Accessing the (CpNi)-Ni-Ar(I) Synthon: Reactions with N-Heterocyclic Carbenes, TEMPO, Sulfur, and Selenium

A reactive “Cp^ArNi^I” surrogate (Cp^Ar = C₅(C₆H₄-4-Et)₅) is accessible via the reduction of the dimer [Cp^ArNi(μ-Br)]₂ with two equivalents of KC₈. A trapping reaction with TEMPO afforded the new nickel(II) complex [Cp^ArNi(η²-TEMPO)] (3), while the addition of N-heterocyclic carbenes gave the new nickel(I) radicals [Cp^ArNi(IPr)] (4a, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and [Cp^ArNi(IiPr₂Me₂)] (4b, IiPr₂Me₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene). EPR spectra supported by DFT calculations on 4a and 4b indicate that the spin density mainly resides at the nickel center. The reaction of the “Cp^ArNi(I) source” with yellow sulfur gave the Ni₂S₆ complex [(Cp^ArNi)₂(μ-S₆)] (5); the “subselenide” [(Cp^ArNi)₂(μ-Se₂)] (6) was formed in the analogous reaction with grey selenium. All new complexes were characterized by NMR, EPR, and UV–vis spectroscopy; their molecular structures were determined by X-ray crystallography.