A Planar Nickelaspiropentane Complex with Magnesium-Based Metalloligands: Synthesis, Structure, and Synergistic Dihydrogen Activation.

The nature of transition-metal-olefin bonding has been explained by the Dewar-Chatt-Duncanson model within a continuum of two extremes, namely, a π-complex and a metallacyclopropane. The textbook rule suggests that a low-spin late-transition-metal-ethylene complex more likely forms a π-complex rather than a metallacyclopropane. Herein, we report a low-spin late-transition-metal- bis -ethylene complex forming an unprecedented planar metalla- bis -cyclopropane structure with magnesium-based metalloligands. Treatment of L MgEt ( L = [(DippNCMe) 2 CH] - , Dipp = 2,6- i Pr 2 C 6 H 3 ) with Ni(cod) 2 (cod = 1,5-cyclooctadiene) formed the heterotrimetallic complex ( L Mg) 2 Ni(C 2 H 4 ) 2 , which features a linear Mg-Ni-Mg linkage and a planar coordination geometry at the nickel center. Both structural features and computational studies strongly supported the Ni(C 2 H 4 ) 2 moiety as a nickelaspiropentane. The exposure of ( L Mg) 2 Ni(C 2 H 4 ) 2 to 1 bar H 2 at room temperature produced a four-hydride-bridged complex ( L Mg) 2 Ni(μ-H) 4 . The profile of H 2 activation was elucidated by density functional theory calculations, which indicated a novel Mg/Ni cooperative activation mechanism with no oxidation occurring at the metal center, differing from the prevailing mono-metal-based redox mechanism. Moreover, the heterotrimetallic complex ( L Mg) 2 Ni(C 2 H 4 ) 2 catalyzed the hydrogenation of a wide range of unsaturated substrates under mild conditions.