NHC-ligated nickel(ii) cyanoborate complexes and salts

Abstract

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[BH₂(CN)₂]₂·H₂O (Ib·H₂O), Ni[BH(CN)₃]₂·0.5H₂O (Ic·0.5H₂O), Ni[B(CN)₄]₂·H₂O (Id·H₂O) were reacted with selected NHCs of different steric size. The reaction of the nickel cyanoborates with small to medium-sized NHCs Me₂Im^Me and iPr₂Im (R₂Im = 1,3-di-organyl-imidazolin-2-ylidene; R₂Im^Me = 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)₄]²⁺ (NHC = Me₂Im^Me (1c–d), iPr₂Im (2c–d)) and cyanoborate counter-anions. Bulkier NHCs such as Mes₂Im and Dipp₂Im afforded complexes trans-[Ni(NHC)₂(CB)₂] (trans-4b, trans-5c). For the combination of the cyanoborate anion [BH₂(CN)₂]⁻ and iPr₂Im^Me the salt of the tris-NHC complex [Ni(iPr₂Im^Me)₃(NC-BH₂CN)][BH₂(CN)₂] (3b) was isolated. Salt metathesis of NHC-ligated nickel(II) halides (Ni(NHC)₂X₂) (X = Cl, Br) with silver(I) and alkali metal cyanoborates were used to synthesize mono- and disubstituted coordination compounds of the type cis- or trans-[Ni(NHC)₂(CB)X] (cis-10c, cis-11c, trans-12b) and cis- or trans-[Ni(NHC)₂(CB)₂] (cis-13b, cis-14a–c, trans-14a–b, trans-15b, trans-5b). Further investigations reveal that NHC-ligated cyanoborate complexes can act as building blocks for coordination polymers, as observed for structurally characterized ¹_∞{trans-[Ni(Mes₂Im)₂(μ²-[NC–BH₂–CN])₂]·2Ag(μ²-[BH₂(CN)₂])} (trans-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.