Cationic Nickel d9-Metalloradicals [Ni(NHC)2]+

Abstract

A series of five new homoleptic, linear nickel d9-complexes of the type [NiI(NHC)2]+ is reported. Starting from the literature known Ni(0) complexes [Ni(Mes2Im)2] 1, [Ni(Mes2ImH2)2] 2, [Ni(Dipp2Im)2] 3, [Ni(Dipp2ImH2)2] 4 and [Ni(cAACMe)2] 5 (Mes2Im=1,3-bis(2,4,6-trimethylphenyl)-imidazolin-2-ylidene, Mes2ImH2=1,3-bis(2,4,6-trimethylphenyl)-imidazolidin-2-ylidene, Dipp2Im=1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene, Dipp2ImH2=1,3-bis(2,6-diisopropylphenyl)-imidazolidin-2-ylidene, cAACMe=1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-yliden), their oxidized Ni(I) analogues [NiI(Mes2Im)2][BPh4] 1+, [NiI(Mes2ImH2)2][BPh4] 2+, [NiI(Dipp2Im)2][BPh4] 3+, [NiI(Dipp2ImH2)2][BPh4] 4+ and [NiI(cAACMe)2][BPh4] 5+ were synthesized by one-electron oxidation with ferrocenium tetraphenyl-borate. The complexes 1+–5+ were fully characterized including X-ray structure analysis. The complex cations reveal linear geometries in the solid state and NMR spectra with extremely broad, paramagnetically shifted resonances. DFT calculations predicted an orbitally degenerate ground state leading to large magnetic anisotropy, which was verified by EPR measurements in solution and on solid samples. The magnetic anisotropy of the complexes is highly dependent from the steric protection of the metal atom, which results in a noticeable decrease of the g-tensor anisotropy for the N-Mes substituted complexes 1+ and 2+ in solution due to the formation of T-shaped THF adducts.