Nickel(II) Complexes of Highly σ‑Donating Cyclic (Alkyl)(Amino)- and Malonate-Carbenes: Syntheses and Catalytic Studies

[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.