Transition Metal Complexes of Heavier Vinylidenes : Allylic Coordination vs Vinylidene-Alkyne Rearrangement at Nickel

Abstract

Transition metal π-allyl complexes are key reagents/intermediates of various catalytic and stoichiometric allylation reactions. We now report the first transition metal complex of a heavier allylic π-system. The η3 -Si2Ge allyl nickel complex is formally obtained by the oxidative addition of the Si−Cl bond of the heavier vinylidene [R2(Cl)Si−(R)Si(NHC)Ge:] to [Ni(COD)2] (R = 2,4,6- triisopropylphenyl; NHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene; COD = 1,5-cyclooctadiene). Due to geometric constraints, the coordination to the Ni(II) center occurs through the formal SiGe double bond instead of the residual lone pair of electrons at germanium. In contrast, the Si−N bond of the analogous vinylidene [R2(Me2N)Si−(R)Si(NHC)Ge:] (obtained by nucleophilic substitution of Cl by NMe2) does not oxidatively add to Ni(0), and a hydridosilagermene-η2 -nickel complex is obtained instead. The formation of this complex necessarily implies the isomerization of the heavier vinylidene to the corresponding heteroalkyne with the SiGe triple bond in the coordination sphere of nickel followed by the activation of a C−H bond of one of the isopropyl groups of an N-heterocyclic carbene (NHC) ligand.