Dual Reactivity of a Geometrically Constrained Phosphenium Cation

A geometrically constrained phosphenium cation in bis(pyrrolyl)pyridine based NNN pincer type ligand (1⁺) was synthesized, isolated and its preliminary reactivity was studied with small molecules. 1⁺ reacts with MeOH and Et₂NH, activating the O−H and N−H bonds via a P-center/ligand assisted path. The reaction of 1⁺ with one equiv. of H₃NBH₃ leads to its dehydrogenation producing 5. Interestingly, reaction of 1⁺ with an excess H₃NBH₃ leads to phosphinidene (P^I) species coordinating to two BH₃ molecules (6). In contrast, [1⁺][OTf] reacts with Et₃SiH by hydride abstraction yielding 1-H and Et₃SiOTf, while [1⁺][B(C₆F₅)₄] reacts with Et₃SiH via an oxidative addition type reaction of Si−H bond to P-center, affording a new P^V compound (8). However, 8 is not stable over time and degrades to a complex mixture of compounds in matter of minutes. Despite this, the ability of [1⁺][B(C₆F₅)₄] to activate Si−H bond could still be tested in catalytic hydrosilylation of benzaldehyde, where 1⁺ closely mimics transition metal behaviour.