Structural Investigations on the Hydrolysis and Condensation

J. Phys. Chem. B 2007, 111, 7501-7518

7501

Structural Investigations on the Hydrolysis and Condensation Behavior of Pure and Chemically Modified Alkoxides. 1. Transition Metal (Hf and Ta) Alkoxides Venkata Krishnan,*,†,§ Silvia Gross,*,‡ Sonja Mu1 ller,† Lidia Armelao,‡ Eugenio Tondello,‡ and Helmut Bertagnolli† Institute of Physical Chemistry, UniVersity of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany, and Istituto di Scienze e Tecnologie Molecolari, Consiglio Nazionale delle Ricerche, and Dipartimento di Scienze Chimiche, UniVersita` di PadoVa and Consorzio InteruniVersitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM), Via Marzolo 1, 35131 PadoVa, Italy ReceiVed: January 23, 2007; In Final Form: April 13, 2007

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.

1. Introduction Metal alkoxides are the most interesting and versatile precursors for the sol-gel process to obtain oxide-based networks.1-4 An extensive description of metal alkoxides and of their structural and physicochemical properties, together with their main synthesis routes, is reported in the classical works of Mehrotra5 and of other authors.6-8 The sol-gel chemistry of the metal alkoxide precursors has been the topic of several studies 1,2,9-11 in which the different factors (nature of the metal, catalyst, hydrolysis ratio, etc.) affecting the evolution of the system from the solution up to the gel formation are thoroughly discussed. In spite of several advantages, the use of transition metal alkoxides as sol-gel precursors also presents some drawbacks. One of the main limitations related to metal alkoxides, which has to be taken into account for their use as sol-gel precursors, is their high reactivity toward water. To circumvent this problem, suitable chemical modifications need to be employed to control the reactivity of the metal alkoxides.4,11,12 The chemical tailoring procedures applied to silicon alkoxides, which are based on the use of organically modified silanes, cannot be tout court extended to other alkoxides, and especially to transition metal alkoxides, since the more reactive M-C bond would be cleaved upon hydrolysis. Therefore, in the case of highly reactive transition metal alkoxides, to prevent the * Authors to whom correspondence should be addressed. E-mail: [email protected] (V.K.); [email protected] (S.G.). Fax: +39 049 827 5161 (S.G.); +1 215 573 2112 (V.K.). † University of Stuttgart. § Current address: Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, PA 19104. ‡ Universita ` di Padova and Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM).

instantaneous hydrolysis of the precursors, the complexation of the metal with suitable ligands is frequently used. The exchange of one or more alkoxy group by a bidentate ligand increases the coordination number of the metal and at the same time lowers the reactivity, thus enabling the control of the polycondensation process through the sol-gel route.11,13 In addition, chemically modified metal alkoxides are also well-suited molecular precursors for the synthesis of inorganic-organic materials.1,14-16 In this framework, complexation by a bidentate ligand is relevant in view of successive embedding of the metal species in a host matrix, since bidentate ligands can in fact allow the introduction of functional groups on the surface of the metal alkoxides.11 These functional groups can in turn be used to introduce metal species into host matrixes. The modification of metal alkoxides with bidentate ligands has been already used as an effective route for the synthesis of hybrid inorganicorganic gels17 as well as for the preparation of transition metal oxoclusters starting from transition metal alkoxides and methacrylic acid.18 Many studies have been devoted to the investigation of the reactivity of chemically modified alkoxides, and several physicochemical approaches to the study of metal alkoxides have been extensively described in a paper by Turova et al.19 For example, the polymerization kinetics of zirconium and titanium alkoxides in the presence of different bidentate ligands has been investigated using electrochemical methods by Cattey et al.20 Acetylacetone (acac) often has been used as a stabilizing agent for different metal alkoxides, such as W(OEt)3,21 Zr(OR)4 (R ) OnPr, OiPr), Al(OsBu)322 and Ti(OR)4 (R) OiPr, OnBu).9,11,22-24 While upon hydrolysis most of the alkoxy groups are quickly removed, the complexes with bidentate ligands are hardly cleaved. Strong complexing ligands are stable toward hydrolysis due to chelating and steric hindrance effects. The hydrolysis

10.1021/jp070566a CCC: $37.00 © 2007 American Chemical Society Published on Web 06/05/2007

7502 J. Phys. Chem. B, Vol. 111, No. 26, 2007 and condensation reactions of Ti(OiPr)4 modified with acetylacetone have been studied by means of NMR, IR, X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopic techniques.21,24 Further studies on the sol-gel reaction of M(OR)4-x(acac)x (M) Ce, Ti, Zr; x ) 0-2) performed by means of NMR, IR, XANES, EXAFS, small-angle X-ray scattering (SAXS), and quasi-elastic light scattering (QELS) methods have also been reported.25 The mechanisms of the hydrolysis and condensation reactions in metal alkoxides have been deeply investigated by several researchers,26-27 where a detailed and systematic study on the hydrolysis and condensation of several M(OR)x and MO(OR)x (M ) Ti, Zr, V, Nb, Ta, Mo, W; x ) 3-5) has been performed by monitoring the reaction progress through 17O NMR spectroscopy. In the past few years, the molecular complexity of alkoxides has been determined by molecular weight measurements or by mass spectrometry analyses.2,5 More recently, further analytical techniques (IR, Raman, and proton and multinuclear NMR spectroscopy) have been used to obtain information on the association degree.5,6,28 Structural information on these systems can also be conveniently obtained by using X-ray absorption fine structure (XAFS) spectroscopy, which enables the selective determination of the short-range order around a specific atom. This method based on the absorption of X-rays by a particular atom is a powerful tool for determining the local atomic environment of the specific atom regardless of the physical state of the sample.29 EXAFS spectroscopy provides information on the coordination number, the nature of scattering atoms surrounding the absorbing atom, the interatomic distance between the absorbing atom and the backscattering atoms, and the Debye-Waller factor, which accounts for the disorders due to static displacements and thermal vibrations.29,30 Due to the short lifetime of the photoelectrons, the maximum range investigated by EXAFS is about 5 Å.29 Thanks to the high sensitivity on the short range, EXAFS spectroscopy can be employed to investigate in detail the structural features of metal alkoxides, especially to distinguish between the terminal and bridging alkoxy groups, which in turn provide information on the molecular complexity.18,31 Furthermore, the XANES region provides information on oxidation states, vacant orbitals, electronic configuration, and coordination geometry of the absorbing atom.32 In particular, in the investigation of chemically modified metal alkoxides, XANES spectroscopy can be used to observe the changes in the coordination geometry occurring due to the complexation of a coordinating ligand. Time-resolved XAFS measurements have also been assessed as a valuable tool to investigate the structural evolution of systems in solution, such as complexation processes or the reaction degree. Moreover, Raman spectroscopy, which provides information on the vibration bands, could be used to obtain additional structural details.33 In the past few years, zirconium and titanium alkoxides have been investigated in detail by means of XAFS spectrscopy.24,31,34-35 These studies have been integrated by investigations on the chemical modification of zirconium alkoxides by acetylacetone and acetic acid.35 The investigations revealed an oligomeric structure for the pure alkoxides and demonstrated that the molecular complexity is influenced by the nature of the alkoxy group. This oligomeric structure is changed upon complexation with acetylacetone, while complexation with acetic acid does not affect the degree of association. In our previous works, the hydrolysis and condensation behavior of single and mixed alkoxides has been extensively studied through electrospray ionization (ESI) and matrix-assisted laser desorption

Krishnan et al. ionization (MALDI) mass spectrometry.36-39 These studies have shown that, depending on the nature of the alkoxide, very different behaviors are observed. In particular, in the first investigations by this approach,40 the polycondensation of Si(OEt)4 was studied by ESI mass spectrometry. Even in absence of water, the formation of polycondensation products was observed together with the occurrence of the alcoholysis reaction. In a further study on the hydrolysis/polycondensation behavior of methanolic solutions containing Ti(OiPr)4 or Ti(OnBu)4, the experimental data showed that these reactions led to the Ti(OMe)4 species, representing the synthon of polycondensation.36 In a further work, MALDI mass spectrometry has been employed for the study of Ti(OnBu)4 polycondensation in presence of Si(OEt)4.37 In a brand new investigation, the MALDI mass spectrometry approach was employed for the study of hydrolysis and polycondensation reactions of Al(OsBu)3 (ASB).41 Understanding of the mechanisms of hydrolysis and condensation of metal alkoxides acquires an important role in the development of rational synthetic routes to polyoxometalates and oxoclusters. The first investigations on the synthesis of early transition metal polyoxometalates by hydrolysis of metal alkoxides was carried out by Jahr and Fuchs in 1963.42 More recently, Kickelbick et al.18 investigated by EXAFS and other methods the mechanistic aspects of the formation of surfacemodified zirconium oxoclusters. Detailed mechanistic investigations on the key steps of the sol-gel process of these transition metal alkoxides can provide information on the role played by the different experimental parameters (hydrolysis ratio, bidentate ligand/alkoxide molar ratio, etc.) on the structural evolution of the systems and would allow us to better orientate the evolution of the system itself. This would in turn enable a fine-tuning of the features of the final system. Once the mechanisms leading from the precursors to the final system have been thoroughly understood, variation of these parameters would allow the tailoring of the structure and the composition of the systems under investigation and, consequently, their properties. In the present work, structural investigations on the hydrolysis and condensation behavior of Hf(OnBu)4 and Ta(OEt)5 under sol-gel conditions performed by means of XAFS and Raman spectroscopy are reported. Time-resolved in situ studies were performed to investigate: (a) the structure and hydrolytic behavior of the alkoxides in solution; (b) the chemical modification of the alkoxides by acetylacetone; (c) the influence of chemical modification on the hydrolysis and condensation behavior of the alkoxides; (d) the structure of the alkoxides in the mixture with prehydrolyzed Si(OEt)4; (e) the effect of prehydrolyzed Si(OEt)4 on the hydrolysis and condensation behavior of the alkoxides. To the best of our knowledge, this is the first comprehensive study on the hydrolysis and condensation behavior of hafnium and tantalum alkoxides as pure alkoxides and in the presence of complexing ligand or other alkoxides performed by the joint use of XAFS and Raman spectroscopic methods. 2. Experimental Details 2.1. Sample Preparation. The commercially available alkoxides Hf(OnBu)4, Ta(OEt)5, and Si(OEt)4 purchased from ABCR GmbH, Karlsruhe, Germany, were analyzed as solutions in anhydrous ethanol. The hydrolysis and condensation reactions were performed by adding to the alkoxide solutions a stoichio-

Transition Metal (Hf and Ta) Alkoxides metric amount of acidified deionized water (using 37% HCl as a catalyst). Typical molar ratios in solutions were M(OR)x/EtOH/ H2O/HCl ) 1:20:2:0.01 (M ) Hf, Ta). Acetylacetone (acac, 2,4-pentandione, Aldrich) was added to ethanol solutions of the alkoxide by using a 1:2 molar ratio of metal alkoxides/acac. The hydrolysis and condensation reactions were carried out by using the same molar ratios already used in the case of alkoxides that are not chemically modified. Mixed solutions of Si(OEt)4Hf(OnBu)4 and of Si(OEt)4-Ta(OEt)5 were prepared by mixing the prehydrolyzed Si(OEt)4 and the transition metal alkoxide in ethanol using a Si/M molar ratio of 1:1. Si(OEt)4 was separately prehydrolyzed by using the molar ratio Si(OEt)4/ EtOH/H2O/HCl ) 1:20:1:0.01 and by stirring this solution for 12 h at room temperature. In all the cases the solutions were stirred for about 5 min and then measured at different time intervals (see infra). Due to the high moisture sensitivity of metal alkoxides, all procedures involved in preparation and structural characterization were performed under an atmosphere of dry nitrogen by using a Schlenk line. 2.2. XAFS Measurements and Data Analysis. The XAFS measurements of the samples were performed at the X-ray absorption spectroscopy (XAS) beamline of Angstroemquelle Karlsruhe (ANKA) at Forschungszentrum Karlsruhe (FZK), Karlsruhe, Germany. The synchrotron beam energy was 2.5 GeV, and the beam current was between 110 and 180 mA during the course of the measurements. The measurements were performed at the Hf LIII-edge at 9561 eV and the Ta LIII-edge at 9881 eV using a Si(111) double-crystal monochromator. Energy calibration was monitored using a hafnium metal foil for the Hf LIII-edge measurements and tungsten metal foil (having a LIII-edge at 10207 eV) for the Ta LIII-edge measurements. All experiments were carried out in transmission mode with nitrogen-filled ion chambers at ambient conditions. The samples, prepared as ethanolic solutions, were filled into a transmission sample cell for liquids specifically designed for XAFS measurements. The concentration of all of the samples was adjusted to yield an extinction of 1.5. Data evaluation started with the removal of background absorption from the experimental absorption spectrum by subtraction of a Victoreen-type polynomial. Then, the spectrum was convoluted with a series of increasingly broader Gaussian functions, and the common intersection point of the convoluted spectrum was taken as energy E0.43 To determine the smooth part of the spectrum, corrected for preedge absorption, a piecewise polynomial was used. It was adjusted in such a manner that the low-R components of the resulting Fourier transform were minimal.44 After division of the backgroundsubtracted spectrum by its smooth part, the photon energy was converted into a photoelectron wave vector, k, and the resulting EXAFS function was weighed with k3. Data analysis in k-space was performed according to the curved-wave formalism of the program EXCURV98 with XALPHA phase and amplitude functions.45 The mean free path of the scattered electrons was calculated from the imaginary part of the potential (VPI set to -4.00), and the Fermi energy term EF was introduced to give a best fit to the data. The amplitude factor (AFAC) was set to a value of 0.8. For clarity reasons, the EXAFS spectra and the Fourier transform plots are shifted along the ordinate axis in all of the cases. The exact zero position of the EXAFS spectra and that of the corresponding Fourier transform can be obtained by observing the value on the ordinate axis at the intersection point. In the fitting procedure, the various parameters, i.e., coordination numbers, interatomic distances, Debye-Waller

J. Phys. Chem. B, Vol. 111, No. 26, 2007 7503

Figure 1. Experimental (solid line) and calculated (dotted line) (a) EXAFS functions and (b) their Fourier transforms for Hf(OnBu)4 measured at the Hf LIII-edge.

factor, and Fermi energy value, were determined by iterations for all of the cases. 2.3. Raman Measurements. Raman spectroscopic measurements were performed on a Bruker RFS 100/S Fourier transform (FT) Raman spectrometer (spectral resolution of 4 cm-1) with an air-cooled near-infrared Nd:YAG laser with a wavelength of 1064 nm. The samples were measured as solutions in ethanol at a laser power level of 1250 mW using a quartz cuvette. The scattered light was collected with a high-sensitivity Ge diode (cooled with liquid nitrogen). For an average measurement, 1024 scans were accumulated. 3. Results and Discussion 3.1. Structure of Pure Alkoxides. The structural investigations were initially performed on pure Hf(OnBu)4, and to the best of our knowledge, there is no description of its structure by EXAFS. In the literature, a dimeric structure was determined for zirconium alkoxides.31,35 However, due to the chemical similarity of hafnium and zirconium and of their compounds, an analogous structure could be expected. The experimentally determined and theoretically calculated EXAFS functions in k-space and their Fourier transformations in real space for Hf(OnBu)4, measured at the Hf LIII-edge, are shown in Figure 1. The corresponding structural parameters are tabulated in Table 1. In the analysis of the EXAFS function, a four-shell model was applied. The first shell with about two oxygen backscatterers at 1.95 Å, the second shell with about four oxygen backscatterers at 2.14 Å, the third shell with about two carbon backscatterers at 3.12 Å, and the fourth shell with a single hafnium backscatterer at 3.42 Å could be fitted. The obtained structural parameters are in good agreement with the single-crystal data on Hf(OiPr)4,46 which revealed a dimeric structure for the

7504 J. Phys. Chem. B, Vol. 111, No. 26, 2007

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TABLE 1: EXAFS-Obtained Structural Parameters for Hf(OnBu)4 sample

A-Bsa

Nb

rc (Å)

σd (Å)

EFe (eV)

k-range (Å-1)

R-factor

Hf(OnBu)4

Hf-O Hf-O Hf-C Hf-Hf

2.4 ( 0.2 4.6 ( 0.5 2.3 ( 0.4 1.1 ( 0.2

1.95 ( 0.02 2.14 ( 0.02 3.12 ( 0.03 3.42 ( 0.04

0.050 ( 0.005 0.087 ( 0.009 0.092 ( 0.014 0.071 ( 0.014

0.738

2.96-15.01

30.05

a

Absorber-backscatterers. b Coordination number. c Interatomic distance. d Debye-Waller factor with its calculated deviation. e Fermi energy.

TABLE 2: EXAFS-Obtained Structural Parameters for Ta(OEt)5 sample

A-Bsa

Nb

rc (Å)

σd (Å)

EFe (eV)

k-range (Å-1)

R-factor

Ta(OEt)5

Ta-O Ta-O Ta-C Ta-Ta

4.5 ( 0.5 2.6 ( 0.3 3.4 ( 0.5 0.8 ( 0.2

1.91 ( 0.02 2.12 ( 0.02 3.07 ( 0.03 3.49 ( 0.04

0.050 ( 0.005 0.077 ( 0.009 0.071 ( 0.011 0.071 ( 0.014

-1.496

3.00-15.01

20.17

a


Figure 4. Proposed molecular structure of Ta(OEt)5. Figure 2. Proposed molecular structure of Hf(OnBu)4.

Figure 5. Raman spectrum of Hf(OnBu)4.

Figure 3. Experimental (solid line) and calculated (dotted line) (a) EXAFS functions and (b) their Fourier transforms for Ta(OEt)5 measured at the Ta LIII-edge.

alkoxide, wherein each hafnium atom is coordinated by three terminal and two bridging alkoxy groups as well as one terminal alcoholic group. The results indicate that Hf(OnBu)4 has a dimeric structure with a 6-fold coordination around each hafnium atom. On the basis of the EXAFS studies and

considering the details on the nature and molecular complexity of hafnium alkoxides reported in the literature5 and the known structure of Hf(OiPr)4,46 the molecular structure proposed for Hf(OnBu)4 is shown in Figure 2. Similar investigations were carried out on tantalum pentaethoxide. Pentavalent tantalum alkoxides have been the subjects of several studies by Mehrotra and Bradley.7 They are highly volatile compounds, and their molecular complexity has been determined, for R ) Me, Et, nPr, to be about 2, with the formation of a Ta2(OR)10 dimer.5 The studies performed on Ta(V) alkoxides by Bradley et al. revealed a dimeric structure for these compounds, and the degree of oligomerization is lowered when they are dissolved in alcohols.47 The molecular structure of Ta(OEt)5 in solution studied by means of 1H NMR investigations revealed that there is a dynamic equilibrium between monomeric and dimeric species.48 Furthermore, the investigations on the electrochemical synthesis of tantalum alkoxides indicated that tantalum pentaalkoxides prefer an octahedral coordination, which is obtained either by dimerization or by

Transition Metal (Hf and Ta) Alkoxides

J. Phys. Chem. B, Vol. 111, No. 26, 2007 7505 coordination of a solvate molecule, such as an alcohol.49 The experimentally determined and theoretically calculated EXAFS functions in k-space and their Fourier transformations in real space for Ta(OEt)5, measured at the Ta LIII-edge, are shown in Figure 3, and the structural parameters are given in Table 2. The analysis of the EXAFS function was performed using a four-shell model in this case as well. The first two coordination shells consist of oxygen atoms, having four backscatterers at 1.91 Å and two backscatterers at 2.12 Å. The obtained distances are in agreement with the Ta-O distances in Ta2(OMe)2(OiPr)8.50 The third shell comprises three carbon backscatterers at 3.07 Å, and the fourth shell has a single tantalum backscatterer at 3.49 Å. The obtained structural parameters suggest a dimeric structure of Ta(OEt)5 with a 6-fold coordination around each tantalum atom, which is in good agreement with the results obtained by ebullioscopic measurements on Ta(OEt)5.51 These results are also in good agreement with other results reported in the literature.7,52 On the basis of the EXAFS investigations and considering the details on the nature and molecular complexity of tantalum alkoxides reported in literature,5,47,49,50

Figure 6. Raman spectrum of Ta(OEt)5.

Figure 7. Experimental (solid line) and calculated (dotted line) (a) EXAFS functions and (b) their Fourier transforms for a Hf(OnBu)4 and water mixture measured at different time intervals at the Hf LIII-edge.

TABLE 3: EXAFS-Obtained Structural Parameters for a Hf(OnBu)4 and Water Mixture Measured at Different Time Intervals time

A-Bsa

Nb

rc (Å)

σd (Å)

EFe (eV)

k-range (Å-1)

R-factor

0 min

Hf-O Hf-C Hf-Hf Hf-O Hf-C Hf-Hf Hf-O Hf-C Hf-Hf

6.7 ( 0.7 5.1 ( 0.8 2.2 ( 0.4 6.5 ( 0.7 5.4 ( 0.8 2.2 ( 0.4 6.5 ( 0.7 5.7 ( 0.8 2.1 ( 0.4

2.12 ( 0.02 3.17 ( 0.03 3.44 ( 0.04 2.13 ( 0.02 3.21 ( 0.03 3.44 ( 0.04 2.13 ( 0.02 3.21 ( 0.03 3.44 ( 0.04

0.095 ( 0.010 0.122 ( 0.018 0.089 ( 0.018 0.092 ( 0.009 0.122 ( 0.018 0.095 ( 0.019 0.095 ( 0.010 0.122 ( 0.018 0.092 ( 0.018

-0.591

2.99-15.00

40.02

-0.651

2.98-15.00

32.41

-0.859

2.97-15.00

24.37

30 min 85 min

a



Krishnan et al.

Figure 8. Experimental (solid line) and calculated (dotted line) (a) EXAFS functions and (b) their Fourier transforms for a Ta(OEt)5 and water mixture measured at different time intervals at the Ta LIII-edge.

the molecular structure proposed for Ta(OEt)5 is depicted in Figure 4. The Raman spectra of Hf(OnBu)4 and Ta(OEt)5 are shown in Figures 5 and 6, respectively. In the spectrum of Hf(OnBu)4, the peak at 397 cm-1 and the weak shoulder at 453 cm-1 could be assigned to Hf-O vibrations53 whereas the bands around 1030 and 1080 cm-1 are attributed to the Hf(-O-C) stretching vibrations.54 In the spectrum of Ta(OEt)5, the peak at 588 cm-1 could be assigned to Ta-O vibrations,55 and the bands around 1080 cm-1 are attributed to the Ta(-O-C) stretching vibrations.54 In both of the spectra, the peak at 1050 cm-1 is attributed to C-C vibrations, and the peaks between 2800 and 2950 cm-1 are ascribed to the C-H vibrations of the alkyl groups.56 3.2. Hydrolysis and Condensation Behavior of Pure Alkoxides. The hydrolysis and condensation reactions were studied by adding to the ethanolic solutions of the metal alkoxides a stoichiometric amount of acidified water. Immediately after the addition of water, the formation of a turbid solution could be observed for both the alkoxides, which could be ascribed to the fast hydrolysis and condensation reactions. EXAFS measurements were performed on the solution immediately after mixing, and subsequent measurements were carried out at different time intervals. The experimentally determined and the theoretically calculated EXAFS functions in k-space and their Fourier transformations in real space for Hf(OnBu)4 and the water mixture measured at the Hf LIII-edge are shown in Figure 7, and the structural parameters are summarized in Table 3. The results show that Hf(OnBu)4 instantaneously undergoes hydrolysis and condensation reactions, as indicated by the change in the coordination number of the hafnium and carbon backscatterers. Moreover, the oxygen shells are no longer separated but occur as a single shell at 2.12 Å, and an increase in Hf-C distance from 3.12 to 3.21 Å could also be noticed in comparison to the pure Hf(OnBu)4. The signal-to-noise ratio is not good for the EXAFS spectra obtained immediately after

Figure 9. Raman spectra of a Hf(OnBu)4 and water mixture measured at different time intervals along with Hf(OnBu)4: (a) Hf(OnBu)4, (b) Hf(OnBu)4 + H2O (0 h), and (c) Hf(OnBu)4 + H2O (48 h).

the addition of water, owing to the inhomogeneities caused in the solution during the initial stages of gelation. Furthermore, the spectra acquired at different time intervals did not show any remarkable variations in the structural parameters in comparison with those obtained immediately after mixing. The experimentally determined and the theoretically calculated EXAFS functions in k-space and their Fourier transformations in real space for Ta(OEt)5 and the water mixture measured at the Ta LIII-edge are shown in Figure 8, and the obtained structural parameters are summarized in Table 4. The results show that Ta(OEt)5 instantaneously undergoes hydrolysis and condensation reactions, as indicated by the change in the coordination number of the tantalum and carbon backscatterers


Figure 10. Raman spectra of a Ta(OEt)5 and water mixture measured at different time intervals along with Ta(OEt)5: (a) Ta(OEt)5, (b) Ta(OEt)5 + H2O (0 h), and (c) Ta(OEt)5 + H2O (48 h).

in comparison to pure Ta(OEt)5. Furthermore, a remarkable shortening of the Ta-Ta distance could also be noticed as a result of the condensation process. The EXAFS spectra acquired at different time intervals did not show any significant variations in the structural parameters in comparison with those obtained immediately after the addition of water, thus confirming that hydrolysis and condensation reactions occur immediately after mixing. Raman spectroscopic investigations were performed to obtain further information on the hydrolysis and condensation mechanisms. Studies were performed on hafnium alkoxide and water in the same molar ratio (1:2) as that used for the EXAFS

J. Phys. Chem. B, Vol. 111, No. 26, 2007 7507 investigations. Raman spectra of Hf(OnBu)4 and water measured immediately and 48 h after mixing are shown in Figure 9 along with the spectrum of pure Hf(OnBu)4. The peak at 397 cm-1, corresponding to Hf-O vibrations, could be observed in both of the spectra, indicating the presence of hafnium species in the solution.53 It is interesting to note the decrease in the intensity of the Hf(-O-C) bands ascribable to the alkoxy groups at 1030 and 1080 cm-1, owing to the formation of the condensation product upon the addition of water. A similar behavior was also observed in the infrared spectroscopic studies on the hydrolysis and condensation behavior of titanium alkoxides.24 Raman spectra of Ta(OEt)5 and water measured immediately and 48 h after mixing are shown in Figure 10 along with the spectrum of pure Ta(OEt)5. The intensity of the Ta-O peak at 588 cm-1 in pure Ta(OEt)5 decreased in the mixture with water, which could be attributed to the decrease of the tantalum species in the solution owing to condensation and subsequent precipitation. In addition, a decrease in the intensity of Ta(-O-C) bands ascribable to the alkoxy groups at 1080 cm-1 is observed in this case as well. In both of the alkoxides, the spectrum measured 48 h after mixing is similar to the spectrum acquired immediately after mixing, indicating that the hydrolysis and condensation reactions occur immediately after the addition of water, in agreement with the EXAFS results. 3.3. Influence of Chemical Modification by Acetylacetone. To study the influence of chemical modification on the reactivity of hafnium and tantalum alkoxides, acetylacetone was added to the alkoxides. Concerning hafnium, β-diketonate derivatives of hafnium and zirconium have been the topic of through investigations by Pinnavaia et al.57 In these studies, the preparation and the IR and Raman characterization of dihalobis(acetylacetonato), halotris(acetylacetonato) and eight-coordinate acetylacetonato complexes of Zr(IV) and Hf(IV) are described,

Figure 11. Experimental (solid line) and calculated (dotted line) (a) EXAFS functions and (b) their Fourier transforms for a Hf(OnBu)4-acac mixture measured at different time intervals at the Hf LIII-edge.


Krishnan et al.

TABLE 4: EXAFS-Obtained Structural Parameters for a Ta(OEt)5 and Water Mixture Measured at Different Time Intervals time

A-Bsa

Nb

rc (Å)

σd (Å)

EFe (eV)

k-range (Å-1)

R-factor

0 min

Ta-O Ta-O Ta-C Ta-Ta Ta-O Ta-O Ta-C Ta-Ta Ta-O Ta-O Ta-C Ta-Ta

4.4 ( 0.5 2.6 ( 0.3 4.6 ( 0.7 1.8 ( 0.3 4.3 ( 0.5 2.7 ( 0.3 4.3 ( 0.7 1.6 ( 0.3 4.3 ( 0.5 2.6 ( 0.3 4.3 ( 0.7 1.6 ( 0.3

1.91 ( 0.02 2.09 ( 0.02 3.02 ( 0.03 3.37 ( 0.04 1.91 ( 0.02 2.08 ( 0.02 3.02 ( 0.03 3.36 ( 0.04 1.91 ( 0.02 2.09 ( 0.02 3.02 ( 0.03 3.36 ( 0.04

0.071 ( 0.007 0.071 ( 0.007 0.097 ( 0.015 0.092 ( 0.018 0.071 ( 0.007 0.071 ( 0.007 0.097 ( 0.015 0.092 ( 0.018 0.071 ( 0.007 0.071 ( 0.007 0.097 ( 0.015 0.092 ( 0.018

-0.790

2.99-15.01

37.73

-0.242

2.98-15.02

33.73

-0.447

2.97-15.02

33.68

35 min

120 min

a


TABLE 5: EXAFS-Obtained Structural Parameters for a Hf(OnBu)4-acac Mixture Measured at Different Time Intervals time

A-Bsa

Nb

rc (Å)

σd (Å)

EFe (eV)

k-range (Å-1)

R-factor

0 min


7.6 ( 0.8 4.8 ( 0.7 1.1 ( 0.2 7.7 ( 0.8 4.7 ( 0.7 1.1 ( 0.2 7.7 ( 0.8 4.9 ( 0.7 1.0 ( 0.2

2.17 ( 0.02 3.21 ( 0.03 3.54 ( 0.04 2.17 ( 0.02 3.20 ( 0.03 3.54 ( 0.04 2.17 ( 0.02 3.20 ( 0.03 3.53 ( 0.04

0.087 ( 0.009 0.063 ( 0.010 0.071 ( 0.014 0.089 ( 0.009 0.067 ( 0.010 0.071 ( 0.014 0.089 ( 0.009 0.071 ( 0.011 0.067 ( 0.013

-1.393

2.99-15.04

27.87

-1.940

2.97-15.04

24.18

-1.097

2.96-15.04

22.17

35 min 120 min

a


TABLE 6: EXAFS-Obtained Structural Parameters for a Ta(OEt)5-acac Mixture Measured at Different Time Intervals time

A-Bsa

Nb

15 min

Ta-O Ta-O Ta-C Ta-Ta Ta-O Ta-O Ta-C Ta-O Ta-O Ta-C Ta-O Ta-O Ta-C

5.9 ( 0.6 2.0 ( 0.2 4.5 ( 0.7 0.0 ( 0.0 5.9 ( 0.6 1.9 ( 0.2 4.3 ( 0.7 5.9 ( 0.6 2.0 ( 0.2 4.4 ( 0.7 5.8 ( 0.6 1.9 ( 0.2 4.3 ( 0.7

50 min 85 min 950 min

a

rc (Å)

σd (Å)

EFe (eV)

k-range (Å-1)

R-factor

1.91 ( 0.02 2.14 ( 0.02 3.06 ( 0.03

0.071 ( 0.007 0.050 ( 0.005 0.097 ( 0.015

-0.285

2.97-15.02

25.25

1.91 ( 0.02 2.14 ( 0.02 3.07 ( 0.03 1.91 ( 0.02 2.14 ( 0.02 3.07 ( 0.03 1.91 ( 0.02 2.13 ( 0.02 3.06 ( 0.03

0.071 ( 0.007 0.050 ( 0.005 0.095 ( 0.014 0.071 ( 0.007 0.050 ( 0.005 0.097 ( 0.015 0.071 ( 0.007 0.055 ( 0.006 0.097 ( 0.015

-0.125

2.98-15.02

24.20

-0.585

2.97-15.03

24.08

0.362

3.00-15.02

25.97


and the stereochemistry and the coordination modes of these complexes are derived on the basis of vibrational spectroscopic data. The studies report that in the case of dihalobis(acetylacetonato) monomeric complexes with a cis-geometrical configuration and a six-coordinated metal are formed, while in the case of monohalide and tetrakis(acetylacetonato) complexes, 7and 8-fold coordination of the metal atom occur, respectively. No remarkable difference was pointed out between zirconium and hafnium, as expected by taking into account the similar chemical features (oxidation state, atomic radius, electronegativity, etc.) of the two metal atoms. The experimentally determined and theoretically calculated EXAFS functions in k-space and their Fourier transformations in real space for a Hf(OnBu)4-acac mixture measured at different time intervals at the Hf LIII-edge are shown in Figure 11, and the obtained structural parameters are given in Table 5. The EXAFS results indicate that the chemical modification with acac takes place immediately after mixing, thus causing an increase in the coordination number of the oxygen and carbon shells. In addition, a substantial increase in Hf-C and Hf-Hf distances

could also be observed (in comparison with Hf(OnBu)4) as a consequence of the chemical modification. Subsequent measurements performed at 35 and 120 min do not show any significant changes in the structural parameters in comparison with those obtained immediately after mixing. As far as chemical modification of tantalum alkoxides is concerned, reactions of tantalum pentaethoxide with different organic ligands have been extensively investigated by Mehrotra et al.58-60 Reactions with β-ketoesters61-62 and β-diketonate were furthermore thoroughly studied.63-64 The studies showed that Ta2(OEt)10 reacts with excess benzoylacetone (bzacH) to give Ta(OEt)2(bzac)3, which may contain eight-coordinate tantalum. Mehrotra and co-workers61,63,65 have also studied the reactions of niobium and tantalum alkoxides with acetylacetone, benzoylacetone, dibenzoylmethane, and methyl and ethyl acetoacetates, and only partial replacements of the alkoxy groups with the bidentate ligands was observed. In particular, in all of these papers, a similar behavior of tantalum pentoethoxide is demonstrated: The reaction with acetylacetone in 1:1 to 1:5 molar ratios leads to the formation of different substitution



Figure 12. Experimental (solid line) and calculated (dotted line) (a) EXAFS functions and (b) their Fourier transforms for a Ta(OEt)5-acac mixture measured at different time intervals at the Ta LIII-edge.

products Ta(OEt)x(acac)5-x (x ) 4, 3, 2), where tantalum is 6-fold (x ) 4), 7-fold (x ) 3), or 8-fold (x ) 2) coordinated, respectively. The reactions proceed quite smoothly up to the formation of bis-derivative compounds, while the formation of the tris-derivatives could be forced by insertion of β-diketone molecules

M(OEt)5 + nacacH / (OEt)(5-n)M(acac)n + nEtOH M) Nb, Ta In conclusion, the complete substitution does not take place, and this can be ascribed (a) to steric hindrance of the larger acac ligands or (b) to a saturated coordination state of the metal in the tris-substituted derivative and to the inability of tantalum to increase the coordination number beyond 8 in these derivatives. As a matter of fact, a general trend observed in these and similar reactions is that the first and second substitutions take place immediately, while the third requires more severe reaction conditions and a larger excess (1:5) of acetylacetone.58-60 These derivatives are generally monomeric and can be separated by in vacuo distillation. The experimentally determined and theoretically calculated EXAFS functions in k-space and their Fourier transformations in real space for a Ta(OEt)5-acac mixture measured at different time intervals at the Ta LIII-edge are shown in Figure 12. For clarity reasons, representative spectra measured at three different time intervals are only shown. The EXAFS-determined structural parameters are given in Table 6. The EXAFS results indicate that the chemical modification with acac occurs immediately (within 15 min), thus causing an increase in the coordination number of the first shell oxygen atoms in comparison with the parameters obtained for pure Ta(OEt)5. As a consequence of chemical modification, one could no longer observe the dimeric structure found in pure Ta(OEt)5, as the tantalum shell at 3.49 Å could no longer be determined. This could be due to the

Figure 13. Comparison of the XANES regions of pure Hf(OnBu)4 (solid line) and acac-modified Hf(OnBu)4 (dashed line).

decrease in the degree of aggregation in Ta(OEt)5 as a result of acac coordination. Subsequent measurements performed at 50, 85, and 950 min did not show any significant changes in the structural parameters in comparison with those obtained immediately after mixing. The XANES regions of Hf(OnBu)4 and Ta(OEt)5 before and after the addition of acac are presented in Figures 13 and 14, respectively. In both cases, a significant increase in the intensity of the white line (the peak of the absorption edge) could be


Krishnan et al.

Figure 16. Raman spectra of a Ta(OEt)5-acac mixture measured at different time intervals along with Ta(OEt)5: (a) Ta(OEt)5, (b) Ta(OEt)5 + acac (0 h), (c) Ta(OEt)5 + acac (24 h), and (d) Ta(OEt)5 + acac (48 h). Figure 14. Comparison of the XANES regions of pure Ta(OEt)5 (solid line) and acac-modified Ta(OEt)5 (dashed line).

Figure 15. Raman spectra of a Hf(OnBu)4-acac mixture measured at different time intervals along with Hf(OnBu)4: (a) Hf(OnBu)4, (b) Hf(OnBu)4 + acac (0 h), (c) Hf(OnBu)4 + acac (24 h), and (d) Hf(OnBu)4 + acac (48 h).

observed upon the addition of acac, which could be attributed to a change in the coordination geometry around the hafnium/ tantalum atom from a 6-fold to an 8-fold coordination. This result is in agreement with an earlier investigation on the chemical modification of zirconium alkoxides by the same ligand.35 Raman spectroscopic measurements were performed on alkoxide-acac mixture in the same molar ratio as that used for EXAFS measurements to further investigate the results observed from EXAFS analysis. The Raman spectra of an alkoxideacac mixture measured at different time intervals are shown along with the spectrum of the corresponding pure alkoxide in Figures 15 and 16 for Hf(OnBu)4 and Ta(OEt)5, respectively. In the Raman spectra of a Hf(OnBu)4-acac mixture and a Ta(OEt)5-acac mixture, the instantaneous coordination of acac to the metal could be demonstrated. As a result of the coordination of acac, a substantial increase in the Raman

intensity was observed for the peaks at 1290 and 1370 cm-1, which correspond to the symmetric C-C stretching modes and the symmetric methyl deformation, respectively.57 The ligand can be attached to one central atom through one coordinating atom (monodentate) or through two coordinating atoms (chelating bidentate).13,66 Generally the latter form of coordination is energetically more favored than the monodentate one. Another possibility for coordination of β-diketones is that the ligand forms a bridge between two different central metal atoms (bridging bidentate). In the literature it is reported that a distinction between monodentate and bidentate can be successfully performed, but in the case of the existence of bidentate no clear distinction between a chelating or bridging attachment of the ligand is possible.24,13,57 In a Hf(OnBu)4-acac mixture, the peak at 1605 cm-1 corresponding to the C-O stretching modes of the free enol form of acac and the bands around 1700 cm-1 corresponding to the CdO stretching vibrations of the free keto form of acac could not be demonstrated. These results indicate that all of the carbonyl groups are coordinated, which was also observed in the studies reported on acac modification of titanium alkoxides.24 Furthermore, the complete disappearance of the bands around 1700 cm-1, in comparison to pure acac, indicates that a bidentate coordination has occurred. Moreover, the spectra acquired at 24 and 48 h after mixing do not show any variations compared to the spectrum collected immediately after mixing, indicating that the coordination of acac is stable even 48 h after mixing. In a Ta(OEt)5-acac mixture, the peak at 1605 cm-1 corresponding to the C-O stretching modes of the free enol form of acac can be demonstrated, indicating the presence of some uncoordinated acac moieties. It can be hypothesized that of the two acac molecules per tantalum atom only one is coordinated. Due to this reason, it could not be unequivocally clarified whether a monodentate or a bidentate coordination has occurred. Furthermore, the spectra acquired at 24 and 48 h after mixing did not show any variations compared to the spectrum collected immediately after mixing, indicating that the coordination of acac is stable even 48 h after mixing, and the persistence of the peak at 1605 cm-1 shows that further coordination of the ligand did not occur during this time period.



Figure 17. Experimental (solid line) and calculated (dotted line) (a) EXAFS functions and (b) their Fourier transforms for a Hf(OnBu)4, acac, and water mixture measured at different time intervals at the Hf LIII-edge.

TABLE 7: EXAFS-Obtained Structural Parameters for a Hf(OnBu)4, acac, and Water Mixture Measured at Different Time Intervals time

A-Bsa

Nb

rc (Å)

σd (Å)

EFe (eV)

k-range (Å-1)

R-factor

0 min

Hf-O Hf-C Hf-Hf Hf-O Hf-C Hf-Hf Hf-O Hf-C Hf-Hf Hf-O Hf-C Hf-Hf Hf-O Hf-C Hf-Hf Hf-O Hf-C Hf-Hf Hf-O Hf-C Hf-Hf

8.0 ( 0.8 5.0 ( 0.8 1.3 ( 0.3 8.0 ( 0.8 5.2 ( 0.8 1.1 ( 0.3 8.0 ( 0.8 5.0 ( 0.8 1.4 ( 0.3 8.0 ( 0.8 5.0 ( 0.8 1.3 ( 0.3 8.0 ( 0.8 4.9 ( 0.8 1.4 ( 0.3 8.0 ( 0.8 4.9 ( 0.8 1.5 ( 0.3 8.0 ( 0.8 5.1 ( 0.8 1.5 ( 0.3

2.15 ( 0.02 3.20 ( 0.03 3.48 ( 0.04 2.15 ( 0.02 3.20 ( 0.03 3.48 ( 0.04 2.15 ( 0.02 3.19 ( 0.03 3.51 ( 0.04 2.15 ( 0.02 3.21 ( 0.03 3.50 ( 0.04 2.15 ( 0.02 3.21 ( 0.03 3.50 ( 0.04 2.15 ( 0.02 3.20 ( 0.03 3.51 ( 0.04 2.15 ( 0.02 3.20 ( 0.03 3.51 ( 0.04

0.089 ( 0.009 0.081 ( 0.012 0.092 ( 0.018 0.089 ( 0.009 0.087 ( 0.013 0.087 ( 0.017 0.089 ( 0.009 0.077 ( 0.012 0.092 ( 0.018 0.092 ( 0.009 0.081 ( 0.012 0.097 ( 0.019 0.092 ( 0.009 0.081 ( 0.012 0.097 ( 0.019 0.092 ( 0.009 0.077 ( 0.012 0.100 ( 0.020 0.092 ( 0.009 0.081 ( 0.012 0.097 ( 0.019

-1.792

2.98-15.04

21.06

-1.737

2.98-15.04

24.36

-1.187

3.00-15.04

24.82

-1.543

2.99-15.04

17.02

-1.539

2.99-15.04

16.74

-1.502

2.99-15.04

16.51

-1.057

2.97-14.79

16.38

15 min 30 min 85 min 165 min 240 min 410 min

a


3.4. Hydrolysis and Condensation Behavior of Chemically Modified Alkoxides. Investigations on the hydrolysis and condensation reactions of Ta and Hf metal alkoxides in the presence of acac were performed to verify whether the addition of the ligand had induced any changes in the condensation behavior. The experimentally determined and theoretically calculated EXAFS functions in k-space and their Fourier transformations in real space for Hf(OnBu)4, acac, and water mixture measured at the Hf LIII-edge are shown in Figure 17. For reasons of clarity, representative spectra measured at three different time intervals are only shown. The structural parameters are summarized in

Table 7. The EXAFS results show that the condensation reaction has not occurred in the chemically modified hafnium alkoxide even 410 min after the addition of water, and the solution remained clear. This is demonstrated by the similarity in the obtained structural parameters in comparison with the parameters obtained before the addition of water. The coordination of the ligand to the hafnium atom has hindered the condensation reaction. The experimentally determined and theoretically calculated EXAFS functions in k-space and their Fourier transformations in real space for Ta(OEt)5, acac, and water mixture measured


Krishnan et al.

Figure 18. Experimental (solid line) and calculated (dotted line) (a) EXAFS functions and (b) their Fourier transforms for a Ta(OEt)5, acac, and water mixture measured at different time intervals at the Ta LIII-edge.

TABLE 8: EXAFS-Obtained Structural Parameters for a Ta(OEt)5, acac, and Water Mixture Measured at Different Time Intervals time 0 min

35 min

120 min

3630 min

a

A-Bsa

Nb

Ta-O Ta-O Ta-C Ta-Ta Ta-O Ta-O Ta-C Ta-Ta Ta-O Ta-O Ta-C Ta-Ta Ta-O Ta-O Ta-C Ta-Ta

5.8 ( 0.6 1.8 ( 0.2 4.1 ( 0.6 0.0 ( 0.0 4.7 ( 0.5 2.3 ( 0.2 3.7 ( 0.5 0.4 ( 0.1 4.6 ( 0.5 2.2 ( 0.2 3.9 ( 0.6 0.4 ( 0.1 4.7 ( 0.5 2.3 ( 0.2 4.1 ( 0.6 0.6 ( 0.1

rc (Å)

σd (Å)

EFe (eV)

k-range (Å-1)

R-factor

1.91 ( 0.02 2.14 ( 0.02 3.06 ( 0.03

0.071 ( 0.007 0.055 ( 0.006 0.095 ( 0.014

-0.572

3.00-15.02

31.46

1.91 ( 0.02 2.12 ( 0.02 3.08 ( 0.03 3.42 ( 0.04 1.91 ( 0.02 2.12 ( 0.02 3.07 ( 0.03 3.41 ( 0.04 1.91 ( 0.02 2.11 ( 0.02 3.06 ( 0.03 3.40 ( 0.04

0.059 ( 0.006 0.067 ( 0.007 0.071 ( 0.011 0.067 ( 0.013 0.063 ( 0.006 0.067 ( 0.007 0.077 ( 0.012 0.077 ( 0.015 0.067 ( 0.006 0.067 ( 0.007 0.084 ( 0.013 0.077 ( 0.015

-0.479

3.00-15.02

27.96

-0.803

2.99-15.01

28.86

-0.349

2.97-15.02

30.28


at the Ta LIII-edge are shown in Figure 18. For clarity reasons, representative spectra measured at three different time intervals are only shown. The EXAFS-obtained structural parameters are given in Table 8. The EXAFS results show that the condensation reaction has slowed down in the chemically modified tantalum alkoxide. The condensation starts in the time interval between 0 and 35 min after the addition of water to the acac-modified tantalum alkoxide, as demonstrated by the appearance of the tantalum backscatterer at 3.42 Å. However it proceeds very slowly, and even after 3630 min a substantial increase in the coordination number of tantalum backscatterers cannot be demonstrated. Raman spectroscopic measurements were performed on solutions with the same molar ratio as that used for EXAFS measurements. Raman spectra measured immediately and after

subsequent time intervals following the addition of water to the alkoxide-acac mixture are shown along with the spectrum of the corresponding alkoxide-acac mixture in Figures 19 and 20 for Hf(OnBu)4 and Ta(OEt)5, respectively. In the Raman spectra of the Hf(OnBu)4-acac-water solution, the peak at 397 cm-1, corresponding to Hf-O vibrations, could be observed in all of the spectra, indicating the presence of hafnium species in the solution. Furthermore, the spectra acquired before and after the addition of water to the Hf(OnBu)4-acac mixture look alike, clearly indicating that the solution is stable even 96 h after preparation. Thus, from the Raman studies, it can be deduced that the chemical modification of Hf(OnBu)4 by acac has hindered the hydrolysis and condensation reaction in agreement with the EXAFS results.


Figure 19. Raman spectra of a Hf(OnBu)4-acac-water solution measured at different time intervals along with a Hf(OnBu)4-acac mixture: (a) Hf(OnBu)4 + acac (0 h), (b) Hf(OnBu)4 + acac + H2O (0 h), (c) Hf(OnBu)4 + acac + H2O (24 h), and (d) Hf(OnBu)4 + acac + H2O (96 h).

Figure 20. Raman spectra of a Ta(OEt)5-acac-water solution measured at different time intervals along with a Ta(OEt)5-acac mixture: (a) Ta(OEt)5 + acac (0 h), (b) Ta(OEt)5 + acac + H2O (0 h), and (c) Ta(OEt)5 + acac + H2O (24 h).

In the Raman spectra of the Ta(OEt)5-acac-water solution, even though the decrease in the intensity of the Ta-O peak at 588 cm-1 could not be clearly observed, it is interesting to note the time-dependent decrease in the intensity of Ta(-O-C) peak at 1080 cm-1 owing to the condensation reaction. Thus, from the Raman studies, it can be inferred that the chemical modification of Ta(OEt)5 by acac has slowed down the hydrolysis and condensation reaction in agreement with the EXAFS results. 3.5. Structural Investigations of a Mixture of Alkoxides. Structural investigations on the mixture of metal alkoxide with prehydrolyzed Si(OEt)4 were carried out by means of EXAFS and Raman spectroscopy to determine whether the addition of another alkoxide alters the structure of the metal alkoxide thereby building a mixed Si-O-M (M ) Hf, Ta) bond. The experimentally determined and the theoretically calculated EXAFS functions in k-space and their Fourier transformations in real space for metal alkoxides mixed with prehydrolyzed Si(OEt)4 (measured at different time intervals) are shown in

J. Phys. Chem. B, Vol. 111, No. 26, 2007 7513 Figures 21 and 22, and the obtained structural parameters are summarized in Tables 9 and 10 for Hf(OnBu)4 and Ta(OEt)5, respectively. For both of the alkoxides, the EXAFS analysis reveals that the structural parameters for the mixture of alkoxides at different time intervals were similar to each other and in agreement with those of the corresponding pure alkoxides. The results indicate that the addition of prehydrolyzed Si(OEt)4 does not change the structure around the metal atoms. Even after aging (300 and 420 min in the cases of Hf(OnBu)4 and Ta(OEt)5, respectively) of the mixture of the two species, they remain as separate entities in the solution. Furthermore, M-O-Si (M ) Hf, Ta) mixed bonds could not be demonstrated from the EXAFS studies. The Raman spectra recorded for a metal alkoxide mixed with prehydrolyzed Si(OEt)4 at different time intervals are shown along with the spectrum of the corresponding pure alkoxide and Si(OEt)4 in Figures 23 and 24 for Hf(OnBu)4 and Ta(OEt)5, respectively. In both cases, the peak at 658 cm-1 characteristic of Si-O vibrations in Si(OEt)4 could be observed along with the corresponding M-O peak from the metal alkoxide (397 cm-1 in Hf(OnBu)4 and 588 cm-1 in Ta(OEt)5). The spectra acquired at different time intervals do not show any remarkable changes in comparison to the spectra obtained immediately after mixing. From the Raman investigations, it could not be unequivocally ascertained whether there are M-O-Si bonds in the mixture of alkoxides. 3.6. Hydrolysis and Condensation Behavior of a Mixture of Alkoxides. Hydrolysis and condensation behaviors of hafnium and tantalum alkoxides mixed with prehydrolyzed Si(OEt)4 were investigated to study the influence of the addition of prehydrolyzed Si(OEt)4 on the formation of the condensation product, especially to investigate whether in the formed gel, homo- or heterocondensation species are present. Si(OEt)4, which is remarkably less reactive than the hafnium and tantalum alkoxides, was prehydrolyzed to match their hydrolysiscondensation reactions. The experimentally determined and the theoretically calculated EXAFS functions in k-space and their Fourier transformations in real space for metal alkoxides, prehydrolyzed Si(OEt)4, and water mixtures measured at different time intervals are shown in Figures 25 and 26, and the obtained structural parameters are summarized in Tables 11 and 12 for Hf(OnBu)4 and Ta(OEt)5, respectively. Again for clarity reasons only three representative spectra are shown for each case. The evaluation of the EXAFS spectra indicates that the hydrolysis and condensation reactions occur instantaneously upon the addition of water to the mixture of alkoxides, as confirmed by the immediate formation of a turbid solution in both cases. It is interesting to note that the obtained structural parameters are in agreement with those obtained when water was added to the corresponding pure alkoxide in the same molar ratio. In particular, a higher backscatterer at a distance of 3.44 Å could be determined in the case of a Hf(OnBu)4-prehydrolyzed Si(OEt)4-water mixture, which could be unequivocally assigned to hafnium, thus proving the formation of Hf-O-Hf species, whereas in the case of a Ta(OEt)5-prehydrolyzed Si(OEt)4-water mixture, an increase in the coordination number of tantalum backscatterers at a distance of about 3.36 Å could be observed, indicating the development of homocondensation species. Moreover, the subsequent measurements performed at different time intervals revealed no remarkable changes in the structural parameters in comparison with the measurements


Krishnan et al.

Figure 21. Experimental (solid line) and calculated (dotted line) (a) EXAFS functions and (b) their Fourier transforms for Hf(OnBu)4 mixed with prehydrolyzed Si(OEt)4 measured at different time intervals at the Hf LIII-edge.

Figure 22. Experimental (solid line) and calculated (dotted line) (a) EXAFS functions and (b) their Fourier transforms for Ta(OEt)5 mixed with prehydrolyzed Si(OEt)4 measured at different time intervals at the Ta LIII-edge.

performed immediately after mixing. Furthermore, in both cases, M-O-Si mixed bonds attributable to the formation of a heterocondensation product could not be demonstrated, which could be due to the differences in the reactivity of correspond-

ing metal alkoxides and the prehydrolyzed Si(OEt)4 with respect to their condensation behavior. The results were further confirmed by Raman spectroscopic investigations performed under similar reaction conditions. The



TABLE 9: EXAFS-Obtained Structural Parameters for Hf(OnBu)4 Mixed with Prehydrolyzed Si(OEt)4 Measured at Different Time Intervals time

A-Bsa

Nb

rc (Å)

σd (Å)

EFe (eV)

k-range (Å-1)

R-factor

0 min

Hf-O Hf-O Hf-C Hf-Hf Hf-O Hf-O Hf-C Hf-Hf Hf-O Hf-O Hf-C Hf-Hf

2.3 ( 0.2 4.5 ( 0.5 1.7 ( 0.3 1.2 ( 0.2 2.2 ( 0.2 4.4 ( 0.5 2.2 ( 0.3 1.1 ( 0.2 2.2 ( 0.2 4.3 ( 0.5 2.2 ( 0.3 1.1 ( 0.2

1.95 ( 0.02 2.13 ( 0.02 3.13 ( 0.03 3.41 ( 0.04 1.95 ( 0.02 2.13 ( 0.02 3.13 ( 0.03 3.42 ( 0.04 1.96 ( 0.02 2.14 ( 0.02 3.13 ( 0.03 3.43 ( 0.04

0.050 ( 0.005 0.081 ( 0.008 0.067 ( 0.010 0.071 ( 0.014 0.050 ( 0.005 0.077 ( 0.008 0.084 ( 0.013 0.074 ( 0.015 0.050 ( 0.005 0.077 ( 0.008 0.077 ( 0.012 0.074 ( 0.014

0.866

2.97-15.01

37.10

1.560

3.00-15.02

36.97

0.782

2.97-15.01

31.23

160 min

300 min

a


TABLE 10: EXAFS-Obtained Structural Parameters for Ta(OEt)5 Mixed with Prehydrolyzed Si(OEt)4 Measured at Different Time Intervals time

A-Bsa

Nb

rc (Å)

σd (Å)

EFe (eV)

k-range (Å-1)

R-factor

35 min


4.6 ( 0.5 2.7 ( 0.3 3.9 ( 0.6 1.0 ( 0.2 4.7 ( 0.5 2.4 ( 0.3 3.7 ( 0.5 1.0 ( 0.2 4.4 ( 0.4 2.6 ( 0.3 3.9 ( 0.6 1.0 ( 0.2

1.91 ( 0.02 2.13 ( 0.02 3.07 ( 0.03 3.50 ( 0.04 1.91 ( 0.02 2.12 ( 0.02 3.07 ( 0.03 3.49 ( 0.04 1.91 ( 0.02 2.12 ( 0.02 3.08 ( 0.03 3.49 ( 0.04

0.063 ( 0.006 0.084 ( 0.008 0.089 ( 0.013 0.087 ( 0.017 0.063 ( 0.006 0.071 ( 0.007 0.087 ( 0.013 0.071 ( 0.014 0.063 ( 0.006 0.071 ( 0.007 0.081 ( 0.012 0.071 ( 0.014

-1.007

2.98-15.01

33.36

-0.341

2.97-15.02

36.39

-2.473

3.00-15.00

43.47

210 min

420 min

a


Figure 23. Raman spectra of Hf(OnBu)4 mixed with prehydrolyzed Si(OEt)4 measured at different time intervals along with pure Hf(OnBu)4 and Si(OEt)4: (a) Si(OEt)4, (b) Hf(OnBu)4, (c) Si(OEt)4 + Hf(OnBu)4 (0 h), (d) Si(OEt)4 + Hf(OnBu)4 (24 h), and (e) Si(OEt)4 + Hf(OnBu)4 (48 h).

Raman spectra measured for metal alkoxide, prehydrolyzed Si(OEt)4, and water mixtures at different time intervals are shown along with the spectrum of the corresponding pure alkoxide-prehydrolyzed Si(OEt)4 mixture in Figures 27 and 28 for Hf(OnBu)4 and Ta(OEt)5, respectively. In the Raman spectra of the metal alkoxide, prehydrolyzed Si(OEt)4, and water mixture, the characteristic peaks belonging to M-O and Si-O vibrations could be noticed. In the spectra of the Hf(OnBu)4, prehydrolyzed Si(OEt)4, and water mixture, the decrease in the intensity of the Hf-O peak at 397 cm-1 and

Figure 24. Raman spectra of Ta(OEt)5 mixed with prehydrolyzed Si(OEt)4 measured at different time intervals along with pure Ta(OEt)5 and Si(OEt)4: (a) Si(OEt)4, (b) Ta(OEt)5, (c) Si(OEt)4 + Ta(OEt)5 (0 h), (d) Si(OEt)4 + Ta(OEt)5 (24 h), and (e) Si(OEt)4 + Ta(OEt)5 (48 h).

the Hf(-O-C) bands at 1030 and 1080 cm-1 could be demonstrated owing to the formation of the condensation product. In the spectra of the Ta(OEt)5, prehydrolyzed Si(OEt)4, and water mixture, the decrease in the intensity of the Ta-O peak at 588 cm-1 and the Ta(-O-C) band at 1080 cm-1 could be observed, which indicates the decrease of tantalum species in the solution owing to condensation. In both cases, the spectrum measured 24 h after mixing does not show any significant variations in comparison to the spectrum acquired immediately after mixing. From the Raman studies, it could not be unambiguously determined whether there are M-O-Si bonds.


Krishnan et al.

Figure 25. Experimental (solid line) and calculated (dotted line) (a) EXAFS functions and (b) their Fourier transforms for a Hf(OnBu)4, prehydrolyzed Si(OEt)4, and water mixture measured at different time intervals at the Hf LIII-edge.

Figure 26. Experimental (solid line) and calculated (dotted line) (a) EXAFS functions and (b) their Fourier transforms for Ta(OEt)5, prehydrolyzed Si(OEt)4, and water mixture measured at different time intervals at the Ta LIII-edge.

4. Conclusions The XAFS and Raman spectroscopic investigations on Hf(OnBu)4 and Ta(OEt)5 reveal that they have a dimeric structure. The results show that both of the transition metal

alkoxides instantaneously undergo hydrolysis and condensation upon the addition of water and no remarkable structural changes take place at subsequent time intervals. The chemical modification of the alkoxides by acetylacetone occurs immediately,



TABLE 11: EXAFS-Obtained Structural Parameters for a Hf(OnBu)4, Prehydrolyzed Si(OEt)4, and Water Mixture Measured at Different Time Intervals time 0 min 100 min 1750 min

a

A-Bsa

Nb

rc (Å)

σd (Å)

EFe (eV)

k-range (Å-1)

R-factor


6.7 ( 0.7 5.3 ( 0.8 2.1 ( 0.4 6.6 ( 0.7 5.4 ( 0.8 2.3 ( 0.4 6.7 ( 0.7 5.3 ( 0.8 2.2 ( 0.4

2.12 ( 0.02 3.19 ( 0.03 3.45 ( 0.04 2.12 ( 0.02 3.18 ( 0.03 3.45 ( 0.04 2.11 ( 0.02 3.17 ( 0.03 3.44 ( 0.04

0.100 ( 0.010 0.120 ( 0.018 0.087 ( 0.017 0.097 ( 0.010 0.120 ( 0.018 0.095 ( 0.019 0.097 ( 0.010 0.114 ( 0.017 0.092 ( 0.018

-0.680

2.98-15.05

32.96

-0.463

2.99-15.05

32.40

-0.465

2.99-15.05

30.23


TABLE 12: EXAFS-Obtained Structural Parameters for a Ta(OEt)5, Prehydrolyzed Si(OEt)4, and Water Mixture Measured at Different Time Intervals time

A-Bsa

Nb

rc (Å)

σd (Å)

EFe (eV)

k-range (Å-1)

R-factor

0 min


4.5 ( 0.5 2.8 ( 0.3 4.9 ( 0.7 1.6 ( 0.3 4.8 ( 0.5 2.5 ( 0.3 4.5 ( 0.7 1.6 ( 0.3 4.4 ( 0.4 2.4 ( 0.2 4.9 ( 0.7 1.6 ( 0.3

1.93 ( 0.02 2.11 ( 0.02 3.06 ( 0.03 3.37 ( 0.04 1.92 ( 0.02 2.11 ( 0.02 3.04 ( 0.03 3.37 ( 0.04 1.91 ( 0.02 2.08 ( 0.02 3.00 ( 0.03 3.36 ( 0.04

0.071 ( 0.007 0.087 ( 0.009 0.095 ( 0.014 0.097 ( 0.019 0.074 ( 0.007 0.087 ( 0.009 0.097 ( 0.015 0.095 ( 0.019 0.074 ( 0.007 0.084 ( 0.008 0.114 ( 0.017 0.100 ( 0.020

-3.480

2.99-14.99

42.20

-1.461

3.00-15.01

36.97

-0.751

2.99-15.01

37.16

35 min

120 min

a


Figure 27. Raman spectra of a Hf(OnBu)4, prehydrolyzed Si(OEt)4, and water mixture measured at different time intervals along with a Hf(OnBu)4-prehydrolyzed Si(OEt)4 mixture: (a) Si(OEt)4 + Hf(OnBu)4 (0 h), (b) Si(OEt)4 + Hf(OnBu)4 + H2O (0 h), and (c) Si(OEt)4 + Hf(OnBu)4 + H2O (24 h).

Figure 28. Raman spectra of a Ta(OEt)5, prehydrolyzed Si(OEt)4, and water mixture measured at different time intervals along with a Ta(OEt)5-prehydrolyzed Si(OEt)4 mixture: (a) Si(OEt)4 + Ta(OEt)5 (0 h), (b) Si(OEt)4 + Ta(OEt)5 + H2O (0 h), and (c) Si(OEt)4 + Ta(OEt)5 + H2O (24 h).

thereby causing a change in the coordination geometry around the metal atom from a 6-fold to an 8-fold coordination. The Raman spectroscopic investigations on Hf(OnBu)4 reveal that no free carbonyl groups are present and that a bidentate coordination has occurred, whereas the studies on Ta(OEt)5 evidence that not all the carbonyl groups from the ligand were involved in the coordination process. The hydrolysis and condensation reactions have been hindered in the chemically modified Hf(OnBu)4 and Ta(OEt)5 solutions due to the coordination of the ligand. This fast and stable coordination with acetylacetone can be chiefly ascribed to the fact that Hf and Ta, being transition metal alkoxides, have d orbitals available for coor-

dination and a strong tendency to form stable coordination compounds. In addition, the structure of the metal alkoxides is not altered by the addition of prehydrolyzed Si(OEt)4, and even after 48 h the alkoxides remain as individual entities in the solution. The investigations on the hydrolysis and condensation reactions of this mixture of alkoxides reveal the formation of homocondensation products in both cases, which could be attributed to the differences in the reactivities of the different alkoxides. Acknowledgment. The Italian Rectors Conference (CRUI), Rome, Italy, and Deutscher Akademischer Austauschdienst (DAAD), Bonn, Germany, are gratefully acknowledged for

7518 J. Phys. Chem. B, Vol. 111, No. 26, 2007 funding the researchers’ mobility in the framework of a Vigoni Programme. S.G. gratefully thanks the DAAD also for financing the research period in Germany. L.A. is indebted to the research program FIRB RBNE033KMA, “Molecular Compounds and Hybrid Nanostructured Materials with Resonant and Nonresonant Optical Properties for Photonic Devices” for financial support. We thank ANKA at FZK, Karlsruhe, Germany, for the provision of synchrotron radiation for XAFS measurements. References and Notes (1) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: New York, 1990. (2) Turova, N. Y.; Turevskaya, E. P.; Kessler, V. G.; Yanovskaya, M. I. in The Chemistry of Metal Alkoxides; Kluwer Academic Publishers: Boston, 2002. (3) Hubert-Pfalzgraf, L. G. New J. Chem. 1987, 11, 663-675. (4) Hubert-Pfalzgraf, L. G. Appl. Organomet. Chem. 1992, 6, 627643. (5) (a) Bradley, D. C.; Mehrotra, R. C.; Gaur, D. P. Metal Alkoxides; Academic Press: London, 1978. (b) Mehrotra, R. C. AdV. Inorg. Chem. Radiochem. 1983, 26, 269. (c) Mehrotra, R. C.; Batwara, J. M.; Kapoor, P. N. Coord. Chem. ReV. 1980, 31, 67. (d) Aegerter, M. A. Sol-Gel: Science Technology, Proceedings of the Winter School on Glasses and Ceramics from Gels, Sa˜o Carlos, Brazil, Aug 14-19, 1989; World Scientific: Singapore, 1989. (e) Mehrotra, R. C. J. Sol-Gel Sci. Technol. 1998, 100, 1-15. (6) Guglielmi, M.; Carturan, G. J. Non-Cryst. Solids 1988, 100, 16. (7) Bradley, D. C.; Holloway, H. Can. J. Chem. 1962, 40, 62-72. (8) (a) Hubert-Pfalzgraf, L. G. In Chemical Processing of Ceramics; Lee, B. I., Pope, E. J. A.; Marcel Dekker: New York, 1994; Chapter 2, pp 23-57. (b) Hubert-Pfalzgraf, L. G. Coord Chem. ReV. 1998, 178-180, 967-998. (c) Hubert-Pfalzgraf, L. G. Polyhedron 1994, 13, 1081. (9) (a) Sanchez, C.; Livage, J. New. J. Chem. 1990, 14, 513-521 and references therein. (b) Sanchez, C.; Livage, J.; Henry, M.; Babonneau F. J. Non-Cryst. Solids 1988, 100, 65-76. (10) Schubert, U.; Bauer, U.; Fric, H.; Puchberger, M.; Rupp, W.; Torma, V. Mater. Res. Soc. Symp. Proc. 2005, 847, 533-539. (11) Schubert, U. Chem. Mater. 2005, 15, 3701. (12) Fric, H.; Kogler, F. R.; Puchberger, M.; Schubert, U. Z. Naturforsch., B: Chem. Sci. 2004, 59, 1241-1245. (13) Mehrotra, R. C.; Bohra, R.; Gaur, D. P. Metal β-Diketonates and Allied DeriVatiVes; Academic Press: London, 1978. (14) Sanchez, C.; Babonneau, F.; Doeuff, S.; Leaustic A. In Ultrastructure Processing of Ceramics, Glasses, and Composites; Hench, L. L., Ulrich, D. R., Eds.; Wiley: New York, 1984. (15) Kickelbick, G. Prog. Polym. Sci. 2003, 28, 83-114. (16) Schubert, U. Chem. Mater. 2001, 13, 3487-3494 and references therein. (17) Sanchez, C.; In, M. J. Non-Cryst. Solids, 1992, 147-148, 1-12. (18) (a) Trimmel, G.; Gross, S.; Kickelbick, G.; Schubert, U. Appl. Organomet. Chem. 2001, 15, 410-406. (b) Moraru, B.; Gross, S.; Kickelbick, G.; Trimmel G.; Schubert, U. Monatsh. Chem. 2001, 132, 993999. (c) Gross, S.; Di Noto, V.; Kickelbick G.; U Schubert Mater. Res. Soc. Symp. Proc. 2002, 726, Q4.1.1-Q4.1.9. (d) Kickelbick, G.; Feth, M. P.; Bertagnolli, H.; Puchberger, M.; Holzinger, D.; Gross, S. J. Chem. Soc., Dalton Trans. 2002, 20, 3892-3898. (e) Gross, S.; Kickelbick, G.; Puchberger, M.; Schubert, U. Monatsh. Chem. 2003, 134, 1053-1063. (f) Moraru, B.; Kickelbick, G.; Schubert, U. Eur. J. Inorg. Chem. 2001, 5, 1295-1301. (g) Moraru, B. A.; Gross, S.; Kickelbick, G.; Trimmel, G.; Schubert, U. Monatsh. Chem. 2001, 132, 993-999. (h) Jupa, M.; Kickelbick G.; Schubert, U. Eur. J. Inorg. Chem. 2004, 9, 1835-1839. (i) Kogler, F. R.; Jupa, M.; Puchberger, M.; Schubert, U. J. Mater. Chem. 2004, 14, 31333138. (j) Fric, H.; Jupa, M.; Schubert, U. Monatsh. Chem. 2006, 137, 1-6. (19) Turova, N. Y.; Turevskaya, E. P.; Yanovskaya, M. I.; Yanovsky, A. I.; Kessler, V. G.; Tcheboukov, D. E. Polyhedron 1998, 17, 899. (20) Cattey, H.; Audebert, P.; Sanchez, C.; Hapiot P. J. Phys. Chem. B 1998, 102, 1193-1202. (21) Unuma, H.; Tokoka, K.; Suzuki, Y.; Furusaki, T.; Kodaira, K.; Matsushida, T. J. Mater. Sci. Lett. 1986, 5, 1248. (22) Deskibar, J. C. J. Non-Cryst. Solids 1986, 86, 231. (23) Deskibar, J. C. J. Mater. Sci. 1985, 20, 231. (24) (a) Leaustic, A.; Babonneau, F.; Livage, J. Chem. Mater. 1989, 1, 240-247. (b) Leaustic, A.; Babonneau, F.; Livage, J. 1989, 1, 248252.

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Structural Investigations on the Hydrolysis and Condensation

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