Going to Extreme Lengths: Bicyclic Diborane(6) Anions and the Limits of Boron–Boron σ Bonding

The reaction of (naphthalen-1,8-diyl)dilithium (NapLi2) with difluoro(mesityl)borane (MesBF2) in Et2O yielded a new 1,5-diborocine, 1, representing a straightforward route to this rare class of eight-membered boracycle. The reduction of 1 with excess Li yielded Li+ salts of [1]2–, whose THF and pyridine solvates possess among the longest crystallographic B–B bonds currently known (1.9909(17) and 2.017(2) Å), and the latter is the first instance of an organoboron compound with an electron-precise B–B bond length beyond 2 Å. The THF solvate comproportionates with 1 to form [Li(THF)4][1] containing a long (2.3–2.4 Å), covalent one-electron σ bond, and both [1]·– and [1]2– demonstrate incredible stability despite geometric strain on the central bond. These experimental findings, combined with theoretical bonding analyses of various one- and two-electron boron–boron bonds using DFT, give valuable insight into the limits of covalent boron–boron bonding and the diversity in bonding observed near these limits.