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

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.