Ch120-L17-13-PdPt-TM atoms-Meta-Feb22

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-Goddard-L19 © copyright 2011 William A. Goddard III, all rights reserved

Goddard-

Last time


2

Compare chemistry of column 10


3

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

2 nd

Ni Pd

Pt 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

Ch120a-Goddard-L19

10

Differential shielding favors n=4 over n=5, stabilize 4d over 5s  d 10

Relativistic effects of 1s huge  decreased KE  contraction  stabilize

4d similar size to 5s and contract all ns 

© copyright 2011 William A. Goddard III, all rights reserved

5 destabilize and expand nd

Mysteries from experiments on oxidative addition and reductive elimination of CH and CC bonds on Pd and Pt

6

Ch120a-Goddard-L19

Step 1: examine GVB orbitals for (PH

3

)

2

Pt(CH

3

)


7

Analysis of GVB wavefunction


8

Alternative models for Pt centers


9


10

energetics

Ch120a-Goddard-L19

Not agree with

11

Possible explanation: kinetics


12

Consider reductive elimination of HH,

CH and CC from Pd

Conclusion:

HH no barrier

CH modest barrier

CC large barrier


13

Consider oxidative addition of

HH, CH, and CC to Pt

Ch120a-Goddard-L19

Conclusion:

HH no barrier

CH modest barrier

14

Summary of barriers

This explains why CC coupling not occur for Pt while CH and HHcoupling is fast

But why?


15

How estimate the size of barriers (without calculations)


16

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

Examine CC coupling at transition state

Can orient the CH

3 can orient the CH

3 to obtain good overlap with Pd sd hybrid OR 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

Examine CH coupling at transition state

Ch120a-Goddard-L19

H can overlap both CH

3 and Pd sd hybrid simultaneously but CH

3 cannot thus get ~ ½ resonance stabilization of TS


19

Now we understand Pt chemistry

But what about Pd?

Why are Pt and Pd so dramatically different


20

new


21

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

Ground state configurations for column 10

Ni Pd Pt


23

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

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

Summary energetics

Conclusion the atomic character of the metal can control the chemistry


26

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

Ground states of neutral atoms

Fe

Co

Ni

Cu

Sc

Ti

V

Cr

Mn

(4s)2(3d)

(4s)2(3d)2

(4s)2(3d)3

(4s)1(3d)5

(4s)2(3d)5

(4s)2(3d)6

(4s)2(3d)7

(4s)2(3d)8

(4s)1(3d)10

Sc + (4s)1(3d)1

Ti +

V +

(4s)1(3d)2

(4s)0(3d)3

Cr + (4s)0(3d)5

Mn + (4s)1(3d)5

Fe + (4s)1(3d)6

Co + (4s)0(3d)7

Ni + (4s)0(3d)8

Cu + (4s)0(3d)10

Sc ++ (3d)1

Ti ++ (3d)2

V ++ (3d)3

Cr ++ (3d)4

Mn ++ (3d)5

Fe ++ (3d)6

Co ++ (3d)7

Ni ++ (3d)8

Cu ++ (3d)10


28

Bond energies MH+

Mo

Re

Cr

Au

Cu

Ch120a-Goddard-L19

Ag


29

Exchange energies:

Mn+: s 1 d 5

For high spin (S=3)

A

[(d

1 a

)(d

2 a

)(d

3 a

)(d

4 a

)(d

5 a

)(s a

)]

Get 6*5/2=15 exchange terms

5Ksd + 10 Kdd

Responsible for Hund’s rule

Ksd Kdd

Mn+ 4.8

19.8 kcal/mol

Tc+ 8.3

15.3

Re+ 11.9

14.1

Form bond to H, must lose half the exchange stabilization for the orbital bonded to the H

A

{(d

1 a

)(d

2 a

)(d

3 a

)(d

4 a

)(sd b a

)[(sd b

)H+H(sd b

)]( ab-ba

)} sd b is a half the time and

Ch120a-Goddard-L19 b half the time


30

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

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

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

Ch120a-Goddard-L19 electronegative


33


34


35


36


37

1 st row


38


39

Schilling


40

Steigerwald


41


42


43

2 nd row


44


45


46


47


48


49


50

3 rd row


51


52


53


54


55


56


57


58


59


60


61


62


63

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?

Ch120a-Goddard-L19

Bonding nonbonding antibonding

0 elect 64

Can we have 2s + 2s reactions for transition metals?

2s + 2s forbidden for organics

X

Cl

2

Ti

2s + 2s forbidden for organometallics?

?

Cl

2

Ti ?

Cl

2

Ti

Cl

2

Ti Me Cl

2

Ti Me

Cl

2

Ti Me

65

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

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

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

GVB orbitals at the TS for

Cl

2

TiH + + D

2

 Cl

2

TiD + + HD


69

GVB orbitals for the Cl

2

TiD + + HD product

Note get phase change for both orbitals


70

Follow the D2 bond as it evolves into the

HD bond


71

Follow the TiH bond as it evolves into the

TiD bond


72

Barriers small, thus allowed

Increased d character in bond  smaller barrier


73

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+0.71Mnsp+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


Show reaction for ClMnH + D2

Show example reactions


76

Olefin Metathesis

2+2 metal-carbocycle reactions

Diego Benitez, Ekaterina Tkatchouk, Sheng Ding


77

OLEFIN METATHESIS

Catalytically make and break double bonds!

R

1

R

1

+

R

2

R

2

2

R

1

Mechanism: actual catalyst is a metal-alkylidene

R

2

R

2

R

2

M

M M

R

2

R

1

R

3

R

1

R

3

R

1

R

3


78

Ru Olefin Metathesis Basics


79

Common Olefin Metathesis Catalysts


80

Applications of the olefin metathesis reaction

Ch120a-Goddard-L21 bulletproof resin

Small scale synthesis to industrial polymers

Acc. Chem. Res. 2001, 34 , 18-29

81

History of Olefin Metathesis Catalysts


82

Well-defined metathesis catalysts

R R

R R i Pr

N

(F

3

C)

2

MeCO

(F

3

C)

2

MeCO

Mo

1

Schrock 1991 alkoxy imido molybdenum complex a

Bazan, G. C.; Oskam, J. H.;

Cho, H. N.; Park, L. Y.;

Schrock, R. R. J. Am.

Chem. Soc. 1991, 113 ,

6899-6907 i Pr

Ph

CH

3

CH

3

Cl

PCy

3 Ph

Ru

Cl

PCy

3

Mes N

Cl

Ru

Cl

N

PCy

3

Mes

Ph

R=H, Cl

3

Grubbs 1999

1,3-dimesityl-imidazole-2-ylidenes

P(Cy)

3 mixed ligand system” c complex b

2

Grubbs 1991 ruthenium benzylidene

Wagener, K. B.;

Boncella, J. M.; Nel,

J. G. Macromolecules

1991 , 24 , 2649-2657

Scholl, M.; Trnka, T. M.; Morgan,

J. P.; Grubbs, R. H. Tetrahedron

Lett. 1999, 40 , 2247-2250.

Mes N

Cl

Ru

Cl

N

PCy

3

Mes

Ph

R=H, Ph,

or -CH

2

-(CH

2

)

2

-CH

2

-

4

GODDARD Ch120-L20 13/11/02

83

Metathesis Catalysts and Mechanism Overview

Mes

Cl

N

Ru

N

Cl

Mes

Ph

PCy

3

Mes

N

Ru

N

Cl

Mes

Cl

i-Pr

O

Mes

Cl

N

Ru

N

Cl

Mes

Ph

Py slow initiating catalyst fast-initiating catalyst ultra-fast-initiating catalyst

General mechanism of Metathesis

IMes

Cl

Ru

IMes

Cl

Ru Initiation

Cl Ph

L

L Cl R

Cl

IMes

Cl

Ru

R

1

Ch120a-Goddard-L21

R

3

R

2

Cl

R

3

IMes

Cl

R

Ru

R

2

Cl

IMes

Cl

Ru

R

3

Propagation

+

R

1


R

2

84

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


85

Ch120-L17-13-PdPt-TM atoms-Meta-Feb22

Lecture 13 February 1, 2011

Pd and Pt, MH

bonding, metathesis

Salient differences between Ni, Pd, Pt

Ru Olefin Metathesis Basics

Related documents

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Ch120-L17-13-PdPt-TM atoms-Meta-Feb22

Lecture 13 February 1, 2011

Pd and Pt, MH

bonding, metathesis

Salient differences between Ni, Pd, Pt

Ru Olefin Metathesis Basics

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