Coordination Properties of Organoborate Ligands – Steric Hindrance Around the Distal Boron Center Directs the Conformation of the Dialkylbis(imidazolyl)borate Scaffold

Abstract

AbstractAlkylmethylbis(imidazolyl)borate ligands, [B(ImN–Me)2(alkyl)Me]– (Lalkyl; alkyl = methyl and n‐butyl), have been synthesized by nucleophilic substitution from the chloride to the corresponding alkyl derivative on the boron center of the chloroborate precursor. In homoleptic complexes of nickel(II) with Lalkyl, [NiII(Lalkyl)2] (1alkyl), one of the two alkyl groups on the boron atom faces the nickel(II) center to form a C–H···M interaction. In the analogous homoleptic nikel(II) complexes of the hydride, phenyl, and methoxy derivatives 1X (X = H, Ph, OMe), the boron‐attached CH3 moieties do not turn towards the metal center. Steric repulsion between the methyl substituent on the imidazolyl ligands and the boron‐attached methyl group directs the orientation of the alkyl groups on Lalkyl. In the solution state the molecular structures of 1alkyl observed in the solid state are retained. Mixed‐ligand complexes, [NiII(Lalkyl)(TpiPr2)] (2alkyl), can be synthesized by reaction of [NiII(Y)(TpiPr2)] (Y = Cl, NO3) with Lalkyl through selective ligand metathesis. In 2Me, one methyl group of LMe approaches the nickel center retaining the bent conformation of the two imidazolyl groups. Conversely, the carbon atoms of the boron‐attached nBu and Me groups of LBu are located away from the nickel center, and the orientation of the two imidazolyl groups is almost coplanar in 2Bu. The structural characteristics of these nickel(II) complexes demonstrate the flexibility of the dialkybis(imidazolyl)borate scaffold.