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[Re2(CO)9]2- on Hydroxylated MgO: Formation from [Re2(CO)10] and Evidence of Ion Pairing at the Surface C. J. Papile,† H. Kno¨zinger,‡ and B. C. Gates*,†,‡,§ Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716, Institut fu¨ r Physikalische Chemie, LMU Mu¨ nchen, Butenandtstrasse 5-13 (Haus E), 81377 Mu¨ nchen, Germany, and Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616 Received December 21, 1999. In Final Form: April 14, 2000 [Re2(CO)10] is physically adsorbed on almost fully dehydroxylated MgO powder, but it is chemisorbed on hydroxylated MgO, forming [Re2(CO)9]2-, as demonstrated by the infrared spectra of the surface species and by extraction of the adsorbed anion into solution by cation metathesis. By comparison with the analogous solution chemistry, the chemisorption on MgO is inferred to involve nucleophilic attack of surface OH groups on CO ligands of [Re2(CO)10] to give [HRe2(CO)9]-, which is deprotonated on the basic surface to give [Re2(CO)9]2-. Comparison of the infrared spectra of the surface species with those of salts of [Re2(CO)9]2in various solvents, combined with ultraviolet-visible spectra of salts of [Re2(CO)9]2- in various solvents, demonstrates that [Re2(CO)9]2- is strongly ion paired with the MgO surface.
Introduction Some organometallic compounds are good probes of oxide surfaces, allowing identification of reactive surface groups and elucidation of adsorbate-surface bonding. Among the most informative of such probes are rhenium carbonyls, including [HRe(CO)5],1,2 [Re2(CO)10],3-5 and [H3Re3(CO)12].1,2,5 Adsorption of each of these on MgO, followed by treatment under oxidizing conditions, leads to supported rhenium subcarbonyls, Re(CO)3, which have been characterized by infrared (IR) and extended X-ray absorption fine structure spectra that agree well with predictions of density functional theory representing Re(CO)3 bonded to three oxygen atoms at a MgO corner site.6 Here we report new chemistry of [Re2(CO)10] on MgO powder, demonstrating that it reacts with surface OH groups to give [Re2(CO)9]2-, which is ion-paired with the MgO surface. The formation of [Re2(CO)9]2- distinguishes MgO from less strongly basic oxides and distinguishes hydroxylated from dehydroxylated MgO. Experimental Section Syntheses and sample handling were done with air-exclusion techniques, as before.1,2,4,5 Solvents were reagent grade and purified before use, usually by vacuum distillation and contacting with molecular sieves or by distillation from sodium benzophenone ketyl under N2.4,5 Some of the reagents and materials have already been described,4,5 including a commercial MgO (MCB Reagents) and MgO made by decomposition of MgCO3.4 Orangeyellow colored K2[Re2(CO)9] was synthesized by a literature procedure7 and purified by recrystallization.8 Mg[Re2(CO)9] was †
University of Delaware. LMU Mu¨nchen. § University of California. ‡
(1) Kirlin, P. S.; van Zon, F. B. M.; Koningsberger, D. C.; Gates, B. C. J. Phys. Chem. 1990, 94, 8439. (2) Kirlin, P. S.; Kno¨zinger, H.; Gates, B. C. J. Phys. Chem. 1990, 94, 8451. (3) Purnell, S. K.; Xu, X.; Goodman, D. W.; Gates, B. C. J. Phys. Chem. 1994, 98, 4076. (4) Papile, C. J.; Gates, B. C. Langmuir 1992, 8, 74. (5) Triantafillou, N. D.; Purnell, S. K.; Papile, C. J.; Chang, J.-R.; Gates, B. C. Langmuir 1994, 10, 4077. (6) Hu, A.; Neyman, K. M.; Staufer, M.; Belling, T.; Gates, B. C.; Ro¨sch, N. J. Phys. Chem. B 1999, 121, 4522. (7) Tam, W.; Marsi, M.; Gladysz, J. A. Inorg. Chem. 1983, 22, 1413.
prepared from K2[Re2(CO)9]; a crown ether, 18-crown-6, was used to bring K2[Re2(CO)9] into solution.8 Commercial MgO was activated at various temperatures by treatment under vacuum then in O2 to vary the density of surface OH groups.1,2,4,5 MgO samples prepared by decomposition of MgCO3 are described elsewhere.4 MgO samples were brought in contact with bipyridine to test for reducing sites, with none being detected.8 Supported rhenium carbonyls were prepared by slurrying [Re2(CO)10] with MgO powder; for example, 0.0338 g of [Re2(CO)10] in dried hexanes (50 mL) was slurried with MgO (made by decomposition of MgCO3 at 390 °C) at an initial temperature of -48 °C, with the color of the powder turning orange-yellow nearly instantaneously; the resultant powder was washed repeatedly with cold hexane, dried at room temperature under vacuum, and stored in a drybox. The resulting samples were treated in the presence of various flowing gases at various temperatures, often in an IR cell. Extractions of surface species were carried out with various solvents in the absence of air.8 IR spectra of solutions and solids were recorded as before.1,2,4,9 Ultraviolet-visible (UV-vis) spectra of solutions were recorded with a Varian-Cary 219 spectrophotometer. Solids were characterized by transmission X-ray absorption, with the height at the Re LIII edge determining the Re content, typically 2 wt %;1,5 the X-ray experiments were carried out at the National Synchrotron Light Source at Brookhaven National Laboratory (Beam Line X-11A). Surface areas of MgO were measured with a standard BET apparatus; data are presented elsewhere.4
Results Physisorption and Chemisorption of [Re2(CO)10] on MgO. When [Re2(CO)10] in hexane solution was slurried at temperatures less than -78 °C with MgO powder that had been activated at temperatures greater than 600 °C to remove most of the surface OH groups (as shown by the IR spectra) and then evacuated, adsorption was evident by the uptake of [Re2(CO)10] from solution. After removal of the solvent by cannula followed by evacuation, the solid incorporated surface species that were readily extracted into hexane under a CO2 blanket. The IR spectrum of the extracted species matches that of [Re2(CO)10] in solution. These results demonstrate that [Re2(CO)10] was phys(8) Papile, C. J. PhD Dissertation, University of Delaware, Newark, DE, 1990. (9) Kirlin, P. S.; DeThomas, F. A.; Bailey, J. W.; Gold, H. S.; Dybowski, C.; Gates, B. C. J. Phys. Chem. 1986, 90, 4882.
10.1021/la991666d CCC: $19.00 © 2000 American Chemical Society Published on Web 06/03/2000
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Figure 1. Infrared spectra of (A) K2[Re2(CO)9] in THF solution and (B) K2[Re2(CO)9] in THF solution with excess 18-crown-6.
isorbed on the largely dehydroxylated MgO. When [Re2(CO)10] in hexanes was slurried at -78 °C with any of the MgO samples, no sign of reaction was evident, as indicated by the lack of color change. We infer that physisorption also occurred on both hydroxylated and largely dehydroxylated MgO samples at this low temperature. However, when the slurry temperature was raised to about -50 °C with MgO samples that had been heated to temperatures less than about 600 °C (retaining a large fraction of their surface OH groups, as shown by the IR peak at approximately 3730 cm-1), the powders became orange-yellow in color. With hydroxylated MgO that had been formed by decomposition of MgCO3 at 390 °C, the uptake of [Re2(CO)10] was almost complete (an IR analysis of the solution indicated the lack of unadsorbed [Re2(CO)10]). The typical resultant sample contained about 2 wt % Re, as determined by the X-ray absorption edge jump. The adsorbate in this sample could not be extracted into hexanes, cyclohexane, or dimethyl sulfoxide (DMSO) solution. However, extraction with a solution of excess tetrabutylammonium bromide in DMSO occurred within about 5 min at room temperature, as shown by the color change of the solid to white and that of the solution to orange-yellow. The νCO IR spectrum of the extracted species in DMSO (2029w, 2009m, 1966s, 1923s, 1881w, 1860m cm-1) matches that of K2[Re2(CO)9] in tetrahydrofuran (THF) (2033w, 2010m, 1966s, 1924s, 1880m, 1860m cm-1) (Figure 1A), consistent with the literature.10 Evidently, the tetrabutylammonium salt of [Re2(CO)9]2- was formed by cation metathesis. Results of a series of experiments show that the conversion of [Re2(CO)10] into [Re2(CO)9]2- on MgO depends on the degree of surface hydroxylation; the conversion is indicated approximately by the disappearance of the [Re2(CO)10] band at 2015 cm-1 and the formation of the [Re2(CO)9]2- band at about 1984 cm-1 characterizing the adsorbed species. Although only [Re2(CO)9]2- was observed for the MgO made from MgCO3 at 390 °C, mixtures of [Re2(CO)10] and [Re2(CO)9]2- generally were obtained on the other MgO surfaces, including that of the commercial sample activated at 390 °C. In summary, the results demonstrate that at temperatures exceeding about -50 °C, [Re2(CO)10] was converted on hydroxylated MgO into adsorbed [Re2(CO)9]2-. The surface-mediated synthesis,11 involving adsorption of [Re2(CO)10] followed by extraction of [Re2(CO)9]2- with a salt (cation metathesis), is a simple method for preparation of salts of [Re2(CO)9]2-. Reactivity [Re2(CO)9]2- on MgO. When [Re2(CO)9]2-/ MgO was heated to 150 °C under vacuum, it was oxidatively fragmented to give rhenium subcarbonyls, (10) Tam, W. PhD Dissertation, University of California, Los Angeles, 1979. (11) Gates, B. C. J. Mol. Catal. 1994, 86, 95.
Figure 2. UV-vis spectra of Mg[Re2(CO)9] in (A) DMSO solution and (B) THF solution.
Re(CO)3, as shown by the IR spectra of the resultant solids and by extended X-ray absorption fine structure spectra, as reported previously.5 Such fragmentation chemistry also occurs in O2; in the absence of O2, the oxidizing agent is evidently surface OH groups.12 When [Re2(CO)9]2-/MgO was treated with CO2 at room temperature and 1 atm, it was converted back into [Re2(CO)10]/MgO, as shown by the color change from orange-yellow to white within about 15 min; physisorbed [Re2(CO)10] was extracted from the MgO with hexanes under CO2 and identified by its IR spectrum. IR Spectra of [Re2(CO)9]2-/MgO and those of [Re2(CO)9]2- Salts. To help elucidate the nature of the bonding of [Re2(CO)9]2- with MgO, the solid samples were characterized by IR spectroscopy. For comparison, the spectra of Mg[Re2(CO)9] in the solid state and in solution with DMSO or THF were also recorded; spectra were also obtained for K2[Re2(CO)9] complexed with 18-crown-6 in THF solution (Figure 1B). The IR band positions are summarized in Table 1. According to Tam et al.,7 K2[Re2(CO)9] exists in solution as two isomers. Our IR spectra of this salt in THF solution agree with those of Tam et al., consistent with two forms of the salt. To investigate what these forms might be, we obtained spectra of the salt (a) complexed with 18-crown-6 in THF solution and (b) in DMSO solution. The results confirm the presence of two different species, one predominant in the former solution and the other predominant in latter. The spectrum of the crown-ether-complexed species in THF nearly matches that of the predominant species in DMSO. Similar results were found for Mg[Re2(CO)9]. (Table 1). UV-Vis Spectra of [Re2(CO)9]2- Salt in Different Solvents. UV-vis spectra are shown in Figure 2 for Mg[Re2(CO)9] in THF and in DMSO. Discussion Chemistry of Chemisorption of [Re2(CO)10] to Form [Re2(CO)9]2-. The data clearly show that [Re2(CO)9]2- forms on MgO incorporating surface OH (12) Lamb, H. H.; Gates, B. C.; Kno¨zinger, H. Angew. Chem., Int. Ed. Engl. 1988, 27, 1127.
[Re2(CO)9]2- on Hydroxylated MgO
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Table 1. Infrared Spectra in the νCO Region Characterizing [Re2(CO)9]2- with Various Counterions, Including MgO Surfacesa sample
band position, cm-1 (w)
band position, cm-1 (m)
band position, cm-1 (vs)b
band position, cm-1 (m)
2086 c 2028 2037 2091 2091 2023
c 2008 1962 1969 c 2012 1952
1980 1979 1922 1930 1978 1979 1901
1882 1878 1838 1866 1882 1883 1834
2098
c
1986
1902
Mg[Re2(CO)9] solid Mg[Re2(CO)9] in THF solution K2[Re2(CO)9] coordinated with 18-crown-6 in THF solution Mg[Re2(CO)9] in DMSO solution [Re2(CO)9]2- on commercial MgO activated at 400 °C [Re2(CO)9]2- on MgO made from MgCO3 at 390 °C [Re2(CO)9]2- on MgO made from MgCO3 at 390 °C and dosed with DMSO [Re2(CO)9]2- on commercial MgO activated at 700 °C a
The bands shown in the table for adsorbed rhenium carbonyls are not the complete νCO spectra of the samples, as the bands characteristic of adsorbed [Re2(CO)10] are not included. For example, the full νCO spectrum of the sample made by adsorption of [Re2(CO)10] on MgO made from MgCO3 at 390 °C is the following: 2091w, 2012sh, 1979vs, 1971sh, 1911s, 1883m cm-1. b The main peak has a small shoulder that is difficult to assign. c Accurate identification of the band position could not be made because of low band intensity or interference with band of another species.
groups, and, as the degree of surface dehydroxylation increases, the formation of this anion is reduced, with the principal adsorbate being physisorbed [Re2(CO)10]. The results identify OH groups as reactants in the conversion of [Re2(CO)10] into chemisorbed [Re2(CO)9]2- and also imply that the reducing sites on highly dehydroxylated MgO are not sufficient for this reaction. The reaction with MgO to form [Re2(CO)9]2- is contrasted with the adsorption of [Re2(CO)10] on less strongly basic oxides, SiO2 and Al2O3,13 each of which has been inferred to give neutral rhenium carbonyls (all presumably dimeric), and not [Re2(CO)9]2-. The structure formed on Al2O3, for example, could not be isolated by extraction,13 as was [Re2(CO)9]2- from MgO. Thus, we conclude that sufficient base strength of the adsorbent is a requirementsalong with OH groupssfor the formation of adsorbed [Re2(CO)9]2-. The surface chemistry on hydroxylated MgO is inferred to be related to chemistry occurring in basic solutions, illustrated by the following reactions:14
[Re2(CO)10] + [N(C2H5)4][OH] f [N(C2H5)4][HRe2(CO)9] + CO2 (1) [N(C2H5)4][HRe2(CO)9] + [N(C2H5)4][OH] f [N(C2H5)4]2[H2Re2(CO)8] + CO2 (2) As neither [HRe2(CO)9]- nor [H2Re2(CO)8]2- was observed in our work, we suggest that the reaction analogous to eq 1, but not that analogous to eq 2, was significant on MgO and that the [HRe2(CO)9]- formed on the surface was deprotonated on basic surface sites (O2- ions) to give [Re2(CO)9]2-. Thus, the chemisorption is regarded as a nucleophilic attack by OH groups on CO ligands of [Re2(CO)10] followed by deprotonation to give [Re2(CO)9]2-. We consider it likely, on the basis of results obtained for organic probe molecules such as pyridine on oxides such as Al2O3,15 that the adsorption first involves a Lewis baseLewis acid interaction of a CO ligand with a Mg2+ ion at the surface, thus polarizing and activating this ligand of [Re2(CO)10] for attack by a neighboring nucleophilic surface OH group. In summary, the contacting of an oxide with a solution of [Re2(CO)10] provides a quick diagnosis of the presence of OH groups combined with strong basicity of the oxide (13) McKenna, W. P.; Higgins, B. E.; Eyring, E. M. J. Mol. Catal. 1985, 31, 199. (14) Beringhelli, T.; D’Alfonso, Ghidorsi, L.; Ciani, G.; Sironi, A.; Molinari, H. Organometallics 1987, 6, 1365. (15) Kno¨zinger, H.; Krietenbrink, H.; Mu¨ller, H.-D.; Schulz, W. Proc. 6th Intl. Congr. Catal. (London) 1977, 1, 183.
surface, indicated simply by the color change accompanying the formation of [Re2(CO)9]2-. The conversion of [Re2(CO)9]2- back into [Re2(CO)10] by treatment with CO2 is a mild oxidation reaction; oxidative fragmentation to give rhenium subcarbonyls apparently requires a stronger oxidizing agent than CO2 (e.g., OH groups at elevated temperatures). Ion-Pairing of [Re2(CO)9]2- on MgO. The IR spectra of Mg[Re2(CO)9] indicate predominantly one species in THF and another in DMSO; the spectrum of K2[Re2(CO)9] coordinated with 18-crown-6 and dissolved in THF nearly matches that of Mg[Re2(CO)9] in DMSO. These results are explained by ion-pairing effects, which are well-known for metal carbonylates in solution16 and have been inferred for metal carbonylates on surfaces, including that of MgO.12 DMSO is a solvent that evidently complexes the anion sufficiently to separate it almost completely from the countercation; masking the K+ ions with the crown ether has the equivalent effect. THF, on the other hand, is not a sufficiently good complexing agent to separate the cations from the anions, and they are largely ion-paired in this solvent. The differences in the positions of the IR bands characterizing the ion-paired and nonion-paired species indicate that the ion pairs are strong. UV-vis spectra are often used to diagnose ion-pairing effects in solution, and the spectra of Figure 1 are consistent with the interpretation stated above, showing one peak (289 nm) dominant for the ion-paired Mg[Re2(CO)9] in THF and another peak (345 nm) dominant for the nonion-paired salt in DMSO. The peaks are slightly shifted, depending on the solvent (Figure 2): 285 nm in DMSO vs 289 nm in THF and 345 nm in DMSO vs 359 in THF. It is typical that the tightly ion-paired species is characterized by a band at a lower wavelength than the nonion-paired species.17 (Reasoning by comparison with results for [Re2(CO)10],18 we infer that the absorption bands are likely associated with Re-Re bonds.18) Thus, the νCO IR spectra of Table 1 distinguish two categories of samples, one consisting of ion-paired salts and the other consisting of nonion-paired salts. Comparison of the spectra of the salts with those of the carbonylate anions formed by adsorption of [Re2(CO)10] on MgO demonstrates that these are ion-paired to the surface. We infer from the band positions that the ion pairing is strong. Probing the Reactivity of Basic Oxides with Organometallics. In summary, the reactivity of [Re2(16) Darensbourg, M. Y. Prog. Inorg. Chem. 1985, 33, 221. (17) Edgell, W. F. Ions and Ion Pairs in Organic Reactions; WileyInterscience: New York, 1972; Vol. 1, Chap. 4. (18) Levenson, R. A.; Gray, H. B. J. Am. Chem. Soc. 1975, 97, 6042.
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(CO)10] with MgO distinguishes hydroxylated from nonhydroxylated surfaces, and in prospect, the relative amounts of [Re2(CO)10] and [Re2(CO)9]2- on the surface could be used to measure the degree of hydroxylation. The results of this work, combined with results characterizing adsorption of other organometallic compounds on surfaces of various metal oxides,19-21 show that organometallic compounds may be sensitive probes of oxide surface reactivity; they deserve more consideration for such applications and may sometimes be preferable to organic probes. (19) Otten, M. M.; Lamb, H. H. J. Am. Chem. Soc. 1994, 116, 1372. (20) Zhao, A.; Gates, B. C. Langmuir 1997, 13, 4024. (21) Triantafillou, N. D.; Gates, B. C. Langmuir 1999, 15, 2595.
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Acknowledgment. The research was supported by the U. S. Department of Energy (DOE), Office of Energy Research, Office of Basic Energy Sciences, Division of Chemical Sciences, Contract No. FG02-87ER13790. B.C.G. thanks the Alexander von Humboldt Foundation for support. We acknowledge the support of DOE, Division of Materials Sciences (Contract No. DE-FG05-89ER45384), for its role in the operation and development of beam line X-11A at the National Synchrotron Light Source, which is supported by DOE, Division of Materials Sciences and Division of Chemical Sciences (Contract No. DE-AC02-76CH00016). LA991666D
