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 PARA(3)Mg(8)H(10) with pyridine initially formed 1,2-DHP and 1,4-DHP product mixtures which converted at 60 degrees 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)](2) with pyridine gave exclusive formation of the 1,2-DHP product NN-[Mg(1,2-DHP)](2).(pyridine)2 (NN = [(2,6-iPr(2)C(6)H(3))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 CH...C p-interactions and p-stacking interactions is found. Whereas PARA3Mg(8)H(10) is hardly active in magnesium-catalyzed hydroboration of pyridines with pinacolborane, [NN-(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.