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.