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

The first chelating bis­(N-heterocyclic silylene)­xanthene ligand [SiII(Xant)­SiII] as well as its Ni complexes [SiII(Xant)­SiII]­Ni­(η2-1,3-cod) and [SiII(Xant)­SiII]­Ni­(PMe3)2 were synthesized and fully characterized. Exposing [SiII(Xant)­SiII]­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 [SiII(Xant)­SiII]­Ni­(PMe3)2, which could activate H2 reversibly to afford the first SiII-stabilized mononuclear dihydrido Ni complex characterized by multinuclear NMR and single-crystal X-ray diffraction analysis. [SiII(Xant)­SiII]­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 SiII atoms of the [SiII(Xant)­SiII] ligand are involved in the key step of H2 cleavage and hydrogen transfer to the olefin.