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 = cAAC^Me, IDipp, IMes, BIMe, BIⁱPr, IⁱPr, and IⁱPr^Me) and [Ni(carbene-CS₂)₂] (3a-c) (carbene = cAAC^Me, IDipp, and IMes) are accessible by alkene substitution using [Ni(IⁱPr)₂(ƞ²-C₂H₄)] or [Ni(COD)₂] as the starting material (cAAC^Me = 1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene, IR = 1,3-diorganylimidazolin-2-ylidene, IR^Me = 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-b^cat). 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.