Structural and spectroscopic characterization of large boron heterocyclic radicals: Matrix infrared spectroscopy and...

Six boron heterocyclic radicals with different conformations or configurations were synthesized in solid neon and identified by matrix isolation infrared spectroscopy as well as quantum-chemical calculations. The ground-state boron atom selectively attacks the C = C bond of cycloheptene forming η² (1,2)-BC₇H₁₂ complex (A), which contains a chair conformation and a boat conformation. Species A isomerizes to the 2,3,4,5,6,7-hexahydroborocine radical (B), which involves an eight-membered boron heterocyclic ring and also has two isomers observed. The 1-(prop-1-en-1-yl)-2,3,4-dihydro borole radical (C) with E-configuration and Z-configuration is generated as the final product under UV light irradiation through ring contraction reaction and the hydrogen atom transfer reaction. The observation of species A and further photo-isomerization to species C is consistent with theoretical predictions that these reactions are thermodynamically exothermic and kinetically facile. This work not only provides a possible route for future design and synthesis of corresponding borole derivatives, but also provides new insights into the structural and spectroscopic information of boron heterocyclic radicals with different conformations and configurations.