Group 3 tetrahydroborates. Part 1. The synthesis and properties of dimethylgallium tetrahydroborate

Abstract

The synthesis, characterization, and properties of Ga(BH₄)Me₂ are reported. Although first prepared via the interaction of GaMe₃ and diborane, the compound is more satisfactorily prepared by the reaction of GaMe₂Cl with Li[BH₄] in the absence of a solvent at –15 °C. Subsequent characterization has been based on the vapour-phase molecular weight, on the mass, vibrational, and ¹H n.m.r. spectra of the material, and on elemental analysis of the ammonia adduct Ga(BH₄)Me₂·2NH₃. The i.r. spectrum of the gaseous tetrahydroborate is consistent with a molecular model Me₂Ga(µ-H)₂BH₂ having C_2v symmetry and containing four-co-ordinate gallium linked to a bi-dentate tetrahydroborate group; such a structure appears to persist in the condensed phases, although the vibra-tional spectra suggest increased polarization in the sense [GaMe₂]⁺[BH₄]^–. Chemical studies have disclosed, inter alia, ready hydrogen–methyl exchange between the boron and gallium centres, as well as instances of symmetrical and unsymmetrical cleavage of the Ga(µ-H)₂B skeleton.