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

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The reactivity of the molecular magnesium hydride [Mg(Me 3 TACD·Al i Bu 3 )H] ( 1 ) featuring a terminal magnesium–hydrogen bond and an NNNN-type macrocyclic ligand, Me 3 TACD ((Me 3 TACD)H = Me 3 [12]aneN 4 = 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(Me 3 TACD·Al i Bu 3 )X] (X = C≡CPh, 3; HN(3,5-Me 2 -C 6 H 3 ), 4; OSiMe 3, 5; OSiPh 3, 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 p K a values of the acids was detected. Oxidation of 1 with elemental iodine gave the iodide [Mg(Me 3 TACD·Al i Bu 3 )I] ( 9 ), and oxidation with nitrous oxide afforded the μ-oxo-bridged compound [{Mg(Me 3 TACD·Al i Bu 3 )} 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 CO 2 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 κ O,κ O ′ coordination in the solid state. Hydride abstraction with triphenylborane gave the ion pair [Mg(Me 3 TACD·Al i Bu 3 )(thf)][HBPh 3 ] ( 16 ), which catalyzed the hydroboration of polar substrates by pinacolborane.