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 °C into PARA -[Mg(1,4-DHP)] 2 ·(pyridine) 2 ( PARA = [(2,6- i Pr 2 C 6 H 3 )NC(Me)C(H)C(Me)N] 2 -( p- C 6 H 4 )). 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- i Pr 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 C–H···C π-interactions and π-stacking interactions is found. Whereas PARA 3 Mg 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.