Retrosynthesis in inorganic crystal structures: application to nesosilicate and inosilicate networks

This paper presents the use of a retrosynthetic tool allowing one to reduce any crystalline structure to a set of two or three simple chemical "tectons" from which the whole network. may be reconstituted. This reduction is performed by probing the susceptibility of any finite secondary building unit to the removal of translation symmetry operators. Atoms displaying atomic charge invariance upon extraction from the network are assumed to belong to the chemical tectons which are at the origin of crystalline growth from molecular species. The chemical bond pattern found around these structural "corner-stones" defines small groups of polyhedra sharing corners or edges and whose topologies are easily understandable in terms of the well-known coordination chemistry of the cationic centers involved. The paper is divided into two parts. The first part is a review of the basic chemical reactions allowing the sharing of corners between two coordination polyhedra from a kinetic point: of view. The second part describes the retrosyntheses of about 50 crystalline structures displaying either insular groups (nesosilicates) or chains (inosilicates) of SiO(4) tetrahedra. This study shows that hydrogen bonding with solvent molecules provides a few eV of stabilization for an anionic species against a few tens eV for hydrogen bonding with a solvated cation and a Few hundreds eV for direct interaction with a naked cation. Tectons which have emerged from a careful analysis of the selected crystalline structures are uncomplexed linear oligomers (SiO(4), Si(2)O(7) Si(3)O(10)), uncomplexed metasilicate rings (Si(3)O(9), Si(4)O(12), Si(6)O(18)) and complexed linear oligomers (TiSiO(4), SbSiO(4), Mn(2)Si(2)O(7), Mn(3)Si(3)O(10), Mn(3)Si(3)O(10), SiBeAlO(10), Si(2)Be(2)O(13)). Some phases cannot yet be studied owing to the non-localization of hydrogen atoms or to the occurence of cationic disorder. The failure to detect any linear oligomer Si(n)O(3n+1) with n>4 or huge Si(n)O(3n) rings (n>9) in solution or in the solid state has been clearly interpreted as the consequence of very unfavorable electrostatic balances for these species. Our observation that such a wide range of silicated crystal structures can be analyzed using just monomeric, dimeric, trimeric or tetrameric tectons is a very encouraging result for future investigations of fully reticulated three-dimensional (3D) networks such as those found in microporous compounds. (C) 1998 Elsevier Science S.A. All rights reserved.