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)] 1, with ^RDipnacnac = HC(RCNDip)₂, Dip = 2,6-iPr₂C₆H₃, R = Me (1a), Et (1b), iPr (1c), readily react with dihydrogen under mild conditions to afford mixed hydride-hydroxide complexes [{(^RDipnacnac)Mg}₂(μ-H)(μ-OH)] 4. Dehydrogenation of complexes 4 is strongly dependent on remote ligand substitution and can be achieved by simple vacuum-degassing of 4c (R = iPr) to regain 1c. Donor addition to complexes 4 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 4b from a magnesium hydride complex have been investigated and the ability of complexes 1 or 4 to act as catalysts for the hydrogenation of 1,1-diphenylethene (DPE) has been tested.