Boronyl as a terminal ligand in boron oxide clusters: hexagonal ring C2v B6O4 and ethylene-like D2h B6O4 -/2-

Considerable recent research effort has been devoted to the development of boronyl (BO) chemistry. Here we predict three perfectly planar boron boronyl clusters: C-2v B6O4 (1, (1)A(1)), D-2h B6O4- (2, B-2(3u)), and D-2h B6O42- (3, (1)A(g)). These are established as the global-minimum structures on the basis of the coalescence kick and basin hopping structural searches and electronic structure calculations at the B3LYP/aug-cc-pVTZ level, with complementary CCSD/6-311+G* and single-point CCSD(T)/6-311+G*// B3LYP/aug-cc-pVTZ calculations for 2. The C-2v B6O4 neutral cluster features a hexagonal B4O2 ring with two terminal BO groups. The D-2h B6O4 (-/2-) clusters have ethylene-like structures and are readily formulated as B-2(BO)(4) (-/2-), in which a B-2 core with double bond character is bonded to four terminal BO groups. Chemical bonding analyses show that B6O4 (1) possesses an aromatic pi bonding system with three delocalized, six-centered p bonds over the hexagonal ring, rendering it an inorganic analogue of benzene, whereas the B6O4-/2- (2 and 3) species closely resemble ethylene in terms of structures and bonding. This work provides new examples for the analogy between boron oxides and hydrocarbons.