Isolobal Analogy

Roald Hoffmann

Organic Structures

O

O

R R

•  Systemization in organic chemistry ! R is any alkyl group CH3 , C2H5, C6H5

! Similarly H can be replaced by any R group. !  =CH2 can be replaced by any CR2 group or

oxygen!

•  Transition metal Chemistry?

•  Organometallic Chemistry?

Common fragments

Organic Fragments •  CH4 saturated organic fragment.

•  Remove hydrogens one by one CH3

. CH2 CH≡

Inorganic Fragments •  Start with Cr(CO)6

Relationship between the two?

Remove hydrogens one by one CH3

. CH2 CH≡

Evolution of metal Orbitals

n+1 p

n+1 s

nd

Evolution of metal Orbitals

n+1 p

n+1 s

nd

s + 3p produced 4 hybrid orbitals sp3

Evolution of metal Orbitals

n+1 p

n+1 s

nd

ML6 orbitals

t2g

ML6 orbitals

t2g

Basis of 18 electron rule

t2g

ML5 orbitals

t2g

Making the fragment

t2g

Mn(CO)5 d7

Not isostructural fragments Not isoelectronic either...

.CH3

A comparison of the frontier orbital...

They are isolobal!

Results of a computation!

What will a methyl radical do?

What will a methyl radical do?

Evolution of metal Orbitals

n+1 p

n+1 s

nd

ML4 fragment

ML4 fragment

t2g

Mendeleev�s Abacus

t2g

a1 b2 a1

b2 π#

σ

4 5 6 7 8 9 10 Ti V Cr Mn Fe Co Ni Zr Nb Mo Tc Ru Rh Pd Hf Ta W Re Os Ir Pt

40

Fe(CO)4

Carbene type reactivity

Fe2(CO)8

Unsaturated molecule like olefin but not stable at RT!

32 However, is stable!

Understanding Complex Structures

ML3

n+1 p

n+1 s

nd

Co(CO)3

t2g

e e a1 a1

Co(CO)3

t2g

e e a1 a1

HOMO (36)

LUMO (37) (38)

Co(CO)3

Structures formed by units isolobal with C-H

Simple analogy...

d7 ML5 CH3 d8 ML4 CH2 d9 ML3 CH

What would happen if you removed two trans ligands?

n+1 p

n+1 s

nd

t2g

a1

z2

d10 ML2

t2g

a1

z2

BUILDING BRIDGES BETWEEN INORGANIC AND ORGANIC CHEMISTRY ROALD HOFFMANN Nobel lecture, 8 December 1981

Angewandte Chemie International Edition in English Volume 21, Issue 10, pages 711–724, October 1982

•  C-H N

– As if the proton has been pushed into the nucleus of the C !

•  BH2 C-H N

Non-carbon Bread..

1 FEBRUARY 2002 VOL 295 SCIENCE 832

Selected bond distances (Å): P1-P2, 2.1608(11); P1-Ti, 2.5617(8);

Fluxional Organometallic Molecules

Molecular Gymnastics

Molecular Gymnastics

•  Static Crystal Structures

•  Walk – Talk

•  Dance..

Mechanisms of Organotransition Metal Reactions

M - C

A. Rearrangements, Isomerizations

B. Ligand Insertion Reactions

M L + X M X L

C. Oxidative Addition Reductive Elimination

+ L M(n+x)+ L

D. Reactivity changes on the Ligand

M L + Y M L' + X

X

Mn+

M - L M - L'

Classification of OM Reactions

MLnMLn

Rearrangememts

Isomerisation

Fe(CO)5

(CO)3Fe

RhCl3. H2OCl

Rh

Cl

Rh

Fluxional behaviour

CH3Fe

CH3Fe

H (s) on unsubstituted ring

3

5

Ligand Rearrangements

Molecular Breathing •  Fe(CO)5

Berry Pseudorotation •  Fe(CO)5

Molecular Breathing •  Fe(CO)5

Carbonyl Hopping

Carbonyl Hopping in Clusters

Metal Hopping

Cis Trans isomerism & CO Hopping

Simple Rotational Isomers

H H

H H

H H

H H

Rh

Rh

H H

H H

Rh

RhH H

H H

H H

H H

Rh

Cl2

.

.

.

.

.

. .

All protons equivalent at RT A

+

B

.

.

.

.

. .

..

At - 25 Rotaion Stops Cd Solvent

2 distinct peaks for inner and outer protons

2JRh-Hi

2JRh-Ho

1.8

2.5

Hz

Hz

At RT Spectrum shows only a 2JRh-H av. 2.1Hz

A Average

B Expect loss of

2JRh-H

Rh-H

0

.

A molecular propeller!

Rotation stops, CD2Cl2

Isomerization of Diastereoisomeric Re-Styrene Complexes

Gladysz, J. Am. Chem. Soc., 1992, 114, 4174.

Gladysz, J. Am. Chem. Soc., 1992, 114, 4174.

Reaction Scheme

Enantiotopic exchange without olefin dissociation

Possible Intermediates

‘Slipped' Carbocation Alkylidene Intermediate

η2-arene intermediate

‘Slipped' Carbocationic Intermediate

Ruled Out

Not Observed

Isomerization can occur by a simple rotation of the =CHR terminus? No

Ph

H

Not Found ----- There is no 1,2-H shift taking place

Alkylidene Intermediate

Ruled Out

Ruled Out

η2-arene complex as an intermediate

Reactions of phenyl complexes – Replacement of benzene by solvent

k(H)/k(=CHDE) = 1.6 --------- Agostic interaction!!!!!!

Carbon-Hydrogen σ bond complex?

Monodendate Bidendate

Molecules are dynamic

•  Watch Jason Gatson on parallel bars…

Allene Fluxionality

-7.5 eV

0.02 eV

HOMO

MOs of allene

Ring Whizzing

Cyclopentadienyl Haptotropy

HH

H

HH

H

M

Importantly, the Cp is a ligand which can adjust the electron donation By sliding the metal along the line shown by the arrow

CH3CH3

CH3CH3 CH3M

M

Allyl like coordination

4 C are interacting Very rare!

PEt3

H

H

H

H

H

SO2

Cu

H

Cu

Cp Cu PEt3Cu

PEt3

Single line at RT

InS

at one obtains a typical A2B2X pattern.

CuPEt3

Cu

Cu

1

23

4

51

2

4

5

Cu

Cu3

4

5

1

2

4

Cu

1,2 Shifts

3

1

2

3

5

-700

1

2

4

3

5

1,3 Shifts

color coded

Ring Whizzing

3

1,2 shifts

Atm Cu at 2 Cu at 1 Cu at 5 1  B A B 2  A B C 3  B C C 4  C C B 5  C B A

C and C will change only ½ the time B,B will change all the time A will change all the time

Atm Cu at 3 Cu at 1 Cu at 4 1  C A C 2  B B C 3  A C B 4  B C A 5  C B B

B and B will change only ½ the time C,C will change all the time A will change all the time

1,2 Shift 1,3 Shift Random

A Nuclear Magnetic Resonance Study of v-Cyclopentadienyl ( triethylphosphine ) copper ( I) George M. Whitesides and John S. Fleming Journal of the American Chemical Society / 89:12, 1967 p 2855-2859

Fluxional Behaviour of Allyls

During Reactions

•  Nucleophilic attacks at a Ru centre

SN2 anti approach

39

Movie of TS2 in SN2 anti approach of a complex

•  Fluxional behaviour is associated with several structures having similar energy

•  Energy of activation for going from one minima to the other is small.

•  18 ve. To 16 ve to 14 v.e is easy..

Steric'and'Electronic'Effects''of''

Suppor3ng'Ligands'

In'which'ligands'are'steric'effects'important?'

•  CO'•  NO+'',''CN@,'''

•  PR3'•  Carbene'complexes'and'NHC''•  Isocyanides'R@NC'

•  H@'

Concepts'•  Steric'influence'exerted'by'the'ligand.''–  A'radius?'Or'just'an'angle?'Is'it'a'volume?'These'are'nebulous'parameters'in'chemistry.'

'•  Tolman’s'cone'angle'•  Nolan’s'Buried'Volume''•  van'der'Waals'radius,'covalent'radius,'ionic'radius''

•  Non@spherical'distribu3ons'of'the'electron''cloud!'

Tolman’s'Cone'Angle'

Phosphines:*Key*Features'•  A'versalite'π'acceptor'PR3'–''•  Electronic'proper3es'of'PR3'can'be'tuned'by'varying'R.'•  Although'not'as'good'as'CO,'it'has'a'variable'steric'influence.'

–  The*STERIC*influence*of*R*groups*in*PR3*gives*us*an*advantage*

SOLUTION:'''''FIX'it'at'an'average'distance'2.28''Å'

Problem:''The'angle'at'M'depends'on'the'M@L'distance!'

Quan3fying'Steric'Effects'•  Cone*angle*measured*by*Tolman*is*the*simplest*/*

fastest*measure**of*the*steric*requirement.*•  Using*space*filling*models,*he*constructed*physical*

models*and*measured*the*angle*using*a*protractor.*****************•  PH3* ************I * *87*•  PF3 * ************I * *104*•  PMe3 * *I * *118*•  PPh3 * *I * *145*•  P(tBu)3 * *I * *182'

COMPLEX* θ* Kd*

P(OEt)3* 109* 10I10*

P(OiPr)3* 114* 2.7x10I5*

P(OIpTol)3* 121* 6x10I10*

PMe3* 118* 10I9*

P(Et)3* 132* 1.2x10I2*

Does'it'ma^er?'

Importance'of'Steric'Effects'

Pd L2Pd L3Pd L4

Me3P PEt3 PCy3 PPhBu2

As'you'put'more'and'more'bulky'groups,'the''complexes'on'the'le`'side'become''less'and'less'stable.''The'equilibrium'shi`s'to'the'right.'

Steric'forces'of'the'ligand'changes'the'structure!!'

•  Extreme'example'is'[CoL4]+''–  Here'L'is'P(OiPr)3'''''–  d8'complexes'prefer'square'planar'structures'

and'not'octahedral'or'tetrahedral.'''''''''''Why'?''•  Difference'in'energy'required'for'Oh'd2sp3'and'SqP'

dsp2'suggests'that'the'la^er'is'be^er'when'M@L'bonds'are'weak!'(not'strong'enough'to'compensate'for'mixing'in'pz)'

'•  A'choice'between'Td'and'SqP'complexes'depends'

on'LFSE'differences''(π'bonding'is'poorer'in'Td,'no.'of'd'electrons)'''and'steric''pressure'favor'Td!'

''–  The'Co'complex'is'tetrahedral'showing'

importance'of'steric'effects'!'

Tolman’s'angle'is'not'just'steric!'It'is'stereoelectronic! ''

–  R' '''''cone'angle'in' '' ' 'PR3'

' ' ''– H''''''' ' ''''87'– OMe'' ' '107'– Me'''''' ' '118'–  Et''''''''' ' '132'–  Ph'''''''' ' '145'–  F'''''''''' ' '104'–  tBu''''''''' '182'– Mesityl''' '212''

Two'ways'in'which'it'is'electronic!''

1.'Smaller'ligands'can'come'closer'while'larger'ligands'bump'into'one'another'and'stay'far'from'the'metal!'Poor'bonding…''2.'If'the'ligand'is'more'π'bonding,'it'will'come'closer'to'the'metal'and'cone'angle'would'increase!'''Unfortunately'it'was'ini3ally'done'with'models''

Tolman’s'cone'angle'from'crystallographic'data.'

'“Determina3on'of'Tolman’s'cone'angle'from'crystallographic…..'Data'Base”'Muller'T.'E.'and'Mingos'D.M.P.'Transi3on'Met.'Chem.'20,'533,'(1995).'*'

Complica3ons'with'P(OR)3''•  “Steric'parameters'of'….Environments”''– Smith'J.M.'and'Coville'N.J.''•  Organometallics''2001,'20,'1210@1215.'

R'='Me'

A'tale'of'two'ligands'

•  'Special'Ligand'no.'1:'N@heterocyclic'carbenes'•  'Special'Ligand'no.'2:'The'hydride'

NHC'are'super'heroes'

N-Heterocyclic Carbenes in Late Transition Metal Catalysis Silvia Dez-Gonzlez, Nicolas Marion and Steven P. Nolan

Chemical Reviews 2009 109 (8), 3612-3676

Beyond catalysis: N-heterocyclic carbene complexes as components for medicinal, luminescent, and functional materials applications Laszlo Mercs and Martin Albrecht Chem. Soc. Rev., 2010, 39, 1903-1912

A'comparison'between'nickel'and'palladium'precatalysts'of'1,2,4@triazole'based'N@heterocyclic'carbenes'in'hydroamina3on'of'ac3vated'olefins'Chandrakanta'Dash,'Mobin'M.'Shaikh,'Ray'J.'Butcher'and'Prasenjit*Ghosh*Dalton'Trans.,'2010,'39,'2515@2524'

Steric'proper3es'of'“NHC”'•  A'neutral'ligand'with'π'acceptor'capability'•  Very'good'sigma'dona3on'

•  Tunable'electronic'and'steric*property*•  Ligand'is'easily'synthesized'in#situ#

• NHC*are*super*heroes!*

Steric'Features'

Will'rota3on'around'the'M=C'bond'change'the'cone'angle?'

3.5'angstroms'

M@C'bond'length'2.0'Angstroms'

Nolan’s'buried'volume'

Steric'Features'

A'por3on'of'this'volume'in''this'imaginary'sphere'of'3.5'Å'radius''is'occupied'by'each'ligand.''%'used'by'the'ligand'is'called'the'buried'volume.'

'Percent'buried'volume'for'phosphine'and'N@heterocyclic'carbene'ligands:'steric'proper3es'in'organometallic'chemistry''Clavier,'Herve;'Nolan,'Steven'P.'Chem.'Commun.'2010,'46,'841@861'''DOI:'10.1039/b922984a'''''

WIRE'FRAME'MODEL'“ADUVOL”'from'the'CSD'

trans@bis(1,3@bis(2,4,6@Trimethylphenyl)imidazolidin@2@ylidene)@dichloro@nickel(ii)'

SPACE'FILLING'MODEL'ADUVOL'

trans@bis(1,3@bis(2,4,6@Trimethylphenyl)imidazolidin@2@ylidene)@dichloro@nickel(ii)'

No.* R* %*buried*volume**(M=Cu)*

%*buried*volume**(M=Ag)**

1' cyclohexyl' 29' 28'

2' Mesityl' 36' 36'

3' Adamantyl' 37'

4' tButyl' 37'

5' Di'@sopropyl'phenyl' 48' 46'

Metal'ion'dependence'

No.* R* %*buried*volume**(M=Cu)*

1' cyclohexyl' 29'

2' Mesityl' 36'

3' Adamantyl' 37'

4' tButyl' 37'

5' Di'@isopropyl'phenyl' 48'

BDE'''in'Kcal'/'mol''

I@Mesityl'' '41'''SI@Mes'''''''' '40''''(S'stands'for'saturated'I'ring)''I@Adam' ''''' '20''I@tButyl'''''' '24'

Steric'Effects''•  Approxima3ons'are'required'to'quan3fy'steric'effects!'''(a)'Angle'subtended'at'the'metal'using'a'fixed'M@L'distance.''(b)'The'%'volume'occupied'by'the'solid'cone'in'an'imaginary'sphere'around'the'metal'and'a'fixed'M@L'distance.''

'

•  Ignore'M@L'bond'distance'effects.'

•  Use'crystallography'to'make'it'more'realis3c:'corrects'for'the'errors'men3oned'above!'

Caveats'in'using'“steric'effects”'•  Size'of'the'balloon'depends'on'how'much'you'squeeze'the'balloon'with'the'

Vernier'calipers'you'are'using'to'measure'the'baloon'with!'P(OR3)3'''

•  Es3ma3ng'the'solid'angle'or'the'cone'angle'can'be'computed'using'a'computa3onal'model':'*SambVca:'A'Web'Applica5on'for'the'Calcula5on'of'the'Buried'Volume'of'*NIHeterocyclic*Carbene*Ligands**Poater*A.,Cosenza*B.,*Correa*A.*,*Giudice*S.,**Ragone*F.,*Scarano*V.*and*Cavallo*L.'#Eur.#J.#Inorg.#Chem.#2009,'1759–1766''URL'h^p://www.molnac.unisa.it/OMtools.php.'

Improved*Algorithm*for*Accurate*Computaeon*of*Molecular*Solid*Angles*B.*Craig*Taverner'h^p://www.gh.wits.ac.za/craig/steric/mul3sol/html/mul3sol.html'

A'brief'comment'on'the'Hydride..'

•  Smallest'ligand'known'in'chemistry.'– What'is'it’s'size?'

Hydrogen'occupies'a'coordina3on'site'on'the'metal.''

Hydride..'

Structures'of'metal'hydrides!'Flexible'steric'effects!'

HRh(CO)(PPh3)3 HRh(PPh3)4

RhHCO(PPh3)3*

P@Rh@P'angle'is'120o'

RhH(PPh3)4*

P@Rh@P'angle'is'~'109o'

RhH(PPh3)4*

Electronic'Effects'

•  Quan3fica3on'of'electron'dona3on:'– Tolman’s'Electronic'Parameter'TEP'•  (See'the'video'lecture'on'subs3tutes'for'CO'and'the'lecture'on'N@heterocyclic'carbenes)'

'''

Quan3fying'Electronic'Effects'•  Be^er'to'look'at'a'series'of'complexes.'

•  Tolman'did'this'with'[Ni(CO)3(L)]'where'the'L'is'a'NHC.'He'quan3fied'the'symmetric'stretch'of'the'3'CO'ligands'as'a'func3on'of'L'

•  L'='CO'''TEP'is'2060'

•  The'average'CO'stretch'is'called'the'''''''Tolman'electronic'parameter.'(TEP)''''

Electronic'Character'of'NHC'•  13C'chemical'shi`'in'a'Pd(II)'complex'is'used'as'a'probe.'Chemical'shi`'increases'with'NCN'angle'

Usually'on'complexa3on,'the'chemical'shi`'goes'upfield'by'30'ppm.'180'ppm.'

Quan3ta3ve'Analysis'of'the'Ligand’s'Electronic'Effects'

•  ECW''Model'of'R.S.'Drago'– Property'='EaEb'+'CaCb''+W'– Does'not'model'the'steric'effects'of'ligands'

'''

Quan3ta3ve'Analysis'of'Ligand'Effects'

•  QALE'method''Developed'by'Giering'and'Prock'•  Property'=''aχ''+'bθ'+'c'(θ@'θst')λ'+'dEar'+'e'''''''//a,'b,'c,'d,'e'is'determined'for'each'property//'–  χ'is'the'electronic'parameter'and'θ'the'cone'angle'(Ar'is'no.'aroma3c'

rings)'*Comments*on*Coupling*Graphical*and*Regression*Analyses*of*Ligand*

Effect*Data*Bartholomew'J.,'Fernandez'A.'L.,'Lorsbach'B.A.,'Wilson'M.R.,'Prock'A.,'and'Giering'W.'P.'•  Organometallics#1996,'15,'295F301'

An'Example'of'QALE'

log'k'

100''''''120''''140''''160''''180''''''200'@6'

@4'

@2'

0'

Kine3cs'of'the'dissocia3on''Plo^ed'against'cone'angle''The'break'at'160°'indicates'a'threshold'for'the'steric'parameter!'

Cone'angle'in'degrees….>'

QALE'and'thermodynamics'

@ΔH'#=##70.73(χ)##±#0.10'θ#@'0.461(θ#7#135)λ#±#0.026'@'1.68Ear#±#0.34'+'46.3'±#0.8''r2#=#0.964######n#=16'

Nega3ve'coefficient'indicates'that'heat'of'reac3on'increase'as'the'phosphines'become'be^er'donors'For'this'reac3on,'there'is'no'steric'effect'3ll'the'Tolman'angle'reaches'135.'Data'from'PR3'and'PAr3''fall'on'two'different'lines'when'χ'is'plo^ed'against'ΔH.''If'there'is'no'aroma3c'ring'effect'the'lines'cross'at'χ#=#4.8#''

QALE'•  Proper3es'can'be:'Thermodynamic'data'(pKa#values,#E°#values,#heats#of#reacIon,#equilibrium#constants),'bond'lengths,'and'NMR,'IR,'UV/vis,'photoelectron,'and'Mossbauer'spectroscopic'data'

•  Recent'example:'“Quan3ta3on'of'the'ligand'effect'in'oxo@transfer'Reac3ons'of'dioxo@Mo(VI)'trispyrazolyl'borate'Complexes”''

'' 'Partha'Basu,'Brian'W.'Kail,'Andrew'K.'Adamsa'and'Victor'N.'Nemykinb'Dalton'Trans.,'2013,'42,'3071–3081'

Quan3fying'S&E'Effects!'•  It'is'possible'to'quan3fy'steric'effects'from'crystallographic'data!'Use'with'cau3on!'

•  Electronic'effects'from'spectroscopic'data!'

•  To'have'predic3ve'value,'one'needs'to'include'both'steric'and'electronic'effects!'