Speaker: Ke An Advisor: Jun Zhu 20 th , Jun. 2014

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 An Advisor: Jun Zhu 20th, Jun. 2014

2. OUTLINE • Introduction & Motivation • Results &Discussion • Conclusion

3. Introduction 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.

4. Introduction 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.

5. Introduction Stephan’s group (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.

6. Introduction 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. Frustrated Lewis Pairs (1) L. J. Hounjet, C. B. Caputo, D. W. Stephan, Angew. Chem. Int. Ed.2012, 51, 4714 “The precise details of the mechanism of CO2 insertion remains unproven.”

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

8. Introduction Further studies indicate that the interplay of ring strain and trans influence determines the reactivity of FLPs. Bond Order P-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)

9. Motivation 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 (1) Y.-R. Luo, Comprehensive Handbook of Chemical Bond Energies, CRC Press: Boca raton, FL, 2007. Software: Gaussian 09 Method: M06-2X Basis set: 6-31+G(d)

10. Results & Discussion 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.

11. 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.

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

13. Results & Discussion Table 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. 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.

14. Conclusion • 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. Part 2

15. Thank you very much! Questions and advice are welcoming!