ISOLATION OF THE FIRST ANIONIC FIVE-COORDINATED SILICATES WITH SIX-AND SEVEN-MEMBERED OXYGEN-CONTAINING RINGS

Abstract

Reaction of PhSi(OMe)₃ with KOMe and naphthalenediol led to the first five-coordinated anionic silicate with an oxygen-containing six-membered ring, [(C₁₀H₆O₂)₂Si(C₆H₅)][K, 18-c-6] (1). Reaction of the tetracoordinate spirocyclic silane (C₁₂H₈O₂)₂Si with either KO-t-Bu or KF in the presence of 18-crown-6 led to the isolation of the first five-coordinated anionic silicates with seven-membered oxygen-containing rings, [(C₁₂H₈O₂)₂SiR][K, 18-c-6], R = O-t-Bu (2), F (3). ²⁹Si NMR spectroscopy shows that the five-coordinated silicates 2 and 3 slowly disproportionate into four-coordinate silicon compounds and organic species lacking any silicon. Syntheses similar to that for 2 and 3 conducted with other ligands containing the isopropoxy group or use of Bu₄N⁺F⁻ in place of potassium 18-crown-4 imparts less stability to the system and leads directly to the formation of anionic organic products consisting of two biphenolate species hydrogen bonded to each other. In the case of [(C₁₂H₈O₂)₂H₃][(n-Bu)₄N] (5) and [(C₁₂H₈O₂)₂H][K, 18-c-6]₃ (6), X-ray studies substantiated these compositions. Although the structure of 6 was disordered, the detailed hydrogen-bonding scheme was revealed in the anionic structure of 5. A hydrolytic cleavage mechanism is proposed indicating enhanced reactivity of pentacoordinated anionic silicates relative to their tetracoordinated silane precursors. An X-ray structural analysis reveals a trigonal-bipyramidal geometry for 1 with the six-membered rings located in axial–equatorial sites similar to that found for related pentacoordinated anionic oxysilicates with five-membered-ring systems and isoelectronic oxyphosphoranes that have varying ring compositions. Silicate 1 crystallizes in the monoclinic space group P2₁/n with a = 9.798 (1) Å, b = 20.919 (4) Å, c = 18.300 (4) Å, β = 101.12 (1)° and Z = 4. The biphenolate derivative 5 crystallizes in the triclinic space group P 1 with a = 9.315 (6) Å, b = 9.467 (3) Å, c = 21.293 (6) Å, α = 78.89 (2)°, β = 84.89 (4)°, γ = 80.84 (4)°, and Z = 2. The final conventional unweighted residuals are 0.058 (1) and 0.055 (5).