Redox-Active, Boron-Based Ligands in Iron Complexes with Inverted Hydride Reactivity in Dehydrogenation Catalysis

For
a series of PBP-type iron­(II) pincer complexes, the central
donor group based on tricoordinate boron is demonstrated to be redox-active,
formally yielding iron(0) and a boronium-species by reversible B–H
reductive elimination. In contrast to common tricoordinate boron compounds,
such as BF3, which are known to act as Lewis acid and Z-type
ligand, the introduction of π-accepting phosphine substituents
at the boron center leads to an umpolung of the bonding situation
from R3B←Fe to L2RB→Fe in the reported complexes. The described iron­(II)
complexes are competent catalysts for the dehydrogenation of Me2NH-BH3. Depending on the substituents a homo- or
hetertopic catalyst is formed. Experimental and quantum chemical investigations
on the most active, homogeneous catalyst indicate that hydrogen liberation
can proceed via different pathways, involving a hydrido ligand as
the proton source or a carbanion bifunctional mechanism. The unprecedented
catalytic mechanism and the unusual reactivity that allows for two-electron
redox steps are attributed to the unique donor properties of the boron-based
ligand.

针对一系列PBP型二价铁钳形配合物，研究证实其基于三配位硼的中心给体基团具有氧化还原活性，可通过可逆B-H还原消除过程，形式上生成零价铁与硼鎓物种。与三氟化硼等常见三配位硼化合物（已知可作为路易斯酸与Z型配体）不同，在硼中心引入π接受性膦取代基后，该类配合物中的成键极性从R3B←Fe发生反转，变为L2RB→Fe。本文所报道的二价铁配合物可作为二甲胺-硼烷（Me₂NH-BH₃）脱氢反应的高效催化剂。根据取代基的差异，可形成同位点或异位位点催化剂。对活性最高的均相催化剂开展的实验与量子化学研究表明，氢气释放可通过多条路径进行：一类是以氢配体作为质子源的路径，另一类则涉及碳负离子双功能机理。这种前所未有的催化机理与可实现双电子氧化还原步骤的特殊反应性，均可归因于该硼基配体独特的给体特性。