ring-Closure of £i§r [ (fl^-DTU) (CB)W(COUl in Chlorobenzene at Various Temperatures. Ordinate = ln(k-i/T), Abscissa = 1/T X 10s K"1 114
43. Plots of In (A* - Sbiank) HS- Time for the Reactions of (fF-DTN)W(COU with P(OMe)a in Chlorobenzene at 43.3 °C at Two Concentrations of P(OMe)a 121
44. Plot of Absorbance 2££. Time for the Reaction of (fF-DTHp)W(COU with P(n-Bu)a (0.1071 M) in Chlorobenzene at 44.5 °C. Ordinate = Absorbance, Abscissa = Seconds X 10~a 125
45. Plots of (Top) In (A* - A i*,-,*) Time for the Third Segment of This Plot, Obtained by Monitoring at 415 run, for Reaction of (rF-DTHp)W(COU with P(n-Bu)a (0.1084 M) at 44.5 °C and (bottom) These Data Plotted as In(A. - At) ys. Time. Ordinate = Absorbance, Abscissa = Seconds X 10~3.. 127
46. 3 1P NMR Spectrum of Cis- and Trans-(P(n-Bu)a )aW(C0U in Chlorobenzene at 35.2 °C 131
47. 3 1P NMR Spectrum of Cis- and Trans -(P(n-Bu)a )eW(C0)<* in Chlorobenzene at 44.5 °C 132
48. 3 1P NMR Spectrum of Cis- and Trans-(P(n-Bu)a )eW(C0U in Chlorobenzene at 54.6 °C 133
49. Proposed Mechanism for the Overall Displacement of DTA, DTA = DTD and DTU, from (fla-DTA)W(COU by L 140
50. Proposed Mechanism for the Overall Displacement of DTHp from (fla-DTHp)W(COU by L 143
XXX
CHAPTER I
INTRODUCTION
History sod general Aspects
The first organometallic compound was prepared in 1827
by W. C. Zeise through a reaction of ethanol with a mixture
of PtCls and PtCl* in the presence of KC1 (D .Zeise
suggested PtCle (CeH*. )KC1 .BiaO as the formula for this new
compound (2, 2). The first organometallic compound having a
direct carbon-metal bond was synthesized by E. Frankland
(it). This compound was obtained by accident. Heating ethyl
iodide with zinc to probe the presence of organic radicals,
Frankland obtained a volatile, colorless liquid that roughly
analyzed as CeH*. At first, he thought that the reaction
product proved the presence of organic radicals. Later,
molecular weight determinations revealed that this compound
was not the ethyl radical but butane that was formed from
decomposition of ethylzinc iodide. Despite their flammable
nature, these compounds were extensively used as alkylating
agents until replaced by Grignard reagents, which were
easier to handle. Later discoveries followed, such as that
of nickel carbonyl by Mond (5.) in 1890, and Grignard
reagents in 1900 (ft).
2
After 1950, with the steady growth of chemical knowledge and
the discovery of ferrocene (2-If)) in 1952 and the invention
of the Ziegler process in 1953 (H), organometallic
chemistry was firmly established as a chemical discipline.
Although main group organometallic chemistry has received a
great deal of attention, this discussion will be centered in
organotransition metal chemistry, in particular, metal
carbonyls.
Metal Carbonvls
Metal carbonyl complexes (12, 11) are molecules
containing a central metal atom coordinated to at least one
carbon monoxide molecule. The coordination number, the
number of 1igauds bonded to the central metal, is determined
by both the nature of the metal and the ligand. Properties
such as the oxidation state, the size of the metal, and the
bulkiness of the ligand will affect the stoichiometries of
these complexes. Furthermore, their molecular formulas can
be predicted by use of the noble gas formalism or
eighteen-electron rule (12). This requires that the number
of valence electrons residing in the metal plus the number
of electrons donated by the ligands equal the number of
electrons of the next noble gas in the periodic table. This
is a consequence of the metal atoms use of its valence nd,
(n + 1)s and (n + D p orbitals as fully as possible when
forming bonds with ligands.
3
Many of these metal atoms are in a low positive,
zero or negative oxidation state. Thus, a high electron
density will develop on the central metal atom when sigma
donation from a bonding ligand takes place. This apparently
anomalous situation is what sets apart the chemistry of
transition metal elements. It can be rationalized by a
close examination of the interacting orbitals between the
central metal and the ligand. Carbon monoxide belongs to
the so-called u-acid ligands and is by far the most
important one. These ligands can act as sigma-electron
donors (Lewis bases) and can also act as Lewis acids in the
sense that energetically accessible u-orbitals are
available to accept electron density from the metal.
Bonding between CO and the central metal thus involves
dative sigma-bond formation through overlap of a slightly
antibonding lone pair on carbon with a dssp3 hybrid orbitals
on the metal. This sigma-interaction is reinforced by "it-
back bonding" between the filled metallic orbitals and the
it-antibonding molecular orbitals on CO. This bonding
mechanism is "synergic" since there is donation of electrons
from the metal to CO via "backbonding"; at the same time,
there is donation of electrons by carbon to the metal.
Thus, sigma-bond formation and it-backbonding enhance each
other and the concerted drift of electrons is such that the
M-C bond approaches electroneutrality (13).
4
Vibrational spectra and Cotton-Khraihanzel force
constants are evidence of the multiple nature of the M-C
bond-lengths (lit). It is expected that as the extent of
bonding increases, the M-C bond becomes stronger at the
expense of the CO bond, which becomes weaker. Thus, as
antibonding orbital occupancy increases, carbonyl
stretching frequencies should become lower.
Although a wide variety of u-acid ligands, L, such
as phosphines, phosphites, arsines, stilbenes, and sulfides
can replace CO, they are relatively poor u-acceptors. Thus,
a replacement of CO by L should lead to a decrease of the
carbonyl stretching frequencies and the force constants.
According to Cotton (Hi), the influence of the substituted
ligand should be twice as great for trans (axial) carbonyls
relative to cis (equatorial) carbonyls, since L and the cis
carbonyls share only one dit orbital while the trans
carbonyls shares two.
Fenske (!&, 12.) has proposed a model to describe the
bonding in substituted carbonyl complexes. This model
involves a through-space direct donation between the p* lone
pair on the ligand and a proper linear combination of the
it*-orbitals of the equatorial carbonyls (Figure 1). The
effect of the substituted ligand on the stretching
frequencies and on force constants of the cis carbonyls is
expected to increase with increasing covalent radius and
L ,0 9
^rf 0
^ 7 ^ "
& ^ / / M
o ° ° C
° 0 °
o O
/
' V o %
Fig. l--Fensko direct donation model
6
decreasing nuclear charge on the donor atom (U£). Dobson
(il-lfl), Keeling (22) and MacDiarmid (22) have presented
experimental evidence that supports a direct
substituent-carbon monoxide interaction.
Reactions
Octahedral metal carbonyls, M(CO)sL, (L = CO,
amines, M = group VIB metals), exhibit a wide variety of
reactions (2k_25.)- These reactions may be classified as
follows: 1. Ligand coordination and dissociation (in this
category ligand-substitution reaction are included). 2.
Oxidative addition and reductive elimination. 3. Insertion
and deinsertion of olefins and other ligands. 4. Reaction
of coordinated ligands. Our attention will be focused on
ligand substitution reactions. Octahedral metal carbonyls
may undergo ligand-substitution reactions through four
different pathways: dissociative (D), dissociative
interchange (Id), associative interchange (I«) and
associative (A).
Dissociative pathway (D).-- This pathway assumes a rate
determining loss of L (figure 2 path a). The rupture of the
M-L bond proceeds far enough to give an intermediate which
can recombine with L governed by k-i or react with the
incoming nucleophile with rate constant ke. The rate law,
assuming that the concentration of the intermediate 2-a is
steady-state, is given by equation (1).
& o
o'
M
C 0
PATH B k2
°o
PATH A
< — ' - 1 a
o c
M
•o
c o
v
-o
k 8 » <? /L
, / .oO N I ' *
^ 1/V M
^l\ c o 0
C O 0
Fig. 2--Substitution reaction mechanism involving rate-determining dissociation of CO; interchange or associative interaction with L.
-dtS]/dt = kikeCL'3)tS]/(k_itL] + k.[L']> (1)
S = M(CO)sL
Although the step governed by k* is the rate-determining
step, the rate will show a dependence on CL'] since the
intermediate will competitively be attacked by L and L'.
Detection of the intermediate would provide the only
unambiguous proof of the mechanism.
Id and la mechanisms.-- These mechanisms may be described as
involving diffusion-controlled cage formation which then
proceeds to the transition state [M L], which positions
the incoming ligand to enter the coordination sphere on
departure of L. The fundamental difference between Id and
I« is that Id involves considerable bond breaking in the
transition state, while I« involves a more advanced M L'
interaction. The assumption made for both Id and I* is that
the initial interaction between the complex and
L1, (governed by k'e in Figure 2), is the rate-determining
step. The rate law is given by equation (2).
-dtS3/dt = kdx-r^k'stS][L'] (2)
9
mechanism (A).Like the interchange mechanism,
this mechanism proceeds to involve diffusion-controlled cage
formation. There is formation of an intermediate of an
increased coordination number. The rate law for this
mechanism is indistinguishable from the rate law of the
interchange pathway. Unambiguous proof for this mechanism
will come from observation of the intermediate.
Reactions of hexacarbonvl complexes.-- Hexacarbonyl
transition metal complexes are relatively inert toward
ligand-substitution reactions. This property makes them
ideal for thermal kinetics studies since reactions will
take place at convenient rates. The most thermally labile
of these complexes is V(CO )<s , f which undergoes CO exchange at
25 °C with a half-life of several hours (£2)• On the other
hand V(CO)a- is inert to CO substitution even in molten
P(CaH= )a (30). Metal carbonyl complexes of the group VIB
metals undergo ligand-exchange in solution and in gas phase
(29). Activation parameters are insensitive to the nature
of the solvent, consistent with a dissociative mechanism
(22)- The order of M-CO lability as indicated by , Mo >
Cr > W, does not parallel the one for the mean bond energies
for M-CO dissociation; W (42.1 kcal/mole) > Mo (35.9) > Cr
(26.1) (31). However, a closer correlation is found with
the M-C force constants: W (2.32 mdyn/A) > Cr (2.10) > Mo
(2.00) (12.).
10
Substitution reactions of M(CO)*,, (M = Cr, Mo, W),
with amines (31), phosphorus ligands (M. !£). and
acetonitrile (36), showed a two term rate law consistent
with a mechanism involving competitive dissociative and
ligand-dependent interchange (Id) pathways. The activation
parameters for the ligand-independent pathway are very
similar to those obtained for CO exchange, which suggests a
common reaction path for both reactions. Two important
trends associated with k'E (figure 2) are observed for the
ligand-dependent pathway. First, there is a progressive
increase of ks as the basicity of L is increased (as given
by &HNP, AHNP = half neutralization potential) (32). The
order of reactivity was found to be: P(OGd>Hs )3 < P(CU,Ife)a <
P(OCHe)aCEt < P(OEt)3 < P(n-Bu)a. Amines and acetonitrile
showed a reactivity comparable to that of P(OCHe)aCMe and
P(C*Hs)a, respectively. Second, the order of reactivity, Cr
< Mo ~ W, parallels the order of increasing covalent radius
of the metal, Cr < Mo ~ W (M) •
Flash photolysis of cis-W(C0)«.LL' , (L = CO,
piperidine (pip), L' = CO, tri(isopropyl) phosphite), and
matrix isolation studies have probed the nature of the
intermediate generated through the dissociative pathway
(Jl£-.5Ji) • The initial photoproduct has not been observed,
but it is presumed to be an excited state of C<w symmetry.
In the matrix, the C-w nature of the intermediate has been
demonstrated (39. 40).
11
The dependence of the visible spectrum on the nature
of the matrix strongly suggests a solvated intermediate.
However» recent work by Dobson et al. (41) and Simon et al.
(42) suggests that upon the flash, a coordinatively-
unsaturated intermediate is initially formed and is rapidly
attacked by the solvent at the unsaturation site.
Competition studies in cyclohexane (Jtl) between a bona fide
ligand, piperidine, and chlorobenzene (CB), a second ligand,
showed that the five coordinate intermediate will
competitively be attacked by chlorobenzene and piperidine.
upon reaction with chlorobenzene, the cyclohexane-
coordinated intermediate will yield the chlorobenzene-
substituted intermediate, which then will undergo further
substitution by piperidine. According to figure 3 and
assuming that the concentration of intermediate 3-a is
steady-state, the rate law for the displacement of CB by pip
from cis-C(CB)(P(O-i-Pr))W(COU] is given by equation (3).
-d[ (L) (CB)W(COU]/dt = kob.d[ (L) (CB)W(CO)^.] (3)
kob.d = k-ask-i [pip]/(k^i [pip] + k=[CB] (4)
1/kot .c = (kss[CB]/k-ssk-i Cpip]) + 1/k-a (5)
According to equation (4), plots of the observed rate
constants vs. [pip]/[CB] are anticipated to be curved. The
reciprocal relationship as dictated by equation (5) predicts
a linear plot of 1/kob.c vs. [CB]/[pip] with intercept equal
to 1/k—ss and slope k=/k-ik-=.
12
Pi p.
o c
W - V'V
y \ k-1 pip ^ \ 1 o co
J? hv
c o
o c
w
J& *
31 C H CH.
O c
w
x?
R5CB
-5
o
c \ l ^
co c o
k : 3
b s
Fig. 3--Generation of coordinatively unsaturated cis-[LW(COU] and attack by CH, CB and pip. CH = cyclohexane, CB = chlorobenzene, pip = piperidine.
13
Curved plots of kot>.c« vs. [pipl/ECB], indicative of the
mechanism depicted in figure 3, were observed for the
reaction of cis-[ (pip) (CB)W(CO)**] with pip. The rate
constant for M-CB bond rupture, obtained as 1/intercept from
the reciprocal plot at 20.2 °C, is 1920 sec-1 (<tl). The
competition ratios, k-i/k«, which describe the relative
rates of attack at 3-a by pip and CB, reflect the poor
selectivity of 3-a toward incoming nucleophiles. For
example, at 20.2 °C kr-i/kas is 2.36. Activation parameters
for k» are consistent with an initial dissociation of the
coordinated solvent (&H^= 13.0(4) kcal/mol, 5.6(11)
cal/deg-mol) (41).
Reactions of chelate complexes.-— Chelate complexes may
undergo ligand-substitution reactions involving either
CO-displacement or complete removal of the bidentate ligand
(56. 60-75). In some instances these two reactions take
place competitively. Early works by Angelici and Graham
showed that Cr(CO)«*(dipy) (dipy = 4,4'-dimethyl-2,2'-
dipyridyl) reacts under fairly mild conditions with a
variety of phosphites, L, in organic solvents to yield
cis-Cr(CO)s(dipy)(L) <jL£). This was an unexpected result in
view of the inertness of Cr(CO)& toward CO exchange (jiZ,
58). The labilizing effect by the hard base, dipy, was
proposed to explain the enhancement in reactivity of the
Cr-C bond ( 59. (jO) .
14
Analogous species , W(CO )* (dipy) and Mo (CO) * (dipy) showed a
more complex behavior (jjl). Reaction of M(CO)*(dipy) (M =
W, Mo) with a variety of phosphites yielded three products
(il): M(C0)3(dipy)L, M(CO)*Ls, and M(CO)3La. The rate law
contains two terms, ascribable to a ligand-dependent pathway
and a ligand-independent pathway (figure 4).
The first term, independent of [L]> suggested a
simple dissociation process analogous to the one observed
for Cr(CO)*(dipy). Although activation parameters (AH = 25
kcal/mole, As^ = 12 to -12 cal/deg-mol) are very similar to
those reported for similar reactions of Cr(CO)*(dipy) (M),
a slight dependency of ki on the nature of L was found.
Despite this observation a pathway involving initial
fission of the M-CO bond was proposed (.61). For the ligand-
dependent pathway, an associative mechanism involving a
7-coordinate intermediate was proposed. Large negative
entropies of activation and small positive enthalpies of
activation are consistent with this mechanism. Dobson, et
al. reinvestigated the reactions of Cr(CO)*(dipy) (£2.) and
W(CO)*(dipy) (63) with L, (L = triethyl phosphite) and
proposed an additional competing reaction pathway which
involves fission of the N-M bond to produce a ring-opened
intermediate. According to the mechanism shown in figure 4,
the rate law, assuming that the concentration of the
intermediate 4-a is steady-state, is given by equation (6).
o c
0 c
M
/ l \ c o
fc 1
M
c o
15
S i -o \
/
/ | \ M
C O
O c
k
c o
M
c o
2 I
'M
% C o
Fig. k--Displacement of bidentate ligand, bonded through nitrogen to the central metal, involving a competitive mechanism.
16
dtS3/dt = k'ltS] + k'.CLHS] + (kikatL] [S] )/(kx + ks[L]) (6)
S = (dipy)Cr(COU
At high ligand concentration, equation (6) reduces to
equation (7).
-d[S]/dt = (k'x + kx )CS] + k'a CL3 CS3 (7)
Equation (7) is consistent with the kinetics behavior for
the reactions of Cr(COU(dipy) with phosphites.
Chelate complexes bonded through gulfuy--. Ligand
substitution reactions of chelate complexes bonded through
sulfur atoms proceed through a complete displacement of the
chelate ring (M-71). However, contrasting kinetics
behavior has been found for systems which are closely
related. The reaction of (De-DTH)M(CO, (DTH =
2,5-dithiahexane), (M = Mo, Cr), with a variety of
phosphites was found to follow a second-order rate law (M)-
Under pseudo-first order conditions, plots of the observed
rate constant vs. [L] were found to be linear with zero
intercept. This behavior is consistent with two closely
related mechanisms which have been shown in some instances
to be competitive (figure 5 path a and b).
4 s
w
c o
PATH B k. f a s t
O C
/ / ' y *
/ | \ + &
PATH A
fast
17 o c
w
I?
4- c I o
o c
w
+ + c o
o c
w
c o
f as t
O c ?
/
w l\ c o
Fig. 5--Displacement of bidentate ligand, bonded through sulfur to the central metal, involving a competitive mechanism.
18
One involves an initial rate-determining ring-opening to
afford a coordinatively-unsaturated five membered
intermediate (path A); the other involves an initial
associative or interchange attack at the metal center
(path B). In order to distinguish between these two
mechanisms an extra-kinetics probe has to be used. The
criterion of entropy of activation was employed to
distinguish between the possible mechanisms, since a
dissociative pathway is expected to show a more positive
entropy of activation. Activation parameters for the
reaction of (fF-DTH)Cr(CO)«. with various phosphites, P(OR)s
(R = methyl, ethyl, isopropyl), in 1,2-dichloroethane were
consistent with a dissociative mechanism <M) (average AH =
24.5 kcal/mole, average As^= 3.39 cal/deg-mol), while
activation parameters for the reaction of (fla-DTH)Mo(CO)«.
under similar conditions suggested an associative^or
interchange mechanism being operative (average Ah = 16.2
kcal/mole, average As* = -19.7 cal/deg-mol) (M>- The
reaction of (fF-BTE)Cr(COU (££) and (ne-BMTB)Cr(COU (46)
(BTE = cis-bis(t-butylthio)ethylene, BMTB = 4-methyl-l,2-
bis(methylthio)benzene), with L = P(OR)a (R = ethyl,
isopropyl) in chlorobenzene and 1,2-dichloroethane showed
similar behavior to that of (De-DTH) Cx*(CO)** in the sense
that linear plots of the observed rate constants vs. EL]
were observed.
19
However, slightly negative entropies of activation were
observed; for (na-BTE)Cr(COU, the average As^(~3-2 cal/deg-
mol); for (fF-BMTB)Cr(CO)*., the average As^(-8.20). This
discrepancy prompted the kinetics studies of the reaction of
(fF-BMTB) Cr(CO)«• with triethyl phosphite over a wide range
of temperatures. This experiment was performed to probe the
possibility of a competing mechanism, since deviations from
linearity for Eyring plots of ln(k/T) vs. 1/T (T = absolute
temperature) are anticipated if a competitive mechanism was
operative. A linear Eyring plot was obtained over an 80
degree temperature range, which suggests a single pathway.
Despite the observed negative entropy of activation a
dissociative mechanism was suggested. The basis for this
interpretation rests on the fact that the observed As^from
the composite rate constant (path a in figure 5), kike/k—x,
is given by As^ = Asi + Ast - Asl*. The rigid backbones of
(n®-BTE)Cr(COW and (fl®-BMTB)Cr(CO)-=» causes few degrees of
freedom to be gained upon ring-opening. Thus, making As»t
small and positive. In addition, upon ring-closure, few
degrees of freedom are lost making A s ! small and negative.
A highly negative ASe is expected for two colliding bodies.
Thus, the summation of all the individual _S might result
in a negative value. Further evidence supporting a
dissociative mechanism comes from volumes of activation
studies of the reaction of (He-BTE)Cr(CO ) . with L (jiZ).
20
The reactions of (fF-DTO)W(COU ( M ) and (n«-DTN)W(C0U (££)
(DTO = 2,2,7,7-tetramethy1-3,6-dithiaoctane, DTN = 2,2,8,8-
tetramethyl-3,7-dithianonane) with a variety of phosphites
have been extensively studied. (fF-DTOJWfCOK undergoes
substitution reactions involving a rate-determining ring-
opening to afford a solvated five-coordinate intermediate
(intermediate 5-a), which then undergoes a competitive ring-
closure and attack by an incoming ligand (figure 5). Plots
of kobad vs. [L] with all ligands employed were curved.
Linear reciprocal plots of 1/kot—c. vs. 1/tL], indicative of
a complex behavior, have common intercepts for all ligands
studied. According to figure 5 the rate law for the
disappearance of the substrate, assuming that Jo-e << ke and
that the concentration of the intermediate 5-a is steady-
state , is given by equation (8).
-dtS]/dt = ((ktke[L])/(k_t + ketL])) [S] (8)
Analogous reactions of ( rF-DTN)W(COU also exhibited
curvature for plots of kob.«. vs. [L] (£2). The reciprocal
plots, although linear, did not show a common intercept for
different ligands as predicted by equation (8). Common
intercepts are anticipated since L is not involved in the
ring-opening step which is governed by k*. An alternative
mechanism was proposed which involves a competitive
rate-determining ring-opening and an initial attack at the
metal center by L (figure 5).
21
According to figure 5 and employing the assumption of the
steady-state concentration of intermediates (5-a) and
(5-b), the rate law for the reaction of (fF-DTN)W(CO)«• with
L is given by equation (9).
-d[S]/dt = kot»d[S] (9)
S = (fF-DTN)W(CO)^
ko b « c
k* (kike + k-ik-a ) tL] + kak.2[LJe
(k-i (k~a + k'alu ) ) + ke (k'a + k*. ) [L]
Assuming that ki >> k'a
ke(k's + k*) k—i(k—a + k-3 + k^)
(10)
(11) kob.et k^.(kik-a + k-ik'e) k^(kike + k-ik'e) [L]
Assuming that k^i >> ke this rate law will assume the same
form as the one observed for (n*-DT0)W(C0U and thus curved
plots of kct>m« vs. tL] are expected. The intercept for the
reciprocal plot is given by equation (11). Since ke, k'a,
k'a, and k*. are ligand-dependent terms and are included in
the intercept of the reciprocal plot, non-common intercepts
for the reciprocal plot are anticipated. It is rather
intriguing that molecules which are closely-related in
structure will show such dramatically different kinetics
behavior. This fact prompted crystal and molecular
structure studies of both (ne-DT0)W(C0U and (ne-DTN)W(C0U
(7(2) .
22
Both systems were found to contain a distorted octahedral
W( CO )«»Se fragment but are quite similar in their structural
parameters.
The problem
As mentioned above, very small but statistically-
significant differences in structural parameters may account
for the dramatic differences in reactivity and reaction
pathway by which (fP-DTO)W(CO)^. and (na-DTN)W(CO)^ undergo
ligand exchange reactions. The kinetics and mechanisms
studies for the reactions of (ns-DTHp)W{CO)«.,
(fF-DTD)W(COU, <fle-DTU)W(COU, (DTHp = 2,2,6, 6-tetramethyl-
3,5-dithiaheptane; DTD = 2,2,9,9-tetramethyl-3,8-dithia-
decane; DTU = 2,2,10,10-tetramethyl-3,9-dithiaundecane) with
phosphites and phosphines could provide a better
understanding about how the molecular structure of these
sulfur chelate complexes may influence the reactivity and
the reaction mechanisms by which the ligand-exchange
reactions take place. These complexes provide an
opportunity to probe this problem since the only difference
between the complexes in the series is the size of the
chelate ring.
CHAPTER BIBLIOGRAPHY
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35. Graham, J. R. ; Angelici, R. J. Inorg. Chem., 1967, 6, 2082 .
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26
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62. Memering, M. N.; Dobson, G. R. Inorg. Chem.. 1973, 12, 2490.
63. McKerley, B. J.; Faber, G. C.; Dobson, G. R. Inorg. Chem., 1975, 14, 2275.
64. Faber, G. C.; Dobson, G. R. Inorg. Chem.. 1968, 7, 584.
65. Halverson, D. E.; Reisner, G. M.; Dobson, G. R.; Bernal, I.; Mulcahy, T. L. Inorg. Chem.. 1982, 21, 4285.
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68. Schultz, L. D. ; Dobson, G. R. jJ. Organomet. Chem. . 1977, 124, 19.
27
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CHAPTER II
EXPERIMENTAL
General
Infrared spectra were obtained employing a Nicolet
20 SXB Fourier transform infrared spectrometer and where
indicated a Perkin-Elmer model 621 grating spectrometer.
Unless indicated, NMR spectra were obtained using a JEOL
FX90Q Fourier transform NMR spectrometer. Reaction rates
were monitored following a decrease in absorbance at 415 nm,
using either a direct reading Beckman DU-2 spectrophoto-
meter, or an in-house built optical rail spectrophotometer.
This in-house built spectrophotometer will be described in a
later subsection. A Haake D8 temperature controlled water
circulator and a refrigerated and heated Forma-Temp Jr model
2095 bath and circulator were employed as temperature
control devices. Elemental analyses were performed by
Midwest Microlab, Indianapolis, IN.
28
29
pirrification of gQlvgPtg
Chlorobenzene (CB; Fisher), and 1,2-dichloroethane (DCE;
Fisher) were stirred and refluxed in the presence of PeOs
under nitrogen for at least twenty-four hours before they
were fractionally distilled. Bromobenzene (BB; Matheson,
Coleman and Bell) was stirred for three hours over anhydrous
MgSO » and then refluxed for four hours over PeOas. It was
then fractionally distilled under nitrogen.
pirrification of lisands
In a typical experiment, the ligands (L) were stirred in
the presence of sodium metal under nitrogen for at least
twenty-four hours, followed by a fractional or a vacuum
distillation.
Constrained phosphite.— The constrained phosphite (CP),
4-methyl-2,6,7-trioxa-l-phosphabicyclo-[2.2.2]octane,
prepared according to the published method (i, j2.), was
twice sublimed under reduced pressure into a wide-mouthed
condenser, and was then recrystallized under nitrogen from
hot n-hexane.
Tri(isopropvl) phosphite.-- To avoid air and moisture upon
breaking the vacuum, tri(isopropyl) phosphite (Aldrich) was
distilled at reduced pressure from sodium metal and under a
bleed of nitrogen, (b 60 °C at 11 torr).
30
Trvimethvl phosphite.— Trimethyl phosphite (Aldrich) was
fractionally distilled from sodium metal under nitrogen
(b 111 °C).
TV\phenyl phosphite.-- Triphenyl phosphite (Aldrich) was
distilled at reduced pressure from sodium metal under a
bleed of nitrogen (b 190 °C at 11 torr).
Tri(n-but.vl) phosphine.-- Tri(n-butyl) phosphine was
distilled at reduced pressure from sodium metal, under a
bleed of nitrogen (b 80 °C at 11 torr).
Syntheses of bidentate liRapds
DTA = DTHp. DTD. DTU.-- DTA were prepared following a
similar method reported by Dobson and others (1, it). In a
typical experiment, 23 g (1 mol) of sodium metal was
allowed to react with 500 mL of ethyl alcohol. After all
the sodium was consumed, 90 g (1 mole) of t-butyl mercaptan
(Aldrich) was slowly added and after addition was completed,
the reaction mixture was stirred for two hours. A pale
yellow solution resulted. Then, 0.5 mol of the appropriate
dibromoalkane (Aldrich) was slowly added under cooling
(dibromomethane for DTHp, 1,4-dibromobutane for DTD, 1,5-
dibromopentane for DTU).
31
Upon addition of the dibromoalkane a milky suspension was
obtained due to the formation of sodium bromide. The
reaction mixture was stirred for several hours, then the
sodium bromide was removed by suction filtration. Ethyl
alcohol was removed from the reaction mixture by
distillation and the reaction product was vacuum distilled.
DTHP.-- DTHp was prepared according to the method described
above. Upon distillation at reduced pressure (1 torr), a
colorless liquid was obtained (b 52-53 °C). The purified
yield was 58%. *H NMR (CDCla) 5 1.28 (18 H, s), 3.62 (2 H,
s); 13C NMR (CDCla) 6 27.83 (t), 30.81 (q), 43.39 (s).
DTD.-- DTD was prepared according to the method described
above. Upon distillation at reduced pressure (1 torr), a
colorless liquid was obtained (b 119-20 °C). The purified
yield was 42%. *H NMR (CDCla) 6 1.24 (18 H, s), 1.67 (4 H,
m), 2.53 (4 H), (t, J = 9 Hz); 13C NMR (CDCla) 6 27.540 (t),
28.970 (t), 30.758 (q), 41.425 (s).
DTU.-- DTU was prepared according to the method described
above. Upon distillation at reduced pressure (1 torr), a
colorless liquid was obtained (b 134-35 °C). The purified
yield was 38%. *H NMR (CDCla) 5 1.251 (18 H, s), 1.50 (6
H, m), 2.45 (4 H), (t, J = 9 Hz); l3C NMR (CDCla) 6 27.778
(t), 28.434 (t), 29.208 (t), 30.70 (q), 41.246 (s).
32
Syntheses of metal complexes
(ng-DTHpiWCCO)^.— Two g of DTHp were added to a solution
of 3 g of W(CO)<s, (Pressure Chemical) in 350 mL of hexanes
(Fisher). The reaction mixture was irradiated using a
Hanovia 450 watt immersion UV lamp. The reaction was
monitored by infrared spectroscopy by observing the growth
of a band at 2017 cm-1. A yellow solid precipitated as the
reaction proceeded. The crude product was recrystallized
from toluene-hexanes. Bright yellow crystals were obtained.
The yield was 20%. The carbonyl stretching spectrum is
shown in figure 6. Anal. Calcd for CiaHsoO^SsW: C, 31.98;
H, 4.13. Found: C, 32.01; H, 4.30.
Cis-(fl1 -DTHP) (CP?W(COU.-- In a 100 mL volumetric flask, 0.3
g (0.61 mmol) of (CF-DTHpJWtCOU were allowed to react with
0.15 g (one mmol) of CP in 1,2-dichloroethane at room
temperature. The reaction was monitored by observing the
relative intensities of the bands at 2017 cm-1 and
2031 cm-1, corresponding to (fle-DTHp)W(CO)^ and
(0*-DTHp)(CP)W(CO)^, respectively. When the intensity of
the band at 2031 cnr1 no longer increased, the reaction
mixture was quenched by immersing the flask in an ice-water
bath. The solvent was evaporated under vacuum and a pale
yellow solid precipitated, which was then recrystallized
from toluene-hexanes. The yield was 53.4%.
33
a
2 0 6 7 2 0 0 0 1933 1866 WAVENUM8ER
1 79S
*>--Carbonyl stretching spectrxim of <2ia-<n«-DTHp)W(C0U in chlorobenzene.
34
The carbonyl stretching spectrum is shown in figure 7.
Anal. Calcd for CisHs^O^SePW: C, 33.97; H, 4.60. Found:
C, 33.90; H, 4.35.
(nB-nTD)W(COU.-- (fle-DTD)W(C0)4 was prepared in a manner
analogous to the one used for (DE!-DTHp)W( CO) *•. The reaction
progress was followed by observing the growth of a band at
2013 cm-1. A yellow solid precipitated as the reaction
proceeded. The crude product was recrystallized from
toluene-hexanes and bright yellow crystals were obtained.
The yield was 25.2%. The carbonyl stretching spectrum is
shown in figure 8. Anal. Calcd for Ci«,He«»04.SeW: C, 36.23;
H, 4.94. Found; C, 36.01; H, 4.99.
Cis-(n*-DTD) (CP)W(COU.-- Cis-(fl1 -DTD) (CP)<CO)<• was
prepared following a method similar to the one used
for cis- (fl'-DTHp) (CP)W(CO)*.. The progress of the reaction
was monitored by observing the disappearance of the band at
2013 cm-1 and the growth of the band at 2028 cm-1. After
completion of the reaction, the reaction mixture was
quenched by immersing the reaction flask in an ice-water
bath. The solvent was then evaporated by bubbling a gentle
stream of nitrogen through the solution. A yellow solid was
obtained. Recrystallization from toluene-hexanes yielded
pale yellow crystals. The yield was 43%. The carbonyl
stretching spectrum is shown in figure 9.
35
(O o <Ti
2077 2015 1953 1891 WAVENUMBER
8 2 ?
Fig. 7--carbonyl stretching spectrum of cis- (n'-DTHp) (CP)W(CO)*. in chlorobenzene.
36
m OD CO
2 0 1 5 1 9 5 3 1 8 9 1
W A V E N U M B E R
1 8 2 9
, ~ ® * ® " ~ C a r b o n y l s t r e t c h i n g s p e c t r u m o f £ i S - < f l s - D T D ) W ( C O ) » i n c h l o r o b e n z e n e .
37
CD O c n
2 0 1 5 1953 1891 WAVENUMBER
1829
^ o f
38
Anal. Calcd for C«iH3*<>*SePW: C, 37.18; H, 5.20. Found:
C, 37.06; H, 5.20.
fne-rmnw(COU.-- (ns-DTU)W(C0U was prepared in a manner
analogous to the one used for (n®-DTD)W(C0)^ and
(fie-DTHp)W(CO)<». The yield was 22%. The carbonyl
stretching spectrum is shown in figure 10. Anal. Calcd for
Ci-rHsoO^SeW: C, 37.51; H, 5.18. Found: C, 37.52; H, 5.13.
Hia-tn^-nTTTWCPiWCCQ)^..-- Cis-(n1-DTU) (CP)W(CO)^ was
prepared in chlorobenzene following a method similar to the
one used for cis-Cfl'-DTHp) (CP)W(COU and cis-Cf^-DTD)
(CP)W(CO)*+. However, this intermediate was not isolated.
It was characterized in situ by infrared spectroscopy. The
carbonyl stretching spectrum is shown in figure 11.
Trfantification of intermediates
The carbonyl stretching spectra of the intermediates
[cis-{n*-DTHp) (L)W(COU] (L = tri(n-butyl) phosphine,
4-methyl-2,6,7-trioxa-1-phosphabicyclo-[2.2.2]octane (CP),
tri(isopropyl) phosphite, trimethyl phosphite, and
triphenyl phosphite) were recorded in situ employing a
Perkin-Elmer 621 grating infrared spectrometer. The
carbonyl stretching frequencies are given in table I.
39
tn m
&
2 0 7 7 2 0 1 5 1 9 5 3 1 8 3 ! i 8 2 9
W A V E N U M B E R
£ i a - ( f t®-DTU?t f^C0^U°in 1 chlorobenzenef e C ^* r U m ° f
40
to o 0)
2 0 7 7 2 0 1 5 1 9 5 3 1 8 9 1
w a V £ N U M B E R
1 8 2 9
c i s - r n ^ l w l T w ™ ^ ™ ? 1 s t r e t c h i n « s p e c t r u m o f H2SL T U ) ( C P ) W ( C O ) ^ x n c h l o r o b e n z e n e .
41
TABLE I
CARBONYL STRETCHING FREQUENCIES OF CIS- (fl1 -DTHp) (L)W(CO)*. IN CHLOROBENZENE
(CO) cnr1
P(n-Bu)s 2008 (w) 1901 (sh) 1980 (s) 1869 (m)
CP 2031 (w) 1925 (sh) 1907 (s) 1883 (m)
P(0-i-Pr)3 2025 (w) 1919 (sh) 1902 (s) 1886 (m)
P(0Me)a 2025 (w) 1917 (sh) 1900 (s) 1872 <m)
PtOCeHsJa 2030 (w) 1927 (sh) 1910 (s) 1886 (m)
THtan-hi fication of reaction products
Reaction products were identified by comparing the
carbonyl stretching spectra of reaction solutions to the
spectra reported for those complexes (JL, &, !)• The
carbonyl stretching spectra for cis-(L)eW(COU, (L =
P(n-Bu)s, CP, tri(isopropyl) phosphite, trimethyl
phosphite, and triphenyl phosphite), are given in figures
12, 13, 14, 15, and 16, respectively (1).
42
M O r-09
2 0 3 3 1977 1921 WAVENUMBER
1865 1809
Fig. 12--Carbonyl stretching spectrum of cis- and trans- (P(n-Bu)a )eW(C0)«» in chlorobenzene.
43
(0 r*. CO N 0)
2 0 8 9 2 0 3 3 1977 1921 1865 WAVENUMBER
eis - (cpf * vfrni C a f b o n^ | stretching spectrum of £A§. v CF )eW( CO )i» m chlorobenzene.
44
o 0 01
2 0 3 3 1977 1921 WflVENUMBER
1865 1809
Fig. 14--Carbonyl stretching spectrum of cis-and trans-(P(O-i-Pr)a)eW(CO)^ in chlorobenzene.
45
O) o O)
2089 2033 1977 1921 WAVENUMBER
1865
Fig. 15--Carbonyl stretching spectrum of cis and trans -(P(OMe)a)eW(CO )*+ in chlorobenzene.
46
CNJ <n <D
2089 2033 1977 1921 WAVENUMBER
1865 1809
CIS Pig. 16--Carbonyl stretching spectrum of
{PfOC ffa )a )rW(C0)<. in chlorobenzene.
47
Recrvstallizatior} of metal complexes
( n e - D T A ) W ( C Q ) ^ . - - All the < n e - D T A ) W ( C O U complexes were
recrystallized from a toluene-hexanes solution. In a
typical experiment, 3g of (fle-DTA)W(COU were dissolved in
25 mL of hot toluene. The solution was then filtered
through celite, and hexanes were added until crystallization
commenced. The solution was shaken and then placed in the
refrigsrator. After 24 hours, yellow crystals were
obtained. After filtration, these crystals were rinsed with
five portions of 10 mL of hot hexanes and dried under vacuum
for 6 hours.
gig- (I11-PTA) (CP)W(CO)^.— All the cis-(n*-DTA) (CP)W(CO)*.
were recrystallized from a toluene-hexanes solution. In a
typical experiment, 0.2 g of cis-(fl1-DTA) (CP)W(COU were
dissolved in 10 mL of toluene. The solution was then
filtered through celite, and hexanes were added until
recrystallization commenced. The solution was shaken and
then placed in the refrigerator. After 24 hours, a pale
yellow solid was obtained. After filtration under a stream
of nitrogen, the product was rinsed with five portions of
hot hexanes and dried under vacuum for 6 hours.
48
Kinetics runs
Fast reactions were monitored employing an in-house
built optical rail spectrophotometer. This spectrophoto-
meter employed a 40-W tungsten lamp, powered by a
Hewlett-Packard (Harrison 6274 A) DC power supply, a Bausch
& Lomb (33-86-20) monochromator, a side-on photomultiplier
tube (Hamamatsu R 136), and an Aminco linear-log-photo
meter, powered by an Aminco dual power supply. The
spectrophotometer's output was digitized employing a Nicolet
2090 Ila oscilloscope.
In a typical experiment, approximately 1.5 mL of a
ligand-solvent solution was placed in a thermostated cell.
Then approximately 2 mg of substrate were dissolved in the
thermally-equilibrated solution. The oscilloscope was then
triggered, and the absorbance of the reaction solution was
measured as a function of time. For slower reactions, a 25
mL volumetric flask containing 25 mL of solvent-ligand
solution was placed in a thermostated bath in which the
temperature was the same as that of the thermostated cell.
Approximately 5 mL of the solvent-ligand solution was
rapidly added to 5 mg of the substrate. The resulting
yellow solution was then quickly placed in the thermostated
cell, and when thermal equilibrium was reestablished
(approximately 10 seconds), the absorbance as a function of
time was recorded.
49
During the early stages of this investigation, the progress
of the slow reactions was monitored by withdrawing samples
and by measuring the absorbance at 415 nm employing a
Beckman DU-2 direct reading spectrophotometer. In a 50 mL
volumetric flask, the solvent was added to a weighed amount
of ligand, which together occupied a volume of approximately
47 mL. The flask was then placed in the thermostated bath
until the solution was thermally equilibrated. Additional
solvent was then added up to the calibration mark.
Approximately 30 mg of the substrate were placed in a 100 mL
volumetric flask equipped with a stopcock and rubber septum.
The solution was added to the substrate and a gentle stream
of nitrogen was bubbled through the solution. The. reaction
mixture was then placed in the thermostated bath and allowed
to equilibrate. Sampling was then begun by withdrawing
samples of 3 mL each and by measuring the absorbance at 415
nm. Three mL of nitrogen were injected into the reaction
flask before removal of the sample. This insured a positive
pressure within the reaction vessel.
Pulsed-laser flash photolysis studies were carried
out using the facilities of the Center for Fast Kinetics
Research (CFKR), University of Texas at Austin (£., 10 ). A
schematic diagram of the equipment employed is shown in
figure 17.
50
LASER (Nd: YAG)
XENON LAMP
• A
- X M
TRIGGER SEQUENCE
GENERATOR
MC
P P
POP 11/70 COMPUTER
BIOMATION 8100
WAVEFORM DIGITIZER
P; P h o t o m u l t i o l i S ? ' k A ' ® t t e " u a t o r : MC, monoch roma to r ; ' D ' P h ° t 0 d i 0 , l e - o n i t c r ; M,
51
A Quantel Q-switched Nd:YAG laser (355 nm irradiating
wavelength, 11 ns FWHI) was used as a source of high
intensity radiation.
In a typical experiment, a solution containing the
substrate, the ligand, and the solvent were placed in a
jacketed observation cell. The temperature of this cell was
controlled by an external circulating bath (Forma Jr. model
2095) and monitored by a Keithley 872 digital thermometer
(FeCuNi thermocouple). Concentrations of the substrate were
in the vicinity of 5 X lO"* M. After the flash, the decay
of the photogenerated transient was followed by a
conventional optical rail spectrophotometer which was
controlled by a pulse generator. The photolysis cell had a
monitoring path of 1.0 cm. Transient absorptions were
monitored at a right angle to the photolyzing beam with an
Oriel xenon lamp (150-W). A Bausch & Lomb monochromator and
a Hamamatsu Corp. 928 photomultiplier were employed as
detection devices. The output wave forms from the
photomultiplier were fed into a Biomation 8100 waveform
digitizer.
During all the experiments the sample and the
detector were protected from the intense monitoring beam by
shutters which were controlled by the programmable sequence
generator.
52
Pseudo-first order rate constants were evaluated
employing an interactive linearized least-square analysis
program (DEC PDP-11/90 computer) and each numerical value is
the average of five or more kinetics runs employing the same
sample.
CHAPTER BIBLIOGRAPHY
1. Wadsworth, W. S.t Jr.; Emmons, W. C. iv Am- £hsm> Soc.,
1962, 84, 610.
2. Verkade, J. G. Inors. Ghem-, 1962, 1, 392.
3. Dobson, G. R.; Faber, G. C. I nor a. Chim. Acta, 1970, 4, 87 .
4. Federov, B. P. ; Savel' eva, I. S. Igvest . Ak^d• » SSSR. 1950, 223.
5. Asali, K. J.; Basson, S. S.; Tucker, J. S.; Hester, B. C.; Cortes, J. E.; Awad, H. H. ; Dobson, G. R. J.. Am. Chem. Soc.. 1987, 109, 5386.
6. Dixon, D. T. ; Kola, J. C.;Howell, J. A. S. jl. Chgffl. Soc. Dal ton Trans.., 1984, 1307.
7. Vandenbroucke, A. C.; Hendricker, D. G.; McCarley, R. E.; Verkade, J. G. Inors. Sbfiffl-. 1968, 1, 1825.
8. The experimental assistance by Mr. David Dumond is gratefully acknowledged.
9. Lindig, B. A.; Rodgers, M. A. J. «I. Phys. Chem. , 1979, 83, 1683.
10. The experimental assistance by the staff of the CFKR is gratefully acknowledged.
53
CHAPTER III
REACTIONS OF (fF-DTA)W(COU
General
The thermal reactions of (na-DTA)W(CO)«., (DTA
2,2,6,6-tetramethyl- 3,5-dithiaheptane, (DTHp), 2,2,9,9
tetramethy1-3,8-dithiadecane, (DTD); 2,2,10,10-tetramethyl-3,9-
dithiaundecane, (DTU)), with L = phosphites and phosphines take
place according to equation (12).
(n*-DTA)W(CO)<. + L > (L)eW(COU + DTA (12)
cis-(n1-DTA) (L)W(CO)*. -> cis-(L)eW(COU + DTA
Plots of In(A* - A») vs. time, in which At and A- are the
absorbances of the reaction solution at a given time (t) and the
absorbances at over ten half-lives, respectively, consisted of
two linear segments. Figure 18 illustrates this plot for the
reaction of (n*-DTHp)W(COU with tri(isopropyl) phosphite (0.2008
M) in chlorobenzene (CB) at 21.1 °C.
During the course of reaction (12), appreciable
formation of cis-(fl1 -DTA) (L)W(COU, (governed by k„*«c), <L =
phosphites, and phosphines) was observed. These intermediates
were characterized when L = CP.
54
55
10 ^
9
8
7
6 -
5 -
4 -<
3 -
1 - i 2
Fig. 18--Plot of (A* - A-.) vs. time for the reaction of (n®-DTHp)W(C0)^ with tri(isopropyl> phosphite in chlorobenzene at 21.1 °C. [L] =0.2008 M. Ordinate = (A* - A-), Abscissa = time X 10—* sec.
56
Further reaction of cis-(fl*-DTA) (L)W(CO)«. with L, governed by
k'ob.d afford the disubstituted product cis-(L)eW(CO)^.
Both the rates at which cis-(fl1 -DTA) (L)W(CO)^ is produced, and at
which it will undergo subsequent reaction to produce
cis-(L)eW(CO)^, are within the same time-scale. Thus these
reactions exhibit biphasic behavior. A discussion of the
reactions of (fle-DTD)W(CO)^. and (ne-DTU)W(CO)*. with L to produce
cis-lfP-DTA)(L)W(CO)^ will be deferred until next chapter.
Reactions of (fF-DTHp)W(CO)^
Formation of Cis-(fl*-DTHp) (L)W(COU
The reactions of (f1e-DTHp)W(C0)^. with L (L =
phosphites, and phosphines) in chlorobenzene (CB) to produce
cis-(fU-DTHp) (L)W(CO)^ were studied under pseudo-first order
conditions. The concentrations of L were at least a twenty-fold
excess relative to that of the substrate. Pseudo-first order
rate constants are given in table II. Plots of the observed rate
constants vs. [L] are linear over a wide range of concentrations.
These plots are illustrated in figure 19 and 20 for L =
tri(isopropyl) phosphite and trimethyl phosphite, respectively.
This behavior is consistent with two previously proposed
mechanisms which are shown in figure 21 (J.-5.).
57
TABLE II
FIRST-ORDER RATE CONSTANTS FOR THE REACTIONS OF ( f F_ D T Hp)W<COU WITH PHOSPHITES IN
CHLOROBENZENE AT VARIOUS TEMPERATURES
ligand O O [L]
(M) 10e kob.d (sec-1)
P(0-i-Pr)a 44.5
0.9230 0.6922 0.6571 0.5455 0.3310 0.2493 0.1316 0.05378
7.19(17) 5.73(6) 5.41(21) 4.89(4) 2.39(1) 2.44(3) 1.28(3) 0.575(11)
35.2 1.1547 0.9315 0.7536 0.6132 0.4769 0.3231 0.1424
4.16(3) 3.45(2) 2.74(2) 2.342(7) 1.556(6) 1.123(5) 0.555(5)
31.1 0.9131 0.6488 0.3984 0.3084 0.1331 0.1021 0.09935 0.07905 0.07321 0.06392 0.05734 0.02837
2.45(3) 1.63(1) 0.996(10) 0.701(7) 0.330(4) 0.278(2) 0.292(3) 0.1815(6) 0.182(1) 0.158(1) 0.159(1) 0.0909(4)
21.1 0.9730 0.7727 0.4848 0.3487 0.3010 0.2674 0.2008 0.1675
1.037(8) 0.840(9) 0.541(6) 0.380(2) 0.317(3) 0.262(2) 0.212(1) 0.164(1)
TABLE II CONTINUED
58
T, [L] (M)
1 0 S kob«d (sec-1)
P(OMe)a 21.1
0.1263 0.125(1) 0.07528 0.0731(6) 0.05462 0.0529(6) 0.04800 0.0449(6)
0.9831 2.72(2) 0.5970 1.64(2) 0.4474 1.23(1) 0.3519 1.04(1) 0.3140 0.841(5) 0.1565 0.463(3)
One involves an associative, or perhaps a dissociative
interchange process (path B), to form a seven-coordinate
transition state or intermediate. The other (path A) involves
rate-determining ring-opening to yield an unsaturated five-
coordinate intermediate (21-a). This intermediate then
undergoes rapid solvation and desolvation governed by ka and
3c-a, respectively, and either ring-closure or competitive attack
by L at the vacant site.
Given the assumption that the concentration of the
intermediates 21-a and 21-b is steady-state, the rate law for
the dissociative pathway is given by equation (14).
59
k T " " [ L I f o r the reactions of in" DTHp)W(CO)-, with tri(lsopropyl) phosphite in cniorobenzene at various temperatures. Ordinate = koo.d X 10E sec-1, Abscissa = [L] X 10 M.
60
2 8
2 6
2 4
2 2 -
2 0
t 8 -
1 6
1 4
1 2 -
1 0 -
Z -
1 0
Fig. 20--Plots of kob.d \£s. [L] for the reactions of (fF-DTHp)W(CO)with L, L = tri( isopropyl) phosphite, trimethyl phosphite, in chlorobenzene at 21.1 °C. Ordinate = ko*..* X 10a sec"1, Abscissa * [L] X 10 M.
s.
0 c
w.
/
- / N S
c - 0
PATH A • *2
r 0 l ' r » * / yCo
\l','
Is*
PATH B
k1
M
CBV
0 C
.W
c 0
cP
'O
b
*3 C 8
- 3
0 C r O
w '
/ \ c 0
2 L
Lv
0 C
\ I • w
S ^ l \ c °o 0
61
Fig. 21--Competitive mechanism for the displacement of one end of DTHp from (fle-DTHp)W{CO)«. by L involving initial rate-determining ring-opening; bimolecular attack by L.
62
-d[S]/dt = ktketLHSl/Oc-i + ka[L]) (14)
Where S = (fF-DTHp)W(COU
kobsd = kike [L]/(k-i + k*[L]) (15)
When k-i >> ke
kob«d kike [L]/k-i
Three cases can be envisioned for this mechanism
(figure 21). The first case involves a competitive ring-
closure and a bimolecular ligand attack at the vacant site,
i.e., k-i ~ ke. The second case involves a rate of ring-
closure which is much faster than the one for the rate of the
bimolecular ligand-attack at the vacant site, i.e, k-i >> ks.
The third case is operative when k-i << ka. The rate law for
the second case is indistinguishable from the rate law for an
associative mechanism.
Entropies of activation have been used as criteria to
distinguish between the two mechanisms just described, because a
dissociative mechanism could show a more positive entropy of
activation (£, 2). The second order rate constants for the
reactions of (fF-DTHp)W(COU with phosphites in CB are given in
table III. The Eyring plot of ln(k/T) vs. 1/T (T = absolute
temperature) for L= tri(isopropyl) phosphite is depicted in
figure 22. An enthalpy of activation of 15.00(4) kcal/mol is
much lower than the enthalpies of activation observed in closely
related systems {5, 8.-JJ2.).
63
•8 -
-9 -
-10
•11 -
1 3.0 3.1 3 2
1 3.3
—r-3 .4
Fig. 22--Eyring plot of ln(k«/T) vg. l/T for the reactions of (f^-DTHpJWtCO)*. with tri( isopropyl) phosphite in chlorobenzene at various temperatures. (Ordinate = ln(ke/T)f Abscissa = (l/T) X 10
3 K~l).
64
This finding should not be surprising, since the
rigidity that the four-membered ring may experience will be
released upon ring-opening. However, a highly negative entropy
of activation is inconsistent with an initial ring-opening.
TABLE III
RATE CONSTANTS AND ACTIVATION PARAMETERS FOR THE REACTIONS OF (fF-DTHp )W( CO )<• WITH PHOSPHITES IN
CHLOROBENZENE AT VARIOUS TEMPERATURES
Ligand T, °C k 10®, M_1. sec-1
P(0-i-Pr)a 44.5 7.8(3) P(0-i-Pr)a 35.2 3.67(11) 31.1 2.61(5) 21.1 1.09(1)
P(MeO)a 21.1 2.73(5)
activation parameters for L = tri(isopropyl) phosphite A S1 = -16.1(1) cal/deg-mol. A H*= 15.00(4) kcal/mole
One can envision a situation in which a negative
entropy of activation will be observed for a dissociative
mechanism, provided there is a competition for the intermediate
21-a between ring-closure and bimolecular ligand attack. For
example, in the limiting case for the dissociative mechanism,
depicted in figure 21, in which k -i >> ks, the observed rate
constant will be given by kiks/k—i (e<juation (15)). The entropy
of activation for such a process will be given by As^ = As^i +
AsJ - A S L .
65
Given a highly constrained four-membered ring, it is not
unreasonable to expect a small and positive entropy of
activation for the ring-opening step, since few degrees of
freedom are gained upon ring-opening. Therefore, few degrees of
freedom are lost upon ring-closure; thus, the observed entropy of
activation for this elementary step should be small and negative.
Hence, the overall entropy of activation is expected to be
negative.
One can see that the nature of the mechanism for the
ring-opening of (rF-DTHp)W(COU cannot be assessed employing the
criterion of the entropy of activation alone. Therefore, a
competitive mechanism cannot be ruled out. This fact prompted
the ring-closure studies of cis-1 (01 -DTHp) (BB)W(CO)^.], and
cis-[ (fl1-DTHp) (DCE)W(CO)*. (BB = bromobenzene and DCE =
1,2-dichloroethane). Both species could be generated via pulsed
laser flash photolysis of (ne-DTHp)W(COU in BB and DCE,
respectively. It is known that upon the flash a coordinatively-
unsaturated intermediate, illustrated in figure 21 (intermediate
21-a), is produced from analogous species (rF-DTA)W(CO )*•, (DTA =
DTO, and DTN) (1&-JL2) • In this case, this species is attacked by
the solvents BB and DCE with rates close to those of diffusion
control C13-18). Since the attack by the solvent at the initial
photoproduct is very fast, the species that is being monitored is
the solvated transient.
66
fl-jpg-nlnsure of cis~ [ (0* -DTHp ) (S )W( CO )^1
The photochemical generation of cis-[(D1-DTHp)(S)
W(CO)4], S = solvent, has been described in chapter II.
According to figure 23, this transient may undergo ring-closure
through a concerted expulsion of the solvent, governed by k'-i ,
or through unimolecular solvent-dissociation followed by ring-
closure. The rate constants for the ring-closure of
cis- [ (H* -DTHp) (BB)W(CO)<»] and cis- [ (H^-DTHp) (DCE)W(CO)*.] are
given in table IV. Plots of ln(k'-i/T) vs. 1/T are given in
figure 24, from which activation parameters have been obtained.
The activation parameters for the ring-closure of cis-nfl1-
DTHp)(BB)W(COU] over eight temperatures suggest that
bromobenzene is either weakly bonded to tungsten (&H = 6.4(3)
kcal/mole) or the W-BB bond fission is assisted by the free end
of the chelate ligand upon ring-closure (figure 23). ¥
Furthermore, a highly negative entropy of activation (&S =
-11.2(8) cal/deg-mol suggests that there is substantial
bond-formation in the transition state leading to the
ring-closed product. •
+ g
A, ,G o w
^ l \ c c o
o
0 c
w
• o
- 1
CB.
0 c
v / w
c o
r O I co C 0
67
0 c
w
s>
/ \ c 0
co
F i g • 2 3 - - R i n g - c l o s u r e o f £ i s ~ [ {f)1 -DTHp) (CB)W(COU] i n v o l v i n g a b i m o l e c u l a r d i s p l a c e m e n t o f CB. (CB = c h l o r o b e n z e n e ) .
68
TABLE IV
RATE CONSTANTS FOR THE RING-CLOSURE OF CIS- t (fl1 -DTHp) ( S )W( CO U ] IN BROMOBENZENE AND CHLOROBENZENE AT VARIOUS TEMPERATURES
S T, °C k-i10~s
(sec-1) (kcal/mole) Asii (cal/deg-mol)
BB
DCE
4 5 . 5 8 . 8 ( 3 ) 4 5 . 5 8 . 2 ( 3 ) 3 9 . 7 7 . 3 ( 6 ) 3 9 . 7 6 . 6 ( 3 ) 3 4 . 6 6 . 1 ( 4 ) 3 4 . 6 5 . 5 ( 4 ) 2 9 . 3 5 . 1 ( 4 ) 2 5 . 7 4 . 5 ( 3 ) 2 5 . 7 4 . 0 ( 2 ) 2 5 . 7 3 . 9 ( 3 ) 1 9 . 9 3 . 2 7 ( 1 0 ) 1 9 . 9 3 . 1 8 ( 2 ) 1 9 . 9 3 . 0 1 ( 1 0 ) 1 6 . 4 2 . 8 5 ( 1 0 ) 1 6 . 4 2 . 8 5 ( 8 )
3 0 . 3 1 0 . 6 ( 3 ) 3 0 . 3 1 1 . 1 ( 4 ) 2 5 . 2 8 . 5 ( 7 ) 2 5 . 2 7 . 8 ( 3 ) 2 5 . 2 7 . 4 ( 6 ) 1 9 . 5 7 . 6 ( 2 ) 1 9 . 5 6 . 3 ( 8 ) 1 9 . 5 6 . 2 ( 6 ) 1 9 . 5 6 . 1 ( 4 ) 1 9 . 1 7 . 2 ( 2 ) 1 6 . 3 6 . 6 ( 4 ) 1 6 . 0 6 . 1 ( 4 ) 1 5 . 9 5 . 8 ( 4 ) 1 5 . 9 5 . 5 ( 4 ) 1 5 . 9 5 . 5 ( 1 ) 1 2 . 3 5 . 2 ( 1 ) 1 2 . 2 5 . 8 ( 1 ) 9 . 5 4 . 7 ( 5 ) 9 . 5 4 . 7 ( 3 ) 9 . 5 4 . 4 ( 3 )
6 . 4 ( 3 ) -11.2(8)
5 . 6 ( 5 ) 1 2 . 7 ( 7 )
S = solvent BB = bromobenzene, DCE 1,2-dichloroethane
69
8 - 5
8.0
7 . 5 H
DC E
. ^ ; s u ^ r ; ^ e n n ? - S T H p , ( s o l v e n t ) w " f c 0 ) ^ a t ° v a r i o u s t e m p e r a t u r e s . b I ^ b U o b e n L n e . DCE - 1 J - d i c h l o r o e t h a n e ( O r d i n a t e = l n ( k - i / T ) , A b s c i s s a = 1 / T X 10 K ) .
70
If there is an initial dissociation of bromobenzene from
cis-[ (fl'-DTHp) (BB)W(CO)*-] to produce the coordinatively-
unsaturated intermediate 23-a, then a low and positive entropy
of activation would be observed (12).
The displacement of CB, CB = chlorobenzene, from
cis-[(CB)(P(0-i-Pr)®)W(C0U] has been shown to be dissociative
in nature (13). The enthalpy of activation (AH = 13.0(4)
kcal/mol) for the CB-W bond-fission suggests there is a strong
interaction between CB and the tungsten metal.
One would expect bromobenzene to be a better Lewis base
than chlorobenzene (11). Thus, there should be an observably
stronger bromobenzene-tungsten interaction. Therefore, a
smaller enthalpy of activation for the displacement of
bromobenzene upon ring-closure argues in favor of an assisted
displacement of BB by the free end of the chelate ligand. This
proposal is further supported by the activation parameters
observed for the ring-closure of cis-[ (ni-DTHp) (DCE)W(CO)*.] (AH*
= 5.6(5) kcal/mole, &S*= -12.7(7) cal/deg-mol. If, indeed, the
solvent is coordinated to the tungsten metal through the halogen
atom, then the dichloroethane-solvated species should resemble
the behavior of its chlorobenzene-solvated analog. Differences
in enthalpies of activation for the ring-closure (DCE = 5.6(5),
BB = 6.4(3)), if significant, can be attributed to the fact that
1,2-dichloroethane is a poorer Lewis base than bromobenzene (19).
However, this is not supported by the entropies of activation.
71
The previous discussion of ring-closure studies clearly indicate
that some kind of an associative pathway is operating during the
ring-closure of the transient 23-b.
It is quite intriguing that the rate for ring-closure
is much faster than the rate for the bimolecular ligand attack
at the solvent coordinated species, especially when one
envisions the formation of a highly strained ring. However,
because DTHp is only seven atoms in length, the free end is
located conveniently close to the reaction site. The
possibility of competition between ring-closure and ligand
attack at cis-[ (fl1 -DTHp) (BB)W(CO)*.] by L = tri( isopropyl)
phosphite thus was tested. The transient cis-[{fl1-DTHp)
(BB)W(CO)*.] was generated in the presence of a large excess of
tri(isopropyl) phosphite (> 1M). In the absence and presence of
tri(isopropyl) phosphite similar rates for the disappearance of
this transient were observed, indicating that ligand attack at
cis-[fl1-DTHp(BB)W(CO)<*.] is not competitive with ring-closure.
With the presently available information, one may propose a
concerted expulsion of the solvent <BB, DCE) upon ring-closure of
cis-(fl1 DTHp) (solvent)W(CO)
Furthermore, one may propose that this associative
pathway takes place because the rate constant for ring-closure,
k'-i, is much larger than k—ak—i/(k—i + kstCB]) (assuming the
steady-state concentration of 23-a). Therefore, if ring-
closure is to take place, the free end of the chelate ligand
must assist the departure of the solvent molecule.
72
MftP-hanism for QlS. formation &£ ?>\ S~ (0* -PTflP) < L )W(CO )<*
The proposed mechanism by which (0s-DTHp)W(CO)^ reacts
with L to afford cis- (fF-DTHp) (L)W(CO)*» (25-c) is shown in
figure 25. According to the principle of microscopic
reversibility and to the results of ring-closure studies of
cis-[ (fl1-DTHp) (S)W(CO)*.], S = solvent, one can propose an
initial solvent-substrate interaction to produce the solvated
intermediate 25-b (figure 25). In view of L being a better
Lewis base than the solvent, another pathway, which involves an
initial ligand-substrate interaction to produce cis-(fl1 -DTHp)
(L)W(COU, can be proposed.
The rate law for the formation of 25-c, assuming that
the concentrations of intermediates 25-a and 25-b (figure 25)
are steady-state and that k'-i >> k-a, k'i, ke; k-x >> ks, k e,
I k ' x / k ' - x ) » k'e >> k'xWk'-i, is given by equation (17).
d[25-cl/dt = R* + R« <17>
Where;
substrate = (ns-DTHp)W(C0)^
S\\ I I co
o
/ i \ c 0
°o
73
Fig. 25--Plausible mechanism for the reactions of (ne-DTHp)W(CO)^ with L to afford cis-(D»-DTHp)(L)W(COU
74
Ri = k'a[L][substrate] (18)
Ra = (k'l/k'-i )ka[L3 [substrate] (19)
Although the parallel photochemical studies showed that
all the steps governed by k'i and k1 —* are operative during the
course of the formation of intermediate 25-c, the most likely
pathway by which intermediate 25-c is formed is by that step
which involves an initial interaction of L with the substrate
governed by k'a.
The basis for this interpretation rests on the relative
magnitudes of the rate constants for the different pathways
involved. One would expect that the rate constant k'» for the
formation of the solvated intermediate 25-b, via association of
chlorobenzene with the substrate, will be similar to k'a, the
rate constant for the process involving the ligand-substrate
interaction. Furthermore, the rate constant for ring-closure of
intermediate 25-b, via a concerted displacement of the solvent
molecule will be much larger than the rate constant for the
expulsion of the ligand coordinated at the metal in 25-c (20).
Therefore, k'a >> k'l/k'-i. Since (k'./k'-i) is included in the
equation (18 ), equation (17) reduces to equation (20), which
describes an associative pathway for the reactions of
(fF-DTHp)W(CO)^. with tri(isopropyl) phosphite.
d[25-c]/dt = k'etL][substrate] (20)
75
pactions of CIS-(n1-DTA) (li)W(QO?**
During the course of "the displacement of DTA, (DTA =
DTHp, DTD, and DTU) from (fle-DTA)W<COU a p p r e c i a b l e formation of
cis- (01 -DTA) (L)W(C0)4, (L = phosphites and phosphines) has been
observed. Cis-(n*-DTA)(L)W(COU will further react with L to
afford cis-(L)eW(COU according to equation (21).
cis- (fl1 -DTA) (L)W(CO)^ + L > cis-(L)«W<COU + DTA (21)
These reactions have been studied under pseudo-first order
conditions. The pseudo-first order rate constants are given in
table V.
The rates of reaction of cis-(n*-DTA) (L)W(CO)*. with all
ligands studied were found to be first-order with respect to the
concentration of cis-(fl1-DTA) (L)W(CO)*., and zero-order with
respect to the concentration of L. The rate law for these
reactions is given by equation (22).
-d[S]/dt = k'ob.ct [S] (22)
k(ob»d — ^4
Where S = cis-(fl*-DTA) (L)W(COU
76
TABLE V
RATE CONSTANTS FOR THE REACTIONS OF CIS- (fl1 -DTA) (L)W(CO). WITH L IN CHLOROBENZENE AT VARIOUS TEMPERATURES
L T, °C [L], M 103 k V , sec"3
P(0-i-Pr)a 44.5 1.235 1.43(1) P(0-i-Pr)a 44.5 0.8958 1.51(2) 0.6922 1.427(6) 0.5565 1.395(5) 0.4376 1.390(4) 0.2259 1.48(1) 0.1985 1.51(2) 0.09449 1.43(2) 0.04127 1.61(5) 0.03640 1.6996) 0.02264 1.64(1)
35.2 1.5065 0.323(3) 0.9998 0.9315
0.326(2) 0.341(5)
33.9 0.2721 0.1736 0.09028
0.290(2) 0.285(1) 0.308(2)
31.1 1.550 1.085 0.5520
0.1750(8) 0.164(2) 0.1714(7)
21.1 0.5840 0.3731 0.1168
0.0392(5) 0.0386(3) 0.0374(3)
P(OMe): 68.1 0.4894 0.3664 0.1438
2.78(3) 3.01(4) 2.52(2)
57.0 0.4925 0.3625 0.2005
0.757(6) 0.707(5) 0.743(3)
44.5 1.929 1.278 0.3325
0.156(1) 0.153(2) 0.151(2)
77
TABLE V CONTINUED
DTA L T, °C [ L J , M 1 0 3 k ^ , s e c - 1
DTHp CP 68.1
5 7 . 5
4 4 . 5
P(OC*tks)3 4 4 . 5
P ( n - B u ) a 4 4 . 5
DTD P ( 0 - i - - P r ) 3 5 2 . 3
4 4 . 5
3 5 . 2
3 1 . 6
21.1
0 . 09346 0 . 7 2 7 ( 5 ) 0 . 0 5 8 3 1 0 . 7 3 6 ( 5 ) 0 . 03186 0 . 6 6 0 ( 6 )
0 . 03222 0 . 1 9 9 ( 2 ) 0 . 02177 0 . 2 0 4 ( 1 ) 0 . 08897 0 . 2 1 3 ( 1 )
0 . 4872 0 . 0 4 5 4 ( 2 0 ) 0 . ,3927 0 . 0 4 4 4 ( 5 ) 0 . 1183 0 . 0 3 9 8 ( 4 )
0 . 9801 1 . 1 5 ( 4 ) 0. ,1555 1 . 2 8 ( 2 ) 0 . ,05351 1 . 1 0 ( 1 )
1. ,059 2 . 8 1 ( 2 ) 0, .7729 3 . 0 4 ( 2 ) 0. ,3958 2 . 8 3 ( 1 )
0. .09968 1 . 7 3 ( 1 ) 0. . 08736 1 . 8 0 ( 6 )
0, .7709 0 . 6 3 9 ( 2 ) 0 .5629 0 . 6 0 1 ( 6 ) 0, .5554 0 . 6 8 7 ( 5 ) 0 . 4494 0 . 6 7 5 ( 6 ) 0, . 2904 0 . 6 5 9 ( 7 ) 0 . 2 1 6 9 0 . 6 3 8 ( 7 )
1 .080 0 . 1 4 8 8 ( 5 ) 0 . 9 0 5 2 0 . 1 8 0 7 ( 9 ) 0 . 7 8 8 3 0 . 1 5 0 0 ( 6 )
0 . 1 3 2 1 0 . , 0 9 6 1 ( 1 0 ) 0 . 0 9 9 9 3 0 . 1 0 8 ( 1 ) 0 . 0 7 5 1 0 0 . , 0 9 7 7 ( 6 )
1 . 3 0 4 0 . , 0 2 2 0 ( 3 ) 0 . 9862 0, . 0 2 2 0 ( 2 ) 0 . 8 3 7 6 0 . , 0 2 4 7 ( 3 ) 0 . 4 4 9 2 0, . 0 2 5 0 ( 3 ) 0 . 1 5 3 2 0. . 0 2 5 8 ( 5 )
78
TABLE V CONTINUED
DTA L o O
[L], M 103 k'^, sec-1
DTD P(0-i~Pr)s 11-1
P(OMe)3 21.1
DTU P(0-i~Pr)a 52.3
44.4
35.2
33.9
21.0
14.2
P(OMe)a 14.2
0. 6584 0. 00422(8) 0. 6347 0. 00398(6) 0. 5310 0. 00457(6) 0. 4457 0. 00432(4) 0. 3892 0. 00447(18) 0. 2938 0. 00481(9)
1. 509 0. 00428(6) 0. 6040 0. ,00356(7) 0. 4354 0. 00349(14) 0. 2823 0. ,00405(6)
0. 5750 1. ,77(1) 0. ,4113 1, .81(3) 0. 3364 1. ,73(2)
0. .2602 0, .625(6) 0. ,1218 0. .63(2)
1. .201 0 .183(1) 0. ,8010 0. .177(1)
1. .123 0 .1516(8) 0. .5418 0 .1612(9) 0 .1524 0 .158(1)
1, .371 0 .0241(3) 0 .2388 0 .02319(2) 0, .2015 0 .0233(2)
1 .051 0 .00780(7) 0 .4471 0 .00749(14) 0 .2468 0 .00746(12)
1 .463 0 .00165(2) 0 .7828 0 .00152(2)
79
The reactions of cis-(fl1-DTHp) (L)W(CO)*. with a wide
variety of phosphites and P(n-Bu)a were studied. A strong
dependency of the rate constant on the steric nature of L was
observed.
In table VI, the rate constants, the Tolman cone
angles, and the Tolman electronic parameters (20.) are given.
The cone angles are an empirical measure of the steric
requirements of the ligands. There are an increase in the
values of the rate constants as the cone angle of the ligand
coordinated cis to DTHp increases.
TABLE VI
RATE CONSTANTS FOR DTHp-DISSOCIATION FROM CIS- (fl1 -DTHp) (L)W(COU IN
CHLOROBENZENE AT 44.5 °C
L 1 0 3 e , Cone Angle , (CO) (seer1 ) (Deg.) cm-1
P(n-Bu)a 2 . 8 9 ( 1 2 ) 132 2060 . 3
P(0-i-Pr)a 1 . 4 3 ( 3 ) 130 2075 . 9
P(OC*ife)a 1 . 1 8 ( 9 ) 128 2085 . 3
P (OMe )a 0 . 1 5 3 ( 4 ) 107 2079 . 3
CP 0 . 0 4 4 8 ( 8 ) 101 2087 . 3
80
Plots of l n ( W T ) vs. 1/T for the reactions of
cis- (DTHp) (L)W(CO)*. with L, L = tri(isopropyl) phosphite,
triinethyl phosphite, and CP are given in figure 26. Analogous
plots for the reactions of cis-ff^-DTD) (L)W(COU and
cis- (nx -DTU) (L)W(CO)* with tri.(isopropyl) phosphite are given in
figures 27 and 28, respectively.
Enthalpies of activation in the vicinity of 25 kcal/mol
and positive entropies of activation strongly suggest a
rate-determining unimolecular dissociation of the anchored end
of the chelate ligand to afford cis~[LW(C0U] (ZX~2k) • Figure
29 shows the proposed mechanism.
The activation parameters for the systems studied are
given in table VII. The enthalpies of activation for the
reactions of cis-<n*-DTHp)(L)W(COU with L, (L = tri(isopropyl)
phosphite, trimethyl phosphite, and CP) are very similar to one
another. But the entropies of activation significantly increase
with the sxze of L. This outcome and the fact that the plot of
Ink* vs. the cone angle of the coordinated ligand is linear
(figure 30) suggest that the increase of the rate constant for
the dissociation of DTHp from cis-(DTHp) (L)W(COU is entropic in
nature. There have been instances in which both the electronic
and steric parameters of the coordinated ligands cis to the
departing ligands are simultaneously affecting the rates of the
unimolecular dissociations (25-21). An empirical equation has
been used by Poe and co-workers to assess the contribution of
each parameter (29).
- 1 0 -
- 1 1
- 1 2
-1 3
-1 4
-1 5 -
-1 6
P(OM«)
2 . 9
P(OP'r),
r e a c t i o n s o f o i ^ n ? - D T O p ) ( L ^ ' S ' r V T f o r t h e
L " t r x { i s o p r o p v l ) ( C ? , < ' W l t h L , S? ,fn chlorobenzene at v a r i o u s P h o s p h i t e , and O r d i n a t e = I ,
- 1 4
- 1 5
- t 6
- 1 7
1 8
- 19
- 2 0 -
82
3*0 3.1 3.2 3 3 3 4 3 5
. ^ g * 27--Eyring plots of ln(k/T) vs. l/T for the reactions of £ A S - ( n i-DTD) (L)W(COU with L
=. ^isopropyl) phosphite, in chlorobenzene at various temperatures. Ordinate = l n < W ? ? Abscissa = l/T X 1053 K~% .
83
-1 2
-1 4
-16-
3 0 32 34
Fig. 28--Eyring plots of ln(k>»/T) ss. 1/T for the reactions of cis- (fi1 -DTU) (L)W(CO )*• with L, L = tri(isopropyl) phosphite( in chlorobenzene at various temperatures. Ordinate = ln(k«»/T), Abscissa = 1/T X 10s K—1.
o
84
^ i/ 'vv
* r / l x
o c
V '
- / i \ v-5
c °o o
o c
u p°
\ /
w
\ c o
Ji£L
8 l \ 1 /
-> W
/ l \ C 0
Fig. 29--Mechanism for the displacement of DTA from cis- (I"!1 -DTA) (L) W (CO)«. by L, L = phosphites, phosphines .
85
- 8
- 9
10
100 11 0 1 2 0 1 30
Fig. 30--Plots of In k*» vs. Tolman cone angles for the reactions of £±£-< fl1-DTHp) <L)W(e0U with L, L = 1, tri(n-butyl) phosphine; 2, triphenyl phosphite; 3, tri(isopropyl) phosphite; 4, trimethyl phosphite; 5, CP.
86
This empirical relationship is given by equation (23).
log k*. = a6 + b2) + c (23)
where:
0 = The Tolman cone angle
y = The Tolman electronic parameter
c = Constant
a = Coefficient for the steric parameter
b = Coefficient for the electronic parameter
The numerical values of a (a = 0.0479 deg-1), b (b = -0.0101
cm), and c (c = 10.9) were determined employing a multivariable
linear regression program (3H). Substitution of these
coefficients into equation (23) yields equation (24).
log k<* = 0.0479 © - 0.01012/ + 10.9 (24)
According to Tolman (2£). the relative contribution of steric
and electronic influence to the value of the rate constant are
given by equation (25) and (26), respectively.
contribution of steric influence = ta/(a + b)] (25)
contribution of electronic influence = [b/[(a + b)] (26)
87
In the present study, it was found that the steric influences of
the coordinated L predominate, with a positive correlation
observed between the rate of departure of DTHp from c i s - < n * -
DTHp) (P(0-i-Pr )a )W(CO)<* and the Tolman cone angle.
The rate constants for the reactions of cis-(H1-DTA)-
(P(0-i-Pr)» )W(CO)«. with L, (L = tri(isopropy 1) phosphite) at
35.2 °C are given in table V. The rate constants for DTA = DTD,
DTU are very similar, 16.0(14) X 10~s sec-1, 18.0(3) X 10
sec-1, respectively. However, the rate constants observed for
DTA = DTHp are almost twice as large (k* = 32.7(1) X 10~s sec"1)
as the ones observed for DTA = DTD, DTU. This outcome is
somewhat surprising, since all the complexes are very similar at
the reaction site.
TABLE VII
ACTIVATION PARAMETERS FOR THE DISSOCIATION OF DTA FROM CIS-((l1 -DTA) (L)W(CO W
DTA Mil AS* He*
(kcal/mole) (kcal/mole)
DTHp P(0-i-Pr)» 28.2(6) 18.1(2)
P(OMe)a 25.8(1) 6.1(2)
CP 25.1(6) 1.4(11)
DTD P(0-i-Pr)» 26.0(3) 11.1(11)
DTU P(0-i-Pr)3 25.8(2) 10.4(5)
88
From table VII it can be seen that the enthalpies of
activation for these reactions are similar.
However, there is a greater difference in the observed entropies
of activation: 18.1(2) cal/deg-mol for the dissociation of DTHp
vs. 11.1(11) and 10.4(5) for the dissociation of DTD and DTU,
respectively.
Cis- [ (P(Q-i-Pr)a) (CB)W(CO)^.]
The generation of cis-[(P(0-i-Pr)3)(CB)W(CO)^] via
pulsed-laser flash photolysis of cis- (pip) (P(0-i-Pr)3 )W(CO)<*
(pip = piperidine) in chlorobenzene provides the opportunity to
study the steps subsequent to DTA dissociation.
Previous studies have firmly established that both the
intermediate which is produced during the thermal dissociation
of pip from cis-(pip) (P(O-i-Pr)a )W(C0)*», and the transient that
is produced after the flash at cis-(pip)(P(O-i-Pr)a)W(CO)^ are
the same (25.). Furthermore, this transient has been
characterized by time-resolved spectroscopy in n-heptane solvent
(11).
As shown in figure 31, the initial ground-state photo
product will undergo very fast attack by CB to yield
cis-C(P(0-i-Pr)3)(CB)W(CO)^]. This transient will establish an
equilibrium involving desolvation-solvation governed by k-= and
k=, respectively.
o ? r ° PiP I
x. / W
/ I V c uo o
h v
89
CB
O C
W
/ v
k5'CB
-5
I y\
,o°
o
\ i w
• i \ o
Fig. 31--Photochemical generation of sis- [LW(COU] in chlorobenzene and attack by L.
90
Assuming that the concentration of intermediate 31-a (figure 31)
is steady-state and that k»[CB] » k*CL], the rate law for the
disappearance of cis-[(P(0-i-Pr).(CB)W(C0U] is given by
equation (27).
-d[42-b]/dt = [42-b] (27)
= K-ck^CLl/tCB] (28)
K-c = k-o/kb (29)
Table VIII contains the pseudo-first order rate
constants for the displacement of CB from cis-t(P(0-i-Pr)»)
(CB)W(CO)*»] by tri(isopropyl) phosphite at 35.2 °C, and figure
32 shows a linear plot of ko*-* vs. [L] from which the value of
the composite rate constant K-ck*,/[CB]. (7.2(3) X 103 M x,
sec-4), is obtained.
Dobson and his co-workers have reported the value of
k*/ks (1.12(20) at 31.1 °C) in CB to be nearly independent of
the temperature (25). The approximate value for the rate
constant for W-CB bond dissociation, k-», can be calculated from
the value of the "competition ratio" k^/ks, and the value of
k-«,k*,/[CB]. This value calculated as k-=s = ([CB]/(k*/k») ) X
k.ck* equals 6.3(13) X 10s sec"1.
It is interesting to note that the calculated value of k-=
is very similar to the value of k—is, (6.3 X 10 sec )
calculated (extrapolated) for the CB dissociation from cis-
t(P(O-i-Pr)a)(CB)W(COU] at 35.2 °C in cyclohexane-CB solvent
(mixed solvents) (2_5).
91
This supports that indeed displacement of CB by L from cis-[P(0-
i-Pr)s)(CB)W(CO)^] takes place involving initial reversible CB
dissociation and followed by attack by L at the non-solvated
intermediate. Activation parameters for CB-W bond dissociation
from cis- E (P(0-i-Pr)a (CB)W(CO)^.] are 13.0(4) kcal/mol for / H—a A
and 5.6(11) cal/deg-mol for AS-s (U.)-
TABLE VIII
RATE CONSTANTS FOR THE DISPLACEMENT OF CHLOROBENZENE FROM CIS-[(P(0-i-Pr)3(CB)W(C0U] BY TRI(ISOPROPYL) PHOSPHITE AT 35.2 °C
[L], M 10~3 kob.d sec-1
1.367 10.05(4) 1.367 9.83(12) 1.367 9.465(8) 1.367 8.65(3)
0.9036 6.6(2) 0.9036 6.1(2) 0.9036 6.13(5) 0.9036 5.28(12) 0.9036 5.76(2)
0.7193 4.24(2) 0.7193 4.21(5)
0.4428 2.89(13) 0.4428 6.68(2)
0.1987 1.28(1) 0.1987 1.146(3)
K«ck<s./[CB] = 7.2(3) X 103 M — 1 , sec"1
92
10 12 14 16
Fig. 32--Plot of kob.d vg.. [L] for the reaction of cis- [ (L) (CB)W(CO)*»] with L at 35.2 °C in chlorobenzene L = tri(isopropyl) phosphite. Ordinate = k X 10~3 sec"1
Abscissa = [L] x 10 M. '
CHAPTER BIBLIOGRAPHY
1. Pardue, J. E.; Dobson, G. R. Inorg. Chim. Acta, 1976, 20, 207.
2. Graham, J. R.; Angelici, R. J. Inorg. Chem., 1967, 6, 2082 .
3. Angelici, R. J.; Graham, J. R. J. Am- Chgm. Sfic. » 1966, 88, 3658.
4. Al-Kathumi, K. M.; Kane-Maguire, L. A. P. J. Inorg. NucI. Chem., 1972, 34, 3759.
5. Dobson, G. R. et al. J. Coord. Chem., 1978, 7, 253.
6. Faber, G. C.; Dobson, G. R. Inprg. Chem., 1968, 7, 584.
7. Dobson, G. R.; Binzet, C. S.; Cortes, J. E. J. Coord. Chem.. 1986, 4, 215.
8. Schultz, L. D.; Dobson, G. R. J. Qrganomet. Chem., 1977, 131, 285.
9. Schultz, L. D.; Dobson, G. R. J. Qrganomet. Chem-, 1977, 124, 19.
10. Dobson, G. R. ; Basson, S. S.; Dobson, C. B. Inorg. CMm-Acta. 1985, 105, L17.
11. Dobson, G. R.; Dobson, C. B.; Halverson, D. E.; Mansour, S. E. J. Drganomet. Chem.. 1983, 253, C27.
12. Dobson, G. R.; Dobson, C. B.; Mansour, S. E. Inorg. Chgm., 1985, 24, 2179.
13. Asali, K. J.; Basson, S. S.; Tucker, J. S.; Hester, B. C.; Cortes, J. E. ; Awad, H. H. ; Dobson, G. R. J. Aju. Chem. Soc.. 1987, 109, 5386.
14. Simon, J. D.; Peters, K. S. Chem. Phys. Lett., 1983, 98, 53.
15. Simon, J. D.; Xie, X. J. J. Phys. Chem-, 1986, 90, 6715.
16. Langford, C. H.; Moralejo, C.; Sharma, D. K. Inorg. Chim. Acta. 1987, 126, Lll.
93
94
17. Seder, T. A. ; Church, S. P.; Weitz, E. J. Am- Ciiem. Sua., 1986, 108, 4721.
18. Bonneau, R. ; Kelly, J. M. J. J. Am. Chem. Sop. , 1980, 102, 1220.
19. Carey, F. A.; Sunberg, R. J. Advanced Organic Chemistry part B: Reactions and Mechanisms: Plenum Publishing Corporation: New York, 1980, pp 281-28 2.
20. Tolman, C. A. Chem. Rev.. 1977, 77, 345.
21. Schultz, L. D.; Dobson, G. R. J. Qreanomet. Chem., 1977, 131, 285.
22. Schultz, L. D.; Dobson, G. R. J. Qreanomet. Chem., 1977, 124, 19.
23. Dobson, G. R.; Basson, S. S.; Dobson, C. B. Triors. Chim. Acta. 1985, 105, L17.
24. Dobson, G. R.; Dobson, C. B.; Mansour, S. E. Inorg• Chem.. 1985, 24, 2179.
25. Asali, K. J.; Basson, S. S.; Tucker, J. S.; Hester, B. C.; Cortes, J. E. ; Awad, H. H. ; Dobson, G. R. J. Sin• Chem.
Soc.. 1987, 109, 5386.
26. Basolo, F. Inorg. Chim. Acta, 1985, 100, 33.
27. Basolo, F. Coord. Chem. Rev., 1982, 43, 7.
28. Darensbourg, D. J.; Graves, A. H. Inorg. Chem., 1979, 18, 1257.
29. Dahlinger, K.; Falcone, F.; Poe, A. Inorg. Chem., 1986, 25, 2654.
30. SAS Institute Inc., Cary, N C
31. Dobson, G. R.; Hodges, M. P.; Healy, A. M.; Poliakoff, M.; Turner, J. J.; Firth, S.; Asali, K. J. Am- Chem.
Soc.. 1987, 109, 4218.
CHAPTER IV
FORMATION OF (fl4 -DTA) (L )W(CO)-DTA = DTD, DTU
Of nis-tn^-DTA) (L)W(COU
The reactions of (fF-DTA)W(COU (DTA = DTD, DTU) with
L = phosphites in CB are similar to the corresponding reactions
Q£ (ns-DTHp)W(CO)^ in the sense that a biphasic behavior is
observed.
Plots of In(At - A„) vs. time are linear up to one half
life, then after curving, another linear segment is observed.
The overall stoichiometric reaction is given by equations 30 and
31.
(fp-DTAJWfCO).* + L > cis-(fl1-DTA) (L)W(CO)^ (30)
k' obwd cis-(IP-DTAWCO)** + L > cis - (L )eW( CO )*. + DTA (31)
Cis-(fl1-DTD) (L)W(CO)*-, L = CP, has been isolated and
characterized by infrared spectroscopy and elemental analysis
(see chapter II).
The reactions were carried out under pseudo-first order
conditions. The first section of the biphasic decay showed rate
constants (kot»-ci) with a strong dependence on the concentration
of L. Observed rate constants are given in table IX.
95
96
TABLE IX
PSEUDO-FIRST ORDER RATE CONSTANTS FOR THE REACTIONS OF (fF-DTA)W(COU WITH PHOSPHITES IN CHLOROBENZENE AT VARIOUS TEMPERATURES
DTA Ligand T, °C CL] (M)
103koto«d (sec-1)
DTD P(0-i-Pr)a 35.2
P(0-i-Pr)a 21.1
P(0-i-Pr)a 11.1
P(OMe): 21.1
DTU P(0-i-Pr)3 35.2
1. 584 5, .05(6) 1. 248 4 .81(3) 0. 9052 3, .90(4) 0. 7883 3 .74(2) 0. 6542 3 .27(3 ) 0. 5198 2 .78(1) 0. 3968 2 .35(1)
1. 7113 0 .772(5) 0. 9862 0 .627(3) 0. 8376 0 .554(5) 0. 6001 0 .462(3) 0. 5395 0 .449(3) 0. 4032 0 .363(3) 0. 3164 0 .315(1)
1. 550 0 .148(2) 0. 6584 0 .1075(9) 0. 6347 0 .1014(7) 0. 5310 0 .1037(6) 0. 4457 0 .0884(9) 0. 3892 0 .0796(7) 0. 2938 0 .0693(8) 0. 2370 0 .0586(4)
1. 509 0 .958(15) 0. 7247 0 .778(11) 0. 6040 0 .681(6) 0. 4354 0 .587(7) 0. 2823 0 .471(3) 0. ,2410 0 .415(4)
1. 4877 8.02(5) 0. .8928 6.60(4) 0. 7316 6.32(2) 0. ,5412 5.48(4) 0. ,3351 3.92(2) 0. .2365 3.05(2)
TABLE IX Continued
97
DTA LI GAUD T, [L] (M)
X 03 ko b*d (sec-1)
P(0-i-Pr)a 21.0
P(0-I-Pr)s 14.2
P(OMe): 14.2
1.592 1.34(2) 1.254 1.278(6) 1.088 1.142(7) 0.6459 0.934(10) 0.5155 0.846(5) 0.3611 0.674(6) 0.2388 0.541(5) 0.2015 0.465(1)
1.150 0.398(3) 0.7282 0.330(4) 0.5216 0.277(3) 0.4471 0.250(2) 0.2468 0.181(1)
1.463 0.419(2) 0.7828 0.382(2) 0.6349 0.330(2) 0.5499 0.308(2) 0.4423 0.293(3) 0.3235 0.251(1)
Plots of the observed rate constants vs. the ligand
concentration, as depicted in figures 33 and 34, are curved.
This kinetics behavior is suggestive of a complex reaction
mechanism. From the previous discussion, one can rule out an
interchange pathway since its rate law will predict linear plots
of vs. [L3. This is an example in which a plausible
mechanism could be ruled out based solely on the form of the rate
law.
98
s -
3 -
1 2 11
3 5 2
Fig. 33--Plots of kob.d vs. [L3 for the reactions of (n®-DTD)W(CO)^ with tri(isopropyl) phosphite in chlorobenzene at various temperatures. Ordinate = kob.d X 10
3 sec-*, Abscissa = [L] X 10 M.
99
2 10
Fig. 34--Plots of kob.d \rs„ [LI for the reactions of (ft®-DTU)W(CO)*. with tri{isopropyl) phosphite in chlorobenzene at various temperatures. Ordinate = kob.d X 10a sec-1, Abscissa = [L] X 10 M.
100
Plots of l/ke»t,„cj vs. 1/[L] (reciprocal plot) were
linear over a wide range of concentrations (figures 35 and 36),
a behavior which is consistent with the mechanism depicted in
figure 37 and predicted by equation (35). The rate law,
assuming the steady-state concentration of the intermediates
37-a and 37-b, is given by equation (32).
d[S]/dt = kiketL][S]/(k—% + ke[L]) (32)
S = (fF-DTA)W(COU
-d[S]/dt = kofcCS] (33)
= kikeEL]/(k-i + ke[L]) (34)
And
1/kcto.a = l/ki + ((Jc-t/kike)/CL3) (35)
Common intercepts for the reciprocal plots were
observed (figure 38) , for two ligands with different steric
properties, (L = tri(isopropyl) phosphite, trimethyl phosphite).
This mechanistic outcome is expected since L is not involved in
the ring-opening step.
101
1 5
1 0
1 11
35--Plots of l/kot».rt vg. 1/[L] for the reactions °£i 0 U w i * h tri^ isopropyl > phosphite in ch 1 orobenz ene at. various temperatures . (Ordinate = l/kot>«d X 10""e sec. Abscissa = 1/[L] M~~l).
102
2 1 . 0
Fig. 36--Plots of l/ko.*»«.<a V £ . 1/[L] for the reactions of <fF-DTU)W(COU with tri(isopropyl) phosphite in chlorobenzene at various temperatures. (Ordinate = l/k^.c X lCTe sec, Abscissa = 1/[L] M~l ).
o o . .0
o 103
o u -
CD a
-uo
CO"
00-
CD
O
00 CO j*:1
- h
° o
CM J*
- > o o -/
J ?
• o O
if)"
CO-
y-4
o
a Q) 0) c o M-4 o
4-> a a> g 0 O a* rH CU to
0
5 . U ^ 0 >1 M-l XX
£ 4 w ~ •jo S H 1 2 2 3 • P 1 I
co e
O O O O
bO £ •H O t*4 U
M-l
Q
104
P I O P r ' ) 3
P f M « o r
4= 3 8 — P l o t s o f l / k r > t > « c j v s . 1 / [ L ] f o r t h e r e a c t i o n s o f ( f l ® - D T D ) W ( C O ) « • w i t h L i n c h l o r o b e n z e n e a t 2 1 . 1 ° C . ( O r d i n a t e = 1 / k o b . d X 1 0 3 s e c , A b s c i s s a = 1 / [ L ] M ~ ~ l )
105
Table X gives the rate constants for ring-opening (k*)
and the competition ratios, ka/k-i, for the reactions of
(0a-DTA)W(C0)^ with L, (L = tri(isopropyl) phosphite and
trimethyl phosphite). The plots of ln(ki/T) vs. 1/T are given
in figures 39 and 40.
TABLE X
RATE CONSTANTS FOR THE RING-OPENING OF (fF-DTD)W(CO)-IN CHLOROBENZENE AT VARIOUS TEMPERATURES
DTA Ligand T, °C 103 kx (seer1 )
ks/k-i M-1
DTD P(0-i-Pr)3 35.2 8.1(3) 1.03(9)
21.1 1.13(4) 1.21(8)
11.1 0.203(11) 1.74(17)
P(OMe)a 21.1 1.26(4) 2.19(20)
DTU P(0-i-Pr)a 35.2 12.1(5) 1.44(9)
21.0 1.78(6) 1.77(9) )
14.2 0.57(3) 1.87(14)
P (OMe )3 14.2 0.53(3) 2.78(33)
Activation parameters for L = tri(isopropyl) phosphite
4 DTD: AJHx = 26.0(7) kcal/mole
&St* = 18.0(23) cal/deg-mol
DTU: &hJ= 24.9(7) kcal/mole 14.7(23) cal/deg-mol
106
- 1 0
- 1 1
- 1 2
-t 3 -
- t 4
6
4
- 2
- 0
3-2 3 .3 3 .4 3-5
Fig. 39—Eyring plots of ln(k/T) v£. i/T for the reactions of (fF-DTD)W(COU with tri < i^propyl) phosphite in chlorobenzene at various temperatures. A: k = ki; B = ke/k-i. Ordinate = ln(k/T). Abscissa = 1/T X 10® Kr*\
107
-1 o
- 11
-1 2
1 3
3.2 3.3 f
3.4
- 1 0
0*5
3 5
Fig. 40--Eyring plots of ln(k/T) vs. 1/T for the reactions of ( r F - D T l D W t C O ) * . with tri( isopropyl) phosphite in chlorobenzene at various temperatures. A = ki, B » ka/k-x. Ordinate = ln(k/T), Abscissa = 1/T X 10a K-t.
108
Highly positive entropies of activation are consistent
with a unimolecular dissociative process, involving a great deal
of bond breaking in the transition state. A comparison of the
activation parameters for the ring-opening of (fle-DTD)W(C0) and
(fF-DTU)W(CO)^ with those of (ne-DT0)W(C0U and (fle-DTN)W(C0)«.
shows that there is an increase in the entropy of activation as
the size of the chelate ring increases (1).
Although, the reactions of (fF-DTO)W(CO)* (g) and
(rF-DTN)WCO)*. (.3.) were carried out in a different solvent
(xylenes), one would expect minimal influence of the solvent in
the step leading to the ring-opened unsaturated intermediate.
The parallel increase in the entropies of activation and the
size of the chelate ring, although expected, is quite high when
one would expect an increase of only four cal/deg-mol for an
increase of one carbon in the chelate ring (it).
The competition ratios, ka/k-i, as given in table X,
suggest an intermediate with a small discriminatory ability.
This small discrimination between incoming nucleophiles should
not be surprising, since these species react with an incoming
nucleophile with rates close to those of diffusion control
The work of Peters and Simon indicates that Cr(C0)s
undergoes solvation within 25 ps (£). Later, Simon and Xie
reported that Cr(C0)ss undergoes solvation by methanol molecules
within 2.5 ps and by cyclohexane within 0.8 ps (7.). The
pseudo-first order rate constant, k = 5 X 1010 sec-1, for the
solvation of W(CO)= by perfluoromethylcyclohexane, has been
109
reported (£).
Ring-closure fif cis-C (n*-DTA) (CB)W(CO)^1
The intermediates 37-bt cis-[ (fl1 -DTA) (CB)W(CO)^] f DTA =
DTO, DTN, have been generated via pulsed-laser flash photolysis
of (fle-DTA)W(CO)*. in chlorobenzene (.!» 5.. U2) • Upon the flash,
the transient 37-a is formed which will enter into a fast
equilibrium involving solvation/desolvation. Furthermore, it
will undergo unimolecular ring-closure. In the absence of an
incoming ligand and assuming that the concentration of the
transient 37-a is steady-state, the rate law is given in
equation (36).
-d[ ( 37-b) ]/dt = (k-slc-! + ka[CB]))[37-b] (36)
k-i ~ ka because of the poor discriminatory ability of 37-a, and
k -i << ka tCB] because tCB] is very high .
Thus equation (36) becomes:
-d[37-b]/dt = k'-i[37-b] (37)
where k'-i = k-iK«c/[CB] (38)
K » q — lt-3/ka
Rate constants for the ring-closure of cis-(fl1-DTA)-
(CB)W(CO)^ at various temperatures are given in Table XI.
110
TABLE XI
RATE CONSTANTS FOR THE RING-CLOSURE OF CIS-(ft*-DTA) (CB )W(CO). IN CHLOROBENZENE AT VARIOUS TEMPERATURES
DTA T, °C lO^kot-.d , (sec-1)
DTD 9.5 1.9(6) 9.5 1.75(11) 9.5 1.71(10) 9.5 1.54(11) 9.5 1.51(6) 9.8 1.54(14)
12.3 1.70(6) 12.3 1.46(9)
15.0 1.8(3) 19.1 2.77(4) 19.1 2.70(4)
19.7 2.52(12) 19.7 2.43(10) 19.7 2.29(13) 19.7 2.25(8)
21.1 2.98(7) 21.1 3.08(8) 21.1 3.04(9)
22.9 3.40(3) 22.9 3.14(4) 22.9 3.1(3)
26.0 5.69(20) 26.0 5.38(10) 26.0 4.6(2)
29.7 6.61(17) 29.7 6.59(13) 29.7 6.6(2)
30.5 6.5(6) 30.5 5.6(5)
Ill
DTU
TABLE XI CONTINUED
DTA T, °C 10^ko (sec-1)
DTD 31.1 5.4(5) 31.1 5.3(4)
35.2 10.6(7) 35.2 8.20(18)
9.5 1.04(7) 9.5 0.96(10)
12.3 1.49(14) 12.3 1.40(9) 12.3 1.14(11) 12.3 1.10(11) 12.3 1.08(8)
15.2 1.78(8) 15.2 1.78(2) 15.2 1.76(3)
19.1 2.21(7) 19.1 2.12(8)
22.1 2.59(9)
22.9 2.74(4) 22.9 2.63(3)
25.9 3.83(13) 25.9 3.44(6)
26.0 3.71(10) 26.0 3.24(10) 26.0 3.5(10)
112
TABLE XI CONTINUED
DTA O O
10"kob«d (sec-1)
28.6 28.6
3.5(2) 3.2(2)
30.2 30.2 30.2 35.2 35.2
4.4(3) 4.1(5) 3.5(4) 6.4(2) 6.01(17)
DTD: bflt = AS* =
11.1(6) kcal/mole 0.2(18) cal/deg-mol
DTU: LJI4 = kS* =
10.5(5) Kcal/mol -2.3(16) cal/deg-mol
The plots of ln(k'-i/T) vs. 1/T are given in figures 41 and 42.
The activation parameters (DTD, /NJT-i = 11.11(6) kcal/mole, * ± *
AS'-i = 0.2(18) cal/deg-mol; DTU, AJi'-! = 10.5(5), =
-2.3(16)) are consistent with a process which involves an initial
rupture of the chlorobenzene-tungsten bond to produce the
coordinatively-unsaturated intermediate cis- [ (fl1 -DTA)W( CO )*. J.
This intermediate will then undergo ring-closure. The value for
ks, the rate constant for the displacement of CB by L =
tri(isopropyl) phosphite from (fl1 -DTA) (CB)W(CO)*. at 35.2 °C, can
be calculated from the competition ratios, ke/3c-i, obtained from
the thermal reactions of (n«-DTA)W(C0U, (DTA = DTD, DTU), with L
and from the value of k-i for the ring-closure of cis-[ (fl1-DTA)-
(CB)W(COUJ.
113
3.3 3-4 3.5
Fig. 41--Eyring plot of ln(k-i/T) vs.. 1/T for the ring-closure of £i&-[ (m-DTD) (CB)W(CO)*.] in chlorobenzene at various temperatures. Ordinate = ln(k-i/T), Abscissa = 1/T x 103 k-1.
114
5-
3 6
Fig. 42--Eyring plot of in(k_,/T) vs. i/t for the
ainvarlm^rt ° f ^ [ ( n * " D T U > < C B > W ( C O U ] in chlorobenzene at various temperatures. (Ordinate = ln(k. ,/T) Abscissa = l/T x 103 k~*).
115
The values of the rate constants k 1 are in agreement
with its values in other systems, in the sense that a decrease
of k"-i is observed as the size of the chelate ring increases
(1). The difference in enthalpy of activation between ring-
closure and bimolecular attack by L suggests a slightly higher
energy barrier for the process involving ring-closure. This
barrier is also indicated by a decrease in the values of the
competition ratios, ke/k-i, as the temperature increases (1).
The difference in the entropy of activation for the
processes just described suggests a greater degree of bond
formation in the transition state for the bimolecular ligand
attack than for the process of ring-closure. Perhaps this
reflects a greater distortion of the octahedral geometry upon
ring-closure, hence there will be less bond formation in the
transition state. This tendency has also been observed in the
photochemically-generated species cis-(fl1-DTA)W(CO)^, (DTA =
DTO, DTN) (lfi.). Activation parameters for the ring-closure and
bimolecular attack by L at the photochemically-generated
transient reflect the same trend.
Summary
Rate constants and activation parameters for the
mechanism described in figure 37 for the reactions of
(f^-DTAWCOU, (DTA = DTO, DTN, DTD, DTU) , with L =
tri(isopropyl) phosphite at 35.2 °C in CB are summarized in
table XII.
116
TABLE XII
RATE CONSTANTS AND ACTIVATION PARAMETERS FOR THE DISPLACEMENT OF DTA FROM (fF-DTA)W(CO)^ at 35.2 °C
Complex "(DTO)W(COU (DTN)W(COU (DTD)W(COU (DTU)W(COU
10s ki (sec-1) -0.16(3 ) b6.5(5) 810(3) 1210(50)
10-= k'-i (sec-1 ) =5.67(2) *=2.04(3) 0.94(12) 0.62(2) 10"® k'a (sec-1) =1.2(2) =1.78(5) 0.97(22) 0.89(9) 10s k* (secr-M 16.0(14) 18.0(3)
ke/k~i (M-1) =0.21(5) =0.87(4) 1.03(9) 1.44(9) *0.28(7) *0.97(9)
*29.0(12) '='25.5(3) 26.0(7) 24.9(7) (kcal/mole)
AS?
(cal/deg-mol) *1.8(37) *-5.2(8) 18.0(23) 14.7(23)
+ i Affe - AHli *-2.3(24) *='-3.8(21) -3.6(8) -2.3(16) (kcal/mole) =-0.7(38)) =-4.9(11) AsJ - AS-!* =-5.6(129) =-15.5(36) -12.3(3) -7.0(5) (cal/deg-mol) *=•-4.5(60) * -11.1(65)
i Afl'-i =8.8(5) =11.5(3) 11.11(6) 10.5(5) (kcal/mole)
(cal/deg-mol)
AH'! (kcal/mole)
As't (cal/deg-mol)
AS'ii =-3.5(6) =2.6(15) 0.2(18) -2.3(16) :al.
AH'! =8.1(33) =6.6(8)
AS't =-9.1(113) =-12.9(21)
-Ref. 1, 2, 10 toRef. 1, 3, 10 =Ref. 10 * Complementary data from thermal ligand-exchange studies, rather than via flash photolysis studies (ref. 10).
117
The value of the rate constant for ring-opening, klf
increases as the size of the chelate ring increases. While
there is a 40-fold increase in the value of the rate constant
when the ring size is increased from five to six atoms, there is
a 120-fold increase when the ring size is increased from six to
seven atoms. However, there is a very small increase in the
value of the rate constant k* for the eight-membered complex in
relation to the seven-membered complex.
Since the evidence points to a transition state with
little bond formation, the acceleration in rates may come from
the degrees of freedom gained by the chelate ligand in the
transition state, plus the distortion that bigger chelate rings
may impose on the octahedral geometry. The conclusion that the
transition state resembles the unsaturated intermediate, cis-
(0* -DTA)W( CO )*», comes from the fact that rate constants for
ring-closure do not change as much as the rate constants for
ring-opening as the ring size is increased.
The values of the competition ratios, ka/k-i, show a
steady increase with increasing ring size. Because rate
constants for the displacement of CB from cis-(fl1-DTA) (CB)
W<C0U by tri(isopropyl) phosphite do not show a definitive
trend, one may conclude that the increase in the values of the
competition ratios as the ring increases is due almost
exclusively to a decrease in the value of ]c-* .
The activation parameters given in table XII can
provide insight into the nature of the transition states for the
118
different steps involved in this mechanism. For example,
negative values for (AHe - Afl-i) indicate that an energy barrier
is imposed upon the ring-closure in relation to the bimolecular
ligand attack. A smaller difference in the enthalpies of
activation, (Afia ~ )» for DTA = DTO suggests a greater
distortion of the octahedral geometry for the bigger rings when
ring-closure of cis-(fl1-DTA)W(CO)«. takes place. There is also
observed a more negative entropy of activation for the
bimolecular attack than for the ring-closure. Furthermore,
negative values for ASe - i S-i obtained from the thermal studies
reinforce these findings.
The rate-determining step for the reactions of (ft®-
DTA)W(CO)** with L will depend on the relative values of kob.d
and k'ob«d (equations (30) and (31)). If k»b.d >> k' ot.mci, the
reaction will be biphasic. Conversively it will be monophasic
when kob.d << k'ob«d. Changes in the rates of the step governed
by kot>.d are dominated by [L] and the value of ki. For a given
L, the value of k'obmd (k'o,tomci = k*.) is independent of [L].
Furthermore, the nature of DTA does not significantly affect the
value of k*. (16.0(14) X 10-= sec-1 and
18.0(3) X 10-= sec"1 for DTD and DTU at 35 °C for L =
tri(isopropyl) phosphite). Thus, for a given [L] the ratios
ki/k'cb.cj will dictate the form of the overall rate law for the
displacement of DTA.
Assuming that the value of k^ (L = tri(isopropyl)
phosphite at 35 °C) for DTO and DTN is 18 X 10"= sec-1, the
119
ratios ki/k'ota.c* can be calculated (DTO (0.0089), DTN (0.36),
DTD (51), DTU (67)). These values indicate that the rate-
determining step for the reactions of (n®-DTA)W(C0)«^ with
tri(isopropyl) phosphite changes as a function of the ring size.
The rate-determining step for the reactions of DTO and DTN is
governed by kob.cs, while for the reactions of DTD and DTU it is
governed by k'ob.d.
Another factor that may influence the rate law for the
reactions of a particular (fP-DTA)W(CO)^ is the effect of the
coordinated L on k*. (k'ot.»ci). The values of k* for the
reactions of cis-((V-DTHp)(L)W(CO)^ with various L at 44.5 °C
were found to depend on the steric properties of L (table VI).
The value of k*. increases with the Tolman cone angle (11) of L
(L = CP, k*. = 4.48(8) X 10~= sec-1, cone angle = 101°; P(0Me)a,
15.3(4) X 10"s sec"1, 107°; P(0-i-Pr)a, 143(3) x 10"= sec"1,
130°; P(OPh)a, 118(9) X 10"a sec-1, 128°; P(n-Bu)3, 289(12) X
10-® sec"1, 132°).
The form of the rate law for the reactions of DTN, for
which kob.cj/k'ote.d is closest to unity, will depend critically
on the steric nature of L. Monophasic behavior is expected for
bulky ligands because k ' o b . d >> kot>.<*. Biphasic behavior is
expected for non-bulky ligands since k'obsc* << kob.d. Biphasic
behavior was observed for the reactions of (rF-DTN)W(CO)«. with
CP (cone angle 101°) 12.). However, for larger ligands (P(OHe)s
(cone angle 107°), P(0Et)a cone angle 109°) non-common intercepts
were observed for the plots of 1/ko.b.c vs. 1/tL] (£).
120
The latter behavior was attributed to competition
between path A and B described in figure 5. For larger ligands
than P(OEt)s, (P(OPh)a (cone angle 128°), P(0-i-Pr)a (130°),
P(Ph)a (145°)), similar intercepts for plots of l/kob»«a vs.
1/[L3 were observed. Thus, indicating that k'eb«> >> kot,«d .
These inconsistencies prompted the preliminary
kinetics reinvestigations of the reaction of (ne-DTN)W(CO)*. with
trimethyl phosphite. The reactions were carried out following
the exact conditions of previous studies (2.), however
chlorobenzene was used as a solvent. In this study, more data
points (which improved the accuracy of the results) were
obtained.
Plots of In (A* - Atoimr,* ), (At, 1 ank s absorbance of the
ligand-solvent solution), vs. time showed a curvature indicative
of a biphasic behavior (figure 43). In previous studies biphasic
behavior was not observed, perhaps due to limitations of data
acquisition. Furthermore, if the data were acquired only at the
early stage of the reaction, the plots of lnfA* - Abx-r,*) vs.
time may have shown a very small curvature or none at all. In
addition, small curvature in this plot can be attributed to a
small difference between the experimentally-determined A. and the
true value (£2).
121
JQ < I
<
0.4
0.3
[P(OMe)3] = 0.3332
[P(OMe)3] = 0.8824
15 20 10"3 TIME, s
30 35
Fig. 43- -Plots of In (At - vs. time for reactions of (DTN)W(C0)<. with P(OMe)a in chlorobenzene at 43.3 °C at two concentrations of P(OMe)3.
122
From figure 43, it can be noticed that the value of
increases with the concentration of P(OMe)a. These plots
are concave downward at low [P(OMe)a] and concave upward at
higher [P(OHe)al. One may conclude, tentatively, that no
interchange pathway (path b in figure 5) is operative in this
system. The mechanism for the reactions of (ne-DTN)W(CO)*, will
be investigated further.
The fact that (ns-DTHp)W(CO)^. undergoes ring-opening
through a different pathway is surprising, especially, when the
kinetics evidence points to an interchange pathway involving a
great deal of bond formation in the transition state.
CHAPTER BIBLIOGRAPHY
1. Dobson, G. R.; Basson, S. S.; Dobson, C. B. Inorg. Chim. Acta. 1985, 105, L17.
2. Schultz, L. D. ; Dobson, G. R. J. Oreanomet. Chem. 1976, 124, 19.
3. Schultz, L. D.; Dobson, G. R. J. Oreanomet. Chem. 1977, 131, 285.
4. Illuminati, G.; Mandolini, L. Acc. Chem. Res.. 1981, 4, 95.
5. Asali, K. J.; Basson, S. S.; Tucker, J. S.; Hester, B. C.; Cortes, J. E.; Awad, H. H.; Dobson, G. R. J. Am. Chem. Soc.. 1987, 109, 5386.
5. Simon, J. D.; Peters, K. S. Chem. Phvs. Lett.. 1983, 98, 53.
6. Simon, J. D.; Xie, X. J. J. Phvs. Chem.. 1986, 90, 6715.
7. Langford, C. H.; Moralejo, C.; Sharma, D. K. Inore. Chim. Acta. 1987, 126, Lll.
8. Seder, T. A.; Church, S. P.; Weitz, E. J. Am- Chem. Soc., 1986, 108, 4721.
9. Bonneau, R. ; Kelly, J. M. J. J. Ajq. Chem. Soc. . 1980, 102, 1220.
10. Dobson, G. R.; Dobson, C. B.; Mansour, S. E. Inore. Chem.. 1985, 24, 2179.
11. Tolman, C. A. Chem. Rev.. 1977, 77, 343.
12. Espenson, J. H. "Chemical Kinetics and Reaction Mechanisms", McGraw-Hill Book Co., New York, 1981, pp. 65-71.
123
CHAPTER V
CIS-TRANS ISOMERIZATION OF (L)eW(COU
Experimental
The cis-trans isomerization of (P(n-Bu)3 )eW(CO)*» was
monitored following the increase of the absorbance at 415 nm
using the in-house built spectrophotometer described in chapter
II. 3 1P NMR spectra were recorded employing a Varian VXR 300
NMR spectrometer (i).
Cis-trans isomerjgation
The plot of absorbance vs. time for the reactions of
(fF-DTHp)W(CO)* with P(n-Bu)a is given in figure 44. The first
and the second segments of the plot can be ascribed to the
formation of cis-(n1-DTHP)(P(n-Bu)a)W(C0)^ and
cis-(P(n-Bu)a)eW(COU, respectively. The third segment, for
which an increase of the absorbance is observed as a function of
the time, can be assigned to the formation of the colored
trans-(P(n-Bu)a )eW(CO)+. (2). The carbonyl stretching spectrum
for the reaction product at t- is given in figure 12. The
strong band at 1870 cm-1 suggests a mixture of the cis and trans
isomers. This carbonyl stretching spectrum is consistent with
the ones reported by Mosbo and his co-workers (2.) arid by Howell
et al (2).
124
125
1 4
OS
• • •
of 44
Fig. 44--Plot of absorbance vg. time for the reaction (fla-DTHp )W( CO)«, with P(n-Bu)a (0.1071 M) in chlorobenzene 5 °C. Ordinate = absorbance, Abscissa = seconds X 10~"-4
at
126
The reactions of (fF-DTA)W(COU, (DTA = DTHp, DTD, and
DTU) with L, (L = CP, trimethyl phosphite, tri(isopropyl)
phosphite, and triphenyl phosphite) did not show a third segment
for the plot of absorbance vs. time. This outcome is due to
either the lack of cis-trans isomerization of the final product,
or to the fact that the trans isomer is colorless. The carbonyl
stretching spectra (figures 13 and 16) of the product of the
reaction of cis-(fl1 -DTHp)W(CO)^. with L, (L = CP and triphenyl
phosphite) show that the final product is exclusively the cis
isomer. However, for L = trimethyl phosphite and tri(isopropyl)
phosphite the carbonyl stretching spectra (figures 14 and 15)
suggest there is a mixture of the cis and trans isomers.
The cis-trans isomerization of (L)sW(CO)*., (L = P(n-
Bu ) a ) , in CB was studied at 35.2 °C, 44.5 °C and 54.6 °C. The
reactions for the isomerization process were monitored after the
reactions for the formation of cis-(L)eW(CO)«* were completed (at
least eight half-lives). Plots of ln(A- - A*) vs. time were
linear over two half-lives (figure 45). These reactions were
carried out in the presence of a high excess of L. The observed
pseudo-first order rate constants, for the transformation
described by the chemical equation (39), were independent of the
concentration of L.
cis-(P(n-Bu)a )aW(CO)f trans-(P{n-Bu)3)aW(CO)*• (39)
127
(At -A b | )
(Aoo-At)
for ^Ki g;v, 4 57 P l o t S ° f ( t o p ) l n { 7 U - vs. time
the third segment of this plot, a t 415 rim, f o r r e a c t i o n o f (IT (0.1084 M) at 44.5 °C; and ln(A- - A*) vs.. t i m e . seconds X 10~3.
obteined by monitoring DTHp)W(C0U with P(n-Bu)3
(bottom) these data plotted as Ordinate = absorbance. Abscissa =
128
The rate law for the reaction reaction described by
equation (39) is given by equation (40).
-d[Substrate ]/dt - kot,«d [ Substrate ] (40)
Substrate = cis-(P(n-Bu)s )eW(CO)<»
kobad = (k? + k-7) (41)
These rate constants (k-7 + k—7) are given in table XIII.
These results are consistent with results observed for other
systems (.2., J., it) •
TABLE XIII
RATE CONSTANTS FOR THE ISOMERIZATION OF (P(n-Bu)a)eW<COU IN CHLOROBENZENE AT
VARIOUS TEMPERATURES
Temp [L] 1 0 " 1 zAt loin
54.6 0.3259 10.49(7) 54.6 0.3259 9.74(5)
44.5 1.616 2.20(2) 44.5 1.059 2.24(1) 44.5 0.9180 2.29(3) 44. 5 0.7729 2.31(1) 44.5 0.7041 2.21(4) 44.5 0.2372 2.34(3) 44.5 0.1080 2.26(1)
35.2 0.8702 0.481(4) 35.2 0.4301 0.471(6)
129
Dixon et al. (.2) have studied the cis-trans
isomerization of (P(n-Bu)a)eW(COK in n-heptane employing 3 1P
NMR spectroscopy. At 46 °C, the rate constant for the
isomerization was reported as 3.5(4) X lO-** sec-1, which is a
slightly higher value than the values obtained in this study at
44.5 °C in CB (2.28(3) X 10~+ sec"1).
The relative concentration of the trans and the cis
isomers can be calculated by 3 1P NMR spectroscopy.
This method is useful in the characterization of complexes of
the type (L)sW(CO)*, (L = phosphines, phosphites) since the cis
and the trans isomers show resonances which are well separated
(1, 1, £, £>•
3 1P NMR studies
In order to determine the ratio of cis and
trans-(L)eW(CO)«., L = P(n-Bu)a, by 3 1P NMR, the solutions
containing the products of the reaction of (fF-DTHp)W(CO)^ with
L were placed in NMR tubes and allowed to thermally equilibrate
in a constant temperature bath (haake ED) for 24 hours at their
respective temperatures (35.2 °C, 44.5 °C and 54.6 °C). To
ensure constancy in the temperature of the product mixture
during the progress of the experiment, the NMR probe was
pre-heated at the equilibrated temperature of the NMR tube which
contained the sample. Then, rapidly the NMR tube was placed in
the probe. The NMR tube was allowed to stand for at least 15
minutes in the NMR probe prior to recording.
130
The NMR spectra were recorded for 30 minutes (pulse intervals = 5
seconds) employing 85% HsaPCW as an external standard. The
spectra at 35 .2 °C, 44 .5 °C and 54.6 °C are shown in figures 46,
47 and 48, respectively.
The resonances at 6 -1.8 and 6 -9.5 were assigned to
phosphorus atoms in trans and cis- (P(n-Bu)a )aW(C0)*.,
respectively. These assignments were made by comparison of
these resonances to resonances of actual species reported by
Mosbo and co-workers (6-2.4, 6-10.6) (2), Grim and Wheatland
(6-2.5, 6-10.0) (5.) and McFarlane et al (6-2.6, 6 -10.4) (.£).
However, these resonances are in disagreement with those
reported by Dixon and co-workers (6-6.03, 6 2.4) (1).
Since, at equilibrium,
k-?[cis-(L)eW(CO)*] = k-v[trans-(L)eW(CO)^], (42)
the equilibrium constant for the cis-trams isomerization is
given by equation (43).
K»d = [trans3/[cis] = k-r/k-r (43)
The ratios, [trans3/[cis], can be calculated from the
relative integrated intensities of the resonances at 6(-1.9) and
6(-9.6), which have been assigned to the trans and the cis
isomers, respectively. Table XIV presents these values,
together with those for K»c*.
131
2: 0-_ IT. Q.
- hit-
L j
.CD I
-ID I
VLB'S*
. r i
91
O .(V U • j 2
to C A3 •
U o -P 0 TJ CM 13 «
A3 uo <T> i
<n -P •H fE3 0 M-l V a 0 a) N gj c p 4) H .0 -M 0 0 h 03 0 04 r-i W J3 a jg a 2 •H 04 J (!)
i o (!)
i u "W
V0 a < 8 dO i •H S3 to CO
i c «w 04
- k <\i
132
}
§ k •P O o
in a • rd <4*
i tO|4J •rfl fd
0
0) e 0) N § G
5 a)
-P 0
ft rH n X 0
ft 41 * 01 O i O I W ®s 5 <> (U
• o
•W 3 En PQ
1 c
13 3
.CO i
i
I- 3 o '
ifS. 5T
-J O)
OJ
CD
—
— d •
" ^ <TJ
H
o 0
<n +> mm <sO
_ 0 D XJ *
_ t d ^ ra m
» ,-p - to a
— 0 I 03 — d
a) 0 N
ru ~*ri
I
I a. CL-yV
o ' *
- I
-J 3
d g 0 3 A H 0 -p h a o 0) H a x to a e d S *H
1 8 I ^ CO 3
92
&! m
c
Ou
134
TABLE XIV
CIS : TRANS RATIOS OF (P(n-Bu)a)aW(eOU IN CHLOROBENZENE AT VARIOUS TEMPERATURES
T, °C [P(n-Bu)»] relative abundances ~K.«, trans cis ttrans]/[cis]
35.2 0.03983 132.3 45.3 2.92 0.03983 131.0 44.2 2.96
44.5 0.03983 89.5 33.4 2.68 0.03983 88.8 31.8 2.79 0.03983 - — 3.17
54.6 0.03983 94.9 34.7 2.73
Average trans/cis = 2.82 * Error limits ca. = ± 6%
The composite rate constant for the isomerization at
44.5 °C (2.28(3) X 10-* sec-1) observed in the present study is
in reasonable agreement with the rate constant under similar
conditions (3.5(4)X10—* sec-1 at 46 °C) reported by Dixon and
co-workers (3). However, the values of K_c found in the present
study (table XVIII), differs significantly from the value of
theirs (K-„ =8.93).
The determination whether this isomerization takes
place in the five coordinate intermediate, [LW(CO)*], on the
time-scale of the ligand-substitution reaction, or through
isomerization of cis-(L)aW(CO)*, has been a subject of
discussion (.2~k.t 7-20). Dobson, Awad and Basson's work
indicated that the five-coordinate intermediates, cis and
trans-[ (P(CAH= )A )W(C0) .3, produced via pulsed-flash photolysis
135
of cis- [ (pip) (PCGsIts )a )W(COU] in chlorobenzene, do not inter-
convert on the time-scale of the bimolecular ligand attack (7.).
a iP NMR studies of the thermal ring displacement of
(tmpa)W(COU, (tmpa = N.N.N',N'-tetramethyl-1,3-diamino-
propane), indicated that only the cis-disubstituted product is
initially produced, and that the production of the trans product
takes place subsequent to this (7). Furthermore, the reaction
rates of the thermal cis to trans isomerization of
cis-[(P(C«,H)a)BW(COU] has been studied and the results are
consistent with a process which does not involve W-P bond
fission (2). In addition, Dixon and co-workers (D have shown
that during the cis-trans isomerization of (P (n-Bu)3 )eW(CO )*. in
n-heptane under an atmosphere of 13C0 there is no formation of
(P(n-Bu)a) (*3CO)W(CO)<». This experimental observation suggests a
non-dissociative process. Similar results have been observed by
Darensbourg amd Gray (4). They reported that after heating
((Et)3 )PW(CO)<* under an atmosphere of iaC0, no incorporation of
iaC0 into (P(Et)a)W(COU was observed. In the present study, the
carbonyl stretching spectrum of a mixture of cis and trans
(P(n-Bu)a)sV(CO)*. in the presence of an excess of piperidine in
CB showed no indication of incorporation of piperidine into the
complex.
From equation (41) and (43) the individual rate constants k- and
1c--?- can be calculated. These rate constants are given in table
XV.
136
TABLE XV
RATE CONSTANTS FOR CIS-TRANS ISOMERIZATION OF (P(n-Bu)a )aW(COU IN CHLOROBENZENE
AT VARIOUS TEMPERATURES
T, °C klssnsr1 cat Ion 10s *kv 10s T, °C 10s (sec-1) (sec-1)
35.2 4.79(2) 3.56 1.21
44.5 22.8(5) 16.9 5.89
54.6 101(5) 74.7 26.4
fcfi-r = 30.9(5) kcal/mole, Atf--* = 31.3(5) kcal/mole A 2 2 . 4 ( 1 4 ) cal/deg-mol , A S ^ - 21.6(12) cal/deg-mol * Error limits for determination of the integrated intensities of the 3 1P NMR absorptions are estimated to be £&. ± 3%.
In this study, the highly positive entropies of i 4
activation (AS- = 22.4(14) cal/deg-mol, AS—^ ~ 21.6(12) cal/deg-
mol) suggest a great deal of reorganization in the transition
state for both pathways.
CHAPTER BIBLIOGRAPHY
1. The experimental assistance by Mr. T. Corby Young is gratefully acknowledged.
2. Boyles, M. L.; Brovm, D. V.; Drake, D. A.; Hostetler, C. K.; Maves, C. K.; Mosbo, J. A. Inore. Chem.. 1985, 24, 3126.
3. Howell, J. A. S.; Dixon, D. T.; Kola, J. C. vj. Chem• SQQ• Dalton Trans.. 1984, 1307.
4. Darensbourg, D. J.; Gray, R. L. Inore. Steffi., 1984, 23, 2993.
5. Grim, S. 0; Wheatland, D. A. Inore. Chem.. 1969, 18, 1716.
6. McFarlane, H. C. E.; McFarlane, W.; Rycroft, D. S. Chem. , Dalton Trans.. 1976, 1616.
7. Dobson, G. R.; Awad, H. H.; Basson, S. S. Inoxs. Ctum. Acta. 1986, 118, L5.
8. Majunke. W.; Leibfrits, T.; Mack, D.; Dieck, H. T. Chem• v., 1975, 108, 3025.
9. Darensbourg, D. J. Inore. Chem.. 1979, 18. 14.
10. Darensbourg, D. J.; Baldwin, B. J. J. Iffi. Chem. goo., 1979, 101, 6447.
11. Darensbourg, D. J.; Kudaroski, R.; Schenk, W. Inore.
Chem.. 1982, 21, 2488.
12. Bailar, J. C., Jr. Inore. Nucl. Chem.. 1958, 8, 165.
13. Cotton, F. A.; Darensbourg, D. J.; Kleins, S.; Kolthammer, B. W. S. Inore. Chem.. 1982, 21, 2661.
14. Darensbourg, D. J.; Kump, R. L. InorK. Chem-. 1978, 17, 2680.
15. Fischer, H.; Fischer, E. 0.; Werner, H. J. J. Qrsanomet. Chem.. 1974, 73, 331.
137
138
16. Hoffmann, R. ; Howell, J. M. ; Ross, A. R. J. Ain. Chem. Soc.• 1976, 98, 2484.
17. Pomeroy, R. K.; Vancea, L.; Calhoun, H. P.; Graham, W. A. G. Inorg. £hm-, 1977, 16, 1508.
18. Ray, P.; Dutt, N. K. £. Indian Chem. SQC., 1943, 20, 81.
19. Springer, C. S. Jr.; Sievers, R. E. Inorg. Chem-. 1967, 6, 852.
20. Vancea, L.; Pomeroy, R. K.; Graham, W. A. G. Chgm. Soc.. 1976, 98, 1407.
CHAPTER VI
CONCLUSIONS
The reactions of (n«-DTA)W<CO)«. with L , (L * CP and
triphenyl phosphite) are biphasic and with L (L = n-butyl
phosphine, tri(isopropyl) phosphite, and trimethyl phosphite)
the reactions are triphasic. During the course of these
reactions, appreciable amounts of cis-(fl1-DTA) (L)W(CO)^. are
produced.
The most likely mechanism by which (fle-DTA)W(CO)^ ,DTA
= DTD and DTU, react with L = phosphites is depicted in figure
49. There is an unimolecular ring-opening of (fle-DTA)W(CO)<* to
afford cis-tn^-DTAWCO)*, which then will undergo an
equilibrium involving solvation and desolvation governed by ka
and k-a, respectively. This intermediate will also undergo a
competitive ring-closure and bimolecular attack by L. Upon
attack of cis-(n*-DTA)W(COU by L, cis-(n*-DTA) (L)W(COU is
produced. Dissociation of DTA from cis-((V -DTA) (L)W(CO)*. will
produce cis-[LW(COU], which will undergo an equilibrium
involving solvation and desolvation governed by k-s and k-a,
respectively. Further attack by L at cis-[LW(CO)«»] will produce
cis-(L)aW(COU.
139
1 4 C
° o
\ x
4 -
r > I
\ 4-
° o
o o - X / >1
% 0° \ S
o o § o o
u +
o o
° a
V O O §
y ' \ +
h -JtL
1
O O
O O -\ / —Y— /\
O O \ /
^ C _ o O
Oo- u o
a o u
o r ;
r—4 c £ r~\ P - l
0 5 Q U t 0 ) -li > C 0 —
< D £ x; o - M p
u - i
0 -
6 a t n
x; q o CD I! C " <
T 3 £ h 0 ) Q c n o -D * c t 0 H M Q
CXf U - J
1 0 I
<x> 4 J C CD
• £ & 0 0
• H o Uu r f l
a J < n
• H " 0 X !
141
Furthermore, cis-(L)eW(COU, L = tri(n-butyl) phosphine,
tri(isopropy1) phosphite, and trimethyl phosphite will undergo a
cis-trans isomerization to afford a final mixture of
cis- (L)eW(COU and trans-(L)aW(COU. Since this isomerization
takes place within the time scale for the formation of
cis-(L)sW(CO)^ and its subsequent reaction with L, the overall
process involves three consecutive reactions. In table XVI, the
rate constants for the overall mechanism just described for L =
tri(isopropyl) phosphite are given.
TABLE XVI
RATE CONSTANTS INVOLVED IN MECHANISM DESCRIBED IN FIGURE 49
Complex (fF-DTD)W(COU (na-DTU)W(COU
ki (sec-1) 8.1(3)X10~3 12.1(5)xl0~3
k'-i (see-31 ) 94(12)X103 62(2)X103
k'e (sec-1) 97(22)X10a 89(9)X103
Jfe/k-! (M-M 1.03(9) 1.44(9)
k*. (sec-1) 0.160(4)X10-3 0.180(3)X10~3
kr-=s (sec-4) 6.17X10** 6.17X10**
ke> (k-=/k= [CB]) (sec-1 )
7.01(2)X103 7.01(2)X103
*k-7 (sec-1 ) 0.0356XZ10-® 0.03 56X10-3
* 1 ( s e c - 1 ) 0.0121X10-3 0.0121X10-3
L = P(n-Bu)3 Temperature = 35.2 °C
142
The proposed mechanism for the reactions of
(n®-DTHp)W(COU with L = phosphites and phosphines is described
in figure 50. According to figure 50, an initial interaction of
L with (na-DTHp)W(COU will afford cis-(n1-DTHp) (L)W(CO)*.. The
mechanism by which cis- (H* -DTHp) (L)W(CO)<* undergoes displacement
of DTHp by L is analogous to the mechanism just described for the
reactions of cis-tf^-DTA) (L)W(COU, DTA = DTD and DTU, with L.
In table XVII, the rate constants for the overall mechanism
described in figure 50 are given.
TABLE XVII
RATE CONSTANTS FOR THE OVERALL MECHANISM DESCRIBED IN FIGURE 50
10—3k'& 103k«. 10~5 k'-i 10-^k-a 10-3k'A 10®k-7 103k-7 sec-1 sec-1 sec-1 sec-1, M-1 sec-1, M 1 sec 1, M 1
36.7(11) 0.327(1) 6.591 6.5(14) 7.2(3) 0.0356s 0.01213
11.62®
*in BB solvent ®in DCE solvent 3L = P(n-Bu)a Temperature = 35.2 °C
0 c
\
.c?
w
c o
to0
143
o c
o °
o c
w
C B c o
*5 i C B
-5
o c
w
l \ c ° o
M .
o c
w
,o°
c o
Fig- 50---Proposed mechanism for the overall displacement of DTHp from (He-DTHp )W( CO )*• by L.
BIBLIOGRAPHY
Al-Kathumi, K. M.; Kane-Maguire, L. A. P. J. Inorg. Nucl.
Chem.. 1972, 34, 3759.
Angelici, R. J. Oreanomet. Chem. Rev.. 1968, 3, 173.
Angelici, R. J.; Graham, J. R. J. £ffi. Chem. Sog., 1965, 87, 5586.
Angelici, R. J.; Graham, J. R. 1. M - Chem. SQC., 1966, 88, 3658.
Asali, K. J.; Basson, S. S.; Tucker, J. S.; Hester, B. C.; Cortes, J. E. ; Awad, H. H.; Dobson, G. R. J. Ajn. Chem. Soc.. 1987, 109, 5386.
Atwood, J. D. Inorganic and Oreanometallie Reaction Mechanisms: Brooks/Cole Pub. Corp: Monterey, Cal., 1986.
Atwood, J. D.; Brown, T. L. M - Chem. SOC-. 1976, 98, 3160.
Bailar, J. C. Jr. Inore. Chem.. 1958, 8, 165.
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