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 pK_a 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.