Lecture 17 February 14, 2013 MH + bonding, metathesis

Size: px

Start display at page:

Download "Lecture 17 February 14, 2013 MH + bonding, metathesis"

Leon Banks
5 years ago
Views:

1 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 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:Sijia Dong <sdong@caltech.edu> Samantha Johnson <sjohnson@wag.caltech.edu> Ch120a-1

2 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 2

3 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 3

4 Bond energies MH+ Re Mo Au Cr Cu Ag 4

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.

5 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 5

6 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 6

7 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 7

8 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 8

9 9

10 10

11 11

12 12

13 3 rd row 13

14 14

15 15

16 16

17 17

18 18

19 19

20 20

21 21

22 22

23 23

24 24

25 25

26 2 nd row 26

27 27

28 28

29 29

30 30

31 31

32 32

33 1 st row 33

34 34

35 Schilling 35

36 Steigerwald 36

37 37

38 38

39 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 39

40 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 40

41 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 41

42 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 42

43 Is Cl 2 TiH + + D 2 è 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 43

44 GVB orbitals at the TS for Cl 2 TiH + + D 2 è Cl 2 TiD + + HD 44

45 GVB orbitals for the Cl 2 TiD + + HD product Note get phase change for both orbitals 45

46 Follow the D2 bond as it evolves into the HD bond 46

47 Follow the TiH bond as it evolves into the TiD bond 47

48 Barriers small, thus allowed Increased d character in bond è smaller barrier 48

49 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 49

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.

50 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

51 Show reaction for ClMnH + D2 Show example reactions 51

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

53 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 53

Ru Olefin Metathesis Basics Ch120a-Goddard-L21

Common Olefin Metathesis Catalysts Ch120a-Goddard-L21

56 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 56

Well-defined metathesis catalysts R R ipr (F 3 C) 2 MeCO (F 3 C) 2 MeCO N Mo Schrock 1991 alkoxy imido

ruthenium benzylidene complex b Mes Ph R=H, Cl Grubbs 1999 1,3-dimesityl-imidazole-2-ylidenes P (Cy) 3

1991, 113, 6899-6907 Wagener, K. B.; Boncella, J. M.; Nel, J. G.

58 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 58

59 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 59

60 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 60

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

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