Small Molecule Activation by Intermolecular Zr(IV)-Phosphine Frustrated Lewis Pairs

We report intermolecular transition metal frustrated Lewis pairs (FLPs) based on zirconocene aryloxide and phosphine moieties that exhibit a broad range of small molecule activation chemistry that has previously been the preserve of only intramolecular pairs. Reactions with D2, CO2, THF, and PhCCH are reported. By contrast with previous intramolecular examples, these systems allow facile access to a variety of steric and electronic characteristics at the Lewis acidic and Lewis basic components, with the three-step syntheses of 10 new intermolecular transition metal FLPs being reported. Systematic variation to the phosphine Lewis base is used to unravel steric considerations, with the surprising conclusion that phosphines with relatively small Tolman steric parameters not only give highly reactive FLPs but are often seen to have the highest selectivity for the desired product. DOSY NMR spectroscopic studies on these systems reveal for the first time the nature of the Lewis acid/Lewis base interactions in transition metal FLPs of this type.

本工作报道了基于茂锆芳氧合物与膦基团的分子间过渡金属受阻路易斯对（frustrated Lewis pairs, FLPs），该类体系展现出此前仅分子内受阻路易斯对才可实现的广谱小分子活化化学活性。本文报道了其与氘气（D₂）、二氧化碳（CO₂）、四氢呋喃（THF）及苯乙炔（PhCCH）的反应。与此前报道的分子内受阻路易斯对体系不同，本研究中的这类分子间过渡金属FLPs可便捷调控路易斯酸与路易斯碱组分的空间与电子特性，本文同时报道了10种新型分子间过渡金属FLPs的三步合成路线。通过对膦类路易斯碱的系统结构调控，本研究解析了空间位阻对该类体系的影响，得到了出人意料的结论：具有较小托尔曼空间参数（Tolman steric parameters）的膦配体不仅可构建高反应活性的FLPs，且往往对目标产物展现出最高的选择性。本研究通过DOSY核磁共振（DOSY NMR）光谱对该类体系进行表征，首次揭示了此类过渡金属FLPs中路易斯酸与路易斯碱相互作用的本质。