Synthesis and Characterization of Tris(catecholato) Silicates, [(C₆H₄O₂)₃Si]^2- with Different Counter Cations – First Pyrolysis Study and X-ray Structure of [{(CH₃)₂CH}₂NH₂]₂[(C₆H₄O₂)₃Si] · 2CH₃CN · H₂O

Abstract

The synthesis of hypervalent tris(catecholato)silicate ion, [(C₆H₄O₂)₃Si]^2- with six different counter cations: (i) diethylammonium (I), (ii) triethylammonium (II), (iii) diisopropylammonium (III), (iv) morpholinium (IV), (v) piperazinium (V) and (vi) N-methylpiperazinium (VI) has been achieved (yield: ca.95%) by the reaction of catechol and tetraethoxysilane in presence of the corresponding amine. Single crystal X-ray structure of compound (III) has been determined. It crystallizes in orthorhombic system [space group P2₁2₁2₁; a = 11.735(2), b = 13.708(3), c = 23.283(5) Å; α = 90°, β = 90°, γ = 90°; Volume = 3745.4(11) ų; Density = 1.165 gcm⁻³; Z = 4; R = 0.0585; wR2(I) = 0.1610 and 3677 reflections] and the structure shows slightly distorted octahedral geometry around silicon. Results of TG, DT and EG analysis of compounds (I)–(VI) reveal the influence of the counter cation on thermal stability. Bench scale pyrolysis of (I) and (II) convert them to [(C₆H₄O₂)₂Si] _n by the quantitative expulsion of ammonium catecholate salt, (M⁺)₂[(C₆H₄O₂)]^2-. CP-MAS ²⁹Si-nmr spectral data suggests that [(C₆H₄O₂)₂Si] _n is a simple mixture.