Oxides heterometal

A 1-methyluracil blue of distinctly different composition, containing (formally) two Ptn and a heterometal ion, Pdm, has likewise been prepared and structurally characterized by X-ray analysis (Scheme 5) [67]. It can be (reversibly) further oxidized to a PtnPdIVPtn state and also reduced to PtnP-d t11. There is no reason why Pt instead of Pd should not behave similarly, although this would require the loss of two NH3 ligands from a cis-[Pt(NH3)2(H20)2]2+entity. 

For example, the research group of Turova et al. (480) found evidence for the formation of such oxide species through the elimination of ether from homo-or heterometal alkoxides ... 

In addition to the use of heterometal alkoxides, metal alkoxides are often associated with more easily available precursors such as acetates for the SG route to multicomponent oxides. A number of such alkoxide acetate precursors [e.g., MNb2(/i-OAc)2(/i-OR)4(OR)6 (M = Cd or Mg), PbZr3(/t4-0)(/i-0Ac)2(/i-OR)5(OR)5, and Gd2Zr6(/i4-O)2(pi-OAc)6(/t-OR)l0(OR)i0 (with R = i-Pr)] were characterized (564) by X-ray crystallography. Their hydrolytic studies indicate their potential use as precursors for the synthesis of electrooptical materials, for example, Pb(ScNb)03 (PSN), and dielectric ceramics, for example, [PbMg1/3Nb2/303] (PNM). 

Heterometal cubanes of general formula, [M 4 M" (LH)4](0Ac)4 (C104), have shown to be versatile singlesource precursors to prepare metal-doped transition metal oxides. The heterocubanes allow several combina-... 

The potential of heterometal alkoxide precursors to preform the ceramic on a molecular level has also been demonstrated for BaTiOs and BaZrOs oxides. The improved homogeneity, phase purity, and lower crystallization... 

The fully oxidized nature of cluster (7) generates significant reactivity properties. The cluster cannot be reversibly reduced, but it does react with FeCl2 to form an Fc4S4 cluster and with reduced metal complexes to afford heterometal cubane-type clusters.Reaction... 

In spite of considerable initial scepticism, the synthesis of an increasing number of well-characterized heterometallic alkoxides is now well established and attempts are being directed towards the synthesis of single-source precursors for preparation of mixed metal-oxide ceramic materials by the sol-gel or MOCVD processes. " " Before proceeding to a brief account of some novel types of heterometal glycolate and aminoalkoxide derivatives, it is appropriate to mention four review articles" summarizing the state of knowledge about the conventional heterometallic alkoxide systems at the end of the twentieth century. 

Apart from the pioneering researches of Bradley and co-workers, who studied the controlled hydrolysis of the alkoxides of some early transition metals, there was little interest in the metal oxo-alkoxides until the sol-gel technique was developed for the conversion of metal alkoxide precursors into metal oxides and heterometal oxides (Chapter 7) A concise review of the chemistry of metal oxo-alkoxides published by Mehrotra and Singh" reveals the rapid growth of interest in this field. The most notable feature has been the dramatic increase in the number of crystal structure determinations in recent years. However, our knowledge of the mechanism of formation of metal oxo-alkoxides leaves much to be desired. 

The catalytic activity is a manifestation of the chemical lability of metal alkoxides especially their reactivity with hydroxyl-containing molecules. The volatility and solubility in common organic solvents of certain metal alkoxides has made them attractive precursors for depositing pure metal oxides by chemical vapour deposition (MOCVD) or by the sol-gel process. The requirement for heterometal oxides as useful materials in the electronics industry has stimulated research in this field in recent years and led to renewed interest in the preparation and characterization of alkoxides of some of the p-block elements which had previously been neglected. In particular, the discovery of the high Tc copper oxide-based superconducting heterometal oxides has made a tremendous impact on this field. 

As an alternative process to MOCVD, which requires a volatile metal alkoxide, there is the technique of depositing a film of metal alkoxide onto a substrate from solution in a volatile solvent followed by thermolysis. Thus oligomeric metal alkoxides [M(OR)jc]n that may not be appreciably volatile can by suitable choice of the alkyl group R be made soluble in volatile organic solvents. This method is especially useful for depositing a multicomponent heterometal oxide because the stoichiometry can be determined by the initial concentration of each component metal alkoxide. This technique is similar in principle to the sol-gel technique which is favoured for the formation of bulk materials (Section 3). 

The sol-gel process is particularly useful for nonvolatile metal alkoxides which cannot be used in the MOCVD process, and for forming heterometal oxides, where control of stoichiometry is important. Since 1980, numerous excellent reviews have been published on the sol-gel chemistry of metal alkoxides to which readers are referred for detailed bibliography. Numerous publications have appeared detailing the preparation of binary metal oxides and heterometal oxides using the sol-gel technique and a few representative examples only are given here. 

The requirement for inorganic electronic materials prepared at low temperatures has stimulated enormous activity in the heterometal oxide field. The sol-gel technique using metal alkoxides has been especially effective in producing ferroelectric, piezoelectric, and pyroelectric materials, e.g. BaTiOz " LiNbOz " LiTaOs " PbTiOs Pb(Zr,Ti)03(PZT) (Pb,La) (Zr,Ti)03(PLZT) 2 Pb(Fe,Nb)03 andPb(Mg,Nb)03. ... 

Considerable attention has been devoted to the preparation of high Tc superconductors based on the heterometal copper oxide systems. Most reports have been concerned with the deposition of films of the 1,2,3 superconductors YBazCusOz- on a variety of different substrates. ... 

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