(Benz-)imidazolin-2-ylidene-benzimidazolatonickel(II) Chelates: Syntheses, Structures, and Tuning of Noninnocent Chelate Ligand Behavior.

Abstract

The deprotonation of 1-(1H-benzimidazol-2-yl)-3-methylbenzimidazolium hexafluorophosphate (2MeH2[PF6]) and 1-(1H-benzimidazol-2-yl)-3-isopropylbenzimidazolium hexafluorophosphate (2iPrH2[PF6]) with potassium tert-butoxide in THF afforded the benzimidazolium-benzimidazolates 2MeH and 2iPrH. The "instant carbene" behavior of these conjugated mesomeric betaines was demonstrated by trapping their carbenic tautomers 2'MeH and 2'iPrH with elemental sulfur and selenium, which afforded the corresponding thio- and selenourea derivatives 2'MeHE and 2'iPrHE (E = S, Se). The treatment of 2MeH and 2iPrH with nickelocene furnished the nickel(II) complexes [NiCp(2'Me)] and [NiCp(2'iPr)], which contain an anionic C,Namido-chelating NHC ligand. The electronic structure and redox behavior of the nickel(II) chelates were investigated, as well as those of the closely related chelates [NiCp(1'Me)] and [NiCp(1'iPr)] derived from the corresponding imidazolium-benzimidazolates 1MeH and 1iPrH. According to DFT calculations, the highest occupied molecular orbital (HOMO) is located over the NiCp moiety and the π system of the chelate ligand with a large contribution from the (benz-)imidazolate moiety. Cyclic voltammetry revealed a reversible oxidation to the monocation [NiCp(L)]+ (E1/2 = 0.315, 0.222, 0.396, 0.265 V vs ferrocene/ferrocenium for L = 1'Me, 1'iPr, 2'Me, 2'iPr, respectively) in CH2Cl2/0.1 M n-Bu4N[B(ArF)4] (B(ArF)4- = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate), and isosbestic behavior was found in UV-vis-NIR spectroelectrochemical experiments. The different redox potentials reflect the different donor/acceptor properties of the NHC part of the chelate ligands, with 1'iPr being the strongest and 2'Me the weakest net electron donor. The EPR spectroscopic signature of [NiCp(2'Me)]+ in CH2Cl2/0.1 M n-Bu4N[B(ArF)4] at 100 K is consistent with a chelate-ligand-based radical with strong spin-orbit coupling to the Ni center. In contrast, the EPR spectra of [NiCp(1'Me)]+, [NiCp(1'iPr)]+, and [NiCp(2'iPr)]+ indicate that these monocations are best described as NiIII complexes, the comparatively higher contribution of the NiIII(L) vs the NiII(L•+) valence tautomer being supported by the results of open-shell DFT calculations.