Multinuclear Magnesium Hydride Clusters: SelectiveReduction and Catalytic Hydroboration of Pyridines.

Multinuclearmagnesium hydride complexes react with pyridine, forming1,2- and 1,4-dihydropyridide (DHP) complexes. Reaction of PARA3Mg8H10with pyridine initiallyformed 1,2-DHP and 1,4-DHP product mixtures which converted at 60°C into PARA-[Mg(1,4-DHP)]2·(pyridine)2(PARA= [(2,6-iPr2C6H3)NC(Me)C(H)C(Me)N]2-(p-C6H4)). Reaction of [NN-(MgH)2]2with pyridine gave exclusive formationof the 1,2-DHP product NN-[Mg(1,2-DHP)]2·(pyridine)2(NN= [(2,6-iPr2C6H3)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 secondaryintramolecular interactions based on mutual communication betweenthe metal coordination geometries: an extended network of C–H···Cπ-interactions and π-stacking interactions is found. Whereas PARA3Mg8H10is hardly activein magnesium-catalyzed hydroboration of pyridines with pinacolborane,[NN-(MgH)2]2shows efficient coupling.However, the regioselectivity of the stoichiometric reaction is nottranslated to the catalytic regime. This result is taken as an indicationfor a potential alternative mechanism in which magnesium hydride intermediatesdo not play a role but the hydride is transferred from an intermediateborate complex. [ABSTRACT FROM AUTHOR]