‘Pincer’ pyridine dicarbene complexes of nickel and their derivatives. Unusual ring opening of a coordinated imidazol-2-ylidene

Abstract

The reaction of NiBr₂(DME), DME = 1,2-dimethoxyethane, with the ‘pincer’ pyridine dicarbene ligands (C-N-C) (2) and (C-N^Me-C) (2^Me), (C-N-C = 2,6-bis-[(DiPP)imidazol-2-ylidene]pyridine, C-N^Me-C = 2,6-bis-[(DiPP)imidazol-2-ylidene]-3,5-dimethylpyridine, DiPP = 2,6-diisopropylphenyl) gave the square planar complexes [Ni(C-N^(Me)-C)Br]Br, 3·(Br)⁻ and 3^MeMe·(Br)⁻ respectively. Transmetallation from [(C-N^Me-C)₂Ag₂](Ag₆I₈), 6^MeMe·(Ag₆I₈)²⁻ to NiBr₂(DME) gave [Ni(C-N^Me-C)Br](AgI₂), 3^MeMe·(AgI₂)⁻. Reaction of 3·(Br)⁻ with KPF₆ resulted only in exchange of the ionic bromide, however the reaction of 3·(Br)⁻ with AgBF₄ in MeCN or AgOTf in THF resulted in the exchange of both coordinated and ionic bromides, giving rise to the square planar 4·(BF₄)⁻₂ and octahedral 5, respectively. In contrast, the reaction of 3^MeMe·(AgI₂)⁻, with excess AgOTf resulted in an unusual ‘reverse transmetallation’ leading to 6^MeMe·(OTf)⁻. The substitution of tmeda in Ni(CH₃)₂(tmeda), tmeda = N,N,N′,N′-tetramethylethylenediamine, by 2 produced the complex 7, in which ring opening of the heterocyclic imidazole ring of one of the NHC functional groups has taken place.