J. Phys. Chem. B 2002, 106, 2109-2116
Osmium Carbonyls in Zeolite NaX: Characterization by Absorption Fine Structure Spectroscopies
129Xe
2109
NMR and Extended X-ray
Bryan Enderle,† Andrea Labouriau,‡ Kevin C. Ott,‡ and Bruce C. Gates*,† Department of Chemical Engineering and Materials Science, UniVersity of California, DaVis, California 95616, and Chemical Science and Technology DiVision, Mail Stop J514, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 ReceiVed: July 23, 2001; In Final Form: NoVember 7, 2001
Osmium carbonyls synthesized at low loadings in the supercages of zeolite NaX, including mononuclear osmium carbonyls and the following clusters, [HOs3(CO)11]-, [H3Os4(CO)12]-, and [Os5C(CO)14]2-, were identified by extended X-ray absorption fine structure and infrared spectroscopies. The samples were also characterized by 129Xe NMR spectroscopy over the temperature range 100-310 K. The 129Xe chemical shifts were greater for samples containing osmium carbonyls than for the bare zeolite over the entire temperature range. At the lowest temperatures, the chemical shifts representing xenon sorbed in the zeolites containing the osmium carbonyl clusters were essentially the same, but at temperatures close to room temperature, the chemical shift increased with decreasing size of the osmium carbonyl. The relatively large chemical shift observed for the smallest osmium carbonyl is consistent with the presence of Xe atoms in the cages with these mononuclear metal complexes, whereas Xe atoms barely fit into the cages with the larger [HOs3(CO)11]-, [H3Os4(CO)12]-, or [Os5C(CO)14]2-.
Introduction 129Xe NMR spectroscopy is a powerful technique for probing the interior spaces of nanoporous materials,1-11 but so far, it has provided little information about species within these materials. Metal complexes and clusters in zeolite cages have been investigated by many techniques, with the most fruitful evidently being transmission electron microscopy (TEM)12 and extended X-ray absorption fine structure (EXAFS) spectroscopy.13-15 However, the smallest clusters are difficult to observe with TEM, even under optimal conditions, and EXAFS spectroscopy provides only average structural information. Because most metal clusters synthesized in zeolite pores are nonuniform, interpretation of the 129Xe NMR chemical shifts of the materials has been less than incisive; thus, there is a strong motivation to determine the usefulness of the technique to zeolites incorporating uniform species. Clusters in zeolite Y approximated as Mo2,10 Ir4,11 Ir6,11 and Rh6,11 made from the metal carbonyl precursors [Mo(CO)6], [Ir4(CO)12], [Ir6(CO)16], and [Rh6(CO)16], respectively, were also characterized by 129Xe NMR spectroscopy, but the chemical shifts alone are not sufficient to determine the metal complex or cluster size. The goal of the research presented here was to use 129Xe NMR spectroscopy to characterize a family of metal carbonyls of systematically varied sizes to determine how sensitive the chemical shifts are to the sizes of the encaged species. The family of osmium carbonyls appears to be nearly ideal to meet this goal, because it includes stable species with a wide range of nuclearities, including mononuclear osmium carbonyls, [HOs3(CO)11]-, [H3Os4(CO)12]-, and [Os5C(CO)14]2-,16 each of which has been made on solid supports by surface-mediated
* To whom correspondence should be addressed. † University of California, ‡ Los Alamos National Laboratory.
organometallic synthesis. We report the syntheses of these osmium carbonyls in zeolite NaX and their characterization by EXAFS, infrared (IR), and 129Xe NMR spectroscopies. Experimental Section Sample Preparation. Zeolite NaX, NaxSi192-xO384Alx, has a cubic unit cell with space group Fd3m and cell parameter a ) 24.85 Å; each unit cell contains eight sodalite cages and eight supercages (R cages). The zeolite sample was obtained from the Davison Division of W. R. Grace and Co. 29Si MAS NMR spectroscopy was used to determine the Si:Al atomic ratio of 1.4, as calculated by the peak area ratios of Si(1Al), Si(2Al), etc. Prior to preparation of the zeolite-supported osmium carbonyls, the zeolite support was heated at 573 K in O2 (Matheson Extra Dry Grade) for 4 h and subsequently evacuated overnight at 10-3 Torr. He, CO, and H2 (99.995%) used in the syntheses of the species in the zeolite were purified by passage through traps containing particles of Cu2O and zeolite 4A to remove traces of O2 and moisture, respectively. [Os3(CO)12] (Strem, 99%) was used as received. Syntheses of osmium carbonyls in the zeolite and sample transfers were performed with exclusion of air and moisture on a double-manifold Schlenk vacuum line and in an N2-filled glovebox (AMO-2032, Vacuum Atmospheres). [H2Os(CO)4] was prepared by reduction of [Os3(CO)12] with Na in liquid ammonia followed by protonation of the resulting Na2[Os(CO)4] with phosphoric acid (85%, Strem).17 To prepare zeolite-supported mononuclear osmium carbonyl, [H2Os(CO)4] vapor was brought in contact with the zeolite powder at room temperature for 36 h. Excess [H2Os(CO)4] was removed by evacuation, and the solid was treated in CO + H2 (1:1 molar) for 12 h at 573 K to form a sample inferred to contain [HOs3(CO)11]-.17 Further treatment of the zeolite-supported [HOs3(CO)11]- in CO at 423 K for 2 h led to the formation of zeolite-entrapped [H3Os4(CO)12]-. Part of this sample was
10.1021/jp0128224 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/02/2002
2110 J. Phys. Chem. B, Vol. 106, No. 8, 2002 converted into [Os5C(CO)14]2- in the zeolite by treatment in CO at 548 K for 4-6 h.16 The mass of [H2Os(CO)4] taken up by the zeolite indicated that the sample contained about 4.0 wt % Os; this relatively high metal loading was chosen to give suitably high signal-to-noise ratios in the NMR experiments. The samples were stored in under N2 in the drybox. Extraction. Extraction experiments were done with the samples under N2 in a glovebox using an excess of [PPN][Cl] (PPN ) (Ph3P)2N) in acetone. The mixture was stirred for several hours. IR Spectroscopy. Transmission IR spectra were collected with a Bruker IFS-66v spectrometer with a spectral resolution of 4 cm-1. Powder samples were pressed into semitransparent, self-supporting wafers and mounted into a cell in a glovebox. Details are as stated elsewhere.18-20 NMR Spectroscopy. Each powder sample characterized by 129Xe NMR spectroscopy was weighed and packed into an 8-mm diameter glass tube (Wilmad) in an N2-filled glovebox. A known amount of Xe was condensed and frozen inside the tube. He (at a pressure of about 16 Torr) was introduced into the tube to increase the rate of heat transfer during the NMR experiments. The tube was then flame-sealed. The amount of Xe was sufficient to give a loading of about 0.5 Xe atoms/supercage in each sample. The 129Xe NMR experiments were performed with a homemade transmission line probe on a Varian Unity 400 spectrometer operating at 110.629 MHz. The typical π/2 pulse width was 10 µs, and a 3 s recycle delay was used for the temperature range investigated. The number of transients per spectrum varied from 200 to 512, depending on the sample temperature. Low temperatures were achieved with an Oxford model CF 1200 cryostat. Temperature was measured with the Au-Fe/constantan thermocouple, which is part of the temperature-sensing and -controlling circuit in the Oxford cryostat. The sample was cooled in steps from room temperature to 100 K and allowed to equilibrate for 20-30 min between temperature steps. Chemical shifts were measured at each step. Furthermore, the chemical shift of an external standard sample of Xe gas at 2.0 atm and room temperature was measured prior to each set of measurements at variable temperatures. The shift was then corrected to the shift at zero pressure with equations given by Jameson et al.21,22 EXAFS Spectroscopy. The EXAFS experiments were performed on beamline 2-3 of the Stanford Synchrotron Radiation Laboratory. The storage ring operated with an energy of 3.0 GeV; the ring current varied within the range of 70-100 mA. Data were collected at the Os LIII edge (10,871 eV) with the samples under vacuum (10-5 Torr) at approximately liquid nitrogen temperature. Details are as reported elsewhere.18-20 Analysis of EXAFS Data The EXAFS data were analyzed with theoretically and experimentally determined reference files obtained from the FEFF 7.0 code and EXAFS data characterizing materials of known structure, respectively. The Os-Os reference file was calculated with the FEFF 7.0 code and structural parameters representing bulk osmium metal. The sample used to collect data for the reference files for analysis of Os-C and Os-O contributions was made from [Os3(CO)12] mixed with BN and scanned at liquid nitrogen temperature The XDAP software was used to estimate the EXAFS parameters from the raw data.23 The methods used to extract the EXAFS function are essentially the same as those reported.17,24 The final normalized EXAFS function characterizing each sample was obtained from the
Enderle et al. TABLE 1: Crystallographic Data Characterizing the Reference Compounds and Fourier Transform Ranges Used in the EXAFS Analysisa crystallographic data
Fourier transform
ref cmpd
shell
N
R, Å
∆k, Å-1
∆r, Å
n
Os [Os3(CO)12]
Os-Os Os-C Os-O Re-O
12 4 4 4
2.62 1.95 3.09 1.74
1.55-19.41 2.07-15.92 2.07-15.92 2.98-15.78
1.98-2.98 0.18-1.91 2.05-3.41 0.19-2.09
3 3 3 3
NH4ReO4
a Notation: N, coordination number; R, distance; ∆k, range of k, the wave vector, in Fourier transformation; ∆r, range of r, distance, in Fourier transformation; n, k weighting in Fourier transformation.
average of 4-6 scans. The main contributions to the spectra were isolated by inverse Fourier transformation of the final EXAFS function. The analysis was done with the raw data without Fourier filtering. The parameters characterizing both Os-low-Z (Os-O and Os-C) and Os-high-Z (Os-Os) contributions were determined by multiple-shell fitting in r space (r is the distance from the absorbing atom, Os) and in k space (k is the wave vector) with application of k1 and k3 weighting in the Fourier transformations. The fit was optimized by use of a difference file technique25 with phase- and amplitude-corrected Fourier transforms.18-20 The parameters used to extract these files from the EXAFS data are summarized in Table 1. Results Lack of Extraction of Sorbed Species. Attempts to extract each of the osmium carbonyl clusters made from [H2Os(CO)4] from the zeolite by using [PPN][Cl] in acetone were unsuccessful, as shown by IR spectra of the extract solutions indicating no νCO bands. The results indicate that the clusters were trapped in the zeolite cages. Sample Colors. The bare NaX zeolite is white in color. After contacting of the zeolite with [H2Os(CO)4], the sample turned light yellow, consistent with adsorption of the osmium complex intact in the zeolite. Treatment to form [HOs3(CO)11]- resulted in a darker yellow color. Further treatment intended to prepare [H3Os4(CO)12]- in the pores caused the sample to turn bright yellow. Still further treatment intended to prepare [Os5C(CO)14]2caused the sample to become slightly orange in color. The colors of the supported samples reported here are broadly consistent with those expected for supported osmium carbonyl clusters. EXAFS Data. The EXAFS parameters determined in the data fitting are reported in Table 2. The software XDAP23 was used to estimate the errors shown in Table 2; these represent precision, not accuracy. The accuracies are estimated to be as follows: coordination number (N), ( 30% (except for Os-Os, ( 10%); distance (R), ( 2% (except for Os-Os, ( 1%); Debye-Waller factor (∆σ2), ( 20%; and inner potential correction (∆E0), ( 20%. The number of statistically justified fitting parameters was typically 42, as estimated on the basis of the Nyquist theorem, n ) (2∆k∆r/π) + 1, where ∆k and ∆r are the respective k and r ranges used in the data analysis (∆k ) 12.8 Å-1; ∆r ) 5 Å). In each of the reported fits, the number of parameters was less than the statistically justified number. Mononuclear Osmium Carbonyls. The EXAFS data (Table 2) are consistent with the conclusion that contacting of the zeolite with [H2Os(CO)4] vapor led to the formation of mononuclear zeolite-supported osmium carbonyls.26-32 No Os-Os contributions were found at typical Os-Os bond distances, consistent with the inference that there were no Os-Os bonds in the sample and that the supported species were mononuclear.
Osmium Carbonyls in Zeolite NaX
J. Phys. Chem. B, Vol. 106, No. 8, 2002 2111
TABLE 2: EXAFS Parametersa Characterizing NaX-Zeolite Encaged Osmium Species
a
103 × ∆σ2, Å2
reference
4.3 ( 0.2
-5.1 ( 0.3
Re-O
[H2Os(CO)4]
2.01 ( 0.01 3.10 ( 0.01 3.04 ( 0.01 2.80 ( 0.01
1.6 ( 0.4 2.3 ( 0.1 1.9 ( 0.3 4.2 ( 0.3
3.0 ( 0.5 1.0 ( 0.3 -1.1 ( 0.2 2.7 ( 0.2
Os-C Os-O Os-Os Os-Os
[HOs3(CO)11]-
3.7 ( 0.1 3.8 ( 0.1 2.5 ( 0.1 3.2 ( 0.1
1.97 ( 0.01 3.05 ( 0.01 2.11 ( 0.01 2.82 ( 0.01
4.9 ( 0.2 -0.1 ( 0.2 1.8 ( 0.1 2.0 ( 0.2
1.3 ( 0.1 1.1 ( 0.1 -7.8 ( 0.4 -0.2 ( 0.1
Os-C Os-O Re-O Os-Os
[H3Os4(CO)11]-
3.4 ( 0.1 3.5 ( 0.1 3.5 ( 0.1
1.90 ( 0.01 3.07 ( 0.01 2.83 ( 0.01
-2.3 ( 0.4 -0.7 ( 0.1 2.6 ( 0.2
0.8 ( 0.1 -1.2 ( 0.2 3.0 ( 0.2
Os-C Os-O Os-Os
[Os5C(CO)14]2-
3.3 ( 0.1 0.5 ( 0.1 3.1 ( 0.1
1.86 ( 0.01 1.97 ( 0.01 3.06 ( 0.01
-2.5 ( 0.3 -9.5 ( 0.2 -0.8 ( 0.1
6.1 ( 0.2 3.5 ( 0.1 -1.2 ( 0.1
Os-C Os-C Os-O
N
R, Å
Os-Osupport Os-CO Os-C Os-O* Os-Os Os-Os Os-CO Os-C Os-O* Os-Osupport Os-Os Os-CO Os-C Os-O* Os-Os Os-CO Os-Ct Os-Cbc Os-O*
1.0 ( 0.1
2.14 ( 0.01
4.6 ( 0.1 5.0 ( 0.1 0.7 ( 0.1 2.3 ( 0.1
Notation as in Table 1.
TABLE 3: EXAFS Resultsa Characterizing Osmium Subcarbonyls on γ-Al2O3 as Reported by Deutsch et al.26 Sample
shell
N
R, Å
osmium tricarbonyl
Os-Osupport Os-CO Os-C Os-O* Os-Osupport Os-CO Os-C Os-O*
2.9
2.17
2.4 3.1 3.9
1.93 3.04 2.17
2.0 2.4
1.85 3.04
osmium dicarbonyl
a
suggested encaged species
∆E0, eV
shell
TABLE 4: Literature Values of Structural Parameters of Unsupported Osmium Compounds Determined by X-ray Diffractiona compound [H2Os(CO)4]
Notation as in Table 1.
Both the Os-C coordination number, 4.6, and the Os-O* coordination number (O* is carbonyl oxygen), 5.0, agree, within the expected error, with the values for [H2Os(CO)4] itself. The presence of osmium subcarbonyls does not appear likely because the coordination number of an osmium atom to a carbon (or oxygen) atom in a CO ligand for an osmium subcarbonyl would typically be in the range of 2-3 (Table 3). Furthermore, the Os-C distance (2.01 Å) agrees well with the value of 1.99 Å determined by X-ray diffraction (XRD) crystallography for [H2Os(CO)4] (Table 4) and is significantly longer than the values of 1.85 and 1.93 Å for supported osmium di- and tricarbonyls, respectively (Table 3). However, an Os-Osupport contribution was found with a coordination number of 1.0 and a distance of 2.14 Å; this is an Os-O bonding distance, typical for metal subcarbonyls bonded to oxide and zeolite supports, although the coordination number is less than typical values for metal subcarbonyls.30 The OsOsupport contribution suggests that a minority of the mononuclear osmium carbonyls were bonded to the zeolite; perhaps some [HOs(CO)4]- formed, consistent with known surface chemistry17 whereby a base abstracts a proton from [H2Os(CO)4]. A small (nonbonding) Os-Os contribution with a coordination number of 0.7 at a distance of 3.04 Å was also observed, but this is too long to indicate Os-Os bonds, which are typically about 2.87 Å in length;24,33,34 instead, the results suggest some mononuclear osmium complexes near each other in the zeolite, consistent with reports31 indicating the interactions of the mononuclear species through supercages in zeolites. Trinuclear Osmium Carbonyls. The EXAFS structural parameters characterizing the sample formed from [H2Os(CO)4] and the zeolite after treatment in CO + H2 (1:1 molar) for 12
[HOs3(CO)11]-
[H3Os4(CO)12]-
[Os5C(CO)14]2-
shell Os-CO Os-C Os-O* Os-Os Os-CO Os-C Os-O* Os-Os Os-CO Os-C Os-O* Os-Os Os-CO Os-Ct Os-Cb Os-Cc Os-O*
N
R, Å
ref
4.0 4.0 2.0
1.99 2.84
29
3.7 3.7 3.0
1.92 3.05 2.87
33
3.0 3.0 3.2
1.86 3.04 2.87
34
2.6 0.2 0.2 2.8
2.05 2.06
28
a Notation as in Table 1; the subscripts b, c, and t refer to bridging, carbido, and terminal, respectively.
h at 573 K (Table 2) are consistent with the formation of trinuclear osmium carbonyls, with the most likely candidate being [HOs3(CO)11]-.17 The first-shell Os-Os coordination number of 2.3 is equal, within the expected error, to the value of 2 for the triangular Os3 frame; the metal-metal distance of 2.80 Å determined by the EXAFS data nearly matches the XRD value of 2.84 Å for crystalline [Me4N][HOs3(CO)11] (Table 4)29 but is less than the value of 2.87 Å obtained by Maloney et al.17 for [HOs3(CO)11]- supported on zeolite NaY.17 The data agree well with EXAFS data representing [HOs3(CO)11]supported on zeolite NaX.17 The Os-C and Os-O* coordination numbers are 3.7 and 3.8, respectively, in good agreement with the XRD values for [HOs3(CO)11]- (Table 4).29 The Os-C distance of 1.97 Å is slightly greater than the XRD value of 1.92 Å for [HOs3(CO)11]-, and the Os-O* distance of 3.05 Å matches the XRD value for [HOs3(CO)11]- (Table 4). An Os-Osupport shell was also determined for this sample at a distance of 2.11 Å, in good agreement with data characterizing iridium carbonyl clusters supported on zeolite NaY, having an Ir-Osupport distance of 2.13 Å.18,19 The distance of 2.11 Å is a bonding distance31 and indicates a strong interaction between the Os atoms in the trinuclear osmium carbonyls and the support,
2112 J. Phys. Chem. B, Vol. 106, No. 8, 2002
Enderle et al.
TABLE 5: IR Spectra of Suggested Zeolite NaX-Supported Osmium Carbonyls in the νCO Region suggested encaged species
νCO, cm-1
[H2Os(CO)4] [HOs3(CO)11][H3Os4(CO)12][Os5C(CO)14]2-
1933m, 2005s, 2022sh, 2050s, 2068w 1930m, 1962sh, 1989s, 2020w, 2054m, 2093w 1930w, 1962s, 1991s, 2025s, 2043s 1930m, 1963s, 1975s, 1990s, 2022sh, 2043m
even though the clusters are coordinatively saturated. An implication of this metal-support interaction is that the clusters are distorted relative to the structure of the crystalline [Me4N][HOs3(CO)11]. In summary, the EXAFS data support the identification of the supported clusters as predominantly [HOs3(CO)11]-. Tetranuclear Osmium Carbonyls. The EXAFS data characterizing the zeolite-supported sample made by treatment of the supported [HOs3(CO)11]- in CO at 423 K for 2 h (Table 2) are consistent with the formation of zeolite-supported tetraosmium clusters, with the most likely species being [H3Os4(CO)12]-. The Os-Os coordination number was found to be 3.2, nearly matching the XRD value of 3 for the tetrahedral Os4 frame of [H3Os4(CO)12]-.33 The Os-Os distance of 2.82 Å is close to the XRD value of 2.87 Å. The Os-C and Os-O* coordination numbers were found to be 3.4 and 3.5, respectively, with distances of 1.90 and 3.07 Å, respectively, also consistent with the XRD values for [H3Os4(CO)12]-. No Os-support shell could be distinguished from the noise in the data. In summary, the EXAFS data provide strong evidence for [H3Os4(CO)12]- in the zeolite. Pentanuclear Osmium Carbonyls. EXAFS data characterizing the sample formed by treatment of zeolite NaX-supported [H3Os4(CO)12]- in CO at 548 K for 4-6 h are consistent with the formation of [Os5C(CO)14]2-. The conclusion is based on a comparison of EXAFS data with XRD data representing [PPN]2[Os5C(CO)14] (Table 2) and EXAFS data representing MgOsupported [Os5C(CO)14]2-.24,34 The Os-Os coordination number, 3.5, is consistent with a square pyramidal metal frame. The Os-Os bond distance of 2.83 Å agrees, within experimental error, with the XRD value of 2.87 Å and is close to the bond distance of 2.90 Å reported24 for MgO-supported [Os5C(CO)14]2-. The Os-C and Os-O* coordination numbers are consistent with the 13 terminal carbonyls, one bridging carbonyl, and a carbido carbon indicated by XRD for [Os5C(CO)14]2-.24,26 The Os-terminal carbon shell, designated Os-Ct, is characterized by a coordination number and bond distance of 3.3 and 1.86 Å, respectively. The Os-bridging carbonyl and Os-carbido carbon (the combination designated by Os-Cbc) contribution (the two are represented together because both have shells at the same distance) is characterized by a coordination number and bond distance of 0.5 and 1.97 Å, respectively. The Os-O* contributions are characterized by a coordination number and bond distance of 3.1 and 3.06 Å, respectively. The weak contribution expected from the Os-Os second coordination shell was not distinguishable from the noise in the spectra and not represented in the fit. IR Spectra. Numerous reports of IR spectra of osmium carbonyls34-37 provide a basis for checking the identifications
of the supported species. The osmium carbonyls inferred to have been synthesized in the pore space of NaX zeolite are listed in Table 5. The precursor [H2Os(CO)4] sorbed in the zeolite gave a sample with νCO bands at 1933m, 2005s, 2022sh, 2050s, and 2068w cm-1. The spectrum resembles that of [H2Os(CO)4] in hexane (Table 6),35 but because of the different environments of NaX zeolite and the solutions referred to in Table 6, the bands of the supported species may be significantly shifted (and broadened) with respect to those of the reference compounds. The bands at 2005 and 2022 cm-1 representing the zeolitesupported sample are close in frequency to the 2016 cm-1 band characterizing [H2Os(CO)4] in solution, and the band at 2050 cm-1 is relatively broad and may be a combination of two bands, observed at 2050 and 2055 cm-1 in the spectrum of [H2Os(CO)4]. Thus, the IR data suggest the presence of [H2Os(CO)4] in the zeolite. However, they are not conclusive, in part because similar spectra have been observed for other mononuclear osmium carbonyls. For example, Deutsch et al.26 observed the following νCO spectrum for osmium subcarbonyls supported on γ-Al2O3: 1970m, 2045s, and 2124m cm-1. A peak at about 2045 cm-1 is common to the spectra of our zeolite-supported sample and Deutsch’s osmium subcarbonyl, but the 1970 and 2124 cm-1 peaks are missing from the spectrum of the zeolitesupported sample; it is possible that a peak near the 1970 cm-1 peak was obscured by other peaks in the spectra. Thus, the spectrum of the zeolite-supported sample is closer to that of [H2Os(CO)4] than that of an osmium subcarbonyl, but the observations are consistent with a mixture of the two; however, we infer that only little, if any, of the osmium subcarbonyl was present because of the absence of a peak near 2124 cm-1 in the zeolite-supported sample. Thus, the IR spectra are supportive of the conclusion based on the EXAFS data. After treatment, the zeolite containing the mononuclear osmium carbonyl with CO + H2 (1:1 molar) at 573 K for 12 h, the νCO region changed, showing bands at 1930m, 1962sh, 1989s, 2020w, 2054m, and 2093w cm-1. The spectrum is similar to that reported36 for [Me4N][HOs3(CO)11] in CH2Cl2 (Table 6), except for the presence of the 1930 and 2054 cm-1 bands, which may indicate some remaining unconverted mononuclear osmium carbonyl. Further treatment of the sample in CO at 423 K for 2 h led to a new IR spectrum similar to that of [Me4N][H3Os4(CO)12] (Table 6),37 but a band was observed at 1930 cm-1 in the spectrum of the zeolite-supported sample that is not characteristic of [Me4N][H3Os4(CO)12]. Thus, the spectrum resembles that of [H3Os4(CO)12]-, but again, the IR results are by themselves not sufficient for identification of the supported species. There may have been some unconverted [HOs3(CO)11]in the sample, as suggested by the peaks at 1930, 1962, and 1991 cm-1; these bands may obscure some of the bands characteristic of [H3Os4(CO)12]-. On the other hand, the 1962 and 1991 cm-1 bands may be indicative of [H3Os4(CO)12]-, with the bands at lower wavenumbers relative to the bands at 1976 and 2000 cm-1 characteristic of [Me4N][H3Os4(CO)12]. Further treatment of the sample in CO at 548 K for 4-6 h yielded a spectrum similar to that reported for [PPN]2[Os5C(CO)14]34 (Table 6). The spectrum of the zeolite-supported
TABLE 6: Literature Values Representing νCO Bands of Osmium Carbonyls in Solution sample/solvent
νCO, cm-1
ref
[H2Os(CO)4]/hexane [Me4N][HOs3(CO)11]/CH2Cl2 [Me4N][H3Os4(CO)12] [PPN]2[Os5C(CO)14]/CH2Cl2
1940w, 2016c, 2050s, 2055s, 2065s 1667w, 1951ms, 1996s, 2021s, 2083w 1976w, 2000s, 2022s, 2048s 1926w, 1945s, 1968vs, 1975vs, 1991s, 2015w, 2040w
35 36 37 34
Osmium Carbonyls in Zeolite NaX
J. Phys. Chem. B, Vol. 106, No. 8, 2002 2113
Figure 2. Dependence of 129Xe NMR chemical shifts measured at room temperature on the nuclearity of the metal carbonyl in the zeolites.
TABLE 7. Approximate Diameters of Xenon, NaX Zeolite Cages and Windows, and Osmium Carbonyls
Figure 1. 129Xe NMR chemical shifts for xenon sorbed in zeolites containing the following osmium carbonyls: (2) [H2Os(CO)4], (9) [HOs3(CO)11]-, (b) [H3Os4(CO)12]-, (4) [Os5C(CO)14]2-; data represented by the symbol 0 refer to xenon in bare zeolite NaX.
sample compares well with that of [PPN]2[Os5C(CO)14], and thus, we infer the formation of [Os5C(CO)14]2- in the zeolite. However, the band at 1945 cm-1 in the spectrum of [PPN]2[Os5C(CO)14] was not observed in the spectrum of the zeolitesupported sample; it was possibly obscured by the broad neighboring band at 1966 cm-1. In summary, the IR results indicate the formation of a family of samples, each containing predominately one osmium carbonyl formed in high yield in NaX zeolite. However, as is typical of IR spectra of metal carbonyls on solid surfaces, the peak locations differ from those of the corresponding metal carbonyl in solution, and the IR data are not sufficient by themselves to determine the structures of the supported species; they are supportive of the structural inferences drawn from the EXAFS data. NMR Data. Figure 1 shows 129Xe NMR chemical shifts as a function of temperature for NaX zeolite as well as for the zeolite containing the following osmium carbonyls: the mononuclear osmium carbonyl, [HOs3(CO)11]-, [H3Os4(CO)12]-, and [Os5C(CO)14]2-. The 129Xe chemical shifts are greater for the zeolites containing the osmium carbonyls than for the bare zeolite over the whole temperature range investigated. At room temperature, the xenon chemical shift values increased with increasing size of the osmium carbonyl (Figure 2). The resonance lines are narrow and well defined, and the error is estimated to be about 0.5 ppm at room temperature; the error increased with decreasing temperature, being about 1.5 ppm at 100 K. As the temperature decreased, the chemical shifts representing xenon sorbed in the zeolite containing [HOs3(CO)11]-, [H3Os4(CO)12]-, and [Os5C(CO)14]2- converged to a common value (Figure 1), within experimental error. At 100 K, the chemical shifts representing the clusters are markedly greater than that observed for xenon adsorbed in the bare zeolite and
structural entity
approximate diameter (Å)
Xe supercage window [H2Os(CO)4] [HOs3(CO)11][H3Os4(CO)12][Os5C(CO)14]2-
4.4 12.5 7.5 6 9 10 11
less than that observed for the zeolite containing the mononuclear osmium complex. Discussion The data reported here, combined with data reported for zeolite NaY-supported iridium and rhodium clusters,11 provide the first systematic investigation of nearly uniform nanostructures in nanopores probed by 129Xe NMR spectroscopy. Chemical Shifts at Room Temperature. The interactions of Xe atoms with the soft carbonyl ligands surrounding the metal are suggested to be approximately independent of the metal frame to which these ligands are attached. These Xe-carbonyl interactions are significant, as shown by the result (Figure 1) that the xenon chemical shift is markedly greater for each sample containing an osmium carbonyl than for the bare zeolite, even though the loading of osmium carbonyls in the zeolite was only about 1 per every 8-10 supercages. To a first approximation, the 129Xe NMR chemical shift data collected at room temperature for the zeolite-supported osmium carbonyl clusters can be interpreted simply on the basis of the geometries of the supported species, the zeolite cages, and the xenon atoms, as follows: At room temperature, Xe exchanges rapidly between zeolite cages and the encaged species, so that the chemical shift reflects an average over all these environments.2 We infer that, in this fast-exchange regime, a significant part of the 129Xe NMR chemical shift should be attributed to the different static polarizabilities of the various adsorption sites and that the different interactions between xenon atoms and the carbonyl groups in the various samples are affected substantially by the accessibilities of the osmium carbonyls in the supercages. The largest cluster of the series, [Os5C(CO)14]2-, which has a diameter of about 11 Å (Table 7), barely fits (Figure 3) into a supercage, which has a diameter of about 12.5 Å (and, like the other clusters, is too large to fit in a sodalite cage). Thus, this cluster leaves little space in the supercage for a xenon atom to penetrate and probe it. The clusters [H3Os4(CO)12]- and
2114 J. Phys. Chem. B, Vol. 106, No. 8, 2002
Enderle et al.
Figure 3. Schematic representation of xenon atom interacting with a pentaosmium carbonyl cluster through the window of a faujasite supercage.
Figure 4. Schematic representation of xenon atom interacting with a mononuclear osmium carbonyl in a faujasite supercage.
[HOs3(CO)11]- are slightly smaller than the penta-osmium carbonyl cluster, having diameters of about 10 and 9 Å, respectively. Again, xenon atoms can hardly intrude into the supercages containing these clusters, but the differences in available space in the supercage, depending on the cluster size, seem to provide a sensitive measure of the cluster size, at least as the cluster nuclearity is varied in the range from 3 to 5. The smallest osmium carbonyl of the series is the mononuclear species, which has an approximate diameter of 6 Å. Thus, xenon fits into the supercage with it (Figure 4), and there is a much better contact between xenon and this osmium carbonyl than between xenon and the osmium carbonyl clusters (Figure 3), and consequently, the chemical shift is substantially greater (Figures 1 and 2). However, we realize that the small differences in the chemical shifts observed close to room temperature for the zeolite containing the tri-, tetra-, and penta-nuclear clusters could also be caused by small sample-to-sample differences, such as, for example, small deviations in the metal carbonyl concentrations
and the distributions of the osmium carbonyls in the cages. Furthermore, our data are restricted to a single xenon concentration (0.5 Xe/cage). A more nearly definitive assessment of the ability of 129Xe NMR spectroscopy to distinguish one of the encaged clusters from another would likely require more precise syntheses than are available and measurement of the 129Xe NMR chemical shifts for a range of xenon concentrations (less than 1 per cage). Chemical Shifts at Low Temperatures. As the temperature decreased, the 129Xe NMR chemical shifts increased for each of the samples investigated. We suggest that this effect arises because the probability of finding a xenon atom at shorter distances from strong adsorption sites increases as the temperature decreases. The lower thermal energy of xenon at lower temperatures reduces and slows the exchange of xenon atoms between cages and encaged species. Thus, at low temperatures, the interactions between a xenon atom and strong adsorption sites become favored over interactions of xenon with weak adsorption sites. In bare zeolite NaX, the lowest-energy positions
Osmium Carbonyls in Zeolite NaX are close to the walls, and we expect that these are associated with larger chemical shifts than the ones observed at room temperature.2 The strong adsorption sites can be assessed in terms of van der Waals interactions between xenon atoms and the zeolite framework and any of the encaged species. The stronger the interaction between a xenon atom and a sorption site, the longer xenon resides at that site. Stronger interactions result in a larger xenon electron cloud deformation, which gives rise to an increase of the 129Xe NMR chemical shift. The xenon-zeolite interaction in bare zeolite NaX is usually modeled in terms of two short-range interactions: xenonoxygen and xenon-sodium. It is usual to neglect the interaction between xenon and the zeolite T atoms (Si and Al), because these are well shielded from xenon by the oxygen atoms. When osmium carbonyls are present in the zeolite, the significant xenon-osmium carbonyl interactions also need to be accounted for. The total static polarizability representing the interaction of a xenon atom with a cluster consisting of N atoms is a factor of N times greater than the static polarizability per atom. Therefore, the total static polarizability characterizing the osmium carbonyl clusters is expected to increase with the number of atoms in the cluster, so that the larger the clusters, the stronger the xenon-cluster interaction. In other words, a strong xenon-cluster interaction would give rise to a large deformation of the xenon electronic cloud, which would then cause a large chemical shift. The 129Xe NMR chemical shift data collected at 100 K for xenon in cages containing [HOs3(CO)11]-, [H3Os4(CO)12]-, and [Os5C(CO)14]2- are at best barely distinguishable from each other within the experimental uncertainty, but they are much less than that representing xenon in cages with the mononuclear osmium carbonyl. On the basis of static polarizability arguments alone, such as that presented above, one would expect the chemical shift of xenon to be greatest for [Os5C(CO)14]2-, because it is larger than the other osmium clusters and the mononuclear osmium carbonyl; thus, some explanation other than cluster size must be invoked. Again we return to the geometric considerations (Table 7). The relatively large osmium carbonyl clusters may so nearly fill the spaces of the supercages they occupy that a xenon atom could interact with the carbonyl ligands of the cluster only through the windows of the zeolite supercage. In this case, it is probably fair to assume that the xenon-cluster interaction would be weaker than if xenon were allowed to interact freely with the osmium carbonyls. Thus, on the basis of such reasoning, the data obtained at low temperatures can be explained in terms of both polarizability of the osmium clusters and excluded volume in the supercage. The results reported here are similar to results reported for iridium carbonyl clusters and rhodium carbonyl clusters supported in the supercages of zeolite NaY.11 The samples used in the reported investigation11 included [Ir4(CO)12] and [Ir6(CO)16] in the zeolite, each containing 3.3 wt % Ir. Just as the chemical shift of xenon for the zeolite NaX-encaged carbonyls was found to decrease with increasing nuclearity of the osmium carbonyl, the chemical shift of xenon in zeolite NaY containing [Ir6(CO)16] is less than that of xenon in zeolite NaY containing [Ir4(CO)12]. In each case, xenon is able to more effectively penetrate and probe the supercages containing smaller metal carbonyl species. The value of the chemical shift for xenon in zeolite NaY containing [Ir6(CO)16],11 for example, is about 10% greater than that for xenon in zeolite NaX containing [Os5C(CO)14]2-, about as expected on the basis of the preceding interpretation and the fact that the observations here were made under conditions
J. Phys. Chem. B, Vol. 106, No. 8, 2002 2115 nearly the same as those reported11 and with nearly the same metal loadings in the zeolites. However, the value of the chemical shift for xenon in zeolite NaY containing [Ir4(CO)12]11 is about 50% greater than that representing xenon in zeolite NaX containing [H3Os4(CO)12]-; the relatively large difference remains to be explained. The important conclusion is that 129Xe chemical shifts are sensitive to variations in sizes of species encaged in zeolites. Conclusions Mononuclear osmium carbonyls [HOs3(CO)11]-, [H3Os4(CO)12]-, and [Os5C(CO)14]2- were each prepared from [H2Os(CO)4] in the pores of zeolite NaX and characterized by EXAFS and IR spectroscopies. The 129Xe chemical shifts characterizing xenon sorbed in the zeolites containing the osmium carbonyls were found to be greater than those observed for xenon in the bare zeolite at temperatures ranging from 100 to 310 K. To a first approximation, the xenon chemical shifts measured at room temperature depend on the sizes of the osmium carbonyls. The largest chemical shift was observed for xenon atoms present in the supercages with the mononuclear osmium carbonyls. The chemical shifts were less for xenon interacting with the osmium carbonyl clusters than for xenon interacting with the mononuclear osmium carbonyls, because, we infer, each of the clusters so nearly filled a zeolite supercage that a xenon atom cannot enter the cage, and the Xe-cluster interaction is largely restricted to the cage windows. This work indicates that 129Xe chemical shifts are sensitive to variations in sizes of the encaged species. Acknowledgment. This research was supported by the U.S. Department of Energy, Office of Energy Research, Office of Basic Energy Sciences, Division of Chemical Sciences, Contract FG02-87ER13790, by the National Science Foundation (IGERT Grant DGE-9972741), and by Los Alamos National Laboratory, U.S. Department of Energy contract W-7405-ENG-36 as part of the Los Alamos Catalysis Initiative. The EXAFS experiments were done at SSRL, which is operated by the Department of Energy, Office of Basic Energy Sciences. The EXAFS data were analyzed with the XDAP software.23,38 References and Notes (1) Ito, T.; Fraissard, J. In Proceedings of the 5th Intternational Conference on Zeolites; Naples, Italy, 1980; Rees, L. V., Ed.; Heyden: London, 1980; p 510. (2) De Menorval, L. C.; Fraissard, J.; Ito, T. J. Chem. Soc., Faraday Trans. 1 1982, 78, 403. (3) Grosse, R.; Burmeister, R.; Boddenberg, B. J. Phys. Chem. 1991, 95, 2443. (4) Ryoo, R.; Cho, S. J.; Pak, C.; Kim, J. G.; Ihm, S. K.; Lee, J. Y. J. Am. Chem. Soc. 1992, 114, 76. (5) Boudart, M.; Ryoo, R.; Valenca, G. P.; Van Grieken, R. Catal. Lett. 1993, 17, 273. (6) Pak, C.; Cho, S. J.; Lee, J. Y.; Ryoo, R. J. Catal. 1994, 149, 61. (7) Moudrakovski, I. L.; Ratcliffe, C. I.; Ripmeester, J. A. J. Am. Chem. Soc. 1998, 120, 3123. (8) Guillemot, D.; Polisset-Thfoin, M.; Fraissard, J. J. Chem. Phys. B 1997, 101, 8243. (9) Hwang, I. C.; Woo, S. I. Stud. Surf. Sci. Catal. 1994, 84, 757. (10) Coddington, J. M.; Howe, R. F.; Yong, Y. S. J. Chem. Soc., Faraday Trans. 1990, 86, 1015. (11) Labouriau, A.; Panjabi, G.; Enderle, B.; Pietrass, T.; Gates, B. C.; Earl, W. L.; Ott, K. C. J. Am. Chem. Soc. 1999, 121, 7674. (12) Carlson, A.; Oker, A.; Bovin, T.; Terasaki, O. Chem. Eur. J. 1993, 5, 244. (13) Gates, B. C. Chem. ReV. 1995, 95, 511. (14) Schwank, J.; Allard, L. F.; Deeba, M.; Gates, B. C. J. Catal. 1993, 84, 27. (15) Alexeev, O.; Gates, B. C. Top. Catal. 2000, 10, 273.
2116 J. Phys. Chem. B, Vol. 106, No. 8, 2002 (16) Lamb, H. H.; Fung, A. S.; Tooley, P. A.; Puga, J.; Krause, T. R.; Kelley, M. J.; Gates, B. C. J. Am. Chem. Soc. 1989, 111, 8367. (17) Maloney, S. D.; Zhou, P.-L.; Kelley, M. J.; Gates, B. C. J. Phys. Chem. 1991, 95, 5409. (18) Kawi, S.; Chang, J. R.; Gates, B. C. J. Phys. Chem. 1993, 97, 10599. (19) Kawi, S.; Chang, J. R.; Gates, B. C. J. Am. Chem. Soc. 1993, 115, 4830. (20) Weber, W. A.; Gates, B. C. J. Phys. Chem. 1997, 101, 10423. (21) Jameson, A. K.; Jameson, C. J.; Gutowsky, H. S. J. Chem. Phys. 1970, 53, 2310. (22) Jameson, C. J.; Jameson, A. K.; Cohen, S. M. J. Chem. Phys. 1975, 62, 4224. (23) Vaarkamp, M.; Linders, J. C.; Koningsberger, D. C. Physica B 1995, 209, 159. (24) Panjabi, G.; Salvi, S.; Allard, L.; Gates, B. C. To be published. (25) Kirlin, P.; van Zon, F. B. M.; Koningsberger, D. C.; Gates, B. C. J. Phys. Chem. 1990, 94, 8439. (26) Deutsch, S. E.; Chang, J.-R.; Gates, B. C. Langmuir 1993, 9, 1284. (27) Kirlin, P. Ph.D. Dissertation, University of Delaware, 1986.
Enderle et al. (28) Bender, B. R.; Norton, J. R.; Miller, M. M.; Anderson, O. P.; Rappe, A. K. Organometallics 1992, 11, 3427. (29) Krause, J. A.; Siriwardane, U.; Salupo, T. A.; Wermer, J. R.; Knoeppel, D. W.; Shore, S. G. J. Organomet. Chem. 1993, 454, 263. (30) Koningsberger, D. C.; Gates, B. C. Catal. Lett. 1992, 14, 271. (31) Zhao, A.; Gates, B. C. J. Am. Chem. Soc. 1996, 118, 2458. (32) Kawi, S.; Gates, B. C. J. Phys. Chem. 1995, 99, 8824. (33) Johnson, B. F. G.; Lewis, J.; Raithby, P. R.; Zuccaro, C. Acta Crystallogr. 1978, B34, 3765. (34) Johnson, B. F. G.; Lewis, J.; Nelson, W. J. H.; Nicholls, J. N.; Puga, J.; Raithby, P. R.; Rosales, M. J.; Schroder, M.; Vargas, M. D. J. Chem. Soc., Dalton Trans. 1983, 11, 2447. (35) Collman, J. P.; Murphy, D. W.; Fleischer, E. B.; Swift, D. Inorg. Chem. 1974, 13, 1. (36) Eady, C. R.; Guy, J. J.; Sheldrick, G. H.; Johnson, B. F. G.; Lewis, J.; Malatesta, M. C. J. Chem. Soc., Chem. Commun. 1976, 602. (37) Jordon, R. F.; Norton, J. R. J. Am. Chem. Soc. 1982, 104, 1255. (38) Vaarkamp, M. Catal. Today 1998, 39, 271.
