Boosting the π-Acceptor Property of Mesoionic Carbenes by Carbonylation with Carbon Monoxide.

We report the room temperature dimerization of carbon monoxide mediated by C4/C5-vicinal anionic dicarbenes Li(ADC) (ADC = ArC{(Dipp)NC} 2 ; Dipp = 2,6-iPr 2 C 6 H 3 ; Ar = Ph, DMP (4-Me 2 NC 6 H 4 ), Bp (4-PhC 6 H 4 )) to yield (E)-ethene-1,2-bis(olate) (i.e. - O-C=C-O - = CO en ) bridged mesoionic carbene (iMIC) lithium compounds CO en -[(iMIC)Li] 2 (CO en -[iMIC] 2 = [ArC{(Dipp)NC} 2 (CO)] 2 ) in quantitative yields. CO en -[(iMIC)Li] 2 are highly colored stable solids, exhibit a strikingly small HOMO-LUMO energy gap, and readily undergo 2e-oxidations with selenium, CuCl (or CuCl 2 ), and AgCl to afford the dinuclear compounds CO on -[(iMIC)E] 2 (E = Se, CuCl, AgCl) featuring a 1,2-dione bridged neutral bis-iMIC (i.e. CO on -[iMIC] 2 = [ArC{(Dipp)NC} 2 (C=O)] 2 ). CO en -[(iMIC)Li] 2 undergo redox-neutral salt metathesis reactions with LiAlH 4 and (Et 2 O) 2 BeBr 2 and afford CO en -[(iMIC)AlH 2 ] 2 and CO en -[(iMIC)BeBr] 2 , in which the dianionic CO en -moiety remains intact. All compounds have been characterized by NMR spectroscopy, mass spectrometry, and X-ray diffraction. Stereoelectronic properties of CO on -[iMIC] 2 are quantified by experimental and theoretical methods.