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 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 (PI) 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 PV 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.