Molecular Structure Of Metal Alkoxide Precursors
- PDF / 762,879 Bytes
- 13 Pages / 420.48 x 639 pts Page_size
- 43 Downloads / 277 Views
MOLECULAR STRUCTURE OF METAL ALKOXIDE PRECURSORS C. SANCHEZ, P. TOLEDANO AND F. RIBOT Laboratoire de Chimie de la Mati~re Condensde URA 302 Universit6 Pierre et Marie Curie 4 place Jussieu 75252 Paris France ABSTRACT Oxo-alkoxides and allied derivatives are the organic counter part of polyanions and polycations. Most of them do not lead to further polymerization and are therefore dead endswhich are grnerally not directly involved in the formation of macromolecular oxide networks. However their well defined structure make them good models for the basic understanding of the first steps of hydrolysis- condensation reactions. The structural characterization of a new metal oxo species Ce 6 (l.3 -O)4(.A3 -OH)4(acac) 12 obtained via the hydrolysis of acetylacetone modified cerium isopropoxide is presented. Possible mechanisms of formation of this compound as well as for some other titanium Ti 6 (.t 2 -O) 2 (It 3 -O) 2 (ýt2 -OAc) 4 (I.t2 -OPri)6(OPri)6 and niobium Nb 8 O10 (OEt)20 based oxo-alkoxides will be discussed. INTRODUCTION Sol-gel chemistry offers unique advantages for making monodispersed powders, multicomponent ceramics, coatings, fibers or even completely new mixed organic-inorganic materials [1,2,3]. It is based on inorganic polymerization reactions. An oxide network can be obtained via hydrolysis and condensation of molecular precursors such as metal alkoxides. This chemistry is controled by parameters such as the hydrolysis ratio, catalysis or molecular structures of precursors [4,5,6]. This latter can be simply modified by oligomerization, solvolysis or addition of nucleophilic chemical additives such as organic acids, 1-diketones or allied derivatives which lead to a tayloring of the coordination shell of the metal. The sol gel chemistry of transition metal alkoxides is very different from that of silicon precursors. Main differences arise from the fact that most of the transition metal atoms exhibit a higher electrophilic character and can have several coordination numbers. Consequently transition metal alkoxides can easily present different molecular structures which depend on the nature of the metal (size, electonegativity, dn or fn configuration, oxidation state) and on the nature of the ligands. Such differences in the structure of the molecular precursors can drastically change their reactivity. The morphology of TiO 2 powders strongly depends on the nature of the alkoxy group. Spherical monodispersed TiO2 particles can be obtained via controled hydrolysis of dilute ethanolic solutions of trimeric titanium ethoxide Ti(OEt)4 [7] while polydispersed particles of irregular shape are obtained from monomeric titanium isopropoxide Ti(OPri) 4 [8]. Oligomerization provides a nice way to promote decoupling between nucleation and growth steps [9]. One of the main parameter that can be easily adjusted is the hydrolysis ratio defined as h = H2 0/M. It does not only affect the mean size of macromolecular species, but also their Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
48
structure,
Data Loading...
