Facile, Reversible Hydrogen Activation by Low-Coordinate Magnesium Oxide Complexes

New approaches to achieve facile and reversible dihydrogen activation are of importance for synthesis, catalysis, and hydrogen storage. Here we show that low-coordinate magnesium oxide complexes [{(^RDipnacnac)Mg}₂(μ-O)] <b>1</b>, with ^RDipnacnac = HC(RCNDip)₂, Dip = 2,6-<i>i</i>Pr₂C₆H₃, R = Me (<b>1a</b>), Et (<b>1b</b>), <i>i</i>Pr (<b>1c</b>), readily react with dihydrogen under mild conditions to afford mixed hydride-hydroxide complexes [{(^RDipnacnac)Mg}₂(μ-H)(μ-OH)] <b>4</b>. Dehydrogenation of complexes <b>4</b> is strongly dependent on remote ligand substitution and can be achieved by simple vacuum-degassing of <b>4c</b> (R = <i>i</i>Pr) to regain <b>1c</b>. Donor addition to complexes <b>4</b> also releases hydrogen and affords donor adducts of magnesium oxide complexes. Computational studies suggest that the hydrogen activation mechanism involves nucleophilic attack of an oxide lone pair at a weakly bound H₂···Mg complex in an S_N2-like manner that induces a heterolytic dihydrogen cleavage to yield an MgOH and an MgH unit. Alternative synthetic routes into complex <b>4b</b> from a magnesium hydride complex have been investigated and the ability of complexes <b>1</b> or <b>4</b> to act as catalysts for the hydrogenation of 1,1-diphenylethene (DPE) has been tested.