Indian Journal of Chemistry Vol. 39A, November 2000, pp. I 164-1168

Photosubstitution kinetics and mechanism of Mo(IV) and W(IV) cyanide complexes with nitrogen heterocycle ligand- 1,3,5,7- tetraazatricyclodecane

S I Ali*

Department ofChemistry,Jamia Millia lslamia, New Delhi 110 025, India

and

Kowsar Majid

Department of Chemistry, Regional Engineering College, Srinagar 100 006, India

Received 3 March 2000; revised 28 June 2000

Photochemical reaction of octacyano-molybdate (IV) and -tungstate (IV) with macrocyclic ligand- I ,3,5,7-tetraazatricyclodecane (hexamethylenetetramine or hexamine, urotropine) has been studied in aqueous medium. Molybdenum and tungsten cyanides form complexes with hexamine in I: I stoichiometry. The substitution of cyanide ligand by the macrocycle is confirmed by the observed shift in the transitions of the electronic spectra. However, the shift in the transitions is significant in the tungsten than the molybdenum system. The rate of formation of the product depends on the concentration of hydronium ions present in the solution. The rate of the reaction increases with increase in ligand and decreases with increase in metal concentration. The rate constant and quantum yield values are higher for tungsten than molybdenum system. A rate law equation on the approximation k3>>k4 has beer. derived which confirms the experimental results . The elementary rate constants k2 and k4 have been calculated. The observed and calculated rate constant values are found in conformity with each other. In the presence of excess of ligand, the observed rate law is as under:

k = kzk4 l,,[Hp+][N 4 (CH 2U olw/ ) + k![J-/30+]

The 1, 3, 5, 7- tetraazatricyclodecane [(CH2) 6N4. commonly known as hexamethylenetetramine, hexamine or urotropine] has wide synthetic applications 1

-4. It is starting material for the

preparation of explosives like HMX and RDX, for the production of technical rubber articles, phenol­formaldehyde resins and deodrants etc.5

-7

• It has biological applications in the manufacturing of a number of medical products and it is also used in treating infection with the HTLV III virus8

. The compound is of significant interest in coordination chemistry, since it contains four equally spaced nitrogen atoms and is capable of acting as a quadridentate ligand in the chemical reactions. Contradictory reports have been published on the coordinating behaviour of this ligand with different transttton metals by the studies of infrared spectroscopy, electrical conductivity and thermal analysis. In the complexes of CuCh HMT A9 N-atom is directly coordinated to the central atom. The same behaviour has been found in cadmium thiocyanate hexamethylenetetramine 10

, showing the ligand to behave as monodentate instead of quadridentate. Very little data is available on the coordination of the ligand with Mo(IV) and W(IV) complexes. However,

the structure of (CH2) 6N4 Mo(C0)5 has shown an axially-distorted octahedron with an apical nitrogen lone pair of the organic ligand coordinated to the metal atom 11

•

The [M(CN)st , where M = Mo(IV) and W(IV) under ligand field excitation undergoes efficient photoaquation of one or more cyanide ligands which is followed by secondary thermal steps depending upon the factors like pH, nature of the ligand, irradiation time, wavelength of radiation etc. For example three primary products, namely, [HM(CNh(H20)] 2

- ,

[M(CNh(H20)] 3-and [M(CNh(OH)( are formed in acid, neutral and alkaline media respectivel/ 2

. The photoreactivity of various ligands with octacyano­metallates of Mo(IV) and W(IV) has been reported with the formation of the final product depending on the number of the cyanide ions removed, e.g., with 1 ,2,3-indane-trione hydrate 13

, 1, 1 0-phenanthrol ine, 2,2'-bipyridyl 14

, 5-nitro 1,10-phenanthroline 15 and diethanolamine 16

, two cyanide ions are removed while with 1 ,2,3-benzotriazole only one cyanide ion is removed with the formation of the final produce 7

[W(CN)7BTt. Collenberg earlier reported that up to maximum 4CN- ions could be replaced by OR in the coordination shell of the central ion when alkaline

ALl et at.: PHOTOSUBSTITUTION KINETICS OF Mo(IV) & W(IV) CYAN IDE COMPLEXES 1165

solution of Mo(CN)84- was irradiated 18

. Since hexamine is a good cr donor but a poor n acceptor, it is supposed to strengthen the bond between cyanide ligand and the metal centre. Keeping in view the interesting coordinating behaviour of the macrocycle organic ligand with different transition metals and the photochemistry of the octacyanometallates, title study was taken up.

Materials and Methods K4M(CN)s.2H20 [where M = Mo (IV) and W (IV)

was prepared by the method of Leipoldt et a/19. The 1,

3, 5, 7- tetraazatricyclodecane (hexamine) was of AR grade. All the solutions were prepared in doubly distilled water. The ionic strength was maintained at !.Ox w-~ M with aqueous NaCI0-1. A photophysics immersion Well qumtz reactor (cat. 3210, model RQ 125) was used for irradiation at ca. 365 nm. The temperature of the reaction was maintained at 25 ± 0.2°C by circulating cold water through it. The absorbance was recorded on Jasco V -570 UV IVIS/NIR spectrophotometer using 1 em quartz cells. The kinetics of the photolysed reaction was studied by recording the absorbance on 20 D spectrophotometer (Milton Roy-3330197069) using quartz cells. The solution pH was recorded on a Control Dynamics pH meter (model APX 175). Cyanide ion estimation was carried on an Elico ion analyser ( 4 model LI-126 ) using cyanide selective electrode. The light intensity was calculated by Murvo's method20 and the quantum yield was determined by the methods of Cal vert and Pitts21

. The

- -- - - -·- ---------~

I-I i

Fig. 1 -Electronic spectra of K4Mo(CN)8 and hexamethylenetetramine N4(CH2) 6 sys tem. (A) Unirradiated solution of Mo(CN)/ and N4(CH 2) 6 (B-F) Mo(CN)8

4- and

N4(CH2) 6 solution exposed for 10, 20. 25 , 32 and 60 minutes respectively.

stoichiometry of the reaction was determined by Job's method of continuous variation and mole ratio method22

•

Results and Discussion In M(CN)8

4- the absorption spectra consists of

metal centered (MC) bands, two very weak singlet­triplet bands at 580 nm and 510 nm (Mo) and 625 and 600 nm (W) as well as three singlet-singlet bands at 427 nm, 367 and 304 nm (Mo) and 433, 369 and 302 nm (W) . In the UV part of the spectra there are two charge transfer bands, metal to solvent band at 267 nm (Mo) and metal to ligand (MLCT) charge transfer at 240 nm (Mo) and 249 nm (W). These are the characteristics when M(CN)s4- is in the ground state23

.

On excitation, reaction occurs under MSCT which can be reached directly or by the internal conversion from MLCT transition. Since the electronic spectra of K4Mo(CN)s and K4W(CN)8 exhibit maxima at 363 nm (27,548 cm- 1

) and 360 nm (27,776 cm- 1) respectively

(Figs 1 and 2) which are assigned as CTTL (MLCT, M ---1 CN). On prolonged irradiation this transition starts decreasing and a new band starts appearing at 610 and 590 nm for Mo(IV) and W(IV) cyanide complexes respectively which is assigned as CTTL (L =solvent H20 or any other ligand) . It is this region which shows shift after reacting with any other ligand and hence ensures photosubstitution. On photoreaction with the ligand MLCT transition shifts to 366 nm

w u z < CXl

"' 0 (f) CXl

""

2

350 400 500 600 700 750 WAVELENGTH (nml

Fig. 2- Electronic spectra of K4W(CN)8 and hexamethylenetetramine N4(CH2) 6 system. (A) Unirradiated solution of W(CN)8

4- and N4(CH2) 6 (B-I) W(CN)8

4- and NiCH2) 6

solutions exposed for 2, 5, 10. 15, 25 , 35 . 55 and 75 minutes respectively.

1166 INDIAN J CHEM., SEC A, j\;QVEMBER 2000

(27,548-727,322 em-') in Mo(IV) and to 368 nm (27,776-727,173 cm-1

) in W(IV) which decreases with the time of irradiation and a new band appears at 608 nm (16,477 -716,393 cm-1

) in Mo(IV) and 562 nm (16,900-717,793 cm-1

) in W(IV) with a shift towards lower wavelength. This transition is assigned as CTTL [M (Mo, W (IV)) -7 HMT A]. The assignment of these transitions is analogus to transitions of other complexes24 such as (CN)5Fe(Pz)3

+

and (NH3)sRu(Pz)2+.

From the comparison of the electronic spectra of the complexes the shift in the case of W(CN)8

4-

system is more pronounced than in molybdenum which can be as a result of effective orbital overlapping between a low lying n* orbital of the ligand and metal t2g orbitals25

'26

. The enhanced spin orbital coupling in the case of heavier elements and the jncreasing value of 10 Dq while going from first to second and to the third transition series are also responsible for the significant transition27

'28

.

The photochemical reaction stoichiometry was found by mole ratio method. The reaction was found to occur in 1:1 stoichiometry, the equation for which can be represented as:

H30 +

M(CN):· + N.(CH 2 ) 6 ~ [HM(CN)7 ••• ••• N4 (CH 2 ) 6 ]2

- +CW

-H 20

Table I -Rate constants and quantum yield values for the reaction of K4Mo(CN)s with hexamethylenetetramine (N4 (CH2)6

[Irradiation wavelength=365 nm; Ia=9.995 X 10'3 s' 1;

Ionic strength= 1.0 X 10'2 mol dm-3; Temp= 25 ± 0.2°C]

[Mo(CN)s4·] [N4(CH2)6] x pH kx 104 s- 1 Quantum

x 103 mol dm·3 102 mol dm-3 yield x 10 (ljl) mol eins- 1

10 10 3.5 0.66 O.Q7 10 10 4.5 1.66 0.17 10 10 5.0 0.53 0.05 10 10 6.0 0.40 0.04 10 10 7.0 0.33 0.03 10 10 10.2 0.26 0.03

10.0 12.0 4.0 1.75 0.18 10.0 10.0 4.0 1.66 0.17 10.0 8.0 4.0 1.53 0.15 10.0 6.0 4.0 1.33 0.13 10.0 4.0 4.0 1.11 0.11 10.0 2.0 4.0 0.68 O.Q7

12.0 10.0 4.0 0.83 0.08 10.0 10.0 4.0 1.66 0.17 8.0 10.0 4.0 2.00 0.20 6.0 10.0 4.0 2.33 0.23 4.0 10.0 4.0 2.66 0.27 2.0 10.0 4.0 3.00 0.30

The substitution of the cyanide is confirmed by the observed increase in the cyanide ion concentration at 365 nm, maintaining the temperature at 25 ± 0.2°C. The ionic strength was kept constant at L.O x 10-2 mol dm-3 and the absorbance of the resulting solutions was measured at 560 nm. The intensity of absorbed radiation , 13 , was found to be 9.995 x 10·3 s-1

. The plot of (A.,-A1) versus time were found linear for at least 2.5 half lives for Mo system and for two half lives for W system. The rate constants and quantum yield values are listed in Tables 1 and 2 for Mo and W systems respectively. The rate constant and quantum yield values increase with increase in H+ concentration up to pH 4.0 and decrease with increase in [Ofr] . The increase in concentrations of metal and ligand causes decrease and increase respectively in the rate constant and quantum yield values. The mechanism proposed to explain the observed results is given in Scheme 1.

[M(CN)s]4- hv [M(CN)s]4-* Ia ... ( l )

[M(CN)s]4-* + [M(CN)s]4-

kl 2[M(CN)8] 4-

.. . (2)

[M(CN)8]4-* + H30+ k2 [HM(CNh (H20 )]2-

+CW . . . (3)

Table 2-Rate constants and quantum yield values for the reaction of K4 W(CN)8 with hexamethylenetetramine (NiCH2)6).

Irradiation wavelength= 365 nm; Ia = 9.995 X w-3 s-1; Ionic

strength= 1.0 X w-2 mol dm-3; Temp= 25 ± 0.2°C

[W(CN)/J [NiCH2)6] pH k x l04 s- 1 Quantum x I 03 mol dm-3 x 102 mol dm·3 yield

(ljl) mol eins·1

10 10 3.0 1.83 0.08 10 10 3.5 1.08 0.11 10 10 4.0 l.l 6 0.12 10 10 5.0 0.66 0.07 10 10 6.0 0.66 0.07 10 10 7.0 0.50 0.05 10 10 8.0 0.41 0.04

10.0 12.0 4.0 1.33 0.13 10.0 10.0 4.0 1.16 0.12 10.0 8.0 4.0 1.16 0.12 10.0 6.0 4.0 1.00 0.10 10.0 4.0 4.0 0.83 0.08 10.0 2.0 4.0 0.55 0.06

12.0 10.0 4.0 2.83 0.08 10.0 10.0 4.0 l.l6 0.12 8.0 10.0 4.0 l.l9 0.12 6.0 10.0 4.0 1.83 0.18 4.0 10.0 4.0 3.1 6 0.32 2.0 10.0 4.0 4.16 0.42

ALI et al. : PHOTOSUBSTITUTION KINETICS OF Mo(IV) & W(IV) CYANIDE COMPLEXES 1167

[HM(CNh(H20)f- + N4(CH2)6 k4

[HM(CN)?(H20) ... ... .. . N(j)N3(CH2)6]2-A# (Activated complex)

A# ks [M(CN)7 N4 (CH2)6]4- + H30+ Product (P)

Scheme I

The rate of the product formation is given by:

d [P]/dt= ks[A#]

0 0 0 (4)

.. 0 (5)

.. 0 (6)

.. 0 (7)

Under the approximation k3>>k4 , the final equation comes out to be:

d[P] k2k4/a[H30+][N4(CH2)6] .. 0 (8) =

dt k3[CN-])(k1[M (CN)8t- + k2 [Hp+])

d[P]I dt l/J=

/a

k2k4 [H 30+ ][N 4 (CH 2 ) 6]

l/J = k3[CN-])(k,[M(CN)8t- + k2[H30+])

.. 0 (9)

According to Eq. (8), rate of the reaction increases with increase in ligand and decreases with increase in [metal]. From Tables 1 and 2, the rate constant and quantum yield values are maximum at pH 4.0 for both the systems. At pH 2:: 4, k1 [M(CN)8]

4- >> k2[H30+],

Eq. 8 takes the form: d[P] k2k4/a[H30+][N 4(CH 2) 6] ---

dt (k3 [CN-])(k,[M(CN)8 t-) .. 0 (10)

According to Eq. (10) the rate constant values are directly proportional to the hydronium ion concentration which is observed in the data. At pH< 4.0, the ligand gets further protonated which lowers the concentration of the reacting species, thus lowering the rate constant and quantum yield. The mechanism involves the excitation of the octacyanometallate in the ligand field band undergoing metal-solvent charge

1.4 1.8

(/)

"'2 en ,., ... 1.4 ;

0 ,;

" "' ,:,... a

:::. :: 0 .6

1.0 ~

0 .2 0 .6

0.1 0.2 0 .3 0.4

If (HMTA) X 102 m;ll dm3 0.5

Fig. 3- Plot of llkobs. versus IIN4(CH2)6 for the reaction of hexamethylenetetramine with (A) Mo(CN)8

4- and (B) W(CN)84-.

Table 3- Elementary rate constants, observed and calculated values for the reaction between hexamethylenetetramine (NiCH2)6) and (A) Mo (CN)/ (B) W(CN)8

4-

[M (CN}g4-] = 10.0x 10'3 mol dm'3; [H30+] =I X 104 mol dm'3; Ionic strength= I X 10'2 mol dm-3; I.=9.995 x 10·3 s- 1

[N4(CH2)6l X k4 mol- 1dm-3 k2 x 104 kobs·x 104 s-1 kcalc· X 104

S-l ( kculc. -kobs. }/ kobs. 102mol dm'3 mol-1 dm3

(A) Mo(CN)8 4' + NiCH2) 6 system

12.0 0.22 2.15 1.75 1.77 0.00798 10.0 0.26 1.81 I. 67 1.67 0.00060 8.0 0.32 1.43 1.54 1.52 -0.00910 6.0 0.43 1.07 1.33 1.34 0.00450 4.0 0.65 0.71 1.11 1.08 -o.02610 2.0 1.30 0.35 0.68 .68 0.00588

Average -o.002009

(B) W(CN)84- + NiCH2) 6 systesm

12.0 1.45 2.79 1.33 1.30 -o.0225 10.0 1.74 2.32 1.16 1.21 0.0431 8.0 2.17 1.86 1.16 1.12 -0.0344 6.0 2.90 1.39 1.00 1.01 0.0100 4.0 4.35 0.93 0.83 0.83 0.0000 2.0 8.70 0.46 0.55 0.55 0.0000

Average -o.000630

1168 INDIAN J CHEM , SEC A, NOVEMBER 2000

transfer reaction (MSCT) forming [HM(CN)?(H20)]2-

in the acidic medium. The insertion of the hexamethylenetetramine in the heptacyano complex is verified by the shift in the absorption peak forming an intermediate complex.

On the basis of the observed results, in the presence of excess of ligand, the first order observed rate law for the reaction can take the form:

kobs= k2k41" [H 30+ ][N 4 (CH2 )6] 1+k2 [H

30+]

l 1

. . . (11)

1 = +-------

kob\ k2 k4 [Hp+][N 4 (CH 2 ) 6 ] k4 /"[[N 4 (CH 2 ) 6 ]

. . . (12) The plots of llkobs. versus l/N4(CHz)6 for both the systems are linear (Fig. 3).

The slope and the intercept were obtained by the programme grapher (Ver. 2.5) and least squares fit method. From these values, elementary rate constants k2 and k4 were calculated at various concentrations of hexamine. The kcaJc. values were found to be in agreement with kobs. values confirming the proposed mechanism (Table 3).

Acknowledgement One of the authors Kowsar Majid is grateful to

CSIR, . New Delhi for the award of research associateship. The facility provided by Prof. Zahid H Khan, Jamia Millia Islamia, for recording the spectra is gratefully acknowledged.

References I Blazevic N, Kolbah D, Belin B, Sunjic V & Kajpez F,

Synthesis, 161 (1979). 2 Siele V I, Warman M, Lecacorvi J, Hutchinson R W, Motto

R, GilbertE E, Benziger T M, Coburn MD, Rohwer R K & Davey R K, Propellants Explos, 6 (1981) 67.

3 Warman M, Siele V I & GilbertE E, J Heterocycle Chern , 10 (1973) 97.

4 Siele VI, Warman M & GilbertE E, J Heterocycle Chern, II (1974) 237.

5 Katsaros N, Tsangaris J M & Tsangaris G M, Montash chem, I (1983) 114.

6 Katsaros N, Trans met chem, 6 (1983) 345. 7 Pannetier G, Salles L & Laurent C, J less comm Metals, 3

(1971) 259. 8 Voigt, Hans, Ger. Offen. DE (3 , 537, 469), II Jun 1987,

Aprpl. 220ct. 1985; 3pp, Chern Abstr, I 08 ( 1988) 44028 . 9 Costisor, Otilia, Policec & Septimia, Chern Bull Tech Univ

Timisoara, 50 ( 1991) 4. 10 Pickandt, Joachim & Taik G G, Z. Naw rforsch b, Chern. Sci,

I (I993) 23. II Hui K Y, Chan PC & Mak T C W, lnorg Chim Acta, 84

(1984) 25 . 12 Permeredi J R, Ph. D Thesis Disserlation, University of

California, USA ( 1962). 13 Ali S I & Kaur H, J Photochem Photobiol A Chern , 6 ( 1991)

183. 14 Ali S I & Kaur H, Bull chem Soc Japan, 63 (1990) 3615. 15 Thaker B T, Kaur H & Thaker P, Trans met Chern, 23 (1998)

33. 16 Ali S I & Majid K, lnt J chem Kinet , II (1997) 819. 17 Ali S I & Kumar A, J Photochem Photobiol A chem, 72

(I993) 29. 18 Collen berg 0, Z Anorg a/leg Chern, 136 (1 924) 245. 19 Leipoldt L G, Bok L D C & Colliers P G, Z Anorg a/leg

Chern, 409 (1974) 343; 407 (1974) 250. 20 Murov S L, Handbook of photochemistry (Marcel Dekker,

New York), pp. 19 (1973) 122. 21 Calvert J G & Pitts J N, Photochemistry (Wiley, New York) ,

(1966) pp. 780, 786, 796. 22 Harrey E & Manning D C, JAm chem Soc, 72 ( 1950) 4488. 23 Samotus A & Szklarzewicz J, Coord Chern Revs, 125 (1993)

63. 24 Johnson C R & Shephered R E, /norg Chem, 22 (1983) 117. 25 Ford P , Rudd D P, Gaunder R & Taube H, JAm chem Soc,

90 (1968) 1187. 26 Craig R J & Shepherd R E, lnorg Chem, 22 (1983) 1117. 27 TomaH E & Malin J M, lnorg Chern, 12 (1973) 1039. 28 Olabe J A, Zerga H 0 & Gentil L A, J chem Soc Dalton

Trans, (1987) 1267.