Lecture 13 February 1, 2011 Pd and Pt, MH + bonding, metathesis

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1 Lecture 13 February 1, 2011 Pd and Pt, MH + bonding, metathesis Nature of the Chemical Bond with applications to catalysis, materials science, nanotechnology, surface science, bioinorganic chemistry, and energy Course number: Ch120a Hours: 2-3pm Monday, Wednesday, Friday William A. Goddard, III, wag@wag.caltech.edu 316 Beckman Institute, x3093 Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics, California Institute of Technology Teaching Assistants: Caitlin Scott <cescott@caltech.edu> Hai Xiao xiao@caltech.edu; Fan Liu <fliu@wag.caltech.edu> Ch120a-God1

2 Last time 2

3 Compare chemistry of column 10 3

4 Ground state of group 10 column Pt: (5d) 9 (6s) 1 3 D ground state Pt: (5d) 10 (6s) 0 1 S excited state at 11.0 kcal/mol Pt: (5d) 8 (6s) 2 3 F excited state at 14.7 kcal/mol Ni: (5d) 8 (6s) 2 3 F ground state Ni: (5d) 9 (6s) 1 3 D excited state at 0.7 kcal/mol Ni: (5d) 10 (6s) 0 1 S excited state at 40.0 kcal/mol Pd: (5d) 10 (6s) 0 1 S ground state Pd: (5d) 9 (6s) 1 3 D excited state at 21.9 kcal/mol Pd: (5d) 8 (6s) 2 3 F excited state at 77.9 kcal/mol 4

Salient differences between Ni, Pd, Pt Ni Pd Pt 2 nd row (Pd): 4d much more stable than 5s Pd d 10 ground state 3 rd row (Pt): 5d and 6s comparable stability Pt d 9 s 1 ground state 4s more stable

5 Salient differences between Ni, Pd, Pt Ni Pd Pt 2 nd row (Pd): 4d much more stable than 5s Pd d 10 ground state 3 rd row (Pt): 5d and 6s comparable stability Pt d 9 s 1 ground state 4s more stable than 3d 5s much less stable than 4d 6s, 5d similar stability 3d much smaller than 4s (No 3d Pauli orthogonality) Huge e-e repulsion for d 10 Differential shielding favors n=4 over n=5, stabilize 4d over 5s d 10 4d similar size to 5s (orthog to 3d,4s Relativistic effects of 1s huge decreased KE contraction stabilize and contract all ns 5 destabilize and expand nd

6 Mysteries from experiments on oxidative addition and reductive elimination of CH and CC bonds on Pd and Pt Why is CC coupling so much harder than CH coupling? 6

7 Step 1: examine GVB orbitals for (PH 3 ) 2 Pt(CH 3 ) 7

8 Analysis of GVB wavefunction 8

9 Alternative models for Pt centers 9

10 10

11 energetics Not agree with experiment 11

12 Possible explanation: kinetics 12

13 Consider reductive elimination of HH, CH and CC from Pd Conclusion: HH no barrier CH modest barrier CC large barrier 13

14 Consider oxidative addition of HH, CH, and CC to Pt Conclusion: HH no barrier CH modest barrier CC large barrier 14

15 Summary of barriers This explains why CC coupling not occur for Pt while CH and HHcoupling is fast But why? 15

16 How estimate the size of barriers (without calculations) 16

17 Examine HH coupling at transition state Can simultaneously get good overlap of H with Pd sd hybrid and with the other H Thus get resonance stabilization of TS low barrier 17

18 Examine CC coupling at transition state Can orient the CH 3 to obtain good overlap with Pd sd hybrid OR can orient the CH 3 to obtain get good overlap with the other CH 3 But CANNOT DO BOTH SIMULTANEOUSLY, thus do NOT get resonance stabilization of TS high barier 18

19 Examine CH coupling at transition state H can overlap both CH 3 and Pd sd hybrid simultaneously but CH 3 cannot thus get ~ ½ resonance stabilization of TS 19

20 Now we understand Pt chemistry But what about Pd? Why are Pt and Pd so dramatically different 20

21 new 21

22 Pt goes from s 1 d 9 to d 10 upon reductive elimination thus product stability is DECREASED by 12 kcal/mol Using numbers from QM 22

23 Ground state configurations for column 10 Ni Pd Pt 23

24 Pd goes from s 1 d 9 to d 10 upon reductive elimination thus product stability is INCREASED by 20 kcal/mol Using numbers from QM Pd and Pt would be ~ same 24

25 Thus reductive elimination from Pd is stabilized by an extra 32 kcal/mol than for Pt due to the ATOMIC nature of the states The dramatic stabilization of the product by 35 kcal/mol reduces the barrier from ~ 41 (Pt) to ~ 10 (Pd) This converts a forbidden reaction to allowed 25

26 Summary energetics Conclusion the atomic character of the metal can control the chemistry 26

27 Examine bonding to all three rows of transition metals Use MH+ as model because a positive metal is more representative of organometallic and inorganic complexes M0 usually has two electrons in ns orbitals or else one M+ generally has one electron in ns orbitals or else zero M2+ never has electrons in ns orbitals 27

28 Ground states of neutral atoms Sc (4s)2(3d) Sc + (4s)1(3d)1 Sc ++ (3d)1 Ti (4s)2(3d)2 Ti + (4s)1(3d)2 Ti ++ (3d)2 V (4s)2(3d)3 V + (4s)0(3d)3 V ++ (3d)3 Cr (4s)1(3d)5 Cr + (4s)0(3d)5 Cr ++ (3d)4 Mn Fe Co Ni Cu (4s)2(3d)5 (4s)2(3d)6 (4s)2(3d)7 (4s)2(3d)8 (4s)1(3d)10 Mn + Fe + Co + Ni + Cu + (4s)1(3d)5 (4s)1(3d)6 (4s)0(3d)7 (4s)0(3d)8 (4s)0(3d)10 Mn ++ Fe ++ Co ++ Ni ++ Cu ++ (3d)5 (3d)6 (3d)7 (3d)8 (3d)10 28

29 Bond energies MH+ Re Mo Au Cr Cu Ag 29

Exchange energies: Mn+: s 1 d 5 For high spin (S=3) A[(d 1 α)(d 2 α)(d 3 α)(d 4 α)(d 5 α)(sα)] Get 6*5/2=15 exchange terms 5Ksd + 10 Kdd Responsible for Hund s rule Ksd Kdd Mn+ 4.8 19.

30 Exchange energies: Mn+: s 1 d 5 For high spin (S=3) A[(d 1 α)(d 2 α)(d 3 α)(d 4 α)(d 5 α)(sα)] Get 6*5/2=15 exchange terms 5Ksd + 10 Kdd Responsible for Hund s rule Ksd Kdd Mn kcal/mol Tc Re Form bond to H, must lose half the exchange stabilization for the orbital bonded to the H A{(d 1 α)(d 2 α)(d 3 α)(d 4 α)(sd b α)[(sd b )H+H(sd b )](αβ βα)} sd b is α half the time and β half the time 30

31 Ground state of M + metals Mostly s1dn-1 Exceptions: 1 st row: V, Cr-Cu 2 nd row: Nb-Mo, Ru-Ag 3 rd row: La, Pt, Au 31

32 Size of atomic orbitals, M + Valence s similar for all three rows, 5s biggest Big decrease from La(an 57) to Hf(an 72 Valence d very small for 3d 32

33 Charge transfer in MH + bonds electropositive 1 st row all electropositive 2 nd row: Ru,Rh,Pd electronegative 3 rd row: Pt, Au, Hg electronegative electronegative 33

34 34

35 35

36 36

37 37

38 1 st row 38

39 39

40 Schilling 40

41 Steigerwald 41

42 42

43 43

44 2 nd row 44

45 45

46 46

47 47

48 48

49 49

50 50

51 3 rd row 51

52 52

53 53

54 54

55 55

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59 59

60 60

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63 63

64 Physics behind Woodward-Hoffman Rules For a reaction to be allowed, the number of bonds must be conserved. Consider H 2 + D 2 2 bonds TS? bonds 2 bonds To be allowed must have 2 bonds at TS How assess number of bonds at the TS. What do the dots mean? Consider first the fragment Have 3 electrons, 3 MO s Have 1 bond. Next consider 4 th atom, can we get 2 bonds? Bonding 2 elect nonbonding 1 elect antibonding 0 elect 64

65 Can we have 2s + 2s reactions for transition metals? 2s + 2s forbidden for organics X 2s + 2s forbidden for organometallics? Cl 2 Ti? Cl 2 Ti? Cl 2 Ti Cl 2 Ti Me Cl 2 Ti Me Cl 2 Ti Me Me Me Me 65

66 Physics behind Woodward-Hoffman Rules Bonding 2 elect nonbonding 1 elect antibonding 0 elect Have 1 bond. Question, when add 4 th atom, can we get 2 bonds? Can it bond to the nonbonding orbital? Answer: NO. The two orbitals are orthogonal in the TS, thus the reaction is forbidden 66

67 Now consider a TM case: Cl 2 TiH + + D 2 Orbitals of reactants GVB orbitals of TiH bond for Cl 2 TiH + GVB orbitals of D 2 67

can we get 2 bonds? Now the bonding orbital on Ti is d-like.

68 Is Cl 2 TiH + + D 2 Cl 2 TiD + + HD allowed? Bonding 2 elect nonbonding 1 elect antibonding 0 elect when add Ti 4 th atom, can we get 2 bonds? Now the bonding orbital on Ti is d-like. Thus at TS have Answer: YES. The two orbitals can have high overlap at the TS orthogonal in the TS, thus the reaction is allowed 68

69 GVB orbitals at the TS for Cl 2 TiH + + D 2 Cl 2 TiD + + HD 69

70 GVB orbitals for the Cl 2 TiD + + HD product Note get phase change for both orbitals 70

71 Follow the D2 bond as it evolves into the HD bond 71

72 Follow the TiH bond as it evolves into the TiD bond 72

73 Barriers small, thus allowed Increased d character in bond smaller barrier 73

74 Are all MH reactions with D2 allowed? No Example: ClMn-H + D2 Here the Mn-Cl bond is very polar Mn(4s-4p z ) lobe orbital with Cl:3pz This leaves the Mn: (3d) 5 (4s+4pz), S=3 state to bond to the H But spin pairing to a d orbital would lose 4*K dd /2+K sd /2= (40+2.5) = 42.5 kcal/mol whereas bonding to the (4s+4pz) orbital loses 5*K sd /2 = 12.5 kcal/mol As a result the H bonds to (4s+4pz), leaving a high spin d5. Now the exchange reaction is forbidden 74

Thus ClMn-H bond is sp-like ClMnH Mn (4s) 2 (3d) 5 The Cl pulls off 1 e from Mn, leaving a d 5 s 1 configuration H bonds to 4s because of exchange stabilization of d 5 Mn-H bond character 0.07 Mnd+0.

75 Thus ClMn-H bond is sp-like ClMnH Mn (4s) 2 (3d) 5 The Cl pulls off 1 e from Mn, leaving a d 5 s 1 configuration H bonds to 4s because of exchange stabilization of d 5 Mn-H bond character 0.07 Mnd+0.71Mnsp+1.20H This cannot overlap the antisymmetric orbital delocalized over the three H atoms in the TS As a result at the Transition state the MnH bond has the character of H 3- with both electrons on the H3. This leads to a high barrier, ~45 kcal/mol

76 Show reaction for ClMnH + D2 Show example reactions 76

Olefin Metathesis 2+2 metal-carbocycle reactions Diego Benitez, Ekaterina Tkatchouk,

78 OLEFIN METATHESIS Catalytically make and break double bonds! + R 1 R 1 R 2 R 2 2 R 1 R 2 Mechanism: actual catalyst is a metal-alkylidene R 2 R 2 R 2 M M M R 1 R 3 R 1 R 3 R 1 R 3 Ch120a-Goddard-L21 copyright 2010 William A. Goddard III, all rights reserved 78

Ru Olefin Metathesis Basics Ch120a-Goddard-L21

Common Olefin Metathesis Catalysts Ch120a-Goddard-L21

81 Applications of the olefin metathesis reaction Small scale synthesis to industrial polymers Ch120a-Goddard-L21 bulletproof resin Acc. Chem. Res. 2001, 34, copyright 2010 William A. Goddard III, all rights reserved 81

Well-defined metathesis catalysts R R ipr (F 3 C) 2 MeCO (F 3 C) 2 MeCO N Mo Schrock 1991 alkoxy imido molybdenum complex a ipr Ph CH 3 CH 3 Cl Cl PCy 3 Ru PCy 3 Ph Mes N Cl Cl Ru N PCy 3 1 2 Grubbs

83 Well-defined metathesis catalysts R R ipr (F 3 C) 2 MeCO (F 3 C) 2 MeCO N Mo Schrock 1991 alkoxy imido molybdenum complex a ipr Ph CH 3 CH 3 Cl Cl PCy 3 Ru PCy 3 Ph Mes N Cl Cl Ru N PCy Grubbs ruthenium benzylidene complex b Mes Ph R=H, Cl Grubbs ,3-dimesityl-imidazole-2-ylidenes P(Cy) 3 mixed ligand system c Bazan, G. C.; Oskam, J. H.; Cho, H. N.; Park, L. Y.; Schrock, R. R. J. Am. Chem. Soc. 1991, 113, Wagener, K. B.; Boncella, J. M.; Nel, J. G. Macromolecules 1991, 24, Scholl, M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H. Tetrahedron Lett. 1999, 40, GODDARD Ch120-L20 13/11/02 83

84 Examples 2 nd Generation Grubbs Metathesis Catalysts Mes Cl N N Mes Cl Ru Ph PCy 3 Mes N N Mes Cl Ru Cl O i-pr Mes N N Mes Cl Ru Ph Cl Py slow initiating catalyst IMes Cl Ru Cl L fast-initiating catalyst General mechanism of Metathesis IMes Cl Ph L Ru Cl R ultra-fast-initiating catalyst Initiation Cl IMes Cl Ru R 1 R 3 R 2 Cl IMes Cl R Ru R 3 R 2 Cl IMes Cl Ru R 3 + Propagation R 1 Ch120a-Goddard-L21 copyright 2010 William A. Goddard III, all rights reserved R 2 84

85 Schrock and Grubbs catalysts make olefin metathesis practical Schrock catalyst very active, but destroys many functional groups Grubbs catalyst very stable, high functional group tolerance, but not as reactive as Schrock Catalysts contain many years of evolutionary improvements Ch120a-Goddard-L21 copyright 2010 William A. Goddard III, all rights reserved 85

Lecture 17 February 14, 2013 MH + bonding, metathesis

Lecture 17 February 14, 2013 MH + bonding, metathesis Nature of the Chemical Bond with applications to catalysis, materials science, nanotechnology, surface science, bioinorganic chemistry, and energy