1 Mechanistic Insight into the CO 2 & CS 2 Capture by the Frustrated Lewis Pairs Amidophosphoranes...

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Mechanistic Insight into the CO2 & CS2 Capture by the Frustrated Lewis Pairs

Amidophosphoranes

Speaker: Jun Zhu15th, Apr. 2014

OUTLINE

1.Introduction & Motivation

2.Results & Discussion

3.Conclusion

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Introduction

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CO2 capture As a greenhouse gas attributed to climate change and global warming, the capture and storage of CO2 are of great importance.

Introduction

4

CO2 capture

Alumina, silica, zeolites, activated carbon and metal-organic frameworks have been developed to sequester this gas.

Introduction

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CO2 captureAn alternative way to address the increasing atmospheric CO2 level is to employ CO2 as a C1 feedstock, and the past few years has seen the concept of “frustrated Lewis pairs” (FLPs) emerge as an effective strategy.

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.

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

Introduction

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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

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. The FLP concept has also been exploited for the development of stoichiometric reductions of anilines to cyclohexylamines and of metal-free catalysts for hydro-genation of polar substrates.

Introduction

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CO2 capture O’Hare et al. demonstrated the non-metal-mediated homogeneous hydrogenation of CO2 to MeOH under forcing conditions.

Angew. Chem. Int. Ed., 2009, 48,

9839.

TMP-H

H-B(C6F5)3

CO2 (1 atm)

<110C

TMP-H

O CH

O

B(C6F5)3

TMP

CH3OH B(C6F5)3

+

160C distill/

TMP/[TMPH]+

decomposition

CH3OH

+H

H

H O

B(C6F5)3

TMP-H

H

HO

B(C6F5)3

TMP-H

H-B(C6F5)3

B(C6F5)3

TMP + H2

O CH

OB(C6F5)3

TMP-H

H-B(C6F5)3

(C6F5)2BOCH3

Introduction

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J. Am. Chem. Soc., 2010, 132, 1796. Angew. Chem. Int. Ed., 2011, 50, 8396.

Stephan et al. reported the stoichiometric conversion of CO2 to methanol or CO using Al/P based FLPs.

Mes3P(AlX3)

X = Cl, BrPMes + AlX3

CO2

AlX3

O O

PMes

X3Al AlX3

Mes

Mes

NH3BH3

15min25C

[Mes3PH][(MeO)nAlX4-n] +(BHNH)n

H2O

C6H5Br

H3COH

PMes3

CO213CO2

[AlX4][AlX4]

CO + 12CO213CO + 12CO2 OO

PMes3

AlX2AlHX2O

AlX3

Introduction

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Piers et al. generated methane via the catalytic deoxygenative hydrosilation of CO2.

J. Am. Chem. Soc., 2010, 132, 10660.

Motivation

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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.”

Stephan’s group recently reported the CO2 capture by the amidophosphorane. 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

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

Results & Discussion

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We firstly examined the pathway on the sequestration by amidophosphorane 1 by using the real model at the M062X/6-31+G(d). The basis set dependence and solvent effect were small.

Figure 1. Gibbs free energy (kcal/mol) profile for the CO2 capture with amidophosphorane 1.

J. Zhu, K. An, Chem. Asian J. 2013, 12, 3147.

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

CO2 capture


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Figure 2. Gibbs free energy (kcal/mol) profile for the CO2 capture with amidophosphorane 3+.

In sharp contrast, when the fluoride is abstracted by Me3SiO3SCF3, the reaction barrier increases significantly and becomes as high as 51.8 kcal/mol.

CO2 capture

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

+

++

+

Although relief of ring strain has been attributed to CO2 capture, the different reactivity of 1 and 3 cannot be rationalized by simply considering the factor.


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To probe the origin of the reactivity of different amidophosphoranes, we examined the geometric and electronic structures of species involved in CO2 by means of natural bond orbital (NBO) analysis.

0.78 (1.715)

1.02 (1.434)

1.32 (1.404)

0.90 (1.789)

0.51 (1.875)1.15 (1.375)

1.28 (1.406)

0.86 (1.806)

1 3+

0.48 (1.688)

Enhanced P-N bond in 3+ should be the main factor in the much higher barrier.

Figure 3. The bond orders and bond lengths for selective key bonds in amidophosphoranes (1 and 3+).

CO2 capture


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The natural bond orbital (NBO) analysis of the transition states.

TS12 TS34

0.11 (2.319) 0.02 (2.976)

1.76 (1.178)

0.16 (2.262)

1.28 (1.201)

0.19 (2.180)

163.9157.4

More energies are required to reach the structure of the transition state TS34.

Figure 4. The bond orders and bond lengths for selective key bonds in the transition states (TS12 and TS34).

CO2 capture


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N

P

R

Ph

Ph

Me

N

P

R

Ph

Ph

Me

N

PPh

Ph

Me

N

PPh

Ph

Me

+ +

G = +16.4 (R = F)

G = +37.0

G = +5.4 (R = OMe)G = -1.1 (R = NMe2)

+

-

With regard to the trans influence, introducing stronger bonds than the P-F bond should be the most straightforward method to stabilize the open-chain FLP in IN12 and IN34 to easily sequestrate CO2.

Scheme 1. Relative Gibbs free energy (kcal/mol) of the open-chain form of a series of amidophosphoranes.

Bond dissociation energy: P-N > P-O > P-F

CO2 capture

J. Zhu, Z. Lin, T. B. Marder, Inorg. Chem. 2005, 44, 9384.


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0.05 (2.840)

1.52 (1.313)

1.16 (1.449)

0.97 (1.756)

0.80 (1.689)

1.23 (1.351)0.40 (1.993)

1.25 (1.413)

0.86 (1.826)

0.64 (1.771)

Figure 5. The bond orders and bond length for selective key bonds in the closed (left) and open form (right) of amidophosphoranes with the amine group.

CO2 capture

The natural bond orbital (NBO) analysis of the closed and open-chain amidophosphoranes with NMe2 group.

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


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Figure 6. Gibbs free energy (kcal/mol) profile for the CO2 capture by amidophosphoranes with different groups.

CO2 capture

Due to the trans influence, the P-O (1.917 Å) bond in the six-membered ring in 2-NMe2 is weaker than that (1.859 Å) in 2-OMe, thus leading to slight exothermicity for CO2 capture.

The P-O bond in 2-F is 1.786 Å, further supporting that trans influence is of great importance in the reaction barrier and exothermicity.

Part 2

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5

0.06

8.6

TS67

38.8

7

14.4

CO2

Me2N

CF3

P

CF3

CH3

CF3

8

-3.6

TS89

6.4

9

-9.5

Me2N

CF3

P

CF3

CH3

CF3

O C O

Me2N

CF3

P

CF3

CH3

CF3

OC O

Me2N

CF3

P

CF3

CH3

CF3

OC

OMe2N

CF3

P

CF3

CH3

CF3

O

CO

Me2N

CF3

P

CF3

CH3

CF3

O

C

O

Me2N

CF3

P

CF3

CH3

CF3OC

O


N

P

F

Ph

Ph

Me

H = -9.0+ NH2Ph

N

P

F

Ph

Ph

MePh

H2N

Figure 7. Gibbs free energy (kcal/mol) profile for the CO2 sequestration by nonstrained amidophosphorane 5.

CO2 capture

What’s the impact on the reaction while only considering the ring strain in FLP ?

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Results & Discussion CS2 capture

Introduction of CS2 capture

The deleterious effects of chronic CS2 intoxication are VB6 deficiency, depletion of the levels of essential trace metals, and an intensification of the development of atherosclerosis.

CS2 capture

Chem. Eur. J. 2009, 15, 7619.

CS2 + Base + ROH Base H+ ROC

S

S

Heldebrant et al. showed CO2BOL (mixtures of amidine or guanidine bases with alcohols)systems could react with CS2, the base/alcohol mixtures show promise for the capture and release of carbon disulfide.

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Kemp et al. used free ligand (Me3Si)(i-Pr2P)NH and its zinc complex to activate CO2 and CS2 to capture these gas.

Ployhedron, 2013, 58, 92.

Given that the bond dissociation energy (BDE) of C=S bond in CS2 is weaker than that of C=O in CO2, the CS2 should be easier to react with those FLPs.

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Figure 8. Gibbs free energy (kcal/mol) profile for the CS2 capture by different amidophosphoranes. Corresponding energies in CO2 capture are given in parentheses.

CS2 capture

1

0.0

TS'12

33.6(22.7)

2

-7.2(-15.5)

CS2

N

P

F

Ph

Ph

Me

PS

N

F

S

Me

PhPh

PS

CN

F

SMe

PhPh

3

0.0

TS'34

25.1(14.3)

4

-5.4(-12.2)

CS2

N

P

OMe

Ph

Ph

Me

PS

N

MeO

S

Me

PhPh

PS

CN

MeO

SMe

PhPh

5

0.0TS'56

16.4(8.2)

6

-4.0(-10.2)

CS2

N

P

NMe2

Ph

Ph

Me

PS

N

Me2N

S

Me

PhPh

PS

CN

Me2N

SMe

PhPh

7

0.0

TS'78

46.0(51.8)

8

-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.

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Table 1. Bond lengths and bond angles at carbon atoms of CO2 and CS2 in the transition states and products via substituted amidophosphoranes. TS’12, TS’34, TS’56 represent the transition states and 2’, 4’, and 6’ represent the products in CO2 capture by the substituted amidophosphoranes.

CS2 capture

Compound BL of X-C Change(%)

XCX Change(%)

TS’12 1.178 1.3 163.9 -8.9

2’ 1.330 14.4 122.8 -31.8

TS12 1.598 2.8 151.0 -16.1

2 1.759 13.2 120.3 -33.2

TS’34 1.177 1.2 163.7 -9.1

4’ 1.316 13.2 124.4 -30.9

TS34 1.595 2.6 151.6 -15.8

4 1.747 12.4 121.4 -32.6

TS’56 1.187 2.1 154.7 -14.1

6’ 1.308 12.5 125.3 -30.4

TS56 1.603 3.0 146.8 -18.4

6 1.738 11.8 122.6 -31.9

0.209 -0.418

-0.528 1.055

CS2

CO2

Charge on S atom

TS12 -0.074

TS34 -0.117

TS56 -0.143

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

TS’78 1.201 3.3 157.4 -12.6

8’ 1.399 20.3 118.7 -34.1

TS78 1.600 3.0 164.0 -8.9

8 1.806 16.2 115.0 -36.1

Table 2. Bond lengths and bond angles at carbon atoms of CO2 and CS2 in the transition states and products via unsubstituted amidophosphoranes. TS’78 represents the transition states and 8’ represents the products in CO2 capture by unsubstituted amidophosphoranes.

Charge on P atom

Charge on O/S atom

TS’78 1.722 -0.481

TS78 1.489 0.048

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.

Conclusion

Part 2 DFT calculations on the mechanism of CO2 capture reveal that the interplay of ring strain and trans influence determines the reactivity of amidophosphoranes.

The stability of the open-chain FLPs can be tuned by choosing different trans influence ligands and a new amidophosphorane with an NMe2 group is predicted to result in more efficient CO2 & CS2 capture.

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

Our findings provide key insights into the mechanism of CO2 & CS2 capture with amidophosphoranes and open a new avenue to the design of FLPs for CO2 & CS2 sequestration.

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CO2 & CS2 capture

Thank you very much!

Questions and advice are welcoming!

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1 Mechanistic Insight into the CO 2 & CS 2 Capture by the Frustrated Lewis Pairs Amidophosphoranes...

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