Multinuclear Magnesium Hydride Clusters: Selective Reduction and Catalytic Hydroboration of Pyridines

Multinuclear magnesium hydride complexes react with pyridine, forming 1,2- and 1,4-dihydropyridide (DHP) complexes. Reaction of <b>PARA</b>₃Mg₈H₁₀ with pyridine initially formed 1,2-DHP and 1,4-DHP product mixtures which converted at 60 °C into <b>PARA</b>-[Mg­(1,4-DHP)]₂·(pyridine)₂ (<b>PARA</b> = [(2,6-<i>i</i>Pr₂C₆H₃)­NC­(Me)­C­(H)­C­(Me)­N]₂-(<i>p-</i>C₆H₄)). Reaction of [<b>NN</b>-(MgH)₂]₂ with pyridine gave exclusive formation of the 1,2-DHP product <b>NN</b>-[Mg­(1,2-DHP)]₂·(pyridine)₂ (<b>NN</b> = [(2,6-<i>i</i>Pr₂C₆H₃)­NC­(Me)­CHC­(Me)­N−]₂). Both products were characterized by crystal structure determinations. The unusual preference for 1,2-addition is likely caused by secondary intramolecular interactions based on mutual communication between the metal coordination geometries: an extended network of C–H···C π-interactions and π-stacking interactions is found. Whereas <b>PARA</b>₃Mg₈H₁₀ is hardly active in magnesium-catalyzed hydroboration of pyridines with pinacolborane, [<b>NN</b>-(MgH)₂]₂ shows efficient coupling. However, the regioselectivity of the stoichiometric reaction is not translated to the catalytic regime. This result is taken as an indication for a potential alternative mechanism in which magnesium hydride intermediates do not play a role but the hydride is transferred from an intermediate borate complex