Synthesis and Reactivity of an Anti-van't Hoff/Le Bel Compound with a Planar Tetracoordinate Silicon(II) Atom.

For a long time, planar tetracoordinate carbon (ptC) represented an exotic coordination mode in organic and organometallic chemistry, but it is now a useful synthetic building block. In contrast, realization of planar tetracoordinate silicon (ptSi), a heavier analogue of ptC, is still challenging. Herein we report the successful synthesis and unusual reactivity of the first ptSi species of divalent silicon present in , supported by the chelating bis(-heterocyclic silylene)bipyridine ligand, 2,2'-{[(4-BuPh)C(NBu)]SiNMe}(CN), ]. The compound resulted from direct reaction of with Idipp-SiI [Idipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]. Alternatively, it can also be synthesized by a two-electron reduction of the corresponding Si(IV) precursor with 2 molar equiv of KCH. Density functional theory calculations show that the lone pair at the ptSi(II) resides almost completely in its 3 orbital, very different from known four-coordinate silylenes. Oxidative addition of MeI to the ptSi(II) atom affords the corresponding pentacoordinate Si(IV) compound , with the methyl group located in an apical position. Remarkably, the reaction of with [CuOBu] leads to the regeneration of the bis(silylene) arms via Si-Si bond scission and induces the Si(II) → Si(IV) oxidation of the central Si(II) atom and concomitant two-electron reduction of the bipyridine moiety to form the neutral bis(silylene)silyl Cu(I) complex .