Electronic and structural effects of stepwise borylation and quaternization on borirene aromaticity

Abstract

Room-temperature photolysis of the aminoboryl complex [(OC)5Cr═B═N(SiMe₃)₂] in the presence of a series of mono- or bis(boryl) alkynes bis{bis(dimethylamino)boryl}ethyne, 1-phenyl-2-bis(dimethylamino)borylethyne, and 1-trimethylsilyl-2-bis(dimethylamino)borylethyne led to the isolation of hitherto unknown borylborirenes in resonable yields, that is, [(RC═CR′)(μ-BN(SiMe3)2)] (7, R = B(NMe2)2, R′ = Ph; 8, R = R′ = B(NMe₂)₂; 9, R = B(NMe₂)₂, R′ = SiMe₃). The borirenes were isolated and spectroscopically characterized by multinuclear NMR, IR, and UV/vis spectroscopy, crystallography, and elemental analysis. Reactivity studies of the borirenes demonstrated their behavior toward different Lewis bases. The isolated adducts and the parent borirenes were compared to 1,3-mesityl-2-phenylborirene and its adducts. To gain insight into the electronic structure and to evaluate the influence of the exocyclic groups, Density Functional Theory (DFT) calculations were carried out.