Divalent Silicon-Assisted Activation of Dihydrogen in a Bis(N-heterocyclic silylene)xanthene Nickel(0) Complex for Efficient Catalytic Hydrogenation of Olefins

Abstract

The first chelating bis(N-heterocyclic silylene)xanthene ligand [Si^II(Xant)Si^II] as well as its Ni complexes [Si^II(Xant)Si^II]Ni(η²-1,3-cod) and [Si^II(Xant)Si^II]Ni(PMe₃)₂ were synthesized and fully characterized. Exposing [Si^II(Xant)Si^II]Ni(η²-1,3-cod) to 1 bar H₂ at room temperature quantitatively generated an unexpected dinuclear hydrido Ni complex with a four-membered planar Ni₂Si₂ core. Exchange of the 1,3-COD ligand by PMe₃ led to [Si^II(Xant)Si^II]Ni(PMe₃)₂, which could activate H₂ reversibly to afford the first Si^II-stabilized mononuclear dihydrido Ni complex characterized by multinuclear NMR and single-crystal X-ray diffraction analysis. [Si^II(Xant)Si^II]Ni(η²-1,3-cod) is a strikingly efficient precatalyst for homogeneous hydrogenation of olefins with a wide substrate scope under 1 bar H₂ pressure at room temperature. DFT calculations reveal a novel mode of H₂ activation, in which the Si^II atoms of the [Si^II(Xant)Si^II] ligand are involved in the key step of H₂ cleavage and hydrogen transfer to the olefin.