Dual Reactivity of a Geometrically Constrained Phosphenium Cation

Abstract

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 Et₂ NH, activating the O-H and N-H bonds via a P-center/ligand assisted path. The reaction of 1⁺ with one equiv. of H₃ NBH₃ leads to its dehydrogenation producing 5. Interestingly, reaction of 1⁺ with an excess H₃ NBH₃ leads to phosphinidene (P^I ) species coordinating to two BH₃ molecules (6). In contrast, [1⁺ ][OTf] reacts with Et₃ SiH by hydride abstraction yielding 1-H and Et₃ SiOTf, while [1⁺ ][B(C₆ F₅ )₄ ] reacts with Et₃ SiH 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(C₆ F₅ )₄ ] to activate Si-H bond could still be tested in catalytic hydrosilylation of benzaldehyde, where 1⁺ closely mimics transition metal behaviour.