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