We report a novel method for the preparation of PBP-pincer complexes from bis(phosphine)boronium salts. The central (RP)HB-moiety in a palladium complex is demonstrated to be a L-type ligand, therewith completing a series of pincer-type complexes with Z-, X- and L-type boron-based ligands, respectively.

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 BF 3 , 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 R 3 B←Fe to L 2 RB→Fe in the reported complexes. The described iron(II) complexes are competent catalysts for the dehydrogenation of Me 2 NH-BH 3 . 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.