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

Alkylmethylbis(imidazolyl)borate ligands, [B(Im N-Me ) 2 (alkyl)-M e ] - (L alkyl ; 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 L alkyl , [Ni II (L alkyl ) 2 ] (1 alkyl ) 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 1 x (X = H, Ph, OMe), the boron-attached CH 3 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 L alkyl . In the solution state the molecular structures of 1 alkyl observed in the solid state are retained. Mixed-ligand complexes, [Ni II (L alkyl )(Tp iPr2 )] (2 alkyl ). can be synthesized by reaction of [Ni II (Y)(Tp iPr2 )] (Y = Cl, NO 3 ) with L alkyl through selective ligand metathesis. In 2 Me , one methyl group of L Me 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 L Bu are located away from the nickel center, and the orientation of the two imidazolyl groups is almost coplanar in 2 Bu . The structural characteristics of these nickel(II) complexes demonstrate the flexibility of the dialkybis(imidazolyl)borate scaffold.