Isolobal Analogy. Roald Hoffmann

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1 Isolobal Analogy Roald Hoffmann

2 Organic Structures O R R O

3 Systemization in organic chemistry! R is any alkyl group CH 3, C 2 H 5, C 6 H 5! Similarly H can be replaced by any R group.! =CH 2 can be replaced by any CR 2 group or oxygen! Transition metal Chemistry? Organometallic Chemistry?

4 Common fragments

5 Organic Fragments CH 4 saturated organic fragment. Remove hydrogens one by one CH 3. CH 2 CH

6 Inorganic Fragments Start with Cr(CO) 6

7 Relationship between the two? Remove hydrogens one by one CH 3. CH 2 CH

8 Evolution of metal Orbitals n+1 p n+1 s nd

9 Evolution of metal Orbitals n+1 p n+1 s nd

10 s + 3p produced 4 hybrid orbitals sp 3

11 Evolution of metal Orbitals n+1 p n+1 s nd

12 t 2g ML 6 orbitals

13 t 2g ML 6 orbitals

14 t 2g Basis of 18 electron rule

15 t 2g ML 5 orbitals

16 t 2g Making the fragment

17 Mn(CO) 5 d 7. CH3 Not isostructural fragments Not isoelectronic either...

18 A comparison of the frontier orbital... They are isolobal!

19 Results of a computation!

20 What will a methyl radical do?

21 What will a methyl radical do?

22 Evolution of metal Orbitals n+1 p n+1 s nd

23 ML 4 fragment

24 t 2g ML 4 fragment

25 Mendeleev s Abacus Ti V Cr Mn Fe Co Ni Zr Nb Mo Tc Ru Rh Pd Hf Ta W Re Os Ir Pt a1 b2 b2 π# a1 σ t 2g

27 40 Fe(CO) 4

28 Carbene type reactivity Fe 2 (CO) 8 Unsaturated molecule like olefin but not stable at RT! 32 However, is stable!

29 Understanding Complex Structures

30 ML 3 n+1 p n+1 s nd

33 Co(CO) 3 a1 e e a1 t 2g

34 Co(CO) 3 a1 e e a1 t 2g

35 LUMO (37) (38) Co(CO) 3 HOMO (36)

36 Structures formed by units isolobal with C-H

37 Simple analogy... d 7 ML 5 CH 3 d 8 ML 4 CH 2 d 9 ML 3 CH

38 What would happen if you removed two trans ligands? n+1 p n+1 s nd

39 a1 t 2g z 2

40 d 10 ML 2 a1 t 2g z 2

41 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 , October 1982

42 C-H N As if the proton has been pushed into the nucleus of the C! BH 2 C-H N

43 Non-carbon Bread..

44 1 FEBRUARY 2002 VOL 295 SCIENCE 832

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

46 Fluxional Organometallic Molecules Molecular Gymnastics

47 Molecular Gymnastics Static Crystal Structures Walk Talk Dance..

48 Mechanisms of Organotransition Metal Reactions Classification of OM Reactions M - C A. Rearrangements, Isomerizations M - L M - L' B. Ligand Insertion Reactions X M L + X M X L C. Oxidative Addition Reductive Elimination M n+ + L M (n+x)+ L D. Reactivity changes on the Ligand M L + Y M L' + X

49 Rearrangememts Ligand MLn Isomerisation MLn Rearrangements (CO) 3 Fe Fe(CO) 5 RhCl 3. 3 H 2 O Rh Cl Rh Cl Fluxional behaviour Fe CH 3 Fe CH 3 5 H (s) on unsubstituted ring

50 Fe(CO) 5 Molecular Breathing

51 Fe(CO) 5 Berry Pseudorotation

52 Fe(CO) 5 Molecular Breathing

53 Carbonyl Hopping

54 Carbonyl Hopping in Clusters

55 Metal Hopping

56 Cis Trans isomerism & CO Hopping

57 Simple Rotational Isomers

58 A molecular propeller!. H. H Rh. H. H Rh All protons equivalent at RT. H. H.. H H A H H B Rh H H Rh + H H. Rh.. H H H.. H. H. H 0 At - 25 Rotaion Stops Cd Cl 2 Solvent Rotation stops, CD 2 Cl 2 2 distinct peaks for inner and outer protons. At RT Spectrum shows only a 2 J Rh-H A Average 2 J Rh-H av. 2.1Hz 2 J Rh-Hi 2 J Rh-Ho Hz Hz B Expect loss of Rh-H

59 Isomerization of Diastereoisomeric Re-Styrene Complexes Gladysz, J. Am. Chem. Soc., 1992, 114, 4174.

60 Reaction Scheme Gladysz, J. Am. Chem. Soc., 1992, 114, 4174.

61 Enantiotopic exchange without olefin dissociation

62 Possible Intermediates Slipped' Carbocation Alkylidene Intermediate η 2 -arene intermediate

63 Slipped' Carbocationic Intermediate Ruled Out Not Observed

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

65 Alkylidene Intermediate Ruled Out Not Found There is no 1,2-H shift taking place

66 η 2 -arene complex as an intermediate Ruled Out Reactions of phenyl complexes Replacement of benzene by solvent

67 Carbon-Hydrogen σ bond complex? k(h)/k(=chd E ) = Agostic interaction!!!!!! Monodendate Bidendate

68 Molecules are dynamic Watch Jason Gatson on parallel bars

69 Allene Fluxionality

71 MOs of allene 0.02 ev -7.5 ev HOMO

72 Ring Whizzing

73 Cyclopentadienyl Haptotropy

74 Importantly, the Cp is a ligand which can adjust the electron donation By sliding the metal along the line shown by the arrow C H 3 C H 3 CH 3 CH3 CH 3 M 4 C are interacting Very rare! M H H H H H H Allyl like coordination M

75 Ring Whizzing Cp Cu PEt 3 Cu Single line at RT PEt 3 In S SO 2 at H one obtains a typical A 2 B 2 X pattern. 3 CuPEt Cu 3 Cu color coded ,2 Shifts H 5 PEt 3 H Cu 3 4 Cu 1,3 Shifts H Cu 1 5 H H 5 Cu Cu

76 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

77 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

79 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

81 Fluxional Behaviour of Allyls

82 During Reactions Nucleophilic attacks at a Ru centre

83 S N 2 anti approach

84 Movie of TS2 in S N 2 anti approach of a complex 39

85 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..

86 Steric'and'Electronic'Effects'' of'' Suppor3ng'Ligands'

87 In'which'ligands'are'steric'effects' CO' NO + important?' PR 3' Carbene'complexes'and'NHC'' Isocyanides'R@NC' '

88 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'' cloud!'

90 Tolman s'cone'angle'

91 Phosphines:*Key*Features' A'versalite'π'acceptor'PR 3 ' '' Electronic'proper3es'of'PR 3 '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*PR 3 *gives*us*an*advantage*

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

94 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.*** ************** PH 3 * ************I * *87* PF 3 * ************I * *104* PMe 3 * *I * *118* PPh 3 * *I * *145* P( t Bu) 3 * *I * *182'

95 Does'it'ma^er?' COMPLEX* θ* Kd* P(OEt) 3* 109* 10 I10* P(O i Pr) 3* 114* 2.7x10 I5* P(OIpTol) 3* 121* 6x10 I10* PMe 3* 118* 10 I9* P(Et) 3* 132* 1.2x10 I2*

96 Importance'of'Steric'Effects' Pd L4 Pd L 3 Pd L 2 Me 3 P PEt 3 PCy 3 PPhBu 2 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.'

97 Steric'forces'of'the'ligand'changes'the' structure!!' Extreme'example'is'[CoL 4 ] +'' Here'L'is'P(O i Pr) 3'''' ' d 8' complexes'prefer'square'planar'structures' and'not'octahedral'or'tetrahedral.'''''''''''why'?'' Difference'in'energy'required'for'O h 'd 2 sp 3 'and'sqp' dsp 2 'suggests'that'the'la^er'is'be^er'when'm@l' bonds'are'weak!'(not'strong'enough'to' compensate'for'mixing'in'p z ) ' ' 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 '!'

98 Tolman s'angle'is'not'just'steric!' It'is'stereoelectronic! R' '''''cone'angle'in' ' ' ' ' 'PR 3' ' '' H''''''' ' ''''87' OMe'' ' '107' Me'''''' ' '118' Et''''''''' ' '132' Ph'''''''' ' '145' F'''''''''' ' '104' t Bu''''''''' '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' '

99 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).'*'

100 Complica3ons'with'P(OR) 3'' Steric'parameters'of'.Environments '' Smith'J.M.'and'Coville'N.J.'' R'='Me'

101 A'tale'of'two'ligands' 'Special'Ligand'no.'2:'The'hydride'

102 NHC'are'super'heroes' ac3vated'olefins' Chandrakanta'Dash,'Mobin'M.'Shaikh,'Ray'J.'Butcher'and' Prasenjit*Ghosh* N-Heterocyclic Carbenes in Late Transition Metal Catalysis 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, Silvia Dez-Gonzlez, Nicolas Marion and Steven P. Nolan Chemical Reviews (8),

103 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!*

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

105 Nolan s'buried'volume' 3.5'angstroms'

106 Steric'Features' ' steric'proper3es'in'organometallic'chemistry'' Clavier,'Herve;'Nolan,'Steven'P.' 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.'

107 WIRE'FRAME'MODEL' ADUVOL 'from'the'csd'

108 SPACE'FILLING'MODEL' ADUVOL'

109 Metal'ion'dependence' No.* R* %*buried* volume** (M=Cu)* 1' cyclohexyl' 29' 28' 2' Mesityl' 36' 36' 3' Adamantyl' 37' 4' tbutyl' 37' 5' 48' 46' %*buried* volume** (M=Ag)* *

110 '' ' ''''' ' t Butyl'''''' BDE'''in'Kcal'/'mol' ' '41' '40''''(S'stands'for'saturated'I'ring)' '20' '24' No.* R* %*buried* volume** (M=Cu)* 1' cyclohexyl' 29' 2' Mesityl' 36' 3' Adamantyl' 37' 4' tbutyl' 37' 5' 48'

111 Steric'Effects'' Approxima3ons'are'required'to'quan3fy'steric'effects!'' ' '(b)'the'%'volume'occupied'by'the'solid'cone'in'an'imaginary' Use'crystallography'to'make'it'more'realis3c:'corrects'for'the' errors'men3oned'above!'

112 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(OR 3 ) 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,' ' 'URL'h^p:// Improved*Algorithm*for*Accurate*Computaeon*of*Molecular*Solid*Angles* B.*Craig*Taverner'h^p://

113 A'brief'comment'on'the'Hydride..' Smallest'ligand'known'in'chemistry.' What'is'it s'size?'

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

115 Structures'of'metal'hydrides!' Flexible'steric'effects!' HRh(CO)(PPh 3 ) 3 HRh(PPh 3 ) 4

116 RhHCO(PPh 3 ) 3* P@Rh@P'angle'is'120 o'

117 RhH(PPh 3 ) 4* P@Rh@P'angle'is'~'109 o'

118 RhH(PPh3)4*

119 Electronic'Effects' Quan3fica3on'of'electron'dona3on:' Tolman s'electronic'parameter'tep' (See'the'video'lecture'on'subs3tutes'for'CO'and'the' '''

120 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)''' '

121 Electronic'Character'of'NHC' 13 C'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.'

122 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' ' ''

123 Quan3ta3ve'Analysis'of'Ligand'Effects' QALE'method''Developed'by'Giering'and'Prock' st ')λ'+'de ar '+'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'

124 An'Example'of'QALE' Kine3cs'of'the'dissocia3on'' log'k' ' The'break'at'160 'indicates'a'threshold' 100''''''120''''140''''160''''180''''''200' Cone'angle'in'degrees. >'

125 QALE'and'thermodynamics' ar #±#0.34'+'46.3'±#0.8' ' r 2# =#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'PR 3 'and'par 3 ''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#' '

126 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:' '' '' 'Partha'Basu,'Brian'W.'Kail,'Andrew'K.'Adamsa'and'Victor'N.'Nemykinb' Dalton'Trans.,'2013,'42,' '

127 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!'

Metal Hydrides, Alkyls, Aryls, and their Reactions

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