Nickel(II) complexes of highly σ-donating cyclic (alkyl)(amino)- and malonate-carbenes: Syntheses and catalytic studies

International audience[NiCl2(PPh3)2] reacted with the cyclic (alkyl)(amino)carbene IPr-CAACMe2-Cy (a) and with the anionic malonate-carbene Mes-maloNHCMe-Mes (b) to give the neutral and zwitterionic complexes [NiCl2(IPr-CAACMe2-Cy)(PPh3)] (1a) and [NiCl(Mes-maloNHCMe-Mes)(PPh3)2] (3b), resulting from the respective substitution of one neutral triphenylphosphine (L-type) and one anionic chloride (X-type) ligand. Complex 1a decomposed in the presence of traces of protons to give the nickelate salt [(IPr-CAACMe2-Cy)·H][NiCl3(PPh3)] (2a). Complex 3b was insoluble in most organic solvents and thus difficult to purify and characterize, but the propensity of b to substitute X-type ligands was further illustrated by the substitution of iodide from the N-heterocyclic carbene complex [NiICp(IMe)] to give the zwitterionic heteroleptic bis-carbene complex [NiCp(IMe)(Mes-maloNHCMe-Mes)] (4b), which was characterized by X-ray crystallography. The latter was postfunctionalized by the addition of methyl triflate on the malonate moiety to give the corresponding cationic species [NiCp(IMe)(Mes-maloNHCMeMe-Mes)](OTf) (5c), in which the ligand electron donicity is greatly reduced. Complexes 3b, 4b, and 5c are the first Ni(maloNHC) species, and 1a joins a very restricted list of Ni(CAAC) complexes. The catalytic activity of 1a, 4b, and 5c was studied in a cross-coupling reaction, a reduction reaction of a C–heteroatom double bond, and a C–H bond functionalization reaction