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 BC 3 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 n Bu 3 SnH was investigated, which is consistent with its radical character.