What are the Molecules of Which the Samples of Metal Alkoxides do Really Consist?
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ABSTRACT On the basis of consideration of the properties of Al, In, Ln, Ti, Zr, Hf, NbO 3+, MoO 4+, WO4+I M002+, WO2 2+, Fe, Co, Ni alkoxide derivatives the main regularities in poly- and oligomerization properties have been deduced: 1) The molecular complexity increases in time (aging) and on action of alcohols. This process is followed by the increase in the melting and boiling points, decrease in solubility in ROH and weakening of colour. In the mass-spectra - in connection with the temperature of transition into the gas phase - increases the stability of products 2) oligomerization occurs on dissolution in with lower molecular masses. aromatic hydrocarbons or on a short thermal treatment (melting, distillation). The oligomerization products possess lower thermal constants, higher solubility in ROH and more intense colour. Their mass- spectra are containing the fragments of higher masses corresponding to oligomers existing apparently also in the condensed phase. the monitoring of molecular permits The mastering of these regularities composition and thus physicochemical properties of the samples which are of importance for their technological application.
The presence of oxoalkoxides in the "M(OR)n" samples should be taken into consideration on the preparation of solutions used in the synthesis of powders and films of pure simple and mixed oxides.
THE COORDINATION POLYMERISM. The irreproducibility of physicochemical characteristics such as melting point, IR and mass-spectra of some metal alkoxide derivatives has been often mentioned in literature, the usual examples being AI(OEt)3, AI(OPri)3, Ga(OPri)3 and Ti(OMe)4. We have found that such varialibility of "constants" for metal alkoxides samples (state of matter, m.p., solubility, volatility, intensity of colour) which turn to be dependent upon their prehistory - e.g. the synthetic route, the nature of solvents they are prepared from, time of storage (the "age") is characteristic of majority of Ill-VII groups metals derivatives. The application of such characterisation techniques as massspectrometry, cryoscopy, IR spectra and X-ray diffraction allowed us to detect the presence of several types of olygomeric associates - up to polymers (having different size, composition and structure) in the same metal alkoxides samples. We 261 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
referred to this phenomenon as "coordination polimerism". A simple example is AI(OPFi)3 which can exist in the form of two kinds of associates - trimeric and tetrameric (see Fig. 1). The freshly disstilled samples of aluminium isopropoxide consist of trimers. In accordance with the 1HHoo NMR data [1] the structure of the 60 latter coincide with that of A13Cl5(OPri)4 studied by X-ray 20 single crystal method [2]. The storage action of alcohol on the melts is converting them into o3,5 40 crystalline solids with the Degree ofassoaon maximal m.p. 140C, built up of 0 8 16 2L4 32 140
40
[AI(OR)3]4
molecules (Fig. 2,3).
Nine at2O.C, days Storage
Fig. I The dependence o
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