Studies on clathrasils VIII. Nonasils-[4¹5⁸], 88SiO₂ · 8M⁸ · 8M⁹ · 4M²⁰: Synthesis, thermal properties, and crystal structure

Abstract

Nonasils-[4¹5⁸], 88SiO₂·8M⁸·8M⁹·4M²⁰, have been synthesized with 2-methylpyrrolidine, hexamethyleneimine, 2-(aminomethyl)-tetrahydrofuran, 1,2-diaminocyclohexane, 2-methylpiperidine, 2-methylpiperazine, 1-aminobutane, 2-aminobutane, and 2-aminopentane as guest molecules, M²⁰. The samples have been prepared from aqueous silicate solutions which were sealed in silica tubes and heated at about 200°C for several weeks. These clathrasils crystallize in space groupFmmm. For the nonasil with 2-aminopentane as the guest molecule and the unit cell dimensionsa _o=22.232(6) Å,b ₀=15.058(4) Å, andc _o=13.627(4) Å, the structure has been refined using 550 non-equivalent single crystal reflexions to a reliability factorR _w =0.125. The 3-dimensional 4-connected silica host framework has three types of cage-like voids, [5⁴6⁴], [4¹5⁸], and [5⁸6¹²], the latter housing the structure-controlling guest molecules, M²⁰. The non-spherical shape of the guest molecules is the most important factor for the formation of nonasils-[4¹5⁸]. On heating nonasils-[4¹5⁸] up to 950°C the organic guest species are driven out and the pure silica form of nonasil is obtained.