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

The coordination chemistry of carbene-CS 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-CS) complexes of the types [Ni(IPr)(carbene-CS)] (2a-g) (carbene = cAAC, IDipp, IMes, BIMe, BIPr, IPr, and IPr) and [Ni(carbene-CS)] (3a-c) (carbene = cAAC, IDipp, and IMes) are accessible by alkene substitution using [Ni(IPr)(ƞ-CH)] or [Ni(COD)] as the starting material (cAAC = 1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene, IR = 1,3-diorganylimidazolin-2-ylidene, IR = 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-CS ligands are coordinated in a κ-,' 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-CS)] (4a-b). 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-CS moieties with a formal -1.5 charge per ligand, further demonstrating the flexible redox active nature of these azolium-2-dithiocarboxylate ligands.