Formation of phosphaethyne dimers: a mechanistic study

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Phosphaacetylene (HCP) is a reactive molecule with limited experimental study.
• Understanding its dimerization is crucial for predicting its reactivity and polymerization.
• Previous theoretical studies on HCP dimerization are scarce.

Purpose of the Study:

• To investigate the dimerization pathways of phosphaacetylene (HCP) using high-level computational methods.
• To identify stable dimer structures and reaction mechanisms.
• To explore the catalytic effect of LiBr on HCP dimerization.

Main Methods:

• High-level ab initio calculations (CCSD(T), CBS-QB3, CASSCF, CASPT2, MR-ACPF, MR-ACPF-2).
• Density functional theory (B3LYP) calculations.
• Exploration of potential energy surfaces to identify low-energy minima and transition states.

Main Results:

• Seventeen low-energy closed-shell and five open-shell HCP dimers were identified.
• Dimerization via closed-shell pathways exhibits high activation barriers, indicating kinetic stability.
• An open-shell pathway with a moderate barrier (95.0 kJ mol(-1)) leads to diphosphacyclobutadienes, suggesting an open-shell polymerization mechanism.
• LiBr catalysis facilitates the formation of 1,4-diphosphatriafulvene with a low activation Gibbs-free energy (44.8 kJ mol(-1)).

Conclusions:

• Closed-shell dimerization of HCP is generally kinetically hindered.
• Open-shell mechanisms are likely involved in HCP polymerization and oligomerization.
• 1,2-diphosphacyclobutadiene formation is favored over 1,3-.
• LiBr catalysis provides a viable route for the synthesis of 1,4-diphosphatriafulvene, consistent with experimental findings.