Structural investigations on the hydrolysis and condensation behavior of pure and chemically modified alkoxides. 1. Transition metal (Hf and Ta) alkoxides

Structural investigations on the hydrolysis and condensation behavior of hafnium and tantalum alkoxides were for the first time performed by means of X-ray absorption fine structure and Raman spectroscopy. The studies reveal that both of the alkoxides are dimeric in nature and instantaneously undergo hydrolysis and condensation reactions upon the addition of water. The results indicate that the chemical reaction of the alkoxides with acetylacetone occurs immediately with an increase from 6- to 8-fold coordination around the metal. As a consequence of the coordination by a bidentate ligand, hydrolysis and condensation reactions are hindered in solutions of the chemically modified Hf(OnBu)4 and Ta(OEt)5. Furthermore, the investigations demonstrate that the structure of metal alkoxides is not altered after mixing with prehydrolyzed silicon tetraethoxide, and even after 48 h, both of the species remain as separate entities in the mixture. The addition of water to this mixture starts the hydrolysis and condensation reactions instantaneously and leads to the formation of a M-O-M homocondensation product due to the different reactivity of the two alkoxides.     

EXAFS study of zirconium alkoxides as precursors in the sol-gel process: II. The influence of the chemical modification

EXAFS studies of primary zirconium alkoxides Zr(OR)₄ with OR = n-propoxide and n-butoxide, dissolved in their corresponding alcohols and chemically modified with acetylacetone (Hacac) and acetic acid (HOAc) in different molal ratios, are presented. The EXAFS-spectroscopic results, supported by FT-IR-studies, indicate a different chemical behavior of the complexing agents. In contrast to acetylacetone, the addition of acetic acid does not change the oligomeric structure of the zirconium alkoxides. Amazingly, the modification with acetic acid leads, in comparison to the pure compounds, to a shortened metal centre distance, whereas in the reaction with acetylacetone the Zr-Zr distance is not changed. With the determined distances and a rough quantitative inclusion of the coordination numbers it was possible to deduce detailed structure models.     

FTIR analysis of the hydrolysis rate in the sol–gel formation of gadolinia-doped ceria with acetylacetonate precursors

This study reveals that Fourier transform infrared is an effective analytical tool in probing the extent of hydrolysis of cerium and gadolinium acetylacetonates dissolved in methanol. It is found that these acetylacetonates have relatively fast hydrolysis rates at the early stage of hydrolysis (<6 h). However, their hydrolysis rates become very slow beyond 6-h of hydrolysis and decrease to near zero after 55-h of hydrolysis. As a result, hydrolysis cannot be completed even after 170 h at room temperature. These phenomena suggest that the first and second acetylacetone ligands chelated to Ce³⁺ and Gd³⁺ ions can actually be hydrolyzed with relative ease at room temperature, similar to alkoxide ligands. It is the last acetylacetone ligand that is very difficult to be hydrolyzed.     

Spray drying of sol-gel precursors for the manufacturing of PZT powders

The preparation of fine PZT-powders via spray drying is described. The precursor solution was composed of Zr- and Ti-n-propoxide and lead acetate trihydrate or basic lead acetate. The hydrolysis and condensation reactions were controlled by the addition of acetylacetone as chelating agent.Depending on water content and solvent in the initial sols, spray dried micronsized gel powders with different morphologies and hydrolytic reactivities were obtained.     

An acetic acid/water based sol-gel PZT process II: Formation of a water based solution

The chemical reactions underlying the formation of a water based alkoxide sol gel process for lead zirconate titanate thin films using solutions of Ti and Zr alkoxides are outlined. Titanium isopropoxide and zirconium propoxide are chemically modified by acetic acid in order to use water as a solvent. The nature of the hydrolysis reactions in the presence of acetic acid is reviewed. It is shown that the formation of ions and charged polymeric species reduces their rates of condensation and aggregation.     

New insight in the role of modifying ligands in the sol-gel processing of metal alkoxide precursors: A possibility to approach new classes of materials

This paper summarizes recent literature data and presents new experimental data on the mechanisms of chemical modification, hydrolysis and polycondensation of the alkoxides and demonstrates possibilities to approach new classes of materials, exploiting these mechanisms. Low reactivity of silicon alkoxides is improved by either basic catalysis exploiting an S_N2 mechanism or acidic catalysis facilitating a proton-assisted S_N1 mechanism as well as by modification with chelating ligands. Metal alkoxides are much stronger Lewis bases compared to silicon alkoxides and the acidity of water is strong enough to achieve their rapid hydrolysis via proton-assisted S_N1 pathway even in the absence of additional catalysts. Introduction of the modifying chelating ligands is leading generally to increased charge distribution in the precursor molecules. Modifying chelating ligands are also appreciably smaller than the alkoxide ligands they replace. The modification with chelating ligands is thus facilitating the kinetics of hydrolysis and polycondensation. The size and shape of the primary particles formed in sol-gel treatment of metal alkoxides are defined not by kinetic factors in their hydrolysis and polycondensation but by the interactions on the phase boundary, which is in its turn directed by the ligand properties. The products of the fast hydrolysis and condensation sequence consist of micelles templated by self-assembly of ligands (mainly oxo-species). This concept provides explanations for commonly observed material properties and allows for the development of new strategies for the preparation of materials. We discuss the formation of inverted micelles, obtained by the appropriate choice of solvents, which allows for the formation of hollow spheres. The modifying β-diketonate ligands act as the surfactant and form an interface between the hollow sphere and the solvent. Retention of ligands inside the gel particles is possible only if ligands possessing both chelating and bridging properties are applied. Application of such ligands, for example, diethanolamine, permits to prepare new transition metal oxide based microporous membranes.     

Formation of GeO2 nanosheets using water thin layers in lamellar phase as a confined reaction field--in situ measurement of SAXS by synchrotron radiation

Highly crystallized GeO2 nanosheets were synthesized by hydrolysis and condensation reactions of germanium alkoxide using a 2-dimensional flat thin lamellar phase water layer containing surfactant molecules at the liquid-liquid interface as a confined reaction field.     

Glycosidation via conjugate addition of anomeric alkoxides to nitroalkenes and nitrosoalkenes

The conjugate addition reactions of protected pyranose alkoxides to both nitroalkenes and nitrosoalkenes, as a route to 2-nitroalkyl, 2-oximinoalkyl and 2-oxoalkyl glycosides, are described.     

3-Oxo-1,2-benzoisothiazoline-2-acetic acid 1,1-dioxide derivatives. I. Reaction of esters with alkoxides

Reaction of 3-oxo-1,2-benzoisothiazoline-2-acetic acid alkyl esters 1,1-dioxide ( 1a-d ) with alkaline alkoxides was carried out under various conditions. Under mild conditions, o-(N-carboxymethylsulfamyl)benzoic acids dialkyl esters ( 2a-d ) were obtained with good yields. Reaction of 1a-d or 2a-d with sodium alkoxides under drastic conditions afforded 4-hydroxy-2H-1,2-benzothiazine-3-carboxylic acid alkyl esters 1,1-dioxide ( 3a-d ). Transesterification was observed when esters 1b-d were treated with sodium methoxide in methanol. Esters 3a-d were hydrolyzed in concentrated aqueous sodium hydroxide affording the acid 6 . Attempts to recrystallize 6 from water resulted in its decarboxylation to give 2H-1,2-benzothiazine-4-(3H)one 1,1-dioxide (7). Compound 6 could not be obtained by acid hydrolysis of esters 3a-d or by rearrangement of 3-oxo-1,2-benzoisothiazoline-2-acetic acid 1,1-dioxide ( 8 ). Different experimental evidence supports the suggestion that rearrangement took place by ethanolysis of the carboxamide linkage affording the open sulfonamides (fast step) followed by a Dieckmann cyclization (slow step). It was demonstrated that transesterification took place in the open sulfonamides 2 .     

Synthesis of yttrium,lanthanum, neodymium,praseodymium, and lutetium alkoxides and acetylacetonates

A convenient and practical method is proposed for the synthesis of lanthanide and yttrium alkoxides. The method involves dissolving the metals or their hydrides in a solution of dry HCl in the corresponding alcohol, with subsequent dehalogenation of the LnCl₃ solution by an equivalent amount of alkali metal (Na, Li), The rareearth alkoxides are easily converted into acetylacetonates Ln(acac)₃ by the action of acetylacetone.A. N. Nesmeyanov Institute of Elementoorganic Compounds, Russian Academy of Sciences, 117813 Moscow. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 11, pp. 2490–2493, November, 1992.     

Reactions of models of the growth center during anionic polymerization of methacrylate esters

Reactions of α-lithioisobutyric acid esters were studied as models of partial reaction taking place during anionic polymerization of methacrylate esters. The rates of selfcondensation reactions and condensations with nonmetalated esters were determined for these models. In both cases ketoesters were the final products. Besides esters of α,α,α′-trimethylglutaric acid expected according to Michael's reaction scheme the addition of methacrylate esters to α-lithio esters also yielded oligomeric compounds due to repeated addition of methacrylate esters. The α-lithio oligomers of methacrylate esters underwent cyclization condensation which gave rise to esters of substituted cyclohexanonedicarboxylic acids. The alkali metal alkoxides slowed down all the condensation reactions of α-lithio esters investigated here. Such effect of alkoxides also appeared in the cyclization condensation of compounds formed by repeated addition of methacrylate esters, because in the presence of alkoxides the reaction mixture contained a higher amount of higher molecular weight compounds than in a system without alkoxide. The effect of alkoxides observed here is in accordance with the view that the cyclization condensation is one of the termination reactions of the anionic polymerization of methacrylate esters.     

Straightforward synthesis of Ti-doped YSZ gels by chemical modification of the precursors alkoxides

Translucent, homogeneous, and monolithic gels of the [(ZrO₂)_0.92(Y₂O₃)_0.08]_1?x(TiO₂)_x system, where x = 0, 0.05, 0.08, and x = 0.10 (mol), have been reliably obtained, for the first time, by a sol–gel route from zirconium (IV) n-propoxide (Zr(OPrⁿ)₄), yttrium acetate hydrate and titanium (IV) isopropoxide (Ti(OPrⁱ)₄). Chemical modification of both alkoxides, zirconium (IV) n-propoxide and titanium (IV) isopropoxide, by acetic acid allows us to change the hydrolysis and condensation behavior of them. Their modification implies the formation of chelating and bridging acetates avoiding the formation of precipitates. The line width and some shoulders in the FT-IR spectra of the solution, resulting of the mixture of the precursors and the catalysts, during the hydrolysis reaction suggest that both coordinations, chelating and bridging, should occur. Furthermore, the separation of the steps of hydrolysis and condensation allows to achieve conditions under which hydrolysis of the molecular precursor is slowed, whereas condensation is promoted under chemical reversibility to ensure a crystalline product at low calcination temperatures. In addition, the formation of metalloxane bondings (M–O–M’, M and M’ = Zr, Y, and Ti) has been confirmed by FT-IR throughout the sol–gel process. At about 630 °C, the crystallization of yttria stabilized zirconia (YSZ) for x = 0 or a titania-doped yttria stabilized zirconia solid solution (Ti-doped YSZ) for x = 0.05, 0.08, and 0.10 is detected by DTA-TG. By SEM-EDX and TEM-EDX the presence of Zr, Y, and Ti elements, in the adequate proportions according to the nominal compositions, has been proven in both dried and calcined gels.     

Crystal phase evolution of TiO2 nanoparticles with reaction time in acidic solutions studied via freeze-drying method

The crystal phase evolution of TiO₂ nanoparticles, during hydrolysis and condensation of titanium tetraisopropoxide, was quenched at various reaction times by a freeze-drying method, followed by various characterizations. Three types of solutions with different acid input times were studied: (1) addition in infinite time (no addition), (2) addition at 24 h after the hydrolysis/condensation reaction started, and (3) addition from the beginning of the reaction. The acid-free solution yielded amorphous TiO₂, which transformed to anatase very slowly. The acid input in 24 h resulted in a fast transformation of amorphous to a metastable anatase having a highly distorted atomic arrangement: thereby its transformation to a more stable phase, rutile, was suitable. The acid addition from the beginning of the reaction yielded the formation of a relatively stable anatase from the hydrolysis seed, thereby the subsequent transformation to rutile was sluggish.     

Lithiation directed by amino alkoxides. Application to the synthesis of new disubstituted dihydrodipyridopyrazines

The synthesis of new 4,6-disubstituted dihydrodipyridopyrazines starting from corresponding carboxaldehydes via lithiation directed by alpha-amino alkoxides is described. The N,N,N'-trimethylethylenediamine was used as amine component for in situ formation of the alpha-amino alkoxides. After optimization, this reaction allowed easy access to new interesting starting materials for further applications by palladium-catalyzed reactions.     

Stability, reactivity, solution, and solid-state structure of halomethylzinc alkoxides.

In this paper, we report our findings regarding the development of a Lewis acid-catalyzed cyclopropanation of allylic alcohols with bis(iodomethyl)zinc. Iodomethylzinc alkoxides can be formed by treatment of an alcohol with bis(iodomethyl)zinc. These species are not prone to undergo cyclopropanation at low temperature but the addition of a Lewis acid in catalytic amounts induces the cyclopropanation reaction. Using this procedure, we demonstrated that the Lewis acid-catalyzed pathway significantly overwhelms the uncatalyzed one. This paper describes fundamental issues regarding the preparation and stability of halomethyl zinc alkoxides in solution as well as their aggregation state in solution and solid-state structures. Furthermore, the competition reaction between the inter- vs intramolecular cyclopropanation will be studied. Finally, we will discuss the possible activation pathways to explain the Lewis acid activation of halomethylzinc alkoxides. These findings provided new insights on the reactivity of ROZnCH(2)I and established the groundwork for the elaboration of an enantioselective version of the reaction.     

Electrochemical synthesis of metal alkoxides. Prospects of commercial alkoxides production

A production method of Ti, Zr, Nb, Ta, Y, Sn, Cu, Ge, Mo, W and some other metals alkoxides at their anode polarization in alcohols (methanol, ethanol, i-propanol, butanol, methoxy-ethanol) in the presence of an electroconductive additive has been developed.As a result of theoretical and experimental studies and engineering development, a universal low-waste technological processes of the electrochemical production of the above high purity alkoxides have been established.Metal efficiency coefficient in the electrosynthesis process is 0.85–0.90, current efficiency is 75–80%, and specific power consumption is within 5–10 kW-h/kg range.The total of impurities provided by this technology does not exceed 10^–3% mass.The developed electrochemical technology has been realized at a pilot-commercial scale at AO NIPIM plant (Russia, Tula) in order to manufacture a high purity titanium tetrabutoxide at 1.5–2.0 t/year plant operation the actual advantages of the electrochemical technology comparing to a traditional chemical one have been revealed.A considerable simplification of the developed electrochemical technology has been achieved at the anode polarization of metals in the methxyethanol due to elimination from the diagram scope of units for the alkoxides separation from after electrolysis solutions and recovery of unreacted raw components.     

IR and NMR time-resolved studies on the hydrolysis and condensation of methacryloxyalkylsilanes

The sol–gel reactions of the two methacrylate-modified silanes methacryloxymethyltriethoxysilane (MAMTES) and methacryloxypropyltrimethoxysilane (MAPTMS) were followed by using two independent time-resolved spectroscopic methods, viz., IR ATR and NMR with the aim to optimise their pre-hydrolysis times and consequently their use as precursors for hybrid materials. Time resolved measurements were carried out on the two systems under sol–gel conditions. Whereas the hydrolysis of both siloxanes is very fast, condensation proceeds gradually and is not completely finished within 5 h.     

Synthesis and characterization of a series of rubidium alkoxides and rubidium-titanium double alkoxides

This report investigates the structural aspects of the products isolated from the reactions of a series of titanium alkoxides [[Ti(OR)4]n n = 2, OR = OCH2C(CH3)3 (ONep) (1); n = 1, OC6H3(CH3)2-2,6 (DMP) (2)] with rubidium alkoxides [[Rb(OR)]infinity where OR = (ONep) (3), (DMP) (4), and OC6H3(CH(CH3)2)2-2,6 (DIP) (5)]. The resultant double alkoxides were determined by single crystal X-ray diffraction to be [Rb(mu-ONep)4(py)Ti(ONep)]2 (6), [Rb(mu-DMP)Ti(DMP)4]infinity (7), and [Rb(mu-DMP)2(mu-ONep)2Ti(ONep)]infinity (8). Compound 1 is the previously reported dinculear species with trigonal bipyramidal Ti metal centers whereas compound 2 is a monomer with a tetrahedral Ti center. Suitable X-ray quality crystals of 3 were not isolated. Compounds 4 and 5 demonstrate extended polymeric networks with Rb coordination ranging from two to five utilizing terminal mu- and mu3-OR ligands and pi-interactions of neighboring OAr ligands. The double alkoxide 6 revealed a simple tetranuclear structure with mu-ONep acting as the bridge, terminal ONep ligands on the Ti, and one terminal py on the Rb. For 7 and 8, the pi-interaction facilitated the formation of extended polymeric systems. All complexes were further characterized by FT-IR and multinuclear NMR spectroscopy.     

钛醇盐电化学合成的研究

采用钛金属为“牺牲”阳极,首次在无隔膜电解槽中,电化学一步法制备了纳米TiO~2前驱体钛醇盐Ti(OEt)~4,Ti(OPr-i)~4,Ti(OBu)~4。产物通过元素分析、红外光谱(FT-IR)、拉曼光谱进行表征。电化学一步法直接制备纳米材料前驱体钛醇盐,克服了传统化学方法合成金属醇盐步骤多、产率低、纯度达不到要求及后续分离繁琐等缺点。本文同时讨论了影响电合成钛醇盐的关键因素及可能的反应机理,实验表明钛在醇溶液中呈点蚀行为,钛醇盐卤化物Ti(Ⅲ)(OR)~nBr~m在阳极形成,然后被氧化为Ti(Ⅳ)(OR)~nBr~m,这种物质在阴极上ROH参与下被还原生成钛醇盐Ti(OR)~4,钛阳极表面拉曼光谱证实了上述观点。防止阳极钝化,温度控制在50～60℃之间,彩有机胺溴化物为导电盐,可以提高电合成收率。     

The catalytic activity of alkali metal alkoxides and titanium alkoxides in the hydrosilylation of unfunctionalized olefins

The catalytic activities of titanium alkoxides and alkali metal alkoxides for hydrosilylation of unfunctionalized olefins have been studied. Titanium(IV) alkoxides showed excellent catalytic activity, while alkali metal alkoxides have low catalytic activity for the hydrosilylation of olefins. However, by using titanocene dichloride as an additive, alkali metal alkoxides showed also excellent catalytic property for hydrosilylation. In comparison with titanium alkoxides, no α-adduct was obtained by using alkali metal alkoxides/Cp₂TiCl₂ as catalysts.