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 [Si-II(Xant)Si-II] as well as its Ni complexes [Si-II(Xant)Si-II]Ni(eta(2)-1,3-cod) and [Si-II(Xant)Si-II]Ni(PMe3)(2) were synthesized and fully characterized. Exposing [Si-II(Xant)si(II)]Ni (eta(2)-1,3-cod) to 1 bar H-2 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 H-2 reversibly to afford the first Si-II-stabilized mononuclear dihydrido Ni complex characterized by multi-nuclear NMR and single-crystal X-ray diffraction analysis. [Si-II(Xant)Si-II]Ni(eta(2)-1,3-cod) is a strikingly efficient precatalyst for homogeneous hydrogenation of olefins with a wide substrate scope under 1 bar H-2 pressure at room temperature. DFT calculations reveal a novel mode of H-2 activation, in which the Sin atoms of the [Si-II(Xant)Si-II] ligand are involved in the key step of H-2 cleavage and hydrogen transfer to the olefin.