Electronic and Structural Effects of Stepwise Borylation and Quaternization on Borirene Aromaticity

Room-temperature photolysis of the amino-boryl complex [(OC)(5)Cr=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')(mu-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.