NHC-ligated nickel(II) cyanoborate complexes and salts.

A comprehensive study on the synthesis and characterization of NHC-ligated nickel(II) cyanoborates (CBs) is presented (NHC = N-heterocyclic carbene). Nickel(II) cyanoborates Ni[BH(CN)]·HO (Ib·HO), Ni[BH(CN)]·0.5HO (Ic·0.5HO), Ni[B(CN)]·HO (Id·HO) were reacted with selected NHCs of different steric size. The reaction of the nickel cyanoborates with small to medium-sized NHCs MeIm and iPrIm (RIm = 1,3-di-organyl-imidazolin-2-ylidene; RIm = 1,3-di-organyl-4,5-dimethyl-imidazolin-2-ylidene) afforded cyanoborate salts containing the rare homoleptic fourfold NHC-ligated nickel(II) cations [Ni(NHC)] (NHC = MeIm (1c-d), iPrIm (2c-d)) and cyanoborate counter-anions. Bulkier NHCs such as MesIm and DippIm afforded complexes -[Ni(NHC)(CB)] (-4b, -5c). For the combination of the cyanoborate anion [BH(CN)] and iPrIm the salt of the tris-NHC complex [Ni(iPrIm)(NC-BHCN)][BH(CN)] (3b) was isolated. Salt metathesis of NHC-ligated nickel(II) halides (Ni(NHC)X) (X = Cl, Br) with silver(I) and alkali metal cyanoborates were used to synthesize mono- and disubstituted coordination compounds of the type - or -[Ni(NHC)(CB)X] (-10c, -11c, -12b) and - or -[Ni(NHC)(CB)] (-13b, -14a-c, -14a-b, -15b, -5b). Further investigations reveal that NHC-ligated cyanoborate complexes can act as building blocks for coordination polymers, as observed for structurally characterized 1∞{-[Ni(MesIm)(μ-[NC-BH-CN])]·2Ag(μ-[BH(CN)])} (-5b·Ag). This study demonstrates the diverse character of cyanoborates in coordination chemistry as both, non-coordinating counter-anions, and weakly to medium coordinating anions forming novel transition metal complexes and salts. It provides evidence that a proper choice of cyanoborate and a proper choice of co-ligand can lead to a rich coordination chemistry of cyanoborate anions.