An Isolable Diboron-Centered Diradical with a Triplet Ground State

Two new diboranes, 2,6-bis(BMes(2))mesitylene (1) and 3,3-bis(BMes(2))bimesitylene (3), were synthesized. Two-electron reduction of 1 with elemental potassium afforded the C-H activation product [(18-c-6)K(THF)(2)](2)(+)2(2-) bearing a BC3 four-membered ring as colorless crystals, whereas the reduction of 3 with potassium led to the isolation of [(18-c-6)K(THF)(2)](2)(+)3(2-..) as dark blue crystals. Both reduction products were characterized by structural and spectroscopic methods. Electron paramagnetic resonance (EPR) spectroscopy and theoretical calculations revealed that the electron spin density of 3(2-..) mainly resides on the two boron nuclei and features a triplet ground state, which was confirmed by superconducting quantum interference device (SQUID) measurements as well as theoretical calculations. 3(2-..) represents the first structurally characterized boron-centered diradical with a triplet ground state. In addition, the reactivity of [(18-c-6)K(THF)(2)](2)(+)3(2-..) toward PhSeSePh and nBu(3)SnH was investigated, which is consistent with its radical character.