'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-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 center dot(Br)(-) and 3(Me)center dot(Br)(-) respectively. Transmetallation from [(C-N-Me-C)(2)Ag-2](Ag6I8), 6(Me)center dot(Ag6I8)(2-) to NiBr2(DME) gave [ Ni(C-N-Me-C) Br](AgI2), 3(Me)center dot(AgI2)(-). Reaction of 3.(Br)(-) with KPF6 resulted only in exchange of the ionic bromide, however the reaction of 3 center dot(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 center dot(BF4)(-)(2) and octahedral 5, respectively. In contrast, the reaction of 3(Me)center dot(AgI2)(-), with excess AgOTf resulted in an unusual ` reverse transmetallation' leading to 6(Me)center dot(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.