Stannyl Cations for the Hydrogenation of Carbon Dioxide to Methanol and Methyl Formate

The principal goal of this proposal is to develop novel stannyl cation-based Lewis pair catalysts [R3Sn(dot)L]+ (L = 2
electron oxygen donor) which are capable of rapidly activating H2 with concomitant reduction of CO2, to selectively form the commodity chemicals methanol (CH3OH) or methyl formate (HCO2CH3), at lower temperatures and pressures than currently used for their industrial-scale production. The activation of H2 by these main-group based systems is based on an extension of 'frustrated Lewis pair' (FLP) chemistry, albeit using catalysts that more closely resemble 'classical' donoracceptor adducts. This research builds upon our extremely promising preliminary results which demonstrate the competence of a Bu3SnH/catalytic [Bu3Sn]+ system for the rapid hydrostannylation of CO2 under mild conditions to methoxide (CH3O-) and oxide (O2-) equivalents, in addition to free HCO2CH3. Excitingly, reaction of this mixture with hydrogen (H2) liberates CH3OH, H2O and HCO2CH3, and regenerates the Bu3SnH reductant. This demonstrates that all stages of a catalytic protocol for CO2 hydrogenation can be achieved, although hydrogenolysis is at present too slow. The approach outlined in this work is to rationally modify the stannyl cation by increasing the size of the pendant alkyl groups and hence increase the amount of 'frustration' in the Lewis pair adducts. This is expected to markedly improve the kinetics of both H2 activation and subsequent CO2 reduction steps. Fundamentally new (water-tolerant) hydrogenation catalysts based on inexpensive and abundant Sn compounds will naturally develop during the course of this work programme, which will undoubtedly find use in other industrially relevant transformations (e.g. organic carbonyl hydrogenation to alcohols).
The specific key objectives to achieve the main aim are:
1. Synthesise new precursors (R3SnH, R3SnX; R = alkyl, X = NTf2, OTf) incorporating bulky R groups.
2. Investigate stannyl cation Lewis pair adducts, [R3Sn(dot)L]+ (L = R3SnOCH3, R3SnOSnR3), for their ability to activate H2, and correlate observed rates with structure.
3. Conduct detailed mechanistic studies on the hydrostannylation of both CO2 and proposed reaction intermediates.
4. Investigate the mechanism of Sn-O bond hydrogenolysis, and establish the relevant experimental parameters (e.g. temperature, pressure) which favour the production of either CH3OH or HCO2CH3.
5. Consolidation of rate data, mechanistic information, and experimental variables obtained from objectives 1-4 in order to optimise the best [R3Sn]+ species for selective catalytic hydrogenation of CO2 to CH3OH and/or HCO2CH3.