1 Mechanistic Insight into the CO 2 & CS 2 Capture by the Frustrated Lewis Pairs Amidophosphoranes Speaker: Jun Zhu 15 th, Apr. 2014

OUTLINE 1.Introduction & Motivation 2.Results & Discussion 3.Conclusion 2

Introduction 3 CO 2 capture As a greenhouse gas attributed to climate change and global warming, the capture and storage of CO 2 are of great importance.

Introduction 4 CO 2 capture Alumina, silica, zeolites, activated carbon and metal-organic frameworks have been developed to sequester this gas.

Introduction 5 CO 2 capture An alternative way to address the increasing atmospheric CO 2 level is to employ CO 2 as a C 1 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.

Introduction 6 FLPs have unprecedented reactivity, including the heterolytic cleavage of H 2 molecules and activation of small molecules, such as CO 2, N 2 O, NO, SO 2, 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 7 CO 2 capture O’Hare et al. demonstrated the non-metal-mediated homogeneous hydrogenation of CO 2 to MeOH under forcing conditions. Angew. Chem. Int. Ed., 2009, 48, 9839.

Introduction 8 J. Am. Chem. Soc., 2010, 132, Angew. Chem. Int. Ed., 2011, 50, Stephan et al. reported the stoichiometric conversion of CO 2 to methanol or CO using Al/P based FLPs.

Introduction 9 Piers et al. generated methane via the catalytic deoxygenative hydrosilation of CO 2. J. Am. Chem. Soc., 2010, 132,

Motivation 10 “The precise details of the mechanism of CO 2 insertion remains unproven.” Stephan’s group recently reported the CO 2 capture by the amidophosphorane. They claimed that the ring strain results in kinetically enhanced reactivity toward CO 2. Frustrated Lewis Pairs L. J. Hounjet, C. B. Caputo, D. W. Stephan, Angew. Chem. Int. Ed. 2012, 51, 4714

Results & Discussion 11 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 CO 2 capture with amidophosphorane 1. J. Zhu, K. An, Chem. Asian J. 2013, 12, CO 2 capture

Results & Discussion 12 Figure 2. Gibbs free energy (kcal/mol) profile for the CO 2 capture with amidophosphorane 3 +. In sharp contrast, when the fluoride is abstracted by Me 3 SiO 3 SCF 3, the reaction barrier increases significantly and becomes as high as 51.8 kcal/mol. CO 2 capture Although relief of ring strain has been attributed to CO 2 capture, the different reactivity of 1 and 3 cannot be rationalized by simply considering the factor.

Results & Discussion 13 To probe the origin of the reactivity of different amidophosphoranes, we examined the geometric and electronic structures of species involved in CO 2 by means of natural bond orbital (NBO) analysis. 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 + ). CO 2 capture

Results & Discussion 14 The natural bond orbital (NBO) analysis of the transition states. 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). CO 2 capture

Results & Discussion 15 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 CO 2. 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 CO 2 capture J. Zhu, Z. Lin, T. B. Marder, Inorg. Chem. 2005, 44, 9384.

Results & Discussion 16 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. CO 2 capture The natural bond orbital (NBO) analysis of the closed and open-chain amidophosphoranes with NMe 2 group.

Results & Discussion 17 Figure 6. Gibbs free energy (kcal/mol) profile for the CO 2 capture by amidophosphoranes with different groups. CO 2 capture Due to the trans influence, the P-O (1.917 Å) bond in the six-membered ring in 2-NMe 2 is weaker than that (1.859 Å) in 2-OMe, thus leading to slight exothermicity for CO 2 capture. The P-O bond in 2-F is Å, further supporting that trans influence is of great importance in the reaction barrier and exothermicity.

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

19 Results & Discussion CS 2 capture Introduction of CS 2 capture The deleterious effects of chronic CS 2 intoxication are V B 6 deficiency, depletion of the levels of essential trace metals, and an intensification of the development of atherosclerosis. CS 2 capture Chem. Eur. J. 2009, 15, Heldebrant et al. showed CO 2 BOL (mixtures of amidine or guanidine bases with alcohols)systems could react with CS 2, the base/alcohol mixtures show promise for the capture and release of carbon disulfide.

20 Results & Discussion CS 2 capture Kemp et al. used free ligand (Me 3 Si)(i-Pr 2 P)NH and its zinc complex to activate CO 2 and CS 2 to capture these gas. Ployhedron, 2013, 58, 92. Given that the bond dissociation energy (BDE) of C=S bond in CS 2 is weaker than that of C=O in CO 2, the CS 2 should be easier to react with those FLPs.

21 Results & Discussion Figure 8. Gibbs free energy (kcal/mol) profile for the CS 2 capture by different amidophosphoranes. Corresponding energies in CO 2 capture are given in parentheses. CS 2 capture Studies of the sequestration of CS 2 by different amidophosphoranes show inconformity with the expectation.

22 Results & Discussion Table 1. Bond lengths and bond angles at carbon atoms of CO 2 and CS 2 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 CO 2 capture by the substituted amidophosphoranes. CS 2 capture CompoundBL of X-CChange(%)  XCX Change(%) TS’ ’ TS TS’ ’ TS TS’ ’ TS Charge on S atom TS TS TS

23 Results & Discussion CS 2 capture CompoundBL of X-CChange(%)  XCX Change(%) TS’ ’ TS Table 2. Bond lengths and bond angles at carbon atoms of CO 2 and CS 2 in the transition states and products via unsubstituted amidophosphoranes. TS’78 represents the transition states and 8’ represents the products in CO 2 capture by unsubstituted amidophosphoranes. Charge on P atom Charge on O/S atom TS’ TS 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 CS 2.

Conclusion Part 2  DFT calculations on the mechanism of CO 2 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 NMe 2 group is predicted to result in more efficient CO 2 & CS 2 capture.  The distortion of CS 2 derived from the charge distribution leads the inconformity between the energy barries and the BDEs.  Our findings provide key insights into the mechanism of CO 2 & CS 2 capture with amidophosphoranes and open a new avenue to the design of FLPs for CO 2 & CS 2 sequestration. 24 CO 2 & CS 2 capture

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