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 Et 2 NH, activating the OÀ H and NÀ H bonds via a P-center/ligand assisted path. The reaction of 1 + with one equiv. of H 3 NBH 3 leads to its dehydrogenation producing 5. Interestingly, reaction of 1 + with an excess H 3 NBH 3 leads to phosphinidene (P I ) species coordinating to two BH 3 molecules (6). In contrast, [1 + ] [OTf] reacts with Et 3 SiH by hydride abstraction yielding 1-H and Et 3 SiOTf, while [1 + ][B(C 6 F 5 ) 4 ] reacts with Et 3 SiH via an oxidative addition type reaction of SiÀ 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(C 6 F 5 ) 4 ] to activate SiÀ H bond could still be tested in catalytic hydrosilylation of benzaldehyde, where 1 + closely mimics transition metal behaviour.