Surface-Mediated Synthesis and Spectroscopic Characterization of

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Surface-Mediated Synthesis and Spectroscopic Characterization of Tantalum Clusters on Silica Sailendra Nemana and Bruce C. Gates* Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, California 95616 ReceiVed April 6, 2006. In Final Form: May 31, 2006 Tantalum clusters were synthesized on the surface of porous silica by treatment of adsorbed Ta(CH2Ph)5 in H2 at temperatures in the range of 523-723 K. The surface species were characterized by UV-vis, far-infrared, and extended X-ray absorption fine-structure (EXAFS) spectroscopies, each of which provided evidence of Ta-Ta bonds similar to those in well-characterized molecular tantalum clusters. The Ta-Ta distance determined by EXAFS spectroscopy was 2.93 Å. The chemistry of the cluster synthesis is similar to that of syntheses of similar tantalum clusters in solution. The supported clusters formed at 523 K are characterized by an EXAFS first-shell Ta-Ta coordination number of nearly 2, indicative of tri-tantalum clusters, although it is expected that a mixture of clusters was present, and reduction in H2 at higher temperatures led to larger tantalum clusters. This is the first example of the surface-mediated synthesis of an early transition metal cluster, and the supported clusters reported here are the first to have been characterized by all three of the spectroscopic methods mentioned above. The similarity of the surface synthesis to that in solution points to opportunities to extend this new class of material to other early transition metal clusters on various supports.

Introduction Among the large class of metal cluster compounds,1 numerous clusters of group 8 metals (and clusters of the group 7 metal Re) have been synthesized on surfaces of oxides.2 Syntheses mediated by the support surfaces typically proceed by chemistry similar to that of the syntheses in solution,3 and researchers have developed methods for modifying surfaces (such as by changing their acid-base properties) to control the synthesis chemistry and modify the yields of various products.4 Although there are many known clusters of early transition metals, there are no reports of the synthesis of such clusters on surfaces. We now report the surface-mediated synthesis of tantalum clusters on SiO2, with the chemistry being analogous to that developed by the Cotton group5 for such clusters in solution. SiO2-supported tantalum clusters were prepared from the precursor Ta(CH2Ph)56 and characterized by UV-vis, far-infrared (far-IR), and extended X-ray absorption fine structure (EXAFS) spectroscopies. The results show that the chemistry of the cluster synthesis on SiO2 closely parallels that in solution, and the X-ray absorption spectra indicate the presence of clusters that are well approximated as tri-tantalum. Experimental Section Materials and Synthesis. Sample synthesis and handling were performed under anaerobic and anhydrous conditions with Schlenk lines and dryboxes. The precursor Ta(CH2Ph)5 was synthesized from TaCl5 (Strem, 99.99%) and C6H5CH2MgCl (Aldrich, 1 M) by the method of Groysman et al.6 The SiO2-supported samples, containing 0.6-1.2 wt % Ta, were prepared by slurrying a solution of Ta(CH2Ph)5 in dry, degassed mixed hexanes with SiO2 powder (fumed, * Author to whom correspondence should be addressed. E-mail: [email protected]. (1) Braunstein, P.; Oro, L. A.; Raithby, P. R. Metal Clusters in Chemistry; Wiley-VCH: Weinheim, Germany, 1999. (2) Guzman, J.; Gates, B. C. Dalton Trans. 2003, 17, 3303. (3) Lamb, H. H.; Fung, A. S.; Tooley, P. A.; Puga, J.; Krause T. R.; Kelley, M. K.; Gates, B. C. J. Am. Chem. Soc. 1989, 111, 8367. (4) Cariati, E.; Roberto, D.; Ugo, R.; Lucenti, E. Chem. ReV. 2003, 103, 3707. (5) Babaian-Kibala, E.; Cotton, F. A.; Shang, M. Inorg. Chem. 1990, 29, 5148.

Degussa Aerosil 200, Brunauer-Emmett-Teller (BET) surface area approximately 200 m2/g, partially dehydroxylated in vacuo at 773 K). After the slurry was stirred overnight (12 h), the solvent was removed by evacuation. The resultant solid was pale yellow; it was nearly white for Ta loadings less than 1 wt %. The Ta contents of the samples were determined by inductively coupled plasma analysis. Each solid sample, handled as air-sensitive, was treated in flowing H2 at 1 bar and 523 K to form the supported clusters, turning brown as a result of the treatment. The time required for the sample to turn brown under H2 at 523 K varied with the Ta loading. Lower loadings required longer treatment times; for example, a sample containing 0.6 wt % Ta required 3 days; that containing 1.2 wt % Ta required only 20 h. IR Spectroscopy. Powder samples in a N2-filled glovebox were pressed between two KBr windows of an environmentally controlled IR cell (International Crystal Laboratories) and transferred to the spectrometer (Bruker IFS 66v) for transmission spectroscopy of the solid samples under vacuum. Measurements of spectra in the midIR region were measured with a triglycine sulfate (TGS) detector. The reported spectra are the averages of 64 scans, obtained with a spectral resolution of 4 cm-1. Spectra were baseline-corrected. For measurements of spectra in the far-IR region (50-220 cm-1), approximately 50 mg of sample was loaded between two highdensity polyethylene windows in the environmentally controlled cell. Spectra were recorded with evacuated samples at room temperature with a TGS detector and a 12 µm Mylar beam splitter. Each spectrum is the average of 512 scans acquired over a period of 1 h with a spectral resolution of 2 cm-1. Spectra were baselinecorrected, and the blank SiO2 spectrum was subtracted from the spectra of the samples. Ultraviolet-Visible Spectroscopy. Transmission UV-vis spectra were collected with a Perkin-Elmer Lambda 2 spectrometer. Optically transparent samples were prepared according to established procedures7 under dry and oxygen-free argon and sealed into polystyrene cuvettes in the glovebox prior to scanning. X-ray Absorption Spectroscopy. The X-ray absorption measurements were performed at beamline 3-2 of the Stanford Synchrotron Radiation Laboratory (SSRL) of the Stanford Linear (6) Groysman, S.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Organometallics 2003, 22, 3793. (7) Carraway, E. R.; Demas, J. N.; Degraff, B. A. Langmuir 1991, 7, 2991.

10.1021/la0609322 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/16/2006

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Table 1. EXAFS parameters characterizing the samples treated in flowing H2 at 523, 623, and 723 Ka temperature of sample treatment in H2 (K)

absorber - backscatterer pair

N

R (Å)

103 × ∆σ2 (Å2)

∆E0 (eV)

523

Ta-Ta Ta-O Ta-Si Ta-Olong

2.1 (0.05) 3.2 (0.1) 3.4 (0.04) 1.2 (0.2)

2.93 (0.006) 1.91 (0.002) 3.22 (0.1) 2.51 (0.01)

6.5 (0.6) 1.9 (0.3) 4.6 (0.6) 3.7 (1)

-0.2 (1) -9.8 (0.3) 2.6 (0.6) 8.5 (4)

623

Ta-Ta Ta-O Ta-Si Ta-Olong

3.8 (0.1) 3.5 (0.05) 2.7 (0.09) 1.3 (0.3)

2.95 (0.005) 1.91 (0.001) 3.22 (0.005) 2.52 (0.01)

5.8 (0.2) 3.8 (0.1) 0.17 (0.2) 6.4 (0.4)

-3.8 (0.7) -7.7 (0.3) 6.3 (0.4) -9.1 (2)

723

Ta-Ta Ta-O Ta-Si Ta-Olong

4.8 (0.3) 3.5 (0.1) 3.8 (0.2) 1.1 (0.2)

2.92 (0.01) 1.91 (0.002) 3.2 (0.01) 2.47 (0.03)

4.6 (1) 1.8 (0.3) 2.9 (1) 8.2 (4)

-1.94 (1) -5.7 (0.3 6.77 (0.8) -12 (2)

a Notation: N ) coordination number; R ) distance between absorber and backscatterer atoms; ∆σ2 ) Debye-Waller factor; ∆E0 ) inner potential correction. Numbers in parentheses are the calculated errors and represent precisions, not accuracies. Estimated accuracies are as follows: Ta-Ta, N ( 20%, R ( 0.02 Å, ∆σ2 ( 20%, ∆E0 ( 20%; Ta-O, N ( 30%, R ( 0.02 Å, ∆σ2 ( 25%, ∆E0 ( 20%; Ta-Si, N ( 50%, R ( 0.03 Å, ∆σ2 ( 30%, ∆E0 ( 20%.22

Acclerator Center, Stanford, CA, and at beamline X-18B of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory, Upton, NY. The storage ring electron energy was 3 GeV at the SSRL and 2.6 GeV at the NSLS; the ring current varied in the range of 50-100 mA at the SSRL and 140-240 mA at the NSLS. Samples were pressed into self-supporting wafers (0.15 mg, 0.6 × 2.0 cm2) in a N2-filled glovebox at each synchrotron and then loaded into a transmission cell equipped with beryllium windows. The cell, described elsewhere,8 was evacuated to a pressure of approximately 10-6 Torr and cooled to liquid-nitrogen temperature prior to data collection. EXAFS data were recorded at the Ta LIII edge (9881 eV). Higher harmonics in the X-ray beam were minimized by detuning of the monochromator by 20-25% at this edge. The resolution of the monochromator was 0.5 eV. Four scans were recorded for each sample. Spectra of tantalum foil were recorded simultaneously with spectra of the samples for calibration of the energy at the Ta LIII edge. Analysis of X-ray Absorption Spectra. With the Athena program contained in the IFFEFIT software,9 the spectra were deglitched, the background was subtracted, the data were normalized, and the EXAFS (chi) data were extracted. The normalized EXAFS function characterizing each sample is the average of four scans. Standard deviations of the experimental EXAFS functions are shown in the Supporting Information. The software XDAP10 was used to analyze the EXAFS data with a difference file technique.11 The functional that was minimized and the function used to model the data are given elsewhere.11 The postulated models used in the data fitting included Ta-Ta, Ta-O, and Ta-Si contributions. Reference backscattering amplitudes and phase shifts for these contributions were calculated with the software FEFF712 from crystallographic coordinates of the unit cells of the following known reference compounds: Ta,13 Ta2Si,14 and Ta2O5.15 The amplitude reduction factor was determined to be 0.95 by fitting the spectrum of tantalum foil by using the FEFFgenerated reference for tantalum metal; this value was used in the analyses of the spectra of the supported samples. (8) Jentoft, R. E.; Deutsch, S. E.; Gates B. C. ReV. Sci. Instrum 1996, 67, 2111. (9) Ravel, B.; Newville, M. J. Synchrotron Radiat. 2005, 12, 537. (10) Vaarkamp, M.; Linders, J. C.; Koningsberger, D. C. Physica B 1995, 209, 159. (11) Koningsberger, D. C.; Mojet, B. L.; van Dorssen, G. E.; Ramaker, D. E. Top. Catal. 2000, 10, 143. (12) Zabinsky, S. I.; Rehr, J. J.; Ankudinov, A.; Albers, R. C.; Eller, M. J. Phys. ReV. B. 1995, 52, 2995. (13) Wyckoff, R. W. G., Ed. Crystal Structures, 2nd ed.; Wiley: New York, 1963; Vol. 1, p 16. (14) Pearson’s Handbook of Crystallographic Data for Intermetallic Phases, 2nd ed.; Villars, P.; Calvert, L. D., Eds.; ASM International: Materials Park OH, 1991; Vol. 4, p 5253. (15) Aleshina, L. A.; Longinova, S. V. Crystallogr. Rep. 2002, 47, 415.

Data analysis was carried out with unfiltered data; the k range was 3.55-13.4 Å-1 (k is the wave vector), and the r range was 0.753.50 Å (r is the absorber (Ta)-backscatterer distance). Iterative fitting was done in both k-space and r-space by using k1-, k2-, and k3-weightings until optimum agreement was obtained between the data and the fits with all the applied k-weightings considered. The statistically justified number of free parameters in the analysis of the EXAFS data of the various samples was estimated on the basis of the Nyquist theorem16 to be approximately 19 for each sample; 16 parameters were used in each of the fits. The goodness of fit parameter, the variances between the data and the fits, and the changes in the goodness of fit parameter resulting from the addition of each shell used in the analysis of the EXAFS data are summarized in Table 2. Residuals are shown in the Supporting Information. Further details are given in the Results section.

Results IR Evidence of OH Groups on SiO2. The IR spectrum of the supported tantalum sample after treatment in H2 at 523 K is shown in Figure 1b, along with the spectrum of the SiO2 sample used in the preparation (Figure 1a). Both spectra show the presence of OH groups on the SiO2, indicated by the sharp band at 3747 cm-1. Because any change in the relative intensities of the 3747 cm-1 absorbance band and the 1750-2050 cm-1 absorbance band (indicative of Si-O combinations or overtones) was too small to detect for the treated sample, these data do not determine whether the surface species consumed any surface OH groups. A very weak band was also present in the 2800-3000 cm-1 region in the spectrum of the supported sample treated at 523 K, inferred to be an indication of C-H stretching bands derived from ligands of the adsorbed precursor. A weak band was also observed in the 3600 cm-1 region for the sample treated at 523 K, which we assign to hydrogen-bonded OH groups interacting with adsorbed tantalum complexes. UV-Visible Spectra: Evidence of Tantalum Cluster Formation. The UV-vis spectrum of the sample formed initially by adsorption of the precursor Ta(CH2Ph)5 on SiO2 is shown in Figure 2, along with the spectrum of the supported sample after treatment in H2 at 523 K. The spectrum of the treated sample includes an absorption band at 317 nm and shoulders at 305 and 328 nm (Figure 2), consistent with the presence of Ta-Ta bonds in the sample. Tantalum cluster compounds are characterized by maxima at similar wavelengths, for example, as follows: [(Me6C6)3Ta3Cl6]+ (Me is methyl; λmax ) 364, 288 nm);17 (16) Stern, E. A. Phys. ReV. B 1993, 48, 9825.

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Table 2. EXAFS Results: Goodness of Fit Parameter and the Variance between Data and Fit Obtained upon Addition of Each Contribution Used in the Analysis temperature of sample treatment in H2 (K)

absorber - backscatterer pair

ν2 a

k3-weighted varianceb (%)

k1-weighted varianceb (%)

523

Ta-O Ta-Si Ta-Ta Ta-Olong

62 42 23 17

9.9 6.8 3.6 1.1

8.2 2.7 1.6 0.8

623

Ta-O Ta-Si Ta-Ta Ta-Olong

37 29 16 10

12 9.6 3.1 1.8

9.1 2.6 1.6 1.2

723

Ta-O Ta-Si Ta-Ta Ta-Olong

16 10 3.7 2.9

10 8.6 3.8 2.2

13 11 2.9 2.1

a Goodness of fit parameter obtained upon addition of the stated absorber-backscatter pair to the model. b Variance between the k-weighted Fourier transform of the data and the fit over the fitting range (0.75-3.5 Å). Stated values are the variance obtained upon addition of the absorber-backscatterer pair to the model.

Figure 2. UV-visible spectra of (a) sample formed initially by adsorption of Ta(C2HPh)5 on SiO2, (b) of that sample after treatment in H2 at 523 K, and (c) of that sample after treatment in H2 at 623 K.

Figure 1. IR spectra of (a) SiO2 dehydroxylated in vacuo at 773 K for 15 h and (b) sample formed from Ta(CH2Ph)5 on SiO2 after treatment in flowing H2 for 3 days at 523 K.

[(Me6C6)3Ta6Cl12]4+ (λmax ) 354 nm);17 and [Ta6Cl12]2+ (n ) 2, 3, or 4; λmax ) 334 ( 10 nm).18,19 The UV-vis spectra of the supported tantalum sample treated in H2 at a higher temperature (623 K) also include absorption bands indicative of Ta-Ta bonds, with maxima in the absorptions at essentially the same positions observed for the sample treated at 523 K. In contrast, the spectrum of the untreated sample does not include any absorption bands indicative of Ta-Ta bonds (Figure (17) King, R. B.; Braitsch D. M.; Kapoor, P. N. J. Am. Chem. Soc. 1975, 97, 60. The maxima at 295 and 291 nm are those of the SiO2 support calcined at 773 K, as shown by the results of a blank experiment. (18) Fleming, P. B.; McCarley, R. E. Inorg. Chem. 1970, 9, 1347. (19) Robbins, D. J.; Thomson, A. J. J. Chem. Soc., Dalton Trans. 1972, 21, 2350.

2). Thus, we infer that the H2 treatments of the supported Ta(CH2Ph)5 led to formation of tantalum clusters on the support. The positions of the absorption bands indicative of Ta-Ta bonds observed in the spectra of our supported samples are shifted relative to those characteristic of [Ta6Cl12]n+ (n ) 2, 3, or 4),18 [(Me6C6)3Ta3Cl6]+,17 and [(Me6C6)3Ta6Cl12]4+.17 In the spectra of [Ta6X12]n+ (X ) halide, n ) 2, 3, or 4), shifts of approximately 25-30 nm resulted when chloride was replaced by bromide.19 Because the ligand sphere of our supported clusters was expected to include oxygen atoms of the support, the position of the maximum and those of the shoulders in the spectrum of our sample are not expected to match exactly those of the tantalum halide compounds listed in this paragraph. In summary, the UV-vis spectra indicate the formation of tantalum clusters on the SiO2 support, but these results, by themselves, are not sufficient to determine the cluster size. EXAFS and XANES Evidence of Ta-Ta Contributions and Formation of Tantalum Clusters on SiO2. EXAFS spectra of the sample treated in H2 at 523 K confirm the presence of tantalum clusters on the SiO2 support, indicated by the Ta-Ta contribution at 2.93 Å. This distance is typical of Ta-Ta bonds in clusters with Ta formally in the +3 oxidation state, exemplified by Ta3Cl10(PEt3)3(HPEt3) (Ta-Ta ) 2.932 Å; Et is ethyl),20 Ta3Cl9(THF)4‚C6H6‚THF (Ta-Ta ) 2.868, 2.850 Å; THF is (20) Cotton, F. A.; Diebold, M. P.; Feng, X.; Roth, W. J. Inorg. Chem. 1988, 27, 3413.

Synthesis and Characterization of Ta Clusters

Figure 3. Ta-Ta phase- and amplitude-corrected k3-weighted Fourier transform of the difference spectrum (s) and fit (- - -) representing the Ta-Ta contribution to the EXAFS characterizing the SiO2-supported tantalum sample treated at 523 K in H2.

tetrahydrofuran),5 and Na[Ta3Cl10(THF)3] (Ta-Ta ) 2.875 Å).5 (For comparison, the Ta-Ta distance in tantalum metal is 2.86 Å.13) The Ta-Ta contribution in the spectrum of our supported sample was identified on the basis of a symmetric Ta-Ta phaseand amplitude-corrected Fourier transform of the difference spectrum representing the Ta-Ta contribution, as shown in Figure 3.21 When the sample was treated in H2 at higher temperatures (623 and 723 K), larger Ta-Ta coordination numbers were observed, 3.8 and 4.8, respectively. These results indicate that treatment at the higher temperature led to the formation of larger tantalum clusters. We infer that the oxidation state of tantalum in the sample treated at 723 K is lower than that in the sample treated at 523 K. This inference is based on the energy of the XANES edge position, which shifted to energies closer to the value of 9881 eV characteristic of tantalum metal as the treatment temperature increased. For example, the sample treated at 723 K has an edge energy of 9884 eV and is closer to the value for tantalum metal than the value of 9886.5 eV measured for the sample treated at 523 K. We lack sufficient information to determine the ligands on the clusters to balance the charges on the Ta atoms. The Ta-Ta contribution in the spectrum of each of the samples treated in H2 was attenuated as a result of partial phase cancellation caused by the presence of an overlapping Ta-Si contribution (Figure 4). The effect of phase cancellation began to disappear at X-ray energies greater than k ≈ 11 Å-1, at which value the Ta-Si backscattering amplitude decreased and began to go out of phase (Figure 4). (In contrast, in typical EXAFS spectra of supported metal clusters,22 the metal-metal contributions are (21) Application of a Ta-O or Ta-Si phase and amplitude correction to the Ta-Ta contribution led to an unsymmetrical Fourier transform that did not include an imaginary part that peaks at the maximum of the magnitude. Fitting of this contribution with Ta-O or Ta-Si contributions was consequently ruled out, because, in general, an amplitude- and phase-corrected Fourier transform for an arbitrary M-X pair has a symmetric Fourier transform with an imaginary part that peaks near the maximum of the magnitude if the EXAFS originates from the M-X pair.11 It is a straightforward matter to detect and identify metal-metal contributions, provided that the data quality is sufficiently high and provided that there is no phase cancellation of the kind encountered here. EXAFS spectroscopy is not, however, able to determine unambiguously the identities of low-atomicnumber backscatterers in samples such as ours. (22) Alexeev, O.; Gates, B. C. Top. Catal. 2000, 10, 273.

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Figure 4. EXAFS data characterizing the sample treated in H2 at 523 K, results showing that when k is greater than approximately 11 Å-1, phase cancellation of the Ta-Ta contribution (dotted line) by the Ta-Si contribution (solid line) begins to disappear. This is reflected in the sum of the two contributions (gray line).

clearly evident even at lower values of k.) Only because the influence of the phase cancellation disappeared at the higher k values could the two contributions be resolved in our spectra. In general, in the fitting of EXAFS data, the Debye-Waller factor of a particular contribution is correlated with the coordination number characteristic of that contribution. Specifically, the analysis of the EXAFS data characterizing our samples showed that the parameters representing the amplitude of the Ta-Ta contribution (namely, the Debye-Waller factor and the coordination number) and the parameters representing the amplitude of the Ta-Si contribution (the Debye-Waller factor and coordination number) were significantly coupled (interdependent). In other words, an increase in the amplitude of one of these contributions could, to some extent, be offset by an increase in the other. To isolate which parameters are most significantly correlated, calculations of the cross correlation coefficients are needed;23 these coefficients provide quantitative estimates of the degree of coupling. A value of this coefficient >0.7 indicates a strong interdependence of two parameters.24 We calculated the various cross correlation coefficients with the software XDAP, finding that only one of the values was nearly as great as 0.7; this was the value of 0.69 found for the Ta-Ta coordination number and the Ta-Si Debye-Waller factor. Thus, the Ta-Ta coordination number was significantly coupled with the Ta-Si Debye-Waller factor, and the error bounds associated with the Ta-Ta coordination number are greater than those typically observed for metal-metal contributions in metal clusters.22 Consequently, instead of the typical uncertainty of a metal-metal coordination number of approximately 10%, we estimate an uncertainty in our Ta-Ta coordination number of 20%; we emphasize that the estimate is rough.22 Far-IR Spectra: Further Evidence of Tantalum Cluster Formation. Far-IR spectra of the sample treated at 523 K include a weak band centered at 148 cm-1 (Figure 5), and the feature (23) Joyner, R. W.; Martin, K. J.; Meehan, P. J. Phys. C: Solid State Phys. 1987, 20, 4005. (24) Error Reporting Recommendations: A Report of the Standards and Criteria Committee, Adopted by the International XAFS Society, Standards and Criteria Committee, July 26, 2000, Ako, Japan. http://ixs.iit.edu/subcommittee_reports/ sc/err-rep.pdf (accessed Feb 15, 2006).

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Figure 5. Far-IR spectrum of the supported sample treated at 523 K (solid line) and the initially formed supported sample (dashed line). The spectra were baseline-corrected, with the spectrum of SiO2 subtracted.

is not present in the spectrum of the untreated supported sample (Figure 5). The observed band is weak relative to the strong absorbance of SiO2 in the far-IR region. The position of the band is broadly consistent with metalmetal stretching frequencies, which have been observed in the range between 120 and 220 cm-1 for a wide variety of metal clusters.25 For example, a band at 140 cm-1 has been assigned to Ta-Ta bonds in hexanuclear halide clusters of tantalum26,27 (although such data are lacking for [(Me6C6)3Ta3Cl6]+, [(Me6C6)3Ta3Cl6]2+, Ta3Cl10(PEt3)3(HPEt3), Ta3Cl9(THF)4‚C6H6‚THF, and Na[Ta3Cl10(THF)3]). Because the band characteristic of our treated sample was observed at 148 cm-1 (Figure 5), we assign it to Ta-Ta bonds in supported tantalum clusters. The small (8 cm-1) difference between our value and that observed by McCarley et al.26 for [Ta6Cl12]n+ (n ) 2, 3, or 4) is attributed to the different structures and compositions of the clusters. EXAFS Evidence of Bonding of Tantalum Clusters to SiO2. In addition to the Ta-Ta contribution discussed above, the EXAFS spectra of the supported clusters formed by treatment of the initially prepared sample in flowing H2 at 523 K also include Ta-O and Ta-Si contributions. Ta-O contributions at a distance of 1.91 Å (indicative of Ta-O bonds) are similar to those characteristic of Ta(OC6H3-2,6-iPr2)3(H)2(PMe2Ph) (iPr ) isopropyl; Ta-O ) 1.912, 1.907, and 1.897 Å)28 and [(silox)2TaH2]2 (silox ) tBu3SiO-; Ta-O ) 1.86 Å).29 Thus, we infer that the supported clusters were anchored to the SiO2 surface through Ta-O bonds. The Ta-Si contribution is characterized by a distance (3.22 Å) that is too long to be a bonding distance, consistent with the bonding of the tantalum to oxygen and not silicon of the support. Difference spectra characterizing this contribution and the fit are shown in the Supporting Information. There are numerous examples of metal-support atom distances in supported metal clusters that are too long to be bonding (25) Finch, A.; Gates, P. N.; Radcliffe, K.; Dickson, F. N.; Bentley, F. F. Chemical Applications of Far Infrared Spectroscopy; Academic Press: London, New York, 1970; pp 159-163. Kno¨zinger, H. In Metal Clusters in Catalysis; Gates, B. C., Guczi, L., Kno¨zinger, H., Eds.; Elsevier: Amsterdam, 1986; p 166. (26) McCarley, R. E.; Meyer, J.; Hughes, B. G.; Converse J. G. Paper presented at the 154th National Meeting of the American Chemical Society, Chicago, IL, Sept 1967. The possibility that the observed band should be assigned to a Ta-O stretching mode is discounted on the basis of the result that these modes occur at higher wavenumbers, for example, at 921, 803, and 704 cm-1 for [(silox)2TaH2]2 (silox ) tBu3SiO- and tBu ) tert-butyl).29 (27) Boorman, P. M.; Straughan, B. P. J. Chem. Soc. A 1966 1514. (28) Visciglio, V. M.; Fanwick, P. E.; Rothwell, I. P. J. Chem. Soc., Chem. Commun. 1992, 20, 1505. (29) Miller, R. L.; Toreki, R.; Lapointe, R. E.; Wolczanski, P. T.; Van Duyne, G. D.; Roe, C. J. Am. Chem. Soc. 1993, 115, 5570.

Figure 6. k3-Weighted EXAFS function (s) and calculated best-fit contributions (- - -) characterizing the sample treated in H2 at 523 K.

Figure 7. Imaginary part and magnitude of the k1-weighted Fourier transform of the EXAFS spectrum (s) and calculated best-fit contributions (- - -) of the sample treated at in H2 523 K.

distances.22 Such contributions are often determined only tentatively and characterized by relatively large errors.22 Especially large errors are associated with the Ta-Si coordination number representing our samples because, as stated above, the Debye-Waller factor characterizing this contribution is strongly correlated with the Ta-Ta coordination number. Moreover, the backscattering phase and amplitudes used in the analysis of the Ta-Si contribution were derived from structure parameters calculated for a Ta-Si alloy reference. The interaction between Ta and Si in the alloy cannot be considered analogous to the interaction between Ta and Si in our sample. The difficulty in analyzing the Ta-Si contribution lies in obtaining a suitable reference that approximates the structure of the interface between tantalum clusters and the surface of SiO2. Another (small) contribution was detected and included in the fit as a single Ta-O contribution at distance of 2.51 Å. This nonbonding distance lies in the range observed for numerous supported metal clusters on metal oxides.22 Difference spectra characterizing this contribution and the fit are shown in the Supporting Information. The EXAFS data and the overall fit, consisting of the sum of the individual contributions, are shown in Figures 6 (the k3weighted raw EXAFS data), 7 (the k1-weighted Fourier trans-

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formed data), and 8 (the k3-weighted Fourier transformed data). Figures for the sample treated at 623 K are given in the Supporting Information.

Discussion There are numerous examples of metal clusters on supports that have structures similar to those of metal clusters in solution or in the crystalline state.30 The supported tantalum clusters reported here add substantially to the examples by being the first such supported clusters of an early transition metal. They are also the first supported clusters to have been characterized by three independent methods that give evidence of the metal-metal bonds. UV-visible and far-IR spectra both indicate Ta-Ta bonds and show strong similarities to the spectra of known tantalum clusters; the EXAFS spectra confirm the evidence of Ta-Ta bonds and provide average Ta-Ta bond distances. The Ta-Ta coordination numbers give an indication of the average cluster nuclearities; the clusters formed on SiO2 after treatment in H2 at 523 K are approximated as Ta3 with a triangular metal frame, corresponding to the Ta-Ta coordination number of nearly 2 (Table 1). Earlier reports2-4 of group 7 and group 8 metal clusters on supports emphasize analogies in the chemistry of their syntheses on surfaces and the chemistry of their syntheses in solution. Our results for SiO2-supported tantalum clusters also give evidence of such an analogy, as follows: The work of Cotton’s group5,20,31,32 on synthesis of group 5 metal clusters shows how the chemistry depends on the precursor, temperature, reducing agent, and solvent. The precursors used by Cotton were mononuclear metal complexes, typically TaCl5,5,20 and the temperature of the solution syntheses was room temperature.5,20 (Higher temperatures were used by other authors19 for solvent-free syntheses.) Cotton’s solvents were THF5 or PEt3.20 The solution syntheses took place over periods such as 20 h.5,20 For comparison, our precursor was also mononuclear, but we avoided halides because they would have remained on the support and complicated the analysis of the EXAFS data; thus, instead, we used an organometallic precursor, Ta(CH2Ph)5. Our synthesis temperature ranged from 523 to 723 K, substantially higher than the temperatures used by Cotton’s group. Our times of synthesis were comparable to theirs, g20 h. The difference in temperature may be attributed to the fact that their reducing agent, sodium amalgam, was much stronger than ours (H2 at atmospheric pressure). (Schrock’s group33 has used H2 as the reducing agent to form dinuclear tantalum clusters from a mononuclear precursor.) A further assessment of the analogy requires a comparison of the chemistry of Cotton’s solvents with that of our support surface. The tantalum clusters formed with sodium amalgam in THF or PEt3 were found to have nuclearities of 2 and higher, and the tri-tantalum clusters were found to incorporate THF or PEt3 in the ligand sphere of tantalum, with the solvent being bonded through oxygen or phosphorus as terminal ligands.5,20 Because siloxane bridges on SiO2 are electron donors34 similar in reactivity to THF, we infer that they react similarly in the cluster synthesis. The EXAFS results characterizing the SiO2-suppported clusters indicate that the support surface is bonded to the clusters through (30) Gates, B. C. J. Mol. Catal. 1994, 86, 95. (31) Mu¨ller, A., Jostes, R.; Cotton, F. A. Angew. Chem., Int. Ed. Engl. 1980, 19, 875. (32) Cotton, F. A.; Shang, M. Inorg. Chem. 1993, 32, 969. (33) Belmonte, B. A.; Schrock, R. R.; Day, C. S. J. Am. Chem. Soc. 1982, 104, 3082. (34) Gillis-D’Hamers, I.; Philippaerts, J.; Van Der Voort, P.; Vansant, E. J. Chem. Soc., Faraday Trans. 1990, 86, 3747.

Figure 8. Imaginary part and magnitude of the k3-weighted Fourier transform of the EXAFS spectrum (s) and calculated best-fit contributions (- - -) characterizing the sample treated in H2 at 523 K.

support surface oxygen atoms, analogous to the incorporation of THF as terminal ligands in some of Cotton’s clusters. The EXAFS data (Table 1) indicate that each Ta atom in the supported clusters was bonded, on average, to three oxygen atoms. SiO2 dehydroxylated at 723 K contains approximately 1.8 terminal silanol groups/nm2.35 Because a triangular tri-tantalum cluster with a Ta-Ta bond length of 2.93 Å has a cross-sectional area of approximately 3.7 Å2, it evident that the surface silanol group density cannot account for the three oxygen atoms bonded to each Ta atom. The solution syntheses of higher-nuclearity clusters were inferred by Cotton and Shang32 to proceed via di- and tri-nuclear cluster intermediates, and the products were mixtures including clusters of various nuclearities. By inference, in view of our increasing Ta-Ta coordination number with increasing temperature of treatment in H2, we suggest that as our samples were treated at higher temperatures, the formation of higher-nuclearity clusters (Table 1) might also have proceeded via small tantalum clusters; similarly, we infer that mixtures of clusters of various nuclearities were generally present on our support surface. Cotton’s group was able to obtain high yields of niobium clusters36 of a particular nuclearity (e.g., 3), by optimizing the time and temperature of the synthesis. We suggest that similar statements pertain to our surface-mediated synthesis; the Ta-Ta first-shell coordination number of 2.1 characterizing the sample formed at 523 K suggests that a high yield of tri-tantalum clusters was obtained, but these results do not rule out mixtures with clusters of higher and lower nuclearity as well as tri-tantalum.

Conclusions This is the first report of clusters of an early transition metal on a support. The synthesis of SiO2-supported tantalum clusters from a mononuclear precursor is analogous to the formation of similar clusters in solution. The similarity of the surface synthesis to that in solution points to opportunities to extend this new class of material to other early transition metal clusters on various supports. Acknowledgment. We thank the National Science Foundation for support (Grant No. CTS-03000982). Portions of this research (35) Zhuravlev, L. T. React. Kinet. Catal. Lett. 1993, 50, 15. (36) The chemistry of tantalum clusters closely resembles the chemistry of niobium clusters.5,17,18,19,20,32

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were carried out at the Stanford Synchrotron Radiation Laboratory, a national user facility operated by Stanford University on behalf of the Office of Basic Energy Sciences, U. S. Department of Energy. Experiments at the National Synchrotron Light Source, Brookhaven National Laboratory, were supported by the Office of Science, Office of Basic Energy Sciences, U. S. Department of Energy, under Contract No. DE-AC02-98CH10886.

Nemana and Gates

Supporting Information Available: EXAFS data, including residuals characterizing the sample treated at 523 K and plots of EXAFS data and the corresponding model fits for the sample treated at 623 K. This material is available free of charge via the Internet at http://pubs.acs.org. LA0609322