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

通过降低反应进行所需的能量障碍，催化剂使化学转化能够以比其他方式更快的速度和更高的能源效率进行。因此，化学制造中约90%的过程依赖于催化来进行有效生产。催化加氢(化合物与氢、氢的反应)在化学生产的所有领域都是常规使用的，但催化剂主要基于贵金属(如Rh、Ru、Pd、Pt)，这些贵金属既昂贵又供应有限，因此有强烈的动机开发不包含这些元素的新催化剂。在过去的十年中，人们发现了一种新的、令人兴奋的化学方法，使用基于廉价和丰富的主族元素的催化剂。这些物质被称为“受挫刘易斯对”(FLP)，由路易斯酸和碱组成(由于空间和/或电子原因)，它们彼此之间不能发生强烈的相互作用，导致未熄灭的反应性，可用于与小分子，最明显的是氢的反应。当氢与Flp反应时，它被转化为更具活性的离子形式(质子型H+和氢化型H-)，随后可以传递到底物，从而实现催化氢化。到目前为止，FLP加氢催化剂几乎只在Lewis酸中心使用硼，这对于还原含氧化合物并不是最优的，因为产物(醇、水：硬碱)与硬Lewis酸结合太强，这对整体反应速度有很强的抑制作用。本建议旨在开发新的FLP加氢催化剂方案，使用锡阳离子(基于[R3Sn]+片段)，明确地将CO2(二氧化碳，温室气体)和H2催化转化为两种重要的商品平台化学品：甲醇(CH3OH)和甲酸甲酯(HCO2CH3)。这些可用作升级为附加值产品的原料，或用作液体燃料。在后一种情况下，假设H2是通过可再生方法(例如，利用太阳能光解水)获得的，这些将是可持续的可储存能源，具有对全球碳平衡产生积极影响的潜力。这种变革性的方法源于非常令人鼓舞的初步结果，这些结果表明，Bu3SnH/催化[Bu3SnH]+(Bu=C4H9)体系在温和的条件下能够还原CO2，此后与H2的反应除了Bu3SnH的再生外，还释放CH3OH、HCO2CH3和水。综上所述，这些结果表明，二氧化碳加氢催化循环的所有阶段都可以实现。值得注意的是，[Bu3Sn]+催化剂对水是热稳定的，显示出比基于硼的Flp有相当大的优势。目前，与氢的反应速度太慢，无法与二氧化碳的转化相媲美；这一问题将通过增加锡基阳离子的体积来解决，从而通过增加“挫折感”来增加它们的反应性。

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'.