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 2 (CN) 2 ] 2 ·H 2 O (Ib·H 2 O), Ni[BH(CN) 3 ] 2 ·0.5H 2 O (Ic·0.5H 2 O), Ni[B(CN) 4 ] 2 ·H 2 O (Id·H 2 O) were reacted with selected NHCs of different steric size. The reaction of the nickel cyanoborates with small to medium-sized NHCs Me 2 Im Me and iPr 2 Im (R 2 Im = 1,3-di-organyl-imidazolin-2-ylidene; R 2 Im 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 = Me 2 Im Me (1c-d), iPr 2 Im (2c-d)) and cyanoborate counter-anions. Bulkier NHCs such as Mes 2 Im and Dipp 2 Im afforded complexes trans -[Ni(NHC) 2 (CB) 2 ] ( trans -4b, trans -5c). For the combination of the cyanoborate anion [BH 2 (CN) 2 ] - and iPr 2 Im Me the salt of the tris-NHC complex [Ni(iPr 2 Im Me ) 3 (NC-BH 2 CN)][BH 2 (CN) 2 ] (3b) was isolated. Salt metathesis of NHC-ligated nickel(II) halides (Ni(NHC) 2 X 2 ) (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(Mes 2 Im) 2 (μ 2 -[NC-BH 2 -CN]) 2 ]·2Ag(μ 2 -[BH 2 (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.