Experimental and Computational Study of a Confirmed Borylene-to-Diborene Dimerization

Area of Science:

• Inorganic Chemistry
• Organoboron Chemistry
• Main Group Chemistry

Background:

• Dimerization of heavier group 13 carbene analogues to alkene analogues is well-established.
• Direct observation of dicoordinate borylene (LRB:) dimerization to diborenes (LRB═BRL) has been lacking.

Purpose of the Study:

• To present the first direct observation of borylene-to-diborene dimerization.
• To elucidate the complex reaction mechanism involving hydroborylene precursors and dihaloboranes.

Main Methods:

• Synthesis of dihydrodiborenes from tricoordinate hydroborylene precursors and Lewis-acidic dihaloboranes.
• Experimental characterization of reaction products and intermediates.
• In-depth computational mechanistic analysis using DFT methods.

Main Results:

• Successful synthesis of dihydrodiborenes via formal borylene dimerization.
• Identification of a complex reaction pathway involving PMe3 ligand abstraction, halide transfers, and redox processes.
• Demonstration of an autocatalytic cycle involving borylene radical cations.

Conclusions:

• The study provides the first direct evidence for borylene dimerization to diborenes.
• The reaction mechanism is complex and autocatalytic, driven by boryl radical intermediates.
• Boryl radicals ([LBXR]•) are proposed as more likely coupling partners than dicoordinate borylenes in diborene formation.