J. Phys. Chem. C 2010, 114, 2685–2693
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Essentially Molecular Metal Complexes Anchored to Zeolite β: Synthesis and Characterization of Rhodium Complexes and Ruthenium Complexes Prepared from Rh(acac)(η2-C2H4)2 and cis-Ru(acac)2(η2-C2H4)2 Isao Ogino and Bruce C. Gates* Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, One Shields AVenue DaVis, California 95616 ReceiVed: October 18, 2009; ReVised Manuscript ReceiVed: December 16, 2009
Mononuclear complexes of rhodium and of ruthenium, Rh(acac)(η2-C2H4)2 and cis-Ru(acac)2(η2-C2H4)2 (acac ) C5H7O2-), were used as precursors to synthesize metal complexes bonded to zeolite β. Infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectra show that the species formed from Rh(acac)(η2C2H4)2 was Rh(η2-C2H4)2+, which was bonded to the zeolite at aluminum sites via two Rh-O bonds. Reaction of this supported rhodium complex with CO gave the supported rhodium gem-dicarbonyl Rh(CO)2+, which was characterized by two νCO bands in the IR spectrum, at 2048 and 2115 cm-1, that were sharp (fwhm of 2115-cm-1 band ) 5 cm-1), indicating a high degree of uniformity of the supported species. Nearly the same result was observed (Liang, A. et al. J. Am. Chem. Soc. 2009, 131, 8460) for the isostructural rhodium complex supported on dealuminated HY zeolite, which was characterized by frequencies of the νCO bands that were 4 and 2 cm-1, respectively, greater than those characterizing the zeolite β-supported complex. This comparison indicates that the Rh atoms in Rh(η2-C2H4)2+ anchored on zeolite β were slightly more electron-rich than those on zeolite Y. This inference is supported by EXAFS results showing shorter Rh-C bonds in the zeolite β-supported rhodium ethene complex than in the zeolite Y-supported rhodium ethene complex. In contrast to these supported rhodium complexes, the zeolite β-supported ruthenium samples were shown by IR and EXAFS spectroscopies to consist of mixtures of mononuclear ruthenium complexes with various numbers of acac ligands; when CO reacted with the supported ruthenium complexes, the resultant ruthenium carbonyls were characterized by νCO spectra characteristic of both ruthenium dicarbonyls and ruthenium tricarbonyls. Introduction Supported metal complexes are an important class of catalyst, being applied on a large scale industrially for alkene polymerization.1 It remains challenging to characterize the supported species in these materials because the supports are typically metal oxides that have intrinsically nonuniform surfaces so that the supported species are also nonuniform. In contrast, when the support is a crystalline material with well-defined bonding sitesssuch as a zeolitesit may provide opportunities for the synthesis of uniform-supported species that can be characterized incisively, as has been demonstrated in investigations with complexes of ruthenium,2,3 rhodium,4-7 and iridium8 anchored to dealuminated zeolite HY and formed from molecular organometallic precursors incorporating reactive acetylacetonate (C5H7O2-, acac) ligands exemplified by Rh(acac)(η2-C2H4)2 (I) and cis-Ru(acac)2(η2-C2H4)2 (II). Earlier results3 demonstrated that (1) a support without aluminum (i.e., silica) resulted in physisorption but not chemisorption of II and (2) the chemisorbed species formed from II in the zeolite was bonded to oxygen atoms near aluminum sites. The bidentate and anionic acac ligands in these metal complexes were readily removed by the oxygen atoms of zeolite, and the resultant metal complexes were bonded to oxygen atoms near aluminum sites of the zeolite.2-8 We infer that the zeolite acts as a bidentate ligand that is electronically analogous to acac. * To whom correspondence should be addressed. E-mail: bcgates@ ucdavis.edu.
This class of well-defined supported metal complexes has been limited to zeolite Y as the support. We now extend the class to another zeolite, zeolite β, demonstrating the strengths and limitations of I and II as precursors for the formation of highly uniform supported metal complexes. Zeolite β is appealing as a support that offers nearly uniform sites for bonding of the complexes, as shown in the recent work of Bare, Corma, and co-workers,9 who used zeolite β incorporating Sn atoms in the framework and demonstrated the uniformity of the tincontaining sites by EXAFS spectroscopy. Specifically, their data showed that Sn atoms in this zeolite occupied nearly identical crystallographic sites (T5/T6 sites in the six-membered rings). Their conclusions were based primarily on a multiple-shell fit of their EXAFS data; their data analysis also led them to infer that Sn atoms were present in the zeolite framework in pairs on opposite sides of the six-membered rings, and they postulated that the specific, unique crystallographic location of the Sn atoms was responsible for the high selectivity of this material as a catalyst for Baeyer-Villiger oxidation reactions.10 Zeolite β in the pure silica or the aluminosilicate form has been used as a support for several metal complexes,11-13 including, for example, complexes of nickel13 and of chromium,11 but the supported species were not uniform, having been prepared from metal salts and consisting of mixtures with metals in more than one oxidation state. The authors probed their samples with CO and characterized them with infrared (IR) spectroscopy, a powerful method of structure determination14 that provides evidencesvia the sharpness of the IR bandssof the degree of uniformity of the supported species.15
10.1021/jp909977n 2010 American Chemical Society Published on Web 01/21/2010
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Figure 1. IR spectra characterizing (A) calcined zeolite β, (B) sample 1, and (C) sample 2. The spectra were normalized with respect to the 1800-cm-1 band characteristic of the zeolite framework.
The samples investigated by these researchers were characterized by relatively broad νCO bands, indicating a low degree of uniformity. We now report samples made from zeolite β and acetylacetonate complexes of rhodium and of ruthenium. The supported samples include metal complexes with nearly uniform structuressand some that are not uniform. Our data (including characterization of the samples by IR and extended X-ray absorption fine structure (EXAFS) spectroscopy) demonstrate properties of the precursors that are essential for the synthesis of uniform supported species. Results Reaction of I or II with Calcined Zeolite β. ObserWations during the Synthesis. The color of the solution containing I or II in n-pentane was initially yellow or light orange, respectively. Each solution gradually became transparent within a few hours after the start of mixing in a slurry with the zeolite, as the precursor reacted with the zeolite. After 24 h, each solution had become colorless, consistent with the complete uptake of I or II by the zeolite. Removal of the solvent by evacuation gave, respectively, a light-yellow solid (incorporating supported rhodium complexes, designated as sample 1) and a light-brown solid (incorporating supported ruthenium complexes, designated as sample 2). IR Spectra of the Zeolites in the νOH Region. IR spectra characterizing the calcined zeolite β include bands at 3609, 3663, 3745, and 3783 cm-1 (Figure 1, spectrum A). The sharp bands at 3609 and 3745 cm-1 are assigned as νOH bands of bridging OH and terminal SiOH groups, respectively.16-19 The weak bands at 3663 and 3783 cm-1 are assigned as νOH bands of AlOH groups.16-19 The 3745-cm-1 band decreased in intensity after adsorption of each of the precursors on the zeolite, as shown in the spectra characterizing samples 1 and 2 (spectra B and C of Figure 1). Adsorption of the precursors was accompanied by the disappearance of the 3783-cm-1 band and the appearance of a broad band centered at approximately 3500 cm-1 (Figure 1, spectra B and C). The decrease in intensity of the band characteristic of nonacidic SiOH groups (3745 cm-1) and the appearance of a broad band at approximately 3500 cm-1 indicate that the precursors and/or species formed from them interacted with
Ogino and Gates
Figure 2. IR spectra in the region of 1475-1625 cm-1 characterizing (A) calcined zeolite, (B) I, (C) sample 1, (D) II, and (E) sample 2.
these SiOH groups, and we infer that the interactions involved noncovalent bonding, because similar results were observed for the physisorption of n-hexane and of benzene on calcined zeolite β.19 Although adsorption of water on the zeolite has also been reported to lead to the appearance of a broad band at approximately 3500 cm-1,19 we exclude the possibility of water formation during the adsorption of our precursors because of the absence of IR bands characteristic of a bending mode of water on zeolites (observed between 1600-1650 cm-1;20 see spectra B and C of Figure 1). Because the 3609-cm-1 band characteristic of acidic silanol groups overlapped the broad 3500-cm-1 band in the spectra characterizing samples 1 and 2 (Figure 1), the IR results are not sufficient to determine whether I or II reacted with the acidic OH groups. Results summarized in the next section provide further insight into how acac ligands in I and in II reacted with the zeolite. IR Spectra of the Samples in the 1475-1625-cm-1 Region: Rhodium Complexes. The IR spectrum characterizing crystalline I (Figure 2, spectrum B) includes bands at 1521, 1554, and 1575 cm-1 that have been assigned as νas(CCC)ring, 2[γ(C-H)], and νs(CO)ring, respectively.6 The disappearance of the bands at 1521 and 1554 cm-1 after the reaction of I with the zeolite (Figure 2, spectrum C) shows that when I reacted with the zeolite, essentially all the acac ligands were removed from the rhodium. As these ligands were removed from the rhodium during the adsorption, two bands appeared in the spectrum, at 1537 and 1590 cm-1; these are assigned as νas(CCC)ring and νs(CO)ring, respectively, of species similar to acac, which we infer formed from the dissociated acac ligands. On the basis of (a) IR spectra of acetylacetone (Hacac) adsorbed on zeolite β21 and on zeolite Y2,6 and (b) the IR spectrum of Al(acac)3,2,22 we suggest that the groups formed from the dissociated acac ligands were bonded to aluminum sites of the zeolite, but we lack sufficient information to determine their structure(s) or compositions. It is plausible to suggest that they might have been formed by reaction with OH groups of the zeolite.23 IR Spectra of the Samples in the 1475-1625-cm-1 Region: Ruthenium Complexes. In contrast to the IR spectrum characterizing sample 1, that characterizing sample 2 (Figure 2, spectrum E) includes a band at 1521 cm-1 as well as bands at 1537, 1575, and 1590 cm-1. The 1521-cm-1 band is charac-
Molecular Metal Complexes Anchored to Zeolite β
Figure 3. IR spectra in the νCO region characterizing the sample formed from I and zeolite β after the sample had been treated in flowing gases for the specified times: (A) helium for 10 min; (B) 0.3 mol % CO in helium for 20 min, followed by a purge of the cell with helium for 10 min.
teristic of acac ligands bonded to ruthenium,2,3 indicating that some of these ligands remained undissociated after the adsorption.2,3 An approximate analysis2,3 of the IR spectrum by deconvolution showed that the ratio of the area of the 1537cm-1 band to that of the 1521-cm-1 band was approximately 7 /3, indicating that approximately 70% of the acac ligands had been removed from II. Thus, the IR results indicate that a fraction of the ruthenium complexes retained at least one acac ligand, and they suggest the presence of a mixture of ruthenium complexes with various numbers of acac ligands. Reaction of Zeolite-Supported Rhodium Complex with CO. To investigate the degree of structural uniformity and the electronic properties of the supported species, each sample was treated with CO in the IR cell that served as a flow reactor while spectra were recorded to characterize the supported species; thus, the CO ligands were used as structural probes. Figure 3 shows IR spectra characterizing sample 1. Spectrum A of Figure 3 was recorded after the sample had been purged with flowing helium for 10 min, and spectrum B was recorded after the same sample had been in contact with flowing 0.3 mol % CO in helium for 20 min, followed by a purge of the cell with helium for 10 min. Throughout the experiment, the effluent gas from the IR cell was monitored periodically by mass spectrometry (Figure 4). After the addition of CO to the stream of helium flowing to the IR cell, which was characterized (after a lag determined by the flow rate and the volume of the lines connecting the IR cell and the mass spectrometer) by a step increase in the intensity of the m/z 28 (CO+) peak in the mass spectrum of the effluent gas (Figure 4), the intensity of the signal m/z 26 (C2H2+, a fragment of C2H4) increased rapidly, reaching a maximum after approximately 1 min and then decreasing to the background level in approximately 2 min (Figure 4). These results indicate that ethene molecules were removed from sample 1 as a result of contact with CO. Correspondingly, almost instantaneously after the COcontaining stream came in contact with sample 1, the IR spectra showed the appearance of sharp νCO bands, at 2048 and 2115 cm-1 (Figure 3, spectrum B), indicating the formation of rhodium dicarbonyl complexes (Rh(CO)2+) on the zeolite.6,7,15,24 Two satellite bands were observed simultaneously, at 2017 and
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Figure 4. Mass spectra of the effluent gases from the flow-through cell containing zeolite-supported rhodium complex: m/z 26 (A, C2H2+) and m/z 28 (B, CO+). The data were normalized with respect to the helium signal (m/z 4).
2083 cm-1, consistent with observations made with samples formed from I and dealuminated zeolite Y (satellite peaks at 2021 and 2101 cm-1).7,15,24 The IR and mass spectral data thus indicate that ethene ligands on the supported rhodium species were readily replaced with CO from the gas phase.7 The νCO band positions (2048 and 2115 cm-1) indicate that the rhodium dicarbonyl complexes were electron-deficient and bonded to oxygen atoms near aluminum sites of the zeolite.7,24 These νCO bands were narrow (Figure 3), with the fwhm of the 2115-cm-1 band being only approximately 5 cm-1;25 we conclude from this observation that the supported rhodium carbonyls were highly uniform, and we infer correspondingly that the rhodium complexes in the initially prepared sample from which these rhodium carbonyls were formed must also have been highly uniform.15 Reaction of Zeolite-Supported Ruthenium Complexes with CO. In contrast to the IR spectra characterizing the rhodiumcontaining sample (sample 1), those characterizing ruthenium (sample 2) after treatment with CO include broad bands at 2022, 2086, 2148, and 2165 cm-1 (Figure 5).26 On the basis of literature data,27 the relatively strong peaks at 2022 and 2086 cm-1 are assigned to νCO of ruthenium dicarbonyl complexes, (Ru(CO)2)2+, bonded to oxygen atoms near Al sites, and the weak bands at 2148 and 2165 cm-1 are assigned to νCO bands of anchored complexes mer-tricarbonyl27 and fac-tricarbonyl ruthenium(II),27 respectively. Other νCO bands characteristic of ruthenium tricarbonyls27 overlapped the strong bands at 2022 and 2086 cm-1 and appeared to cause broadening of these bands (Figure 5). These IR spectra indicate the presence of at least three species: one ruthenium dicarbonyl and two ruthenium tricarbonyls. Therefore, we infer that the supported species from which these formed must have included ruthenium complexes with various numbers of acac ligands. EXAFS Characterization of Supported Metal Complexes. Samples 1 and 2 were also characterized by EXAFS spectroscopy. Tables 1 and 2 are summaries of the fit parameters for the recommended models representing samples 1 and 2, respectively. Figures 6 and 7 show the raw EXAFS data and Fourier transforms of the EXAFS data with the best fits for samples 1 and 2, respectively. Detailed analyses of the EXAFS data were performed by procedures similar to those reported.2,5 On the basis of the IR
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Figure 5. IR spectra in the νCO region characterizing sample 2 after the following treatments at atmospheric pressure, listed in sequence: (A) helium for 10 min, (B) 0.3 mol % CO in helium for 10 min, and (C) 0.3 mol % CO in helium with the temperature of the IR cell ramped to 423 K at a rate of 5 K/min.
TABLE 1: EXAFS Parameters at the Rh K Edge Characterizing Sample 1a absorber-backcatterer pair
N
103 × ∆σ2/Å2
R/Å
∆E0/eV
Rh-Rh Rh-Os Rh-C Rh-Al
2.3 4.3 1.2
8.8 4.9 3.7
2.21 2.10 3.02
3.4 -1.0 -3.2
Figure 6. Results of EXAFS data analysis for sample 1: (A) k1-weighted EXAFS data (solid line) and the best fit (dotted line). (B) Magnitude and imaginary part of Fourier-transform of k1-weighted EXAFS data (solid lines) and the best fit (dotted lines). Fit range: 3.68 < k < 14.51 Å-1; 0.8 < R < 3.0 Å.
a Errors: coordination number N, (20%; interatomic distance R, (0.02 Å; Debye-Waller factor, ∆σ2, (20%; inner potential correction ∆E0, (10%. The subscript “s” stands for short. The fit was performed in R space. Fit range: 3.68 < k < 14.51 Å; 0.8 < R < 3.0 Å.
TABLE 2: EXAFS Parameters at the Ru K Edge Characterizing Sample 2a absorber-backcatterer pair
N
103 × ∆σ2/Å2
R/Å
∆E0/eV
Ru-Ru Ru-O Ru-Cs Ru-Al
3.5 4.2 0.3
6.5 7.7 2.0
2.03 2.16 3.02
0.9 6.0 8.7
Errors: coordination number N, (20%; interatomic distance R, (0.02 Å; Debye-Waller factor, ∆σ2, (20%; inner potential correction ∆E0, (10%. The subscript “s” stands for short. The fit was performed in R space. Fit range: 3.70 < k < 14.57 Å; 0.8 < R < 3.0 Å. a
and mass spectrometry data, various candidate models of the supported species were constructed. The candidate models and comments about them, with explanations of why all but one of the models for each supported sample were rejected, are provided in the section entitled as EXAFS Data Analysis and in the Supporting Information. The following two sections include EXAFS results characterizing samples 1 and 2 separately. EXAFS Results Characterizing Zeolite-Supported Rhodium Complex. Analysis of the EXAFS data characterizing sample 1 indicated no significant Rh-Rh contribution (Table 1)sthus, no detectable rhodium clusters; we conclude that the supported species were essentially all mononuclear rhodium complexes. The EXAFS data (Table 1) indicate a Rh-Os contribution (s stands for short to contrast this contribution with a longer Rh-O
Figure 7. Results of EXAFS data analysis for sample 2: (A) k1-weighted EXAFS data (solid line) and the best fit (dotted line). (B) Magnitude and imaginary part of Fourier-transform of k1-weighted EXAFS data (solid lines) and the best fit (dotted lines). Fit range: 3.70 < k < 14.57 Å-1; 0.8 < R < 3.0 Å.
contribution that includes support oxygen atoms that may interact with Rh atoms through noncovalent interactions; see Table S2 of Supporting Information) with a coordination number of approximately 2 and a bonding distance (2.21 Å), consistent with the inference that each Rh atom was bonded to 2 oxygen atoms. Because the IR results (Figure 2) demonstrate the essentially complete removal of the acac ligands from the Rh atoms, we interpret these results as evidence that the two oxygen atoms bonded to each Rh atom (on average) were part of the zeolite and not of acac ligands. Consistent with this result, the Rh-O bond distance characterizing sample 1 (2.21 Å, Table 1) is significantly greater than those characterizing the bonds between Rh atoms and oxygen atoms of acac ligands in I itself, as determined by X-ray diffraction crystallography (2.051 Å).28
Molecular Metal Complexes Anchored to Zeolite β The EXAFS-determined Rh-O distance characterizing the zeolite β-supported rhodium complex is rather close to those characterizing the analogous zeolite Y-supported rhodium complex, as determined by EXAFS spectroscopy (2.14 or 2.19 Å, depending on the EXAFS model)4,5 and also by calculations at the level of density functional theory (DFT) (2.12, 2.17, and 2.19 Å, depending on basis sets and also on zeolite models used in the calculations).4,5 The EXAFS data also indicate a Rh-C contribution with a coordination number of approximately 4 and a bond distance of 2.10 Å. This Rh-C distance is less than those charactering I, as determined by X-ray diffraction crystallography (2.125 and 2.129 Å); it is characteristic of π-bonded ethene ligands on the supported rhodium complexes (2.12 or 2.13 Å).4,5 The EXAFS evidence of ethene ligands in sample 1 is consistent with the mass spectra showing the displacement of ethene by CO (Figure 4). The EXAFS data also give evidence of a Rh-Al contribution, with a coordination number of approximately 1 and a distance of 3.02 Å, essentially matching the Rh-Al distances determined by EXAFS spectroscopy for zeolite Y-supported rhodium complexes (2.92 and 2.95 Å)4,5 and those calculated by DFT (2.99 or 2.89 Å).4,5 In summary, the EXAFS results are consistent with the IR results and show that the supported rhodium complexes formed from I incorporate ethene ligands and were bonded to oxygen atoms near Al sites in the zeolite, as shown schematically below (where the π-bonding of the ethene ligands to the Rh is not depicted, but instead dotted lines are included to indicate the orientation of these ligands with respect to the Rh atom):
J. Phys. Chem. C, Vol. 114, No. 6, 2010 2689 ligands bonded the ruthenium to be evident in the EXAFS spectra or that the acac ligands bonded to the Ru atoms were not present in structures that were uniform enough to give rise to a clear EXAFS contribution. The EXAFS results indicate a Ru-O contribution with a coordination number of 3.5 with a distance of 2.03 Å (Table 2). Because the precursor II has four Ru-O bonds (two bidentate acac ligands) and the IR results indicate that approximately 70% of the acac ligands were removed from the Ru atoms after II had reacted with the zeolite, we infer that some of the adsorbed species formed from II had become bonded to oxygen atoms of the zeolite. Consistent with this inference, the EXAFS results indicate a small Ru-Al contribution with a coordination number of 0.3, indicating that some of the adsorbed complexes II were bonded to the zeolite near Al sites. The EXAFS data also indicate a Ru-C contribution with a coordination number of approximately 4 and a distance of 2.16 Å; this is a bonding distance characteristic of π-bonded ethene ligands on ruthenium.29,30 The coordination number indicates that approximately two ethene ligands were bonded, on average, to each Ru atom. Thus, we infer that these ligands remained intact as the ruthenium complex II was bonded to the zeolite. In summary, the EXAFS results are consistent with the IR results showing that most of the acac ligands had been removed from the Ru atoms when II reacted with the zeolite. Because the supported species were not uniform in composition, the EXAFS data do not provide sufficient information to determine their structures, although they clearly demonstrate the presence of ethene ligands. Thus, the chemistry of chemisorption of the ruthenium-containing precursor II was markedly less simple than that of the rhodium-containing precursor I. Discussion
The sites at which the rhodium complexes were bonded are those at which one would expect electron-deficient metal complexes. We suggest that the electron deficiency of the Rh atoms in the supported complex relative to I resulted from the replacement of the bidentate acac ligands in I with oxygen atoms of the zeolite (which is a bidentate ligand); we infer that the Lewis basicity of the zeolite is less than that of the acac ligands. This inference is consistent with the long Rh-O distance characteristic of sample 1 (Table 1) relative to that in I and the correspondingly reduced back-donation of electron density from Rh atoms to ethene ligands, leading to the shorter Rh-C distance in the supported complex relative to I (Table 1). EXAFS Results Characterizing Zeolite-Supported Ruthenium Complex. Analysis of the EXAFS data characterizing sample 2 led to the inference that there was no significant Ru-Ru contribution, within error, indicating that the supported species were essentially all mononuclear. The EXAFS data do not indicate a significant Ru-Cl contribution (l stands for long to contrast this contribution with a shorter Ru-Cs contribution, which is characterized by a bonding distance) that would be expected for ruthenium bonded to acac ligands, in qualitative agreement with the IR spectra (Figure 2) showing that approximately 70% of acac ligands were removed from II after it reacted with the zeolite; we infer that there were too few acac
Requirements for Formation of Nearly Uniform ZeoliteSupported Metal Complexes. Investigations of the structures of supported metal complexes and the determination of relationships between structure and catalytic performance are markedly facilitated when the supported metal complexes are uniform. There are still only a few examples of supported metal complexes shown to have a high degree of uniformity. For example, well-defined supported metal complexes were prepared from precursors such as I, II, and Ir(acac)(η2-C2H4)2 (III), with dealuminated zeolite HY as a support.8 The facile and complete removal of the bidentate acac ligands from these precursors and their replacement with support oxygen atoms led to the genesis of nearly uniform species; the uniformity was demonstrated by (a) IR spectra showing narrow νCO bands when the ethene ligands were replaced by CO3,6,8 and (b) in the case of the supported rhodium diethene complex by variable-temperature 13C NMR spectra4 showing that at a particular temperature (approached from either a higher or a lower temperature) the 13C resonance broadened to the point of being absent, which indicates that the 1H-13C dipolar decoupling was compensated by the random anisotropic reorientation of the 1H-13C bond vector. The results imply a dynamic uniformity of the supported species and hence, by inference, evidence of their structural uniformity. In contrast, typical oxide-supported metal complexes are characterized by a lack of uniformity associated with the intrinsic nonuniformity of the support surfacessurface nonuniformity is a characteristic typical of amorphous solids. Even when the supports have been crystalline (i.e., zeolites12,13), the supported species have often been nonuniform, depending on the synthetic
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methods and conditions.12,13,31,32 The goal of determining relationships between catalytic properties and the structures of nonuniform supported species remains a major challenge and motivates our work with the better-defined structures as an alternative. We regard the zeolite β-supported rhodium complexes as meeting a high standard of uniformity and worthy of further work, whereas the zeolite β-supported ruthenium complexes do not meet this standard. Supported metal complexes that are nearly uniform allow manipulation of the structure and reactivity by simple ligand replacement,7 and, furthermore, modeling of the structure and reactivity by calculations at the DFT level.7 In the modeling, the supported species are regarded as molecular analogues with ligands, including the support, that can be represented as a simple fragment of the surface; thus, calculations that match experiment well have been done for species such as Al(OH)4Rh(H)(η2C2H4) and Al(OH)4Rh(CO)(η2-C2H4)7 to represent the zeolite Y-supported metal complexes. Now, working with another zeolite, zeolite β, we have found essentially the same simple synthesis chemistry of the rhodium complex precursor I with the zeolite as in the earlier work with zeolite Y. However, the chemistry of the ruthenium complex II in zeolite β is complex, resulting in mixtures of ruthenium complexes with various numbers of acac ligands removed per Ru atom, even at the low loading of 1.0 wt % Ru in the zeolite. Simpler chemistry was observed with zeolite Y as the support for similar ruthenium complexes. Specifically, our earlier results with zeolite Y-supported ruthenium complexes3 formed from II and dealuminated zeolite HY showed that the number of acac ligands removed from II was determined by the Al/Ru ratio. When the supported sample had an Al/ Ru atomic ratio of 3, then a mixture of species formed, including ruthenium complexes with one and with two acac ligands. When the Al/Ru ratio was increased to 6, so that more bonding sites were available for dissociated acac ligands, then nearly one acac per Ru was removed from II upon adsorption, and so we infer that this sample was nearly uniform.3 When the Al/Ru ratio was increased to 11, then more acac ligands were removed from the ruthenium, and again a mixture of species formed, including ruthenium complexes with zero acac ligands and with one acac ligand.33 We have not yet succeeded in preparing a supported sample from II with removal of all the acac ligands from the ruthenium. Thus, formation of nearly uniform supported ruthenium complexes from II, at least with our methods, is not straightforward. We regard it as more challenging to prepare uniform supported metal complexes from precursors having more than one acac ligand than from precursors having only one acac ligand. Thus, when the goal is to prepare uniform supported metal complexes from metal complexes incorporating acac ligands, we favor the use of precursors incorporating only one such ligand, including Rh(acac)(η2-C2H4)2 and Ir(acac)(η2-C2H4)2. Furthermore, as supports we recommend zeolites that have a sufficient density of framework ions such as aluminum that can accept acac ligands readily (although the chemistry of the reaction of the acac ligands with the aluminum sites is not fully elucidated). We suggest that the density of such sites in the zeolite should be low enough to ensure good separation of the metal complexes bonded at these sites (site-isolation) while still being high enough to accept all the acac ligands from the precursor and also to anchor the metal complexes.
Ogino and Gates Structures Zeolite-Supported Rhodium Ethene Complexes: Comparison of Zeolite Y and Zeolite β as Supports. On the basis of the IR and EXAFS data representing sample 1, we infer that the reaction of I with zeolite β led to the replacement of one acac ligand with two oxygen atoms of the support, producing surface-bound rhodium complexes with two ethene ligands intact. The same chemistry was observed with zeolite Y as the support. If one assumes that the zeolite surface oxygen atoms are twoelectron donors and provide a formal charge of -1 per AlO2 unit, then the supported rhodium complexes are coordinatively unsaturated, formally 16-electron species (8 e- from Rh+, 4 efrom the zeolite, and 4 e- from two ethene ligands). The mass spectrometry data show that C2H4 ligands in such species were readily replaced with CO ligands at room temperature (Figure 4). The resultant supported rhodium gem-dicarbonyl species in zeolite β are characterized by νCO bands at 2048 and 2115 cm-1, with satellite bands at 2017 and 2083 cm-1 (Figure 3). These frequencies are slightly lower than those characterizing the analogous zeolite Y-supported rhodium complex (sharp bands at 2052 and 2117 cm-1 with satellite bands at 2021 and 2101 cm-1).6,7,15 This comparison indicates that Rh atoms in zeolite β are more electron-rich than those in zeolite Y, consistent with greater donation of electron density from the Rh atoms to the electron-accepting ligands CO and C2H4 in zeolite β than in zeolite Y.34,35 Consistent with this inference, the Rh-C bond of the zeolite β-supported rhodium ethene complex (sample 1) determined by the EXAFS data (2.10 Å, Table 1) is slightly shorter than those characterizing the zeolite Y-supported rhodium complex (2.12 or 2.13 Å).4,5 These comparisons illustrate a subtle difference between the two zeolites as ligands, which is explained by differences in the local structure around the bonding sites, such as the T-O-T angles (T ) Si or Al). Although Bare et al.9 demonstrated the locations of Sn atoms in zeolite β, their work lacked information about how the local structure of the zeolite affects the electronic properties of the Sn atoms; their conclusion regarding the location of the Sn atoms in the zeolite framework was based on differences in coordination shells at distances greater than 3.5 Å. Therefore, it remains an open question how the zeolite may modify the electronic properties of Sn atoms in the framework. In contrast, our results show subtle differences in the local structure and electronic properties of the Rh atoms anchored to the zeolite framework (not in it), suggesting that we may be able to tailor the electronic properties of supported metal complexes by using zeolites with various compositions and structures as supports. In contrast to the zeolite β-supported rhodium complexes, which lend themselves to a straightforward comparison with zeolite Y-supported complexes that are structurally analogous, comparisons of the zeolite β-supported ruthenium complexes with the zeolite Y-supported ruthenium complexes3 are challengingsbecause the supported ruthenium species in zeolite β incorporate various numbers of acac ligands, we are not yet able to resolve the roles of the zeolite and acac ligands. The contrast between the rhodium complexes and the ruthenium complexes reinforces the importance of the uniformity of the supported species for incisive determination of structure and electronic properties of the supported metal complexes. The success in the synthesis with the rhodium complex in both zeolite β and zeolite Y demonstrates a generality of the
Molecular Metal Complexes Anchored to Zeolite β synthesis methodswe expect that other aluminosilicate zeolites with pores large enough to accommodate I may also work as supports. Conclusions The reaction of I with zeolite β led to essentially complete removal of acac ligands from the Rh atoms of I, yielding siteisolated and highly uniform Rh(η2-C2H4)2+ species bonded to the zeolite via two Rh-O bonds at aluminum sites of the zeolite. The Rh atoms in this species are more electron-rich than those in the isostructural zeolite Y-supported rhodium complex, reflecting subtle differences in the local structures around the anchoring sites of the two zeolites. In contrast to the zeolite β-supported rhodium complexes, the zeolite β-supported ruthenium complexes consisted of a mixture with various numbers of acac ligands per Ru atom. These results illustrate the importance of the uniformity of the supported species for precise characterization of structure, and they also suggest further opportunities for using I for preparation of similar metal complexes in other types of zeolites. Experimental Section Materials and Procedures. Samples were handled with standard air-exclusion techniques, including Schlenk techniques, and stored in a glovebox under dry argon; the O2 and moisture contents in the glovebox were less than 0.1 ppm each. Glassware was dried at 393 K overnight prior to use. n-Pentane solvent (Fisher Scientific) was purified in chromatographic columns containing activated Al2O3 and Al2O3-supported activated copper (MBraun, MB-SPS). The rhodium complex I (Strem, 99%) was used as received. The ruthenium complex II was synthesized according to a reported method.36 The β zeolite (CP814C, Zeolyst International, Si/Al atomic ratio ) 19) was treated as described in the following section. Synthesis of Zeolite-Supported Rhodium Complexes (Sample 1). The zeolite was calcined by heating in flowing dry O2 from room temperature to 773 K over a period of 3 h, then held in flowing O2 at 773 K for 4 h, and then under vacuum at 773 for an additional 16 h. The zeolite under vacuum was then cooled to room temperature and used immediately for sample preparation, as follows: to the calcined zeolite (1.0 g) in a 100-mL Schlenk flask with a stir bar was added complex I (0.025 g), giving 1.0 wt % Rh on the zeolite (Rh/Al atomic ratio ≈ 1/8). To this mixture was added freshly distilled n-pentane (ca. 20 mL). The mixture was stirred at room temperature, and after 24 h the n-pentane was removed by evacuation, giving a powder with a light-yellow color. Synthesis of Zeolite-Supported Ruthenium Complexes (Sample 2). To 2.0 g of the zeolite that had been calcined under the conditions described in the preceding section was added 0.072 g of complex II [Ru/Al ≈ 1/6 (atomic)]. To this mixture was added approximately 30 mL of freshly distilled n-pentane. The mixture was stirred at room temperature, and after 24 h, the n-pentane was removed by evacuation, giving a powder with a light-brown color. IR Spectroscopy of Zeolite-Supported Metal Complexes and Analysis of Effluent Gases. Spectra of solid samples were collected in transmission mode with a Bruker IFS 66v Fourier transform spectrometer with a spectral resolution of 2 cm-1. Samples were loaded into the cell in the glovebox in the presence of argon, and then the cell was sealed, transferred to the sample chamber of the IR instrument without exposure of the sample to air, and evacuated at room temperature under dynamic vacuum (
