Carbone stabilized B2 and B22+ – isoelectronic analogues to diborabutyne and diborabutatriene

Abstract

It has been reported that various unusual main group compounds can be stabilized by coordinating with ligands. Here, we report the use of carbone ligands in stabilizing diboron in its neutral and dicationic states by computational quantum mechanical calculations. The neutral [(L₂C)·B₂·(CL₂)] (L = CO, NHC, PMe₃, and cAAC) has singlet non-planar cumulenic-type equilibrium geometry where CL₂ groups are almost orthogonal to each other. MO analysis indicates that the [(L₂C)·B₂·(CL₂)] can be considered as formed by the interaction of the B₂ fragment in the ¹Σ_g⁺ excited state with two CL₂ ligands having σ- and π-type lone pairs. Accordingly, the π delocalization in the C–B–B–C skeleton consists of two mutually orthogonal allylic anionic-type delocalizations along the C–B–B chain. Since one of the π-delocalized MOs of allylic anionic C–B–B is majorly localized on the carbone carbon atom, the carbone ligands formally act as two-electron ligands. On the other hand, the ground state of [(L₂C)·B₂·(CL₂)]²⁺ shows a singlet planar/pseudo-planar cumulenic geometry when L = NHC and PMe₃. The MO analysis indicates that the C–B–B–C skeleton is similar to that of butatriene, viz. one localized B–B π MO, and two delocalized C–B–B–C π MOs, indicating that each carbone acts as a four-electron ligand. Since CO and cAAC are good π-acceptor ligands, [(L₂C)·B₂·(CL₂)]²⁺ ions (L = CO and cAAC) have triplet non-planar cumulenic ground states.