Three-coordinate beryllium β-diketiminates: Synthesis and reduction chemistry

A series of mononuclear, heteroleptic beryllium complexes supported by the monoanionic β-diketiminate ligand [HC{CMeNDipp}] (L; Dipp = 2,6-diisopropylphenyl) have been synthesized. Halide complexes of the form [LBeX] (X = Cl, I) and a bis(trimethylsilyl)amide complex were produced via salt metathesis routes. Alkylberyllium β-diketiminate complexes of the form [LBeR] (R = Me, Bu) were obtained by salt metathesis from the chloride precursor [LBeCl]. Controlled hydrolysis of [LBeMe] afforded an air-stable, monomeric β-diketiminatoberyllium hydroxide complex. [LBeMe] also underwent facile protonolysis with alcohols to form the corresponding β-diketiminatoberyllium alkoxides [LBeOR] (R = Me, Bu, Ph). High temperatures and prolonged reaction times were required for protonolysis of [LBeMe] with primary amines to yield the β-diketiminatoberyllium amide complexes [LBeNHR] (R = Bu, CHPh, Ph). No reactions were observed between [LBeMe] and silanes, terminal acetylenes, or secondary amines. All compounds were characterized by H, C, and Be NMR spectroscopy and, in most cases, by X-ray crystallography. Reduction of the beryllium chloride complex with potassium metal resulted in apparent hydrogen-atom transfer between two β-diketiminate backbones, yielding two dimeric, potassium chloride bridged diamidoberyllium species. X-ray analysis of a cocrystallized mixture of the 18-crown-6 adducts of these species allowed unambiguous identification of the two reduced diketiminate ligands, one of which had been deprotonated at a backbone methyl substituent and the other reduced by hydride addition to the β-imine position. It is proposed that this process occurs by the formation of an unobserved radical anion species and intermolecular hydrogen-atom transfer by a radical-based hydrogen abstraction mechanism.