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

The reaction of NiBr2(DME), DME = 1,2-dimethoxyethane, with the ‘pincer’ pyridine dicarbene ligands (C-N-C) (2) and (C-NMe-C) (2Me), (C-N-C = 2,6-bis-[(DiPP)imidazol-2-ylidene]pyridine, C-NMe-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 3MeMe·(Br)− respectively. Transmetallation from [(C-NMe-C)2Ag2](Ag6I8), 6MeMe·(Ag6I8)2− to NiBr2(DME) gave [Ni(C-NMe-C)Br](AgI2), 3MeMe·(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 3MeMe·(AgI2)−, with excess AgOTf resulted in an unusual ‘reverse transmetallation’ leading to 6MeMe·(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.