Reactivity of a Molecular Magnesium Hydride Featuring a Terminal Magnesium–Hydrogen Bond

The reactivity of
the molecular magnesium hydride [Mg­(Me3TACD·AliBu3)­H] (1) featuring
a terminal magnesium–hydrogen bond and an NNNN-type macrocyclic
ligand, Me3TACD ((Me3TACD)H = Me3[12]­aneN4 = 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane),
can be grouped into protonolysis, oxidation, hydrometalation, (insertion),
and hydride abstraction. Protonolysis of 1 with weak
Brønsted acids HX such as terminal acetylenes, amines, silanols,
and silanes gave the corresponding derivatives [Mg­(Me3TACD·AliBu3)­X] (X = CCPh, 3; HN­(3,5-Me2-C6H3), 4; OSiMe3, 5; OSiPh3, 6; Cl, 7; Br, 8). Single-crystal
X-ray diffraction of anilide 4 showed a square-pyramidal
coordination geometry for magnesium. No correlation with the pKa values of the acids was detected. Oxidation
of 1 with elemental iodine gave the iodide [Mg­(Me3TACD·AliBu3)­I]
(9), and oxidation with nitrous oxide afforded the μ-oxo-bridged
compound [{Mg­(Me3TACD·AliBu3)}2(μ-O)] (10) with a
linear Mg–O–Mg core, as characterized by single-crystal
X-ray diffraction. The Mg–H bond reacted with benzaldehyde,
benzophenone, fluorenone, and CO2 under insertion but not
with the olefins 1,1,2-triphenylethylene, tert-butylethylene,
and cyclopentene. The unstable formate, prepared also by salt metathesis
of iodide 9 with potassium formate, revealed κO,κO′ coordination in the
solid state. Hydride abstraction with triphenylborane gave the ion
pair [Mg­(Me3TACD·AliBu3)­(thf)]­[HBPh3] (16), which catalyzed
the hydroboration of polar substrates by pinacolborane.