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

1. Mechanistic Insight into the CO2 & CS2 Capture by the Frustrated Lewis Pairs Amidophosphoranes Ring Strain Trans-influence CO2 Ring Strain Speaker: Jun Zhu 15th, Apr. 2014

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

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

4. Introduction CO2 capture Alumina, silica, zeolites, activated carbon and metal-organic frameworks have been developed to sequester this gas.

5. Introduction CO2 capture An 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.

6. 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. 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.

7. Introduction 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.

8. Introduction Stephan et al. reported the stoichiometric conversion of CO2 to methanol or CO using Al/P based FLPs. J. Am. Chem. Soc., 2010, 132, 1796. Angew. Chem. Int. Ed., 2011, 50, 8396.

9. Introduction Piers et al. generated methane via the catalytic deoxygenative hydrosilation of CO2. J. Am. Chem. Soc., 2010, 132, 10660.

10. Motivation Stephan’s group recently reported the CO2 capture by the amidophosphorane. They claimed that the ring strain results in kinetically enhanced reactivity toward CO2. Frustrated Lewis Pairs 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.”

11. Results & Discussion CO2 capture 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.

12. Results & Discussion CO2 capture In sharp contrast, when the fluoride is abstracted by Me3SiO3SCF3, the reaction barrier increases significantly and becomes as high as 51.8 kcal/mol. Figure 2. Gibbs free energy (kcal/mol) profile for the CO2 capture with amidophosphorane 3+. 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.

13. Results & Discussion CO2 capture 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. Figure 3. The bond orders and bond lengths for selective key bonds in amidophosphoranes (1 and 3+). Enhanced P-N bond in 3+ should be the main factor in the much higher barrier.

14. Results & Discussion CO2 capture The natural bond orbital (NBO) analysis of the transition states. Figure 4. The bond orders and bond lengths for selective key bonds in the transition states (TS12 and TS34). More energies are required to reach the structure of the transition state TS34.

15. Results & Discussion CO2 capture 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. Bond dissociation energy: P-N > P-O > P-F Scheme 1. Relative Gibbs free energy (kcal/mol) of the open-chain form of a series of amidophosphoranes. J. Zhu, Z. Lin, T. B. Marder, Inorg. Chem.2005, 44, 9384.

16. Results & Discussion CO2 capture The natural bond orbital (NBO) analysis of the closed and open-chain amidophosphoranes with NMe2 group. 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.

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

18. Results & Discussion CO2 capture What’s the impact on the reaction while only considering the ring strain in FLP ? Part 2 Figure 7. Gibbs free energy (kcal/mol) profile for the CO2 sequestration by nonstrained amidophosphorane 5.

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

20. Results & Discussion CS2 capture 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.

21. Results & Discussion CS2 capture Studies of the sequestration of CS2 by different amidophosphoranes show inconformity with the expectation. Figure 8. Gibbs free energy (kcal/mol) profile for the CS2 capture by different amidophosphoranes. Corresponding energies in CO2 capture are given in parentheses.

22. Results & Discussion CS2 capture 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.

23. Results & Discussion CS2 capture 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. 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.

24. Conclusion CO2 & CS2 capture • 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. Part 2

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