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.