Boron-based frustrated Lewis pairs (FLPs) have become well-established catalysts for the hydrogenation of a wide range of functional groups. Conversely, aluminium-based FLP hydrogenation catalysts are less common, especially for CO2 reduction. They are mostly confined to the hydrogenation of imines, alkenes, and alkynes even though aluminium is much more abundant than boron and forms structurally related compounds. Moreover, aluminium forms penta- and hexa-coordinated complexes, which remain untested in FLP hydrogenation catalysis. Herein, we demonstrate that cationic, hexa-coordinated diaqua-meso-tetraphenylporphyrin aluminium complexes [Al(TPP)(OH2)2]X and [Al(tBuTPP)(OH2)2]X (X = Cl-, OTf-, ClO4-) form FLPs with nitrogen bases, activate H2, and reductively couple CO2 to amines, yielding N-formylamines and water. Our experimental results and DFT analysis indicate that H2 activation involves the formation of an FLP, base-promoted CO2 reduction and formate salt elimination from the FLP, as proposed for transition metal-catalyzed N-formylations. These similarities in the reaction mechanism and structure of aluminium complexes brings Al-based FLPs closer to transition metal catalysis and may enable us to apply this knowledge to ligand design to enhance main group metal-promoted hydrogenations.

中文翻译：

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铝卟啉催化 CO2 与 H2 的氢化反应


硼基挫折路易斯对 （FLP） 已成为多种官能团氢化的成熟催化剂。相反，铝基 FLP 加氢催化剂不太常见，尤其是用于 CO2 还原。它们主要局限于亚胺、烯烃和炔烃的氢化，尽管铝比硼丰富得多，并形成结构相关的化合物。此外，铝形成五配位和六配位配合物，这在 FLP 加氢催化中仍未得到测试。在此，我们证明了阳离子、六配位二水-中旋-四苯基卟啉铝络合物 [Al（TPP）（OH2）2]X 和 [Al（tBuTPP）（OH2）2]X（X = Cl-、OTf-、ClO4-）与氮碱形成 FLP，活化 H2，并将 CO2 还原偶联到胺上，生成 N-甲酰胺和水。我们的实验结果和 DFT 分析表明，H2 活化涉及 FLP 的形成、碱促进的 CO2 还原和从 FLP 中消除甲酸盐，如过渡金属催化的 N-甲酰化所提出的。铝配合物的反应机理和结构的这些相似性使 Al 基 FLP 更接近过渡金属催化，并可能使我们能够将这些知识应用于配体设计，以增强主族金属促进的氢化反应。