The Structure of Bis(catecholato)silanes: Phase Adaptation by Dynamic Covalent Chemistry of the Si–O Bond

Abstract

Catechols occupy a unique role in the structural, bio-, and geochemistry of silicon. Although a wealth of knowledge exists on their hypercoordinate complexes, the structure of tetracoordinate bis­(catecholato)­silane, Si­(catH)2 1, has been enigmatic since its first report in 1951. Indeed, the claim of a planar-tetracoordinated silicon in 1 triggered a prominent debate, which is unsettled to this day. Herewith, we present a comprehensive structural study on 1 and derivatives in the gas phase by electron diffraction, in a neon matrix by IR spectroscopy, in solution by diffusion NMR spectroscopy, and in the solid-state by X-ray diffraction and MAS NMR spectroscopy, complemented by high-level quantum-chemical computations. The compound exhibits unprecedented phase adaptation. In the gas phase, the monomeric bis­(catecholato)­silane is tetrahedral, but in the condensed phase, it is metastable toward oligomerization up to a degree controllable by the type of catechol, temperature, and concentration. For the first time, spectroscopic evidence is obtained for a rapid Si–O σ-bond metathesis reaction. Hence, this study sorts out a long-lasting debate and confirms dynamic covalent features for our Earth’s crust’s most abundant chemical bond.

Synthesis and Characterization of Hypercoordinated Silicon Anions: Catching Intermediates of Lewis Base Catalysis

“…Solid-state MAS 29 Si NMR measurements of the isolated powder revealed a chemical shift of À 103.5 ppm, well in line with a polymeric [1] n . [19] Although [1] n is insoluble in common non-coordinating organic solvents, it rapidly dissolved upon adding the respective PPh 4 -or NBu 4 -salts or by reaction with potassium or lithium salts in the presence of suitable crown ethers (Scheme 1b, for details, see Supporting Information). Hence, polymeric [1] n acts as a source of "free" 1 by SiÀ O metathesis depolymerization.…”

“…The averaged bond length to the catecholato ligands SiÀ O cat provides a metric parameter for the degree of "structural activation" (Rule 1). A SiÀ O cat bond elongation is noted for all adducts (Table 3, col 4) with reference to the "free" Lewis acid in the gas phase (1.644(1) Å), [19] but these values neither correlate with the SiÀ X bond lengths (Table 3, col 3, for computed trends, see Table S3.1) nor with ion affinity (Table 1). Interestingly, the acetate induces the most pronounced SiÀ O cat bond elongation.…”

Bis(catecholato)silicates: Synthesis and Structural Data

“…Contrary to other pentacoordinate silicates such as those originating from Martin's spirosilane (bearing the dianion of hexafluorocumyl alcohol), alkyl and aryl bis(catecholato)silicates cannot be prepared from the corresponding spirosilane by addition of nucleophiles onto the electrophilic silicon center [4,5] . Bis(catecholato)silane indeed exists only in solution as an insoluble oligomer [6] . Of note, the tris(catecholato)silicate dianion has also been described but despite a recent highly interesting report by Greb [7a] its chemistry has so far been limited in terms of applications (Figure 2).…”

“…[4,5] Bis(catecholato)silane indeed exists only in solution as an insoluble oligomer. [6] Of note, the tris(catecholato)silicate dianion has also been described but despite a recent highly interesting report by Greb [7a] its chemistry has so far been limited in terms of applications (Figure 2).…”

First Evidence of Tris(catecholato)silicate Formation from Hydrolysis of an Alkyl Bis(catecholato)silicate

“…The chemistry of catechol and silicon is very intriguing. As an example, the structure of the simple bis(catecholato)silane has been debated since 1951 [1], and only in 2021, a detailed discussion about its structural features has been reported [2]. Moreover, the chemistry of hypervalent penta-and hexacoordinated (catecholato)silicates is worthy of attention, both from a synthetic and biological standpoint, and still needs to be deeply investigated [3].…”