Tetrakis(-heterocyclic Carbene)-Diboron(0): Double Single-Electron-Transfer Reactivity.

The use of 1,3,4,5-tetramethylimidazol-2-ylidene (IMe) to coordinate with diatomic B species afforded a tetrakis(-heterocyclic carbene)-diboron(0) [(IMe)B-B(IMe)] (). The singly bonded B moiety therein possesses a valence electronic configuration 1σ1π1π* with four vacant molecular orbitals (1σ*, 2σ, 1π', 1π') coordinated with IMe. Its unprecedented electronic structure is analogous to the energetically unfavorable planar hydrazine with a D symmetry. The two highly reactive π antibonding electrons enable double single-electron-transfer (SET) reactivity in small-molecule activation. Compound underwent a double SET reduction with CO to form two carbon dioxide radical anions CO, which then reduced pyridine to yield a carboxylated pyridine reductive coupling dianion [OCNC(H)-C(H)NCO] and converted compound to the tetrakis(-heterocyclic carbene)-diborene dication [(IMe)B═B(IMe)] (). This is a remarkable transition-metal-free SET reduction of CO without ultraviolet/visible (UV/vis) light conditions.