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 η 3 -Si 2 Ge allyl nickel complex is formally obtained by the oxidative addition of the Si−Cl bond of the heavier vinylidene [R 2 (Cl)Si−(R)Si(NHC)Ge:] to [Ni(COD) 2 ] (R = 2,4,6triisopropylphenyl; 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 [R 2 (Me 2 N)Si−(R)Si(NHC)Ge:] (obtained by nucleophilic substitution of Cl by NMe 2 ) does not oxidatively add to Ni(0), and a hydridosilagermene-η 2 -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.

Reactivity of free N-heterocyclic carbenes in dichloromethane

“…5 Me IPr acts as a crucial support ligand in a Ni 0 stabilised silyliumylidene ion 6 and in heavier vinylidene complexes of nickel. 7 The carbene was used as donor ligand for Jones’ magnesium( i ) complexes to facilitate the trimerisation of CO, 8 to stabilise magnesium amidoboranes 9 and to trap the highly reactive [BO 2 ] − ion in situ . 10 Me IPr also allowed the isolation of the first neutral Ge 0 octahedron, 11 digermavinylidenes 12 and dimetallodigermenes.…”

Reduction of a Bicyclo[1.1.0]tetrasil-1(3)-ene with LiAlH4 Leading to an Isolable Cyclotrisilenide

“…Although halogen abstraction of 1a from alkyl halides provided 1,3-dihalobicyclo[1.1.0]­tetrasilanes that contain an unsupported bridgehead Si–Si π-bond, the reactivity of 1 for other reagents remains to be elucidated. Herein, we report that the reduction of 1a with lithium aluminum hydride (LiAlH 4 ) provides unexpectedly cyclotrisilenide 2 as the first disilenide with a cyclotrisilene moiety. , …”