Azolium-2-dithiocarboxylates as redox active ligands in nickel chemistry

The coordination chemistry of carbene-CS2 adducts of selected NHCs and cAACs and their redox active nature in nickel complexes is reported. These azolium-2-dithiocarboxylate ligands can be considered as 1,1-isomeric dithiolene analogues bearing a 2π electron reservoir. Depending on the co-ligands attached to nickel, square planar mono- or bis-(carbene-CS2) complexes of the types [Ni(IiPr)2(carbene-CS2)] (2a-g) (carbene = cAACMe, IDipp, IMes, BIMe, BIiPr, IiPr, and IiPrMe) and [Ni(carbene-CS2)2] (3a-c) (carbene = cAACMe, IDipp, and IMes) are accessible by alkene substitution using [Ni(IiPr)2(ƞ2-C2H4)] or [Ni(COD)2] as the starting material (cAACMe = 1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene, IR = 1,3-diorganylimidazolin-2-ylidene, IRMe = 1,3-diorganyl-4,5-dimethylimidazolin-2-ylidene, and BIR = 1,3-diorganylbenzimidazolin-2-ylidene). In the complexes 2a-g and 3a-c, all carbene-CS2 ligands are coordinated in a κ2-S,S′ fashion to nickel(ii) and are ligated either in their formally one electron reduced (3a-c) or two electron reduced (2a-g) redox states. The complexes 3a-c reveal intense NIR absorptions, which shift upon metallic reduction to the nickelate salts of the type [Cat]+[Ni(carbene-CS2)2]˙− (4a-bcat). In these nickelates, an additional electron is shared across a ligand-centered SOMO of π-symmetry which is delocalized over both azolium-2-dithiocarboxylate ligands and results in carbene-CS2 moieties with a formal −1.5 charge per ligand, further demonstrating the flexible redox active nature of these azolium-2-dithiocarboxylate ligands.