1

Why Activation of Weaker C=S Bond in the CS2 by FLPs Requires More Energies than That of the C=O

Bond in CO2: A DFT Study

Speaker: Ke AnAdvisor: Jun Zhu20th, Jun. 2014

1

0.00.0

TS1222.7

2-15.5

CX2

N

PPh

Ph

Me

PX

N X

Me

PhPh

PX

CN XMe

PhPh

33.6

-7.2

X = SX = O

BDE: O=C > S=C

BDE

EA

F

F

F

OUTLINE

1.Introduction & Motivation

2.Results & Discussion

3.Conclusion

2

Introduction

3

Frustrated Lewis pair: a concept for new reactivity and catalysis

A FLP is an intra- or intermolecular combination of a Lewis acid and a Lewis base in which steric hindrance inhibits the formation of a classical Lewis donor-acceptor adduct.

(1) D. W. Stephan, Org. Biomol. Chem. 2008, 6, 1535-1539. (2) D. W. Stephan, Dalton Trans. 2009, 3129-3136. (3) D. W. Stephan, Dalton Trans. 2012, 41, 9015.

Classical Lewis Adducts

A

B

[CPh3]+[B(C6F5)4]- Me3P[Me3P CPh3] [B(C6F5)4]-

Me3P(C6F5)3B(THF) Me3P B(C6F5)3

Me3P B(C6F5)3

Frustrated Lewis Pair Products

C

D

[CPh3]+[B(C6F5)4]-t-Bu3P

[B(C6F5)4]-

t-Bu3P(C6F5)3B(THF)

E

CPh2

Ht-Bu3P

[B(C6F5)4]-CHPh2t-Bu3P

t-Bu3PO

B(C6F5)3

(C6F5)3BCy3P

BF(C6F5)2Cy3P

FF

F F

Introduction

4

FLPs have unprecedented reactivity, including the heterolytic cleavage of H2 molecules and activation of small molecules, such as CO2, N2O, NO, SO2, alkenes and alkynes.

FLPs have been demonstrated an effective strategy to sequestrate CO2 for the carbon atom is electrophilic while the O atom is nucleophilic though CO2 has overall thermodynamic stability.

Specifically, B/P, B/N, P/N and Al/P-based FLPs have shown the capacity for the conversion of CO2 into C1 feedstock such as carbonic acid derivatives, methanol, methane, or CO by the groups of Stephan, O’Hare and Piers.

Introduction

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Stephan’s group

PtBu3 + B(C6F5)3CO2, 25°C

80°C, vacuum -CO2

OO

B(C6F5)3tBu3PH

B(C6F5)2

(C6H2Me3)2P

(C6H2Me3)2P B(C6F5)2

CO2, 25°C

CH2Cl2, > -20°C-CO2

(C6H2Me3)2PO

O

B(C6F5)3

(1) C. M. Momming, E. Otten, G. Kehr, R. Frohlich, S. Grimme, D. W. Stephan, G. Erker, Angew. Chem. Int. Ed. 2009, 48, 6643.(2) G. Menard, D. W. Stephan, J. Am. Chem. Soc. 2010, 132, 1796.(3) G. Menard, D. W. Stephan, Angew. Chem. Int. Ed. 2011, 50, 8396.(4) L. J. Hounjet, C. B. Caputo, D. W. Stephan, Angew. Chem. Int. Ed. 2012, 51, 4714.(5) M. J. Sgro, D. W. Stephan, Chem. Commun. 2013, 49, 2610.(6) A. E. Ashley, A. L. Thompson, D. O’Hare, Angew. Chem. Int. Ed. 2009, 48, 9839.(7) A. Berkefeld, W. E. Piers, M. Parvez, J. Am. Chem. Soc. 2010, 132, 10660.

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Me3SiO3SCF3

N

P

F

Ph

Ph

Me

CO2

PO

N

F

O

Me

PhPh

1 2

FSiMe3

N

PPh

Ph

Me

PO

N O

Me

PhPh

4

Me3SiO3SCF3

FSiMe3

CO2+ +

O3SCF3- O3SCF3

-

3

THF, RT

“The precise details of the mechanism of CO2 insertion remains unproven.”

In 2012, Stephan’s group reported the CO2 capture by the N/P based FLPs experimentally. They claimed that the ring strain results in kinetically enhanced reactivity toward CO2.

N

P

R

Ph

Ph

Me

N

P

R

Ph

Ph

Me

N

P

R

Ph

Ph

Me

A B C

Frustrated Lewis Pairs

(1) L. J. Hounjet, C. B. Caputo, D. W. Stephan, Angew. Chem. Int. Ed. 2012, 51, 4714

Introduction

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DFT studies into the mechanism were performed by using the real model at the M062X/6-31+G(d) level.

Introduction

1

0.0IN12

19.5

TS12

22.7

2

-15.5CO2

N

P

F

Ph

Ph

Me

PO

N

F

O

Me

PhPh

PO

CN

F

OMe

PhPh

PO

CN

F

OMe

PhPh

3

0.0

IN34

39.4 TS34

51.8

4

-14.2

CO2

N

PPh

Ph

Me

PO

N O

Me

PhPh

PO

CN OMe

PhPh

PO

CN

OMe

PhPh

+

+

+

+

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Introduction

Further studies indicate that the interplay of ring strain and trans influence determines the reactivity of FLPs.

1-R

0.00.0

IN12-R

8.9

TS12-R

14.3

2-R-12.2

CO2

N

P

R

Ph

Ph

Me

PO

N

R

O

Me

PhPh

PO

CN

R

OMe

PhPhP

O

CN

R

OMe

PhPh

0.0

8.2

-10.2

R = OMeR = NMe2

Bond OrderP-R : 0.48, 0.56, 0.64 (R = F, OMe, NMe2) P-N: 0.51, 0.46, 0.40 (the bond order of P-N bond in 3 is 0.78)

Motivation

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As the analogue of CO2, CS2 is also a pollutant in environment and can cause physical damage, such as deficiency of vitamin B6, depletion of essential trace metals, intensification of the atherosclerosis, chronically but in a small amount.

Bond dissociation energy (BDE) of C=S in CS2 is weaker than that of C=O in CO2, and CS2 is less stable, so it should be easier to react with those amidophosphoranes.

BDE: C=S 105.3 kcal/mol C=O 127.2 kcal/mol

Software: Gaussian 09Method: M06-2XBasis set: 6-31+G(d)

(1) Y.-R. Luo, Comprehensive Handbook of Chemical Bond Energies, CRC Press: Boca raton, FL, 2007.

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Results & Discussion

1-F

0.0

TS-F

33.6(22.7)

2-F

-7.2(-15.5)

CS2

N

P

F

Ph

Ph

Me

PS

N

F

S

Me

PhPh

PS

CN

F

SMe

PhPh

1-OMe

0.0

TS-OMe

25.1(14.3)

2-OMe

-5.4(-12.2)

CS2

N

P

OMe

Ph

Ph

Me

PS

N

MeO

S

Me

PhPh

PS

CN

MeO

SMe

PhPh

1-NMe2

0.0TS-NMe2

16.4(8.2)

2-NMe2

-4.0(-10.2)

CS2

N

P

NMe2

Ph

Ph

Me

PS

N

Me2N

S

Me

PhPh

PS

CN

Me2N

SMe

PhPh

3

0.0

TS

46.0(51.8)

4

-15.0(-14.2)

CS2

N

PPh

Ph

Me

PS

N S

Me

PhPh

PS

CNS

Me

PhPh

Studies of the sequestration of CS2 by different amidophosphoranes show inconformity with the expectation.

Corresponding Gibbs free energies of CO2 capture are given in parenthese.

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Results & Discussion

Table 1. Bond lengths and bond angles at carbon atoms of CO2 and CS2 in the transition states and products via substituted amidophosphoranes. TS’-R (R = F, NMe2, OMe) represent the transition states and 2’-R (R = F, NMe2, OMe) represent the products in CO2 capture.

Compound BL of X-C Change(%) XCX Change(%)

TS’-F 1.178 1.3 163.9 -8.9

2’-F 1.330 14.4 122.8 -31.8

TS-F 1.598 2.8 151.0 -16.1

2-F 1.759 13.2 120.3 -33.2

TS’-OMe 1.177 1.2 163.7 -9.1

2’-OMe 1.316 13.2 124.4 -30.9

TS-OMe 1.595 2.6 151.6 -15.8

2-OMe 1.747 12.4 121.4 -32.6

TS’-NMe2 1.187 2.1 154.7 -14.1

2’-NMe2 1.308 12.5 125.3 -30.4

TS-NMe2 1.603 3.0 146.8 -18.4

2-NMe2 1.738 11.8 122.6 -31.9

PS

N

R

SMe

PhPh

XCX

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Results & Discussion

Table 2. NBO analysis of natural charge on P, S, and O atoms in TSs.

0.209 -0.418

-0.528 1.055

CS2

CO2

Charge P S Change of S (%)

TS-F 2.07 0.018 -91.4

TS-OMe 2.04 0.028 -86.6

TS-NMe2 1.96 0.013 -93.8

PS

N

R

SMe

PhPh

Charge P O Change of O (%)

TS’-F 2.08 -0.609 15.3

TS’-OMe 2.04 -0.604 14.4

TS’-NMe2 1.96 -0.621 17.6

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Results & Discussion

BL of X-C Change(%)

XCX Change(%)

TS’ 1.201 3.3 157.4 -12.6

4’ 1.399 20.3 118.7 -34.1

TS 1.600 3.0 164.0 -8.9

4 1.806 16.2 115.0 -36.1Table 3. Bond lengths and bond angles at carbon atoms of CO2 and CS2 in the transition states and products via unsubstituted amidophosphorane. TS’ represents the transition states and 4’ represents the products in CO2 capture by unsubstituted amidophosphorane.

Charge P X Change of X (%)

TS’ (O) 1.72 -0.64 21.2

TS (S) 1.51 0.22 5.3

The positive charge on phosphorus and sulfur make the two atoms repulsive and they cannot be attracted by each other like P and O atoms, indicting a smaller distortion of CS2.

3

0.0

TS

46.0(51.8)

4

-15.0(-14.2)

CS2

N

PPh

Ph

Me

PS

N S

Me

PhPh

PS

CNS

Me

PhPh

Conclusion

Part 2 DFT calculations on the mechanism of CS2 capture reveal that the interplay of ring strain and trans influence still determines the reactivity of amidophosphoranes.

The distortion of CS2 derived from the charge distribution leads the inconformity between the energy barriers and the BDEs.

Our findings provide key insights into the mechanism of CS2 capture with amidophosphoranes and open a new avenue to the design of FLPs for CS2 sequestration.

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Thank you very much!

Questions and advice are welcoming!

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