Understanding the Unexpected Product Distribution in the Aerial Oxidation of Carbene-Stabilized Diphosphorus Complex.

Abstract

Oxidation of nonmetallic singlet molecules by oxygen has its own share of intricacies. Herein, by means of DFT and ab initio techniques, mechanistic details of the aerial oxidation of an N-heterocyclic carbene (NHC) stabilized diphosphorus complex are revealed. This particular oxidation process is known to produce an unexpected P-P bond containing diphosphorus tetroxide complex, instead of the more thermodynamically stable oxo-bridged (P-O-P) compound. These findings suggest that the P-P bond containing less stabilized species is a kinetically controlled product (KCP) and obtained due to the presence of lower lying transition states (TSs) in the pathway leading to its formation, relative to the higher lying corresponding minimum-energy crossing points (MECPs) present in the pathway involved in the formation of the oxo-bridged species, which is the thermodynamically controlled product (TCP). Thus, an intriguing variant of the well-known KCP/TCP phenomenon is presented here, in which the KCP is formed not by merely traditionally known lower barrier heights of TSs involved in the formation of KCP, but by faster transmission of a system through a low barrier TS relative to a higher lying MECP. Additionally, the faster kinetics of an irreversible unimolecular O-O dissociation step, which avoids the formation of the TCP is a contributing factor in dictating the fate of the reaction. The insights provided herein may help to understand the oxidation of other P-P-containing species, such as black phosphorene.