Synthesis and structural characterization of a new class of organoborato ligands containing imidazolyl functional groups [MeB(ImN-Me)2(X)]−

Abstract

A series of alkylborate compounds containing imidazolyl functional groups [MeB(Im^N-Me)₂(X)]⁻ (X = OPrⁱ1, Cl 2, Pz 3 or Ph 4) has been synthesized and characterized. Reaction of di(isopropoxy)methylborane with 2 equivalents of 2-lithio-1-methylimidazole yielded the lithium isopropoxymethylbis(2-methyl-1-imidazolyl)borate, Li[1]. The chlorobis(imidazolyl)methylborate 2, which was obtained by treatment of 1 with HCl, could be transformed to the pyrazolyl and phenyl derivatives 3 and 4, respectively, by nucleophilic substitution of the B-bound chlorine atom. The molecular structures of {H₂[2]}Cl, H[3] and Ni[4]₂ have been successfully determined by X-ray crystallography. In {H₂[2]}Cl both the imidazolyl groups are protonated, forming an intermolecular hydrogen bonding interaction with the chloride anion and giving a dimeric structure. In H[3] an intramolecular hydrogen bond between the the non-co-ordinated nitrogen atoms of the two imidazolyl groups is formed. Structural properties of the protonated imidazole rings in {H₂[2]}⁺ and H[3] indicate delocalization of positive charge over the rings, which results in strong interaction with the negatively charged boron center. Formation of Ni[4]₂ clearly indicates the chelation ability of the bis(imidazolyl)borate framework. Therefore, the imidazolyl groups in [MeB(Im^N-Me)₂(X)]⁻ act as a proton acceptor and metal-binding site as found for poly(pyrazolyl)borates.