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

Abstract

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.