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

Abstract

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 3, supported by the chelating bis(N-heterocyclic silylene)bipyridine ligand, 2,2′-{[(4-tBuPh)­C(N t Bu)]2­SiNMe}2­(C5N)2, 1]. The compound resulted from direct reaction of 1 with Idipp-SiI2 [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 2 with 2 molar equiv of KC10H8. Density functional theory calculations show that the lone pair at the ptSi(II) resides almost completely in its 3p z orbital, very different from known four-coordinate silylenes. Oxidative addition of MeI to the ptSi(II) atom affords the corresponding pentacoordinate Si(IV) compound 4, with the methyl group located in an apical position. Remarkably, the reaction of 2 with [CuO t Bu] 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 5.

Ligand-enforced geometric constraints and associated reactivity in p-block compounds

“…Another example utilizing a rigid tetradentate ligand to access planar silicon species is Driess’ planar silicon( ii ) complex 33 supported by a bis(N-heterocyclic silylene) bipyridine ligand. 130 While not strictly within this review's scope due to the planarity being a result of additional coordination, it is noteworthy that this species shows not only planarity but also a very wide angle between the amidinate groups of approximately 111°. This dication features a planar central silicon atom with a lone pair of electrons localized in the silicon p z orbital, in contrast to most silylenes in which the lone pair is localized in an s orbital.…”

“…This dication features a planar central silicon atom with a lone pair of electrons localized in the silicon p z orbital, in contrast to most silylenes in which the lone pair is localized in an s orbital. 130 The compound is reversibly interconverted with a hexacoordinate silicon( iv ) species by reaction with I 2 and a reduction to return to 33 . 130…”

Pyramidanes: newcomers to the anti-van’t Hoff–Le Bel family

“…By contrast, for heavier tetrel derivatives, several organosilicon compounds featuring planar (or nearly planar) geometry at the Si centre were very recently reported. They include: (a) tetrasilabicyclo[1.1.0]butanes with an exceptionally long Si–Si bridge (treated as either π-single bond 13,14 or 1,3-biradicaloid interaction 15 ) and planarity around the bridgehead Si atoms; (b) Mo and W bimetallic complexes in which a planar tetracoordinate Si centre is embedded in a tricyclic trapezoidal M–C–C–M (M = Mo, W) core; 16 (c) bis(NHSi)silylene, in which a planar tetracoordinate silylene centre is intramolecularly stabilized by a bipyridine ligand; 17 (d) organosilicon compound, in which a planar tetracoordinate Si atom is rigidly fixed within the calix[4]pyrrolato ligand, thus representing a unique example of a square-planar silicon( iv ) derivative with remarkably small HOMO–LUMO energy gap achieved upon planarization at Si. 18…”

Chemical Bonding and Activity of Atomically Dispersed Silicon in Two- and Three-Dimensional Materials

“…In the same year, a variety of compounds containing anti-van’t Hoff/Le Bel ptSi were successfully synthesized, exhibiting remarkable stability at room temperature . Just recently in 2023, Shan et al reported the first experimental synthesis of isolable metal-free ptSi­(II) dication . Moreover, first-principles calculations have confirmed that dispersed Si with a ptSi motif can act as a fundamental building block for 2D sheets in extended systems, and the incorporation of ptSi gives rise to remarkable electronic properties in 2D nanomaterials. , …”

“…20 Just recently in 2023, Shan et al reported the first experimental synthesis of isolable metalfree ptSi(II) dication. 21 Moreover, first-principles calculations have confirmed that dispersed Si with a ptSi motif can act as a fundamental building block for 2D sheets in extended systems, and the incorporation of ptSi gives rise to remarkable electronic properties in 2D nanomaterials. 22,23 Despite the notable advancements mentioned above, two intriguing and pivotal aspects of ptSi remain to be clarified.…”