Fabian Ebner and Lutz Greb are reporting a planar silicon(IV) complex in the journal Chem this month (DOI). Planar silicon(IV) is unusual because like carbon, the element prefers a tetrahedral geometry when it is to have 4 neighbors.
Greb has a background among other things in catalysis, in 2017 he was the inventor of bis(perchlorocatecholato)silane (DOI), a Lewis acid with at its core silicon with 4 oxygen substituents that is also a super Lewis acid based on its ability to abstract fluorine from the hexafluoroantimonate ion. The motif was inspired by earlier work (2015) by Tilley (DOI). An early example of a stable pentacoordinated SiO4F compound can be found in a 1981 publication by Harland (DOI) , a reminder that silicon is not like carbon and the better candidate when it comes to designing exotic molecules.
Also in 2017 Greb got a first glimpse of a planarized silicon compound in working with certain amino silanes (DOI) . By replacing oxygen with nitrogen in a roughly similar structure as bis(perchlorocatecholato)silane, a solid state tetrahedral angle (usually 109 degrees) was now a record flatter 126 degrees.
In 2018 he and Ebner published work on a hydridosilicate (DOI). This anionic Si-H compound was prepared by deprotonating methyl-calix(4)pyrrole with benzyl potassium and a subsequent reaction with trichlorosilane. It should be a super hydride donor but was found to resist hydrolysis and in general behaved as a very reluctant reducing agent. A tentative explanation was found in a computational simulation of the neutral deprotonated compound that revealed a planar geometry at silicon.
In 2020 this neutral compound was subject of a computational study by Zhang, Zhang, Mo and Cao (DOI). According to this study the pyrrole groups stabilize the central silicon atom by electron donation giving it a carbenic character. The partially filled Pz orbital also prevents reverting to the tetrahedral geometry.
In the new Ebner / Lutz work the neutral compound has finally materialized. It was synthesized from meso-octaethyl-calix(4)pyrrole by deprotonation with n-butyl lithium and reaction with tetrachlorosilane in dimethoxyethane to a DME salt, The tetraphenylphosphine salt was then reacted with sodium tetrakis(pentafluorophenyl)borate. The new compound is stable and truly flat at silicon. At the detail level two of the methyl groups of the outer rim of the calixarene point with their hydrogen atoms inwards towards the central silicon atom in an agnostic theme. Reduction of the compound is easy and indicative of a low-lying LUMO, in a reaction with decamethylcobaltocene and chloroform it acts as a radical anion. The HOMO energy is raised and a low LUMO-HOMO gap according to the authors explain the reaction with phenylacetylene in a 1,2-addition.
More researchers have been and are on the planar silicon trail. The end of 2020 (DOI) already saw the publication of another article on the subject. In it, Filippou et al. also claim the isolation of a planar silicon compound, one with silicon bonded to two molybdenum centers and also part of a silacyclopropene ring system. Ebner and Lutz though write that their system is the only one with silicon in its natural oxidation state.