Formation of Phosphaethyne Dimers: A Mechanistic Study

Abstract

High‐level ab initio (CCSD(T), CBS‐QB3 and CASSCF, CASPT2, MR‐ACPF, MR‐ACPF‐2) and density functional theory (B3LYP) calculations were carried out to study the dimerization of phosphaacetylene or phosphaethyne (HCP). Seventeen low energy closed‐shell and five open‐shell phosphaacetylene dimers were found on the potential energy surface. Two head‐to‐head, one head‐to‐tail and three other dimerization reaction pathways were determined, all with high activation barriers, suggesting that closed‐shell minima are usually kinetically stable. An open‐shell head‐to‐head reaction pathway has also been found with moderate initial barrier (95.0 kJ mol⁻¹) leading to 1,2‐ and 1,3‐diphosphacyclobutadiene, suggesting that polymerization of HCP and oligomerization of its derivatives have open‐shell mechanism. Formation of 1,2‐diphosphacyclobutadiene is both thermodynamically and kinetically favored over 1,3‐diphosphacyclobutadiene. A head‐to‐head reaction involving LiBr as a catalyst was also studied. It has been pointed out that LiBr catalyze the closed‐shell mechanism. All the four possible reaction channels of this reaction yield 1,4‐diphosphatriafulvene with a fairly low activation Gibbs‐free energy (44.8 kJ mol⁻¹), suggesting that this compound could be synthesized. This finding fully supports the experimental results.