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 (PI ) 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 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(C6 F5 )4 ] to activate Si-H bond could still be tested in catalytic hydrosilylation of benzaldehyde, where 1+ closely mimics transition metal behaviour.