Diborane(6) as a H-bridged dimer of monoborane can be converted cleanly by two-electron reduction into diborane(6) dianion, which is isoelectronic with ethane, through B–B σ-bond formation when each boron atom has a bulky ligand on it. The existence of the B–B σ bond is supported by the X-ray molecular structure [B–B bond length of 1.924(3) Å], NMR studies, magnetic susceptibility measurements, and DFT calculations. Stepwise hydride abstraction reactions of the diborane(6) dianion produce the corresponding H-bridged diborane(5) anion and doubly H-bridged diborane(4) without B–B bond scission.
Theory has long predicted that by pumping extra electrons into a compound such as diborane, the boron–hydrogen–boron structure should break down to form a boron–boron single bond. Until now, however, all such attempts to make and isolate such a structure had failed, instead generating clusters or single boron species.
Matsuo and Tamao’s strategy for generating the boron–boron bond was to start with a borane precursor where each boron atom was fitted with a bulky side-group known as an Eind group. The researchers suspected that previous attempts probably succeeded in generating the boron–boron single bond but failed to protect that structure from quickly falling apart through over-reaction. Using the bulky side-groups, they were able to block these over-reaction processes, and successfully isolate the desired boron–boron single bond