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HANDBOOK of SOL–GEL SCIENCE and TECHNOLOGY Processing, Characterization and Applications VOLUME I SOL–GEL PROCESSING HANDBOOK of SOL–GEL SCIENCE and TECHNOLOGY Processing, Characterization and Applications edited by Sumio Sakka VOLUME I SOL–GEL PROCESSING Volume editor: Hiromitsu Kozuka Kansai University Suita, Osaka, Japan Professor Emeritus of Kyoto University Hirakata, Osaka, Japan KLUWER ACADEMIC PUBLISHERS NEW YORK, BOSTON, DORDRECHT, LONDON, MOSCOW Contents Preface by Sumio Sakka Preface by H. Kozuka List of Contributors vii I. SOL–GEL PRECURSORS The Synthesis and Solution Stability of Alkoxide Precursors V. G. Kessler Reactions of Alkoxides Toward Nanostructured or Multi-Component Oxide Films K. Kato Sol-Gel Processing of Thin Films with Metal Salts K. Nishio and T. Tsuchiya Chemistry and Applications of Polymeric Gel Precursors V. Petrykin and M. Kakihana Aqueous Precursors Y.Ohya 31. 2. 3. 4. 5. II. PROCESSING OF POWDERS AND BULK MATERIALS Sol–Gel Formation of Bulk Glasses S. Sakka Sol–Gel Derived Powders and Bulk Ceramics D. Ganguli PROCESSING OF NON-OXIDE MATERIALS Oxynitride Glasses and Nitrides K. Kamiya 6. 7. 9. Oxycarbide Glasses and Carbides K. Kamiya 10. Sol–Gel Processing of Fluoride and Oxyfluoride Materials S. Fujihara 11. Sol–Gel Processing of Sulfide Materials R. M. Almeida and J. Xu IV. PROCESSING OF THIN FILMS III. 12. Fundamental Issues on Sol–Gel Coatings: Stress Evolution, Cracking and Radiative Striations H. Kozuka 13. Ultrasonic Pulverization of an Aerosol: A Versatile Tool for the Deposition of Sol–Gel Thin Films M. Langlet ix xi 41 59 77 105 129 149 171 185 203 225 247 289 8. CONTENTS 14. Electrophoretic Sol–Gel Deposition A. Matsuda and M. Tatsumisago 15. Low Temperature Processing of Sol–Gel Thin Films in the Binary System M. Langlet 16. Self-Standing Thick Films M. Yamane 17. Synthesis of Ferroelectric Thin Films with Preferred Orientation S. Hirano and W. Sakamoto 309 331 361 371 V. PROCESSING OF FIBERS AND MONODISPERSE PARTICLES 18. Processing of Fibers K. Kamiya 19. Processing of Monodisperse Particles S. Ramakrishnan and C. F. Zukoski VI. ENCAPSULATION OF ORGANIC MATERIALS 20. Entrapment of Organic Molecules K. Matsui 21. Encapsulation of Enzymes, Antibodies and Bacteria J. Livage and T. Coradin 401 417 VII. PROCESSING OF CATALYSTS, POROUS MATERIALS, AND AEROGELS 22. 23. 24. 25. Processing of High Performance Catalysts A. Ueno Macroporous Morphology Control by Phase Separation K. Nakanishi Formation of Ordered Mesoporous Thin Films Through Templating K. J. Edler Aerogel Processing J. Phalippou, T. Woignier, F. Despetis and S. Calas VIII. SPECIAL TECHNIQUES USED IN SOL–GEL PROCESSING 26. 27. Non-Hydrolytic Sol–Gel Technology A. Vioux and P. H. Mutin Ultraviolet (UV) Irradiation H. Imai Index vi 459 485 507 529 541 599 621 639 651 Preface to the Handbook (Sol–Gel Science and Technology) This three-volume Handbook “Sol–Gel Science and Technology” was planned with the purpose of providing those who are interested in processing, characterization and applica- tion of materials with comprehensive knowledge on sol–gel science and technology. Around 1970, three different groups in the field of inorganic materials published re- search results on preparation of glass and ceramics via solutions or sol–gel route. H. Dislich prepared a pyrex-type borosilicate glass lens by heating a compact of metal alkoxide derived powder at temperatures as low as 650°C. R. Roy prepared a millimeter-size small piece of silica glass via sol–gel route at temperatures around 1000°C. Mazdiyasni et al. showed that well-sintered, dense ferroelectric ceramics can be obtained at temperatures as low as 900°C, when sol–gel powders prepared from solutions of metal alkoxides are employed for sintering. Those works stimulated people’s interest in sol–gel preparation of inorganic materials, such as glasses and ceramics. Materials scientists and engineers paid attention to the pos- sibility of this method in giving shaped materials directly from a solution without passing through the powder processing and the fact that the maximum temperature required for processing is very low compared with conventional technology for preparing glasses and ceramics. Thus, many efforts have been made in preparing bulk bodies, coating films, membranes, fibers and particles, and many commercial products were born. The significant characteristics unique to the sol–gel method became evident, when organic-inorganic hybrid materials were prepared by H. Schmidt and silica materials con- taining functional organic molecules were prepared by Avnir in early 1980’s. Such mate- rials are produced at low temperatures near room temperature, where no decomposition of organic matter takes place. Low temperature synthesis and preparation of materials is the world of chemists. Therefore, the sol–gel method was propagated to the wide area including not only glasses and ceramics, but also organic and biomaterials. In 1990, an excellent book entitled “Sol–Gel Science” was written by Brinker and Scherer, obtaining a very high reputation. However, the remarkable scientific and tech- nological development and broadening in the sol–gel field, together with an enormous increase in sol–gel population, appeared to demand publication of a new, comprehensive Handbook on sol–gel science and technology. Thus, it was planned to publish the present Handbook, which consists of the following three volumes: Volume 1 Sol–Gel Processing Volume editor: Prof. Hiromitsu Kozuka Volume 2 Characterization and Properties of Sol–Gel Materials and Products Volume editor: Prof. Rui M. Almeida Volume 3 Applications of Sol–Gel Technology Volume editor: Prof. Sumio Sakka Volume 1 compiles the articles describing various aspects of sol–gel processing. Consid- ering that the sol–gel method is a method for preparing materials, the knowledge on sol–gel processing is of primary importance to all those who are interested in sol–gel science and viii PREFACE TO THE HANDBOOKS technology. Articles describing processing of some particular property as well as general basics for sol–gel processing are collected. Volume 2 consists of the articles dealing with characterization and properties of sol–gel materials and products. Since materials exhibit their functional properties based on their microstructure, characterization of the structure is very important. We can produce useful materials only when processing-characterization-property relationships are worked out. This indicates the importance of the articles collected in Volume 2. The title of Volume 3 is “Applications of Sol–Gel Technology”. The sol–gel technology is one of the methods for producing materials and so there are many other competitive methods, whenever a particular material is planned to be produced. Therefore, for the development of this excellent technology, it is important to know the sol–gel science and technology in producing new materials as well as already achieved applications. This is the purpose of Volume 3. Sol–gel technology is a versatile technology, making it possible to produce a wide variety of materials and to provide existing materials with novel properties. I hope this three- volume Handbook will serve as an indispensable reference book for researchers, engineers, manufacturers and students working in the field of materials. Finally, I would like to express my sincere thanks to all the authors of the articles included in the Handbook for their efforts in writing excellent articles by spending their precious time. As general editor I extend my thanks to Prof. H. Kozuka and Prof. R. Almeida for their difficult work of editing each Volume. I have to confess that this Handbook would not have been realized without enthusiastic encouragement of Mr. Gregory Franklin, senior editor at Kluwer Academic Publishers. Sumio Sakka Preface to Volume 1 (“Sol–Gel Processing”) Volume 1 entitled “Sol–Gel Processing” has 27 chapters, concerning techniques for sol–gel processing of materials of specified shapes, structure, and chemistry, including chemistry of precursors and special processing techniques. Sol–gel technology has already long history, starting with processing of oxide materials including glass and ceramics about 30 years ago. However, since then, the technology has been employed in preparation not only of oxides, but also of non-oxide materials including nitrides, carbides, fluorides, and sulfides as well as oxynitride and oxycarbide glasses. Processing of organic-inorganic materials is now a very active field of research, which has been expanded even in the field of biotechnology as is represented by research on encapsulation of enzymes, antibodies and bacteria. The sol–gel technology started with processing of dense, bulk materials, and great efforts have been made on how to densify porous gels into glasses and ceramics. However, recently sol–gel processing of mesoporous and macroporous materials has also attracted much attention, including materials with well-controlled pore characteristics and highly porous materials, which have excellent chemical and photonic functions. As far as the shape of the products are concerned, powders and fibers are also important products via sol–gel processing, the techniques of which are still in progress. Thin films or coatings are other shaped materials that can be prepared by sol–gel method, which also has already long history. Although dip- and spin-coating methods are very familiar techniques, and science on sol–gel thin film deposition seems to have been established already, there are still technical issues to be solved scientifically for practical fabrication of industrial products. Deposition techniques have now a variety, such as ultrasonic pulverization of aerosols and electrophoretic deposition, and those allowing coating of plastic materials and self-standing thick films have also been developed. There are also special techniques like non-hydrolytic sol–gel technique, which produces unique materials, and UV irradiation that activates the chemical bonds of organic and inorganic components in sol–gel films. Metal alkoxides are the most important precursors employed in sol–gel processing. The development of synthesis technique is often the key for preparing materials of excel- lent functions with sophisticated nanostructures. Then, how about other precursors than alkoxides? Sometimes people say that “sol–gel method” exclusively represents processing that undergoes hydrolysis and polycondensation of metal alkoxides, ending up with for- mation of metalloxane bonds. And also some people say that “sol” represents “colloidal solutions,” not “polymer solutions.” Prof. Pierre-Gilles de Gennes, who was awarded the Nobel Prize in Physics in 1991, wrote a famous book entitled “Scaling Concepts in Polymer Physics” (Cornel University Press, Ithaca, 1979). In that book, first he precisely describes theoretical aspects of “polymer solutions,” which he never calls “sols.” Then, he describes the conversion of “polymer solutions” into “gels,” which he calls “sol–gel transition,” however. No other terms than “sol–gel transition” can represent this kind of conversion, and we should recognize that “sol–gel method” is a processing that passes through “sol–gel transition” irrespective of the kinds of precursors. Science and technology have been greatly developed on (i) metal salt routes for thin film deposition, (ii) polymeric x PREFACE TO VOLUME 1 gel precursors for materials with high homogeneity, and (iii) aqueous precursors ideal for material production in industries. The current state of science and technology on all of these are covered in Volume 1, and the chapters are grouped in eight parts; (1) Sol–gel precursors, (2) Processing of powders and bulk materials, (3) Processing of non-oxide materials, (4) Processing of thin films, (5) Processing of fibers and monodisperse perticles, (6) Encapsulation of organic materials, (7) Processing of catalysts, porous materials and aerogels, and (8) Special techniques used in sol–gel processing. Each chapter has been written by a leading expert in the field. I hope that Volume 1 will provide great information on the current state of sol–gel processing of materials of specified shapes, structure, and chemistry, including chemistry of precursors and special processing techniques. Finally I would like to thank all the authors for spending their precious time, and making much efforts to make great contribution to the Handbook. I also thank Mr. Gregory Granklin, senior editor in Kluwer Academic Publishers and Prof. Sumio Sakka for continuous encouragement. Hiromitsu Kozuka PART I Sol–Gel Precursors This page intentionally left blank CHAPTER 1 The Synthesis and Solution Stability of Alkoxide Precursors Vadim G. Kessler INTRODUCTION The development of sol–gel technology has at very early step put forward a request on development of precursor compounds—chemical substances that have high solubility in organic solvents, are easily transformed into chemically reactive forms of hydrated ox- ides on hydrolysis. They should display considerable stability in solution to guarantee the reproducibility of the materials preparation and, last but not the least, be easy to be purified to provide sufficient chemical quality of the final products. Metal alkoxides, are derivatives of alcohols, ROH, which are usually easily accessible and inex- pensive organic compounds, and are extremely weak as acids, easily removable via hy- drolysis and thermal treatment, leaving high purity hydrated oxides. This circumstance made metal alkoxides the most common candidates for the role of molecular precursors (Veith, 2002; Jones, 2002; Hubert-Pfalzgraf, 2003; Kessler, 2003). The works in this field during the last 20 years, including both the studies of the molecular and crystal structure and the reactivity of these compounds, have considerably changed their image in the eyes of both chemists and the materials scientists: it turned out that sometimes the compounds that are the most stable products in the reactions of synthesis of metal alkox- ides and that were earlier considered to be are in fact oxoalkoxides In many cases, especially for the preparation of complex solutions, including derivatives of several metals, it turned out impossible to use only the derivatives of aliphatic alco- hols, because of their poor solubility, stability or reactivity. This gave rise to development of two new types of alkoxide precursors—derivatives of functional alcohols (alkoxyalcohols and aminoalkohols), on one hand, and heteroleptic alkoxides—including other ligands (such as carboxylate and aminoalkoxide ones) in addition to common aliphatic alkoxide groups, on the other. This change in the understanding of nature ofalkoxide complexes has been reflected even by the titles of the modern textbooks on this topic called “Alkoxide and Phenoxide Derivatives of Metals” (Bradley, 2001) and “The Chemistry of Metal Alkoxides” (Turova, 2002). The complexity of situation has been increased even more by the rise of a still quite small but quickly growing family of alkoholates—highly soluble complexes of metal carboxylates or with func- tional alcohols. The latter do not contain formally the alkoxide ligands but are related to metal alkoxides in many of their properties and find the increasing application in sol–gel technology. The aim of this chapter is to serve as a guide in understanding the principles in the chemical approaches to and stability of the metal alkoxide precursors known already in fact for all the elements of the Periodic Table, excluding only the highly radioactive ones. The major accent is made on the laboratory approaches to the soluble and chemically reactive alkoxide derivatives, applicable in sol–gel technology. The chapter includes synthesis of homometallic precursors, synthesis of heterometallic precursors, solution stability with respect to formation ofoxoalkoxides, solution stabilitywith respect to solvolysis and, finally, a short review summarizing the literature data on individual alkoxide complexes applied as precursors considering separately the homometallic species and the heterometallic ones— so-called single-source precursors. 4 CHAPTER 1 SYNTHESIS OF HOMOMETALLIC PRECURSORS Reactions of Metals with Alcohols (Method 1.1) Direct Reaction in Inert Atmosphere (Argon or Nitrogen). Direct reaction with alcohols with evolution of hydrogen gas and formation of metal alkoxides is possible only for the most electropositive metals such as alkali, magnesium and alkaline earth, rare earth metals and aluminum. In fact the reaction proceeds readily enough at room temperature only for the alkali metals and most acidic alcohols such as MeOH, EtOH or functional ones such as 2- methoxyethanol. Heating is usually indispensable to lead reaction to the completion even in the case of alkaline and alkaline earth metals. The readiness of an alcohol to react with metals may depend strongly on its purity. Thus comparably small water contents, such as less than 0.5 wt.%, may, for example, leave ethanol almost inert in reaction with magnesium or calcium even on reflux, while ethanol with less than 0.01 wt.% water has an observable reactivity toward these metals even at room temperature. To increase the reaction temper- ature in the case of magnesium or rare earth metals, the reaction is often carried out in a mixture of a parent alcohol (usually in the latter case) and a high-boiling point hydrocarbon (toluene or even xylene, in 1:1 or 1:2 volume ratio to alcohol). The reaction requires the use of a catalyst for the alkaline earth metals, rare earth metals and aluminium. The most common approaches are the use of (in the laboratory practice only) the salts of mercury(II) such as or Very small portions of these salts cause amalgamation of the metal surface (and thus clean it from the oxide layer) and facilitate the reaction with alcohols. The larger scale synthesis (and thus the industrial one—in the scope of pollution danger) uses the initial addition of solid iodine (1 g or less per 100 g of alkoxide to be prepared). Formation of metal iodide serves both for cleaning the surface and increases also slightly the acidity of alcohols via formation of solvate complexes. In the case of barium, the application of dry ammonia gas has been reported for this purpose (Caulton, 1990; Drake, 1992). The major factor facilitating the reaction of metals with alcohols is the solubility of the alkoxides formed. Insoluble alkoxides form a protective layer on the surface of the metal and it hinders the reaction. Even the reaction of sodium with in toluene may be almost stopped by the formation of poorly soluble It is to be mentioned that the reaction of metals with an excess of alcohol leads normally, except for aluminium, to formation of solvates with corresponding alco- hols, such as Li(OEt)·2EtOH, etc. (see “Solution Stability with respect to Solvolysis”). To obtain non-solvated species the reaction should be carried out with a stoichiometric amount of alcohol in a different solvent (most often, toluene) (Fisher, 1934). It is also important to notice that the reaction products quite often contain impurities of oxoalkoxides resulting from the presence of residual oxygen in the solvents, or quite complicated redox side reac- tions. If such oxoalkoxides possess considerable thermodynamic stability, as, for example, (Caulton, 1993) or where Ln = Sc, Y, or lanthanides (Hubert-Pfalzgraf, 1997), their formation can not be avoided, and they will in any case be isolated as the major reaction product and may be purified further by recrystallization. The reaction of metals with alcohols in inert atmosphere (except for the alkali ones) leaves very often a dark residue of unreacted small particles of metal or metal sub-oxides. SYNTHESIS AND SOLUTION STABILITY OF ALKOXIDE PRECURSORS 5 This kind of residue is almost inevitable for aluminium and rare earth metals, and can be simply removed by decantation at the end of the reaction. Oxidation of Metals by Oxygen Gas in Alcohol Media. This approach offers only extremely highly soluble and stable alkoxide complexes with rather high resistance to hydrolysis. It was first applied to the preparation of thallium(I) ethoxide, carried out in a Soxhlet filter: The reaction results in formation of a double-layer system, where the bottom layer is a 95 wt.% solution of TlOEt in EtOH, while the upper one is containing almost all T1OH (Turevskaya, 1975). Oxidation by oxygen in air and solvents turned also to be a useful tool in approach to the copper(II) derivatives of aminoalcohols. The hydrolytic stability of Cu(II) aminoalkoxides appears to be its driving force. It is also apparently the same for the reaction of copper metal with alcohols in the presence of N-donor ligands, L, and considerable excess of alcohol (Kovbasyuk, 1998): Anodic Oxidation of Metals (Method 1.2) Considered initially as, in general, a simple extension of the direct reaction with alcohols for less active metals by application of an anodic potential, the anodic oxidation of metals turned to be a much more complicated process. At present, at least three different oxidation mechanisms have been proposed for different groups of metals: (1) (2) The most active metals, such as lanthanides, receive really just a support for the direct interaction with alcohols (2-propanol in this case) from the applied anodic potential supposedly via the elimination of the oxide barrier. The electric current yields (the ratio of the alkoxide obtained to the total charge that passed through the system) often exceed 100%. High concentrations of soluble conductive additives (LiX or where X = Cl, Br), which contaminate the product have to be removed by repeated recrystallization from hydrocarbon solvents. The late transition and main group metals follow the anodic oxidation pathway anal- ogous to that in aqueous solutions. The minimal oxidation potentials in these cases can in fact be very low (up to max. 3.0 V), while higher ones are readily applied to accelerate the process. The anodic reaction consists of dissolution of metal ions in the form of anionic halide complexes, which are later transformed into insoluble alkoxides by reaction with alkoxide anions generated at the cathode, for example (Lehmkuhl, 1975): Cathode: Anode: Solution: 6 CHAPTER 1 Only insoluble alkoxides can be obtained by this method because the soluble ones are normally reduced at the cathode, transforming the process into the electrochemical transport of the metal from anode to cathode. The products again are usually heavily polluted by halide admixtures and should be then washed repeatedly with alcohols to remove adsorbed conductive additives (Hubert-Pfalzgraf and Kessler, 1997). It has, however, been reported that application of amines (such as dipyridyl, phenantroline), giving rather stable insoluble complexes with Cd and Cu alkoxides, allows alkoxides free from halide admixtures to be isolated (Banait, 1986). The early transition metals are dissolved via a complex mechanism involving oxidation of alkoxide ligands with formation of extremely reactive alkoxo-radicals that in turn attack the metal, forming soluble alkoxide complexes already at the anode: (3) The reaction has highest speed in the alcohols displaying highest electric conductivity, such as, for example, MeOH or Low concentrations of conductive additives applied in this case assure high purity of the final product. It is in fact very important to keep the concentrations of the additives in the interval 0.01–0.05 M as the high potentials applied cause the formation of free halogens that oxidize the alcohols and provide finally water as byproduct, leading to formation of oxoalkoxide impurities. The other impurity formed simultaneously is the alkoxide derived from the conductive additive, for example, lithium alkoxides from lithium halides. On interaction with the metal alkoxide they provide heterometallic complexes. Thus, a whole series of different bimetallic Li–Mo and Li–W alkoxides have been isolated and characterized as byprod- ucts of the electrochemical syntheses of M(VI) alkoxides (Kessler and Panov, 1998). Another source of oxoalkoxide impurities is the cathodic reduction, which transforms low oxidation state impurities into oxoalkoxides via subsequent re-oxidation by oxy- gen dissolved in solvents. Following the optimized procedures it is possible, however, to produce rather high quality methoxide derivatives of Nb (Turevskaya, 1995), Ta (Turova and Korolev, 1996), Mo (Kessler, 1993), W (Seisenbaeva and Kloo, 2001) and Re (Seisenbaeva and Shevelkov, 2001). Reactions of Metal Oxides or Hydroxides with Alcohols (Method 1.3) This reaction is useful for preparation of alkoxides from most basic or most acidic oxides and hydroxides. The alkoxides obtained should have quite high hydrolytic and thermal stability, because water formed during the reaction, is removed by distillation as an azeotrope with an aromatic hydrocarbon solvent (usually toluene). In the laboratory practice it can be applied for the preparation of phenoxides of alkali or alkaline earth metals, for example: Recent crystal structure studies have shown that the interaction of basic hydroxides with aliphatic alcohols does not lead to metal alkoxides, but to alcohol solvates of the hy- droxides. For example, the reaction of with MeOH was found to provide (Turova, priv.commun.). This reaction has been in contrast successfully applied for the synthesis of alkoxide derivatives of acidic oxides, as the whole homologous series of vanadium alkoxides (Orlov, 1959; Prandtl, 1913), and for the preparation of a number of hydrocarbon soluble com- plexes with diols of molybdenum(VI) (Bishop, 1979), rhenium(VI) (Edwards, 1998) and SYNTHESIS AND SOLUTION STABILITY OF ALKOXIDE PRECURSORS 7 osmium(VI) (Lehtonen, 1999): The reaction achieves completeness, when the alkoxides thus formed display quite con- siderable resistance to hydrolysis and can then be purified by some efficient technique (distillation for vanadium derivatives and recrystallization for the diolates). Alcoholysis of Metal Derivatives of Weak or Volatile Acids (Method 1.4) The action of alcohols on the metal derivatives of extremely weak and, which is of special importance, highly volatile acids, for example, alkyls, carbides, nitrides, amides, alkyl amides, silazides, hydrosulfides, hydrides etc. provides an approach to high purity samples of metal alkoxides, usually under extremely mild conditions. The reaction is usually carried out in a volatile hydrocarbon solvent (such as hexanes or pentane) and the products are purified by evaporation of the byproducts and the solvent in vacuum, leaving the target alkoxide as the residue. Hydrides can be used as sources of alkaline metal alkoxides (LiH, NaH) in the reactions with halogenated alcohols, such as, for example, to avoid the danger of condensation of Wurtz type (Dear, 1970). Metal alkyls have been applied earlier for the preparation of a number of early transition metal derivatives, for example, CuOMe (Costa, 1965), (Chisholm, 1979), (Razuvaev, 1977), but are themselves extremely unstable and normally not commercially available, which precludes their application in common laboratory practice. The most broadly applied laboratory approach of this type is the reaction of metal alkyl amides, usually bis-(trimethylsilyl)-amides with the stoichiometric amounts of alcohols. The starting reagents even in this case are not available commercially, but can be obtained more-or-less easily by reaction of the corresponding metal chlorides with commercially available in anhydrous diethyl ether: can then be purified—for the main group derivatives (for application in the synthesis of alkoxides, see Zn (Goel, 1990), Cd (Boulmaaz, 1992), Pb (Matchett, 1990; Papiernik, 1989), Bi (Massiani, 1990; Goel, 1990))—by sublimation direct from the reaction mixture, after removal of the in vacuum, and—for the early transition metal compounds (Cr(II), Mn(II) (Horvath, 1979)), after the removal of ether—by the extraction from the residue with pentane or hexanes, separating LiCl by decantation. It should be mentioned that this approach is hardly practically applicable for the synthesis of the derivatives of late transition metals such as Co, Ni or Cu because of poor stability of their amide derivatives (Bryndza, 1988). It should be mentioned that the reaction of metal chlorides with alcohols could not be applied for the synthesis of metal alkoxides—precursors of oxide materials. Its products are usually quite complex mixtures of alkoxide chlorides and alcohol solvates of metal oxochlorides (Turova, 2002; Turevskaya, 1989). 8 CHAPTER 1 Formation of alcoholates—solvate complexes with functional alcohols can be consid- ered as a variety of this synthetic approach. Metal or carboxylates are reacted with amino- or alkoxy-alcohols in stoichiometric amounts in organic solvents (both non- polar such as toluene or hexane or polar, such as methanol or ethanol can be applied (Williams, 2001; Seisenbaeva, in press): The advantage of the alcoholate complexes lies in their high solubility in organic solvents. They provide also a possibility to avoid more complicated dehydration procedures neces- sary for the derivatives of late transition metals to be used for the preparation of complex solutions together with metal alkoxides. Metathesis Reactions with Alkali Alkoxides and Ammonia or Amines (Method 1.5) This approach remains the most commonly applied in the synthesis of metal alkoxides. The starting reagents are the anhydrous metal halides, most often chlorides, or other anhydrous metal salts, such as nitrates or acetates: The traditional technique using ammonia gas has been applied for the preparation of the alkoxides of titanium (Demarcay, 1875), zirconium and hafnium (Bradley, 1952), cerium(IV) (Bradley, 1962), niobium and tantalum (Bradley, 1956). In this approach a halide or a pyridinium halogenometallate salt, for example, is dissolved in a mixture of toluene with the parent alcohol, and ammonia gas is bubbled through the solution for several hours. The voluminous precipitate of is removed by filtration and washed with the alcohol on the filter to improve the yield of the soluble alkoxide. To avoid the use of ammonia gas and simplify the procedure as a whole and specifically the separation of the ammonium salts, there has been proposed to use the amines such as triethylamine or pyridine in the same purpose. This route provided access, for example, to the stable samples of from (Chisholm, 1984) and those of from (Edwards, 1980). It is necessary to mention that neither ammonia nor amines can be applied for the preparation of pronouncedly basic alkoxides— derivatives of alkali, alkaline earth or rare earth metals (their formation is impossible in the presence of acidic ammonium salts). A specific problem in application of ammonia or amines lies in the need of intro- ducing a metal halide into this reaction as a solution in a solvent mixture including the parent alcohol. Strong Lewis acids such as metal halides are at room temperature prone to convert the alcohols, especially the ramified ones, into alkyl halides and transforming themselves into oxohalides (Turova, 2002). This side reaction decreases the yield of the target products and, when the tertiary (Bradley, 1978) or aromatic (Niederberger, 2002) alcohols are used, can lead (in not completely anhydrous conditions) even to formation of oxides or hydrated oxides. This effect can be avoided if the halides are introduced as solutions in aprotic solvents (toluene, ether, THF) into the solutions of alkali alkoxides, for SYNTHESIS AND SOLUTION STABILITY OF ALKOXIDE PRECURSORS 9 example: Strong cooling is always recommended at the initial step of this process. Then the reaction mixtures are usually warmed to room temperature after the complete addition of the halide, and then even often subjected to reflux in order to destroy the possible heterometallic im- purities. The heterometallic impurities are sometimes so stable, for example, that they can even be distilled in vacuum without decomposition (Bartley, 1958). It is important to avoid the possible deviations in the reaction stoichiometry, but even the perfect one does not guarantee the purity of the obtained samples because the alkoxide chlorides or bimetallic alkoxide chlorides can sometimes display really high stability. For example, or have been isolated as the major products in the reaction of the corresponding trichlorides with three equivalents of NaOR (Evans, 1988; Andersen, 1978). In many cases larger halide ligands (Br or I instead of Cl) or a larger alkaline metal atoms (K instead of Na or Li) can help to avoid the side reactions of this kind, for example in (Kritikos, 2001): Metal acetates have been used as reagents in the metathesis with alkali alkoxide mainly in order to produce the main group metal derivatives, for example, the homologous series of lead alkoxides (Papiernik, 1989; Turevskaya, 1982). The reaction produces the insoluble sodium acetate that can be removed by filtration or decantation. It should be mentioned that, when carried out in toluene (on reflux) it could very easily lead to oxoalkoxide derivatives via ester or ether elimination side reactions (see Method 1.3). Application of the nitrate complexes has been proposed in the metathesis-based ap- proaches to the derivatives of Ce(IV) in the view of their much higher stability and com- mercial availability (Gradeff, 1985). It is necessary to mention that during the development of metathetic approaches a num- ber of alkoxylating agents other than the alkali alkoxides have been tested in this purpose. For example, the gas phase co-condensation of volatile metal fluorides or chlorides with alkylsiliconalkoxides has been reported for the preparation of W and Re (Jacob, 1982; Bryan, 1991). This technique requires a special equipment and provides rather small quantities of these products that can be obtained much more easily by the anodic oxidation of the corresponding metals. Another example of a different alkoxylating agent is the soluble which has been used to produce the methoxides from the metal fluorides (Bryan, 1991). Alcohol Interchange Reactions (Method 1.6) are often very flexible. It is important to keep this fact in mind in developing the procedures for sol–gel applications of the alkoxide precursors: when dissolved in other alcohols than the parent one they would undergo a ligand exchange that can change their molecular structure and hydrolytic properties and, in case of the heterometallic compounds (to be used as single-source precursors), even their chemical compositions. This method is useful as a synthetic procedure, when an efficient synthetic approach or The equilibrium reactions of metal alkoxides with alcohols: 10 CHAPTER 1 commercial availability provide a different homologue than the one desired for further applications. The completeness of the reaction is achieved more easily if the desired product possesses much lower solubility in the new parent alcohol, for example: or if the alcohol to be introduced has a considerably higher boiling point, which facilitates the removal of the other alcohol, formed in the reaction, by vacuum distillation, for example (Johansson, 2000): The treatment with the new alcohol must be repeated several times (with complete disso- lution of the crude intermediate product) to insure the completeness of transformation. It should be mentioned that the completeness in the exchange of the alkoxide groups might not always be achieved: the stability of molecular structures including small bridging ligands or functional chelating alkoxide ligands prevent in many cases the possibility to replace them, for example (Bradley, 1978; Johansson, 2000): Replacement can also be achieved by other sources than alcohols, for example, esters. This is of interest if the boiling points of the two alcohols are very close (e.g., and b.p. 82.4°C) or when the alcohol to be applied is highly unstable (e.g. silanols, unsaturated alcohols etc.): The reactions with esters are most often carried out in aromatic hydrocarbon solvents to decrease the reaction temperature by removing a more volatile azeotropic mixture of a new ester with, for example, toluene or (in the past) benzene (Bradley, 1959, 1978). A specific class of the ligand exchange reactions and, in some cases, of alcohol interchange improves the solubility and behavior in the hydrolysis and subsequent gelation processes. The reaction can in general be written as follows: where HZ represents aminoalcohols or other functional alcohols, or carboxylic acids. These reactions have been rather thoroughly studied for the derivatives of M(IV) such as titanium, zirconium and cerium and are described in a number of recent review articles (Ribot, 1991; Hubert-Pfalzgraf, 2003; Jones, 2002). Redox Processes in Approach to Alkoxide Precursors (Method 1.7) The redox reactions do not in fact belong to the common approaches in prepara- tion of precursors for the sol–gel technology. The only example worth noting here is the oxidation of low-valent chromium derivatives (dibenzene-chromium, by the t- butylperoxide, providing access to chromium(IV) alkoxides—highly soluble and volatile compounds (Krauss, 1967). On the other hand, the redox reactions are in many cases responsible for the transformation of metal alkoxides in solutions leading to formation of SYNTHESIS AND SOLUTION STABILITY OF ALKOXIDE PRECURSORS 11 oxoalkoxides and will be discussed below in “Solution Stability with respect to Formation of Oxoalkoxides”. SYNTHESIS OF HETEROMETALLIC ALKOXIDE PRECURSORS The special interest in heterometallic alkoxide complexes is due to the possibility of their application as single-source precursors in the preparation of complex inorganic materials (oxides, sulfides, metal alloys and even nanocomposites) (Veith, 2002). The single-source precursor represents a compound containing the necessary elements in desired stoichio- metric ratio. Synthesis and properties of the heterometallic alkoxides have been described in detail in a number of recent reviews (Veith, 2002; Jones, 2002; Hubert-Pfalzgraf, 2003; Caulton, 1990; Kessler, 2003). The formation of heterometallic complexes in general can occur due to one of the three following factors: (1) (2) (3) Lewis acid–base interaction (exploiting the difference between two or several metal atoms in electronegativity, which permits to consider one metal center as a stronger acceptor of the electron density and the alkoxide or other ligands at the other as a better donor of it). Formation of a heterometallic metal-metal bond, which in this case should also provide a donor-acceptor interaction. Isomorphous substitution, which might in some cases not lead to formation of the true heterometallic species, but provides in any case the homogenization at the molecular level. The synthetic approaches to heterometallic complexes will be classified here below accord- ing to these three principles providing their formation. Heterometallic Alkoxides formed via Lewis Acid–Base Interaction Complex Formation Between Two Alkoxides (Method 2.1). The pronounced Lewis basicity of the alkoxide ligands of the alkali and alkaline earth metal alkoxides ex- plains their capacity to form heterometallic complexes in solution with the vast majority of high-valent transition or main group metal alkoxides, for example: The chemical composition of the products is determined by a number of important factors, such as the nature of metal atoms involved, the nature of alkoxide groups, the ratio of homometallic reactants applied and, in certain cases, even on the solvent in which the reaction is carried out. For example, the reaction of barium ethoxide and titanium ethoxide in ratio in oxygen-free solutions provides different products in alcohol and in hydrocarbon media (Yanovsky, 1995; Kessler, 1994): Formation of different complexes at different ratios of reactants in the same solvent (THF in this case) can be illustrated by the examples from barium–zirconium isopropoxide system (Vaartstra, 1991) (the authors used barium metal in the presence of solvating alcohol, but application of the ready alkoxide gives the same result (Turevskaya, 1995)): 12 CHAPTER 1 Synthesis of heterometallic complexes by direct interaction of homometallic alkoxides has been reported in many cases also for the rare earth metals, but in this case it is neces- sary to keep in mind that the commercial usually contain the oxoalkoxide complex, as their major component. The reactivity of the latter toward other alkoxides is comparably low, and prolonged refluxing in toluene or the reaction in a melt is recommended to insure the completeness of transformation (Poncelet, 1989). The only reaction between the two high-valent metal alkoxides, not involving specific mech- anisms with formation of oxoalkoxides, is the formation of the aluminium and hafnium isopropoxide (Turevskaya, 1997): It is also important to notice that this reaction takes place even in the alcohol media, but gives in this case far not quantitative yields of the product, whose formation took several days. Metathesis of a Metal Halide with a Bimetallic Alkoxide of another Metal and an Alkali Metal (Method 2.2). This approach has been proposed for the case, where the alkoxide of one of the metals is not easily accessible or is an insoluble and inert solid as, for example, the alkoxides of late transition and some main group metals (Mn(II), Fe(II), Fe(III), Co, Ni, Cu, Zn, Cd, Sn(II), Pb(II)). It has been applied also for the preparation of heterometallic derivatives of rare earth elements (see Bradley, 1978,2001; Turova, 2002). It is necessary to take into account that the reaction: not always follows the simplified reaction formula given by the equation (1-34). The so called “alkoxometallate ligands”, existing in the structures of heterometallic alkoxides of alkali metals, such as or are not the ultimately thermodynamically stable ionic units. They are therefore very rarely just transferred by this reaction from an alkali metal cation to a less electropositive metal cation. The deviations may occur both due to formation of more stable oxocomplexes (Boulmaaz, 1994) and even more stable homoleptic alkoxide complexes (Kessler and Daran, 1994; Yanovsky, 1994), for example: A serious problem that can also be associated with application of this technique lies in the possibilities of formation of byproducts including the halides or alkali metals (or even both as in case of formation of (Hubert-Pfalzgraf, priv. commun.). A family of the techniques described below is based on the introduction of new ligands such as oxo-groups or different organic residues such as carboxylate or aminoalkoxides groups. Successful development of new approaches of this kind needs prediction of the chemical composition and even the structure of the new complexes to be prepared and choice of the proper reaction stoichiometry. This prediction can be done using the Molecular Structure Design Concept, described in detail in Kessler (2003), and including the following steps: 1. 2. Choice of the structure type to be used (see Fig. 1-1), Calculation of the necessary number of the donor atoms, SYNTHESIS AND SOLUTION STABILITY OF ALKOXIDE PRECURSORS 13 Figure 1-1. Schematic views of the most important building blocks in the structures of metal alkoxide aggregates. Choice of the ligands with proper composition and sterical requirements that provide both the right number of donor functions and the protection of the chosen core of metal and donor atoms (placing the metal atoms into the thermodynamically preferred coordination polyhedra). The most complete classification of the stable structure types for the alkoxide complexes can be found in (Turova, 2002). The practical principles in application of this concept lie either in decreasing the number of donor atoms by replacing two OR groups with an oxo- ligand and thus increasing the strength of the Lewis acids involved or by providing some additional donor atoms from bidentate (usually) chelating ligands, which are necessary to support the chosen structure type. Microhydrolysis of Alkoxides of Different Metals in Solution (Method 2.3). This approach has been first applied to access to the heterometallic alkoxides of bismuth, because the homoleptic usually do not form any heterometallic complexes with the alkoxides of other high-valent metals (Parola, 1997): This reaction was carried out successfully in an alcohol media. It is necessary to men- tion, however, that alcohols very often destroy the heterometallic complexes derived from elements with close Lewis acidity. The efficiency of microhydrolysis as approach to het- erometallic species is much better in inert media (hydrocarbon solvent). Micropyrolysis of Alkoxides of Different Metals in Solutions (Method 2.4). When an alkoxide derivative can easily decompose thermally in solution forming an oxoalkoxide (see “Solution Stability with respect to Formation of Oxoalkoxides”), this reaction might be exploited in approach to heterometallic species. This reaction is useful especially for the synthesis of heterometallic derivatives of molybdenum (Johansson, 2000; Johansson and Kessler, 2000) and zirconium (Kessler and Borissevitch, 1998) (see also in 3. 14 CHAPTER 1 Method 2.5), for example: It is important to underline that this approach is not applicable in the preparation of temperature-sensitive alkoxide derivatives such as those of Ni, Cu, Zn, Cd, Pb or Bi, where the heating results in formation of metals or oxides (see Elimination”). Interaction of Metal Complexes with Organic Ligands and Metal Alkoxides or Chemical Modification of Complexes in Solutions (Method 2.5). This approach pro- vides an important alternative to the Method 2.2 in the preparation of derivatives of late transition elements (the homometallic alkoxides of those being insoluble and not reactive polymeric solids). The reaction stoichiometry and conditions are dependent on the nature of reactants and on the composition of the product to be obtained. In some cases the reaction is facile and provides the desired products as the result of mixing the reactants in proper ratio (usually in toluene) and subjecting them to short reflux, for example (Boul- maaz, 1997; Kessler, 2003): Preparation of the heterometallic complexes of zirconium from re- quires prolonged refluxing in order to generate the reactive oxo-species (see Method 1.3) (Hubert-Pfalzgraf, 1994): When the number of additional donor atoms of chelating ligands remains insufficient for the stabilization of the proper structure, an additional modification by chelating ligands is required (Kessler, 2003; Kessler and Parola, 2003): The procedure should be carried out in separate steps including mixing of the homometallic reagents, refluxing them in toluene, cooling down below the room temperature and only then—the addition of the necessary extra amount of acidic chelating agent or carboxylic acid). Introduction of an organic acid into a warm solution would result in an instant gelation (due to the condensation with released alcohols producing water in situ) instead of formation of heterometallic mixed-ligand complexes. Heterometallic Alkoxides formed via Formation of Heteronuclear Metal–Metal Bonds or Isomorphous Substitution No specific techniques have been elaborated for these particular cases. The majority of compounds of these two classes are obtained by complex formation between homometallic species (most often in hydrocarbon solvents) at ambient conditions or via short-term reflux (Method 2.1). SYNTHESIS AND SOLUTION STABILITY OF ALKOXIDE PRECURSORS 15 The formation of a metal–metal bond requires interaction of electron rich low-valent derivatives of one metal with electron deficient, high-valent derivatives of the other, for example (Chisholm, 1981; Kessler and Seisenbaeva, 1995): In the simplest cases, the isomorphous substitution can be achieved via mixing the isostruc- tural but chemically different species in solution (Hubert-Pfalzgraf, 1978): When the bonding parameters of two metal atoms are analogous, but the molecular aggre- gates observed for the homometallic species are different, the formation of heterometallic complexes, following the structure type observed for only one of two elements, can be achieved sometimes applying the solution thermolysis (Method 2.4), for example (Seisen- baeva and Shevelkov, 2001): It is important to mention that the same reactants (even in the same ratio) can pro- vide different heterometallic products if different reaction temperatures are applied. For example, the interaction between and in toluene at room temper- ature provides while the reflux of the same reaction mixture gives (Shcheglov, 2002). SOLUTION STABILITY WITH RESPECT TO FORMATION OF OXOALKOXIDES One of the most important requirements put on application of molecular precursors in different technological procedures, and in sol–gel technology in particular, is the de- mand of stability during the application procedure itself and on storage. It is, therefore, very important to know, what mechanisms can lead to the changes in the properties and molecular structures of precursors and what measures should be undertaken to support the stability of solutions and to achieve reproducibility in their application. We distinguish here between the reactions leading to transformation of alkoxide ligands resulting in for- mation of oxoalkoxides discussed in this part and the solvent-supported reactions of ligand redistribution (solvolysis), presented in the next one. The most important reaction pathways leading to oxoalkoxides are partial hydroly- sis, oxidation by oxygen from the atmosphere and dissolved in solvents, ether and ester elimination, elimination, and thermal desolvation. Partial Hydrolysis Almost all metal alkoxides (with the exception of the kinetically rendered derivatives of precious metals and the most sterically hindered complexes) are extremely moisture sensitive. Interaction with water molecules from moist atmosphere or not properly dried solvents results in drastic changes in molecular complexity and chemical composition, for example (Ibers, 1963; Bradley, 1968): 16 CHAPTER 1 Different hydrolysis ratios, (number of water molecules per correspondent alkoxide formula unit, provide different aggregates. For example, for the titanium ethoxide, different conditions of partial hydrolysis have also provided such aggregates as (Day, 1991), (Day, 1991) and (Mosset, 1988). It is not always pointed out directly, but in the complex solutions the microhy- drolysis can turn out rather selective, transforming into oxoalkoxide species only one or few of the components and changing the stoichiometry of molecular precursors. The risk of uncontrolled hydrolysis should be then eliminated as thoroughly as possible: all the operations in the preparation and weighing the samples of alkoxides are to be carried out in dry atmosphere using a Schlenk line or a dry box. The solvents dried according to reliable techniques (see Errington, 1997) have to be applied. It is necessary to take into account that the water molecules can appear not only due to improper drying of the system, but can even be products of different side reactions. For example, they are formed on modification of (warm) alkoxide solutions with carboxylic acids (Stenou, 1998): Strict control of the reaction temperature and stoichiometry (modification ratio, number of modifying ligand molecules per alkoxide formula unit) is very important to insure the reproducibility of further application of such solutions. Oxidation by Oxygen from Atmosphere or Dissolved in Solvents The alkoxide groups possessing a hydrogen atom in i.e. at the first carbon atom connected to the oxygen one—all primary and secondary ones—react with oxygen in basic media, forming the products of oxidation such as carbonyl compounds and water (Turova, 2002). This means that the homoleptic (alkoxide-only) derivatives of alkali, alkaline earth and rare earth metals are very sensitive in solution to the presence of even very small traces of oxygen. The reaction is proceeding with a radical mechanism, which results in a very intensive yellowish orange (in case of high concentrations of both basic alkoxide and oxygen–even brown) coloration of solutions. The reaction speed increases with the basicity of media (alkali > alkaline earth rare earth elements). It is much higher in alcohols than in hydrocarbon solvents, and much higher for homometallic than for the heterometallic derivatives of these elements. Really rigorous precautions can (under laboratory condi- tions) provide formation of the samples free from oxidation products. For the alkaline earth or rare earth elements these are always solvates with O-donor ligands such as alco- hols, THF or dme (dimethoxyethane) (Turova, 2002) as desolvation itself produces the oxo-species (see below). In order to provide the samples more stable in solutions there have been reported numerous attempts of their chemical modification using acidic ligands such as (Arunasalam, 1995) or aminoalkoxides (Poncelet, 1991). One of the major trends in the recently reported sol–gel preparations of the derivatives of these elements lies in application of other organic precursors than alkoxides SYNTHESIS AND SOLUTION STABILITY OF ALKOXIDE PRECURSORS 17 2-ethylhexanoates) for the preparation of solutions or application of stable heterometallic alkoxide or heteroleptic complexes (Kessler, 2003; Veith, 2002; Hubert-Pfalzgraf, 2003). Ether and Ester Elimination The ether elimination reaction is a spontaneous decomposition process characteristic of, in the first hand, high-valent early transition elements, such as Mo, W, Re, Nb and possibly Ta. The reaction mechanism involves at the first step a redistribution of electron density with a heterolytic cleavage of a O–C bond as a result. The liberated alkyl-cation is transferred to a neighboring terminal alkoxide group, forming an ether molecule: The reaction speed increases in the series of homologues It is catalyzed by Brönsted acids (proton donors), neutralizing the negative charge, which ap- pears at the oxygen atom due to heterolytic bond cleavage (Bradley, 1956; Turova, 2002; Gibson, 2002). Molybdenum alkoxides, for example, are transformed into “molybdenum blues” (reduced and often hydrated molybdenum oxides) on action of water or organic acids. It is also accelerated by the basic (alkali, alkaline earth) metal alkoxides, facilitating the departure of the liberated cation. Thus the alkoxides of Mo(VI) and W(VI) are converted directly into mixed oxides (inorganic molybdates and tungstates) on action of alkali or alkaline earth metal alkoxides in excess (Turova, 1991). The ether elimination re- action is also strongly accelerated by heating, which should then be avoided in treatment of solutions containing the derivatives of elements stated above. Heating under solvothermal conditions (at temperatures over 200°C and at high pressure) has been proved to provide oxoalkoxides via ether elimination even for the derivatives of titanium (Mosset, 1988). The ether elimination was considered also to be the reason of formation of oxoalkoxide derivatives of heavy main group elements such as Pb and Bi (Papiernik, 1989; Turevskaya, 1996). An analogous reaction mechanism has been supposed for the reaction of ester elimi- nation that was documented in the systems containing alkoxide ligands together with the carboxylate ones (Tyssie, 1973; Caruso, 1995). This reaction has even been applied for the preparation of single-source heteroleptic oxoalkoxide precursors, for example (Caruso, 1995): It is necessary to note that, if heating is applied for the homogenization of solutions prepared from metal alkoxides together with metal carboxylates, it is necessary to control very thoroughly the thermal conditions to insure the reproducibility in subsequent sol–gel applications. This family of processes includes two kinds of reactions: transfer and transfer. The reaction of transfer is typical for strongly electronegative high-valent metals such as V(V) (Nabavi, 1991; Kessler, 2000), Mo(VI) (Kessler and Shevelkov, 1998), rhenium (detected even in +III oxidation state (Hoffman, 1989)) and Elimination 18 CHAPTER 1 precious metals and also Bi(III) (and, probably, Sb(III), Pb(II)). It generates at the first step a metal hydride (transformed then into low-valent (oxo)alkoxide or even elementary metal) and a carbonyl compound: The transfer accompanies often the other partial decomposition processes such as hydrolysis (Kessler, 2000) and ether elimination (Kessler and Shevelkov, 1998): It is necessary to avoid heating of solutions of heavy late transition and heavy main group elements (especially important for Bi(III)!) to avoid the formation of metal powders via reduction through the transfer mechanism (Parola, 1997). The reaction of transfer is well known in organic chemistry as the reaction of dehydration of alcohols (especially typical for the ramified ones). It is discussed in the majority of textbooks on organic chemistry, and is supposed to be catalyzed by acidic reagents and may be the reason of spontaneous decomposition of many alkoxides on microhydrolysis with formation of oxo-species with much higher yield than the one to be expected from the added amount of water. Thermal Desolvation Derivatives of certain metals being of primary interest in sol–gel technology, for exam- ple, zirconium, hafnium and lanthanides, do exist as non-oxo species exclusively as solvates with alcohols or other O- or N-donor ligands (such as or An attempt of their desolvation, sometimes even under rather mild condi- tions (for lanthanides) (Helgesson, 1991) leads to formation of oxoalkoxides. The samples of “non-solvated” or (Turova, 2002; Turevskaya, 1995) are in practice always contaminated with oligonuclear oxoalkoxide impurities. The same process is responsible apparently for the formation of heterometallic oxoalkoxides when, for example, the zirconium alkoxide solvates are reacted with other precursors (alkoxides, and, partly at least also due to this reaction, carboxylates) in hydrocarbon media on heating (Shmid, 1991; Hubert-Pfalzgraf, 1994): SOLUTION STABILITY WITH RESPECT TO SOLVOLYSIS The molecular composition of the solutions aimed for applications in sol–gel technology is of primary importance for the reproducibility of the results in materials preparation. SYNTHESIS AND SOLUTION STABILITY OF ALKOXIDE PRECURSORS 19 Simple dissociation into components (for the heterometallic species) is quite rare. Only very few cases, where the homometallic complexes were found as major species in solutions obtained from the heterometallic solid reactants, have been reported, for example, the dissociation of into and (Boulmaaz, 1991). Changes in the nature of precursors can much more often be caused by the ligand transfer reactions, facilitated in solutions. It is possible to distinguish two principal directions in these processes leading to separation of a derivative with one ligand type in one case and to separation of homometallic derivatives in the other. The first type of transformations is characteristic of both homo- and heterometallic derivatives and leads to formation of stable fully substituted complexes with chelating ligands from the mixed-ligand derivatives. It has been first observed and described in detail by Wengrovius (1986) for the alkoxide derivatives of aluminium and then was studied also by Errington (1998) for the same type of titanium derivatives: Formation of poorly soluble and extremely thermally stable is quite a con- siderable danger in application of for the modification of alkoxide solutions. The latter is a powerful tool in stabilization of sols and gels produced from very hydrolysis sensitive alkoxides and also in regulation and improvement of the particle size in xerogels produced from them (Livage, 1992). All the studies of the kinetics of this reaction show (Fig. 1-2) that it has normally only minor effect at room temperature and below, but is strongly accelerated by heating. One should then avoid the subsequent heat treatment of solutions subjected to modification by or containing the species that can be transformed to stable (like via solvolysis. The other type of transformations on solvolysis is due to the fact that solvents, acting as Lewis bases can cause the reactions, leading to separation of homometallic species and often to decomposition of heterometallic single-source precursors. Application of alcohols and functional alcohols as solvents improves the sol–gel preparation procedures due to increased viscosity and good surface affinity (in preparation of films and coatings) but can sometimes keep the homometallic species in complex systems away from the chemi- cal interaction with each other or lead to formation of homometallic products from the heterometallic ones (Labrize, 1996; Kessler and Parola, 2003), for example: The transformation of the UV–Vis spectrum of the reaction system above (Fig. 1-3) shows clearly the slow change of coordination geometry for nickel(II) atoms from purely tetra- hedral (bluish violet color, spectrum 1) to purely octahedral (bright green, spectrum 4) revealing the transformation of the initial heterometallic species into solvated homometallic ones. The possibility of such transformations should be taken into account in considera- tion of solution stability: the homogeneity can be hold either via application of solvents providing increased solubility and/or viscosity (functional alcohols) or via application of hydrocarbons, in the first hand, toluene, as solvent in preparation of coatings and films. INDIVIDUAL ALKOXIDE COMPLEXES APPLIED AS PRECURSORS In this chapter we will try to summarize shortly the information on complexes that have been applied or proposed for application as molecular precursors in sol–gel processes. In 20 CHAPTER 1 Figure 1-2. Transformation of NMR spectra of the solutions of on beating: * Indicates the top corresponding to formed on solvolysis. some cases even the data on alkoxide precursors, proposed for MOCVD applications but comparably easy to prepare in the laboratory conditions, are also included. The data are presented in the table form (see Tables 1-1 and 1-2) and include the chemical formulation, appearance and physicochemical properties, characterization techniques applied and the literature references. Homometallic Precursors The homometallic derivatives listed in Table 1-1 are classified according to posi- tions of the corresponding metals in the Periodic Table, starting from the alkaline met- als (Group 1) and finishing with heavy main group elements (Group 15). Abbrevia- tions applied are cryst. for a crystalline solid, hc for hydrocarbon solvents, ROH for SYNTHESIS AND SOLUTION STABILITY OF ALKOXIDE PRECURSORS 21 Figure 1-3. Transformation of the UV–vis spectrum of the solution of in toluene: 2-propanol = 1:1 volume ratio on storage (1—-freshly prepared sample, 2—1 h of storage, 3—3 h of storage, 4—2 weeks of storage). alcohols. Abbreviation comm. stays for commercial products available from large-scale producers such as Aldrich, STREM or Fluka. Well-characterized products, whose existence is supported by literature references are provided also by Epichem Ltd. Broader choice of more specific precursor products is offered by Gelest Inc. and Chemat Inc. The chemical nature and composition of the products not otherwise reported in literature is a matter of responsibility of the producers. Heterometallic Precursors The preparation and properties of heterometallic precursors are described in a number of very recent reviews providing a more general insight in the Chemistry of these com- pounds (see Caulton, 1990; Veith, 2002; Jones, 2002; Hubert-Pfalzgraf, 2003; Kessler, 2003). The heterometallic derivatives listed here below in Table 1-2 are, again, exclusively those proposed for application in sol–gel technology. They are classified by the nature of the more electropositive component and by the nature of materials, for which they serve as precursors. It should be mentioned that heterometallic precursors are known for far not all complex materials prepared by sol–gel technology. For example, an extensive search for such precursors for compounds displaying the Giant Magnetoresistance (GMR), such as has so far not provided any such complexes. In many other systems there have been found numerous heterometallic complexes but possessing compositions inappropriate for the preparation of desired materials. It is usually better to avoid the for- mation of such species (named in a list at the end of each block devoted to the discussed material as not single-source precursors, “not SSP”) as they can cause contamination with impurities possessing correspondent inappropriate compositions (as, for example, 22 CHAPTER 1 SYNTHESIS AND SOLUTION STABILITY OF ALKOXIDE PRECURSORS 23 24 CHAPTER 1 SYNTHESIS AND SOLUTION STABILITY OF ALKOXIDE PRECURSORS 25 26 CHAPTER 1 SYNTHESIS AND SOLUTION STABILITY OF ALKOXIDE PRECURSORS 27 28 CHAPTER 1 SYNTHESIS AND SOLUTION STABILITY OF ALKOXIDE PRECURSORS 29 in obtained from ethoxide precursor solutions containing as one of the major species present). Table 1-2 reports only the properly structurally characterized compounds. References Abbati G.L., Caneschi A., Cornia A., Fabretti A.C., Gatteschi D. synthesis, solid-state characterization and reactivity of a new molecular ferric wheel. Inorg. Chim. 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A soluble copper(II) alkoxide for solution-based syntheses of yttrium barium copper oxides
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