Group 12 Metal Complexes of N‑Heterocyclic Ditopic Carbanionic Carbenes

Reaction of the N-heterocyclic carbene 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene (IPr) with M­(mes)2 (M = Zn, Cd) in diethyl ether afforded the Lewis acid–base adducts [M­(IPr)­(mes)2] (M = Zn (1), Cd (2)) in quantitative yields. An analogous reaction between Hg­(mes)2 and IPr failed to form the desired 1:1 adduct, [Hg­(IPr)­(mes)2], as evidenced by NMR spectroscopy. Reduction of 1 and 2 with KC8 afforded K+ salts of the anionic complexes [{:C­[N­(2,6-iPr2C6H3)]2(CH)­C}2M­(mes)]− (M = Zn (3), Cd (4)). By contrast, reduction of a THF solution of a mixture of Hg­(mes)2 and IPr gave rise to the homoleptic anionic species [{:C­[N­(2,6-iPr2C6H3)]2(CH)­C}3Hg]− (5). Species 3–5 display abnormally bonded anionic N-heterocyclic “dicarbene” ligands (or ditopic carbanionic carbenes) in which IPr has been deprotonated at the C4/C5 position. The vacant C2 atoms retain carbenic character, allowing for further coordination to Lewis acids. This was demonstrated by reaction of 3 with H3B:SMe2, AlEt3, and CO2 (in the presence of the appropriate cation-sequestering agents), which afforded salts of the [{LA:C­[N­(2,6-iPr2C6H3)]2(CH)­C}2Zn­(mes)]− anions (LA = BH3 (6), AlEt3 (7), and CO2 (8)).