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}2]− (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, nBu) 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, tBu, 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 = nBu, CH2Ph, Ph). No reactions were observed between [LBeMe] and silanes,
terminal acetylenes, or secondary amines. All compounds were characterized
by 1H, 13C, and 9Be 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.