Softer Frustrated Lewis Pair Catalysis for Harder Substrates: Stannyl Cations for the Hydrogenation of Carbon Dioxide to Methanol and Methyl Formate

点击翻译按钮获取中文摘要

By lowering the energy barrier required for reactions to proceed, catalysts enable chemical transformations to be conducted at faster rates and with greater energy efficiency than would otherwise be possible. As such, around 90% of all processes in chemical manufacturing rely upon catalysis for effective production. Catalytic hydrogenations (the reaction of compounds with hydrogen, H2) are routinely employed in all areas of chemical production, yet the catalysts are predominantly based on precious metals (e.g. Rh, Ru, Pd, Pt) which are both expensive and of limited supply; there is therefore a strong motivation to develop new catalysts which do not incorporate such elements.In the last decade a new and exciting chemical methodology using catalysts based on inexpensive and abundant main group elements has been discovered. Known as 'frustrated Lewis pairs' (FLPs), these consist of a Lewis acid and base which (for steric and/or electronic reasons) cannot interact strongly with one another, leading to unquenched reactivity that can be exploited for the reaction with small molecules, most notably H2. When H2 reacts with FLPs it is converted into a much more reactive ionic form (protic H+ and hydridic H-) which can subsequently be delivered to substrates, hence effecting catalytic hydrogenation. To date, FLP hydrogenation catalysts almost exclusively use boron at the Lewis acidic centre, which is not optimal for the reduction of compounds containing oxygen, since the products (alcohols, water: hard bases) bind too strongly to the hard Lewis acid, which has a potent inhibitory effect on the overall rate of reaction.This proposal aims to develop new FLP-hydrogenation catalyst protocols using stannyl cations (based on [R3Sn]+ fragments), explicitly for the catalytic conversion of CO2 (carbon dioxide, a greenhouse gas) and H2 to two important commodity platform chemicals: methanol (CH3OH) and methyl formate (HCO2CH3). These can be used as feedstocks for upgrading to added-value products, or as liquid fuels. In the latter case, assuming the H2 is obtained by renewable means (e.g. photo-splitting of water using solar energy) these would represent sustainable sources of storable energy, with the potential to impact positively on the global carbon balance.This transformative approach stems from extremely encouraging initial results which show that a Bu3SnH/catalytic [Bu3Sn]+ (Bu = C4H9) system is competent for the reduction of CO2 under mild conditions, thereafter reaction with H2 liberates CH3OH, HCO2CH3 and water, in addition to the regeneration of Bu3SnH. Taken together, these results demonstrate that all stages of a catalytic cycle for CO2 hydrogenation can be achieved. Notably, the [Bu3Sn]+ catalysts are thermally stable to water, demonstrating a considerable advantage over boron-based FLPs. Currently the rate of reaction with H2 is too slow to be comparable with CO2 conversion; this will be addressed by increasing the bulk of the stannyl cations, and hence increasing their reactivity via augmented 'frustration'.