In the Laboratory
Photochemical Synthesis and Ligand Exchange Reactions η2-alkene) Compounds of Ru(CO)4(η
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Jason Cooke* Department of Chemistry, University of Alberta, Edmonton, AB T6G 2G2, Canada; *
[email protected] David E. Berry and Kelli L. Fawkes Department of Chemistry, University of Victoria, Victoria, BC V8W 3V6, Canada
Substituted mononuclear metal carbonyls play a central role in the discussion of organometallic chemistry. In the undergraduate laboratory, these compounds are most often prepared by thermal reaction of the parent binary metal carbonyl with a suitable ligand, L (1). When thermal routes fail, photochemical synthesis is a viable alternative for carbonyl ligand replacement (2), but, with a few notable exceptions (3–7), this approach can often be too technically demanding for the conventional undergraduate laboratory. Even in these instances, the use of sophisticated equipment can be required (4, 5), potentially limiting the applicability of the experiment to only the most well-equipped institutions. Furthermore, the inherent challenges of carrying out synthetic photochemistry on metal carbonyl compounds have thus far limited undergraduate laboratory investigation to the group six metals (Cr, Mo, W) (3–7). Ruthenium pentacarbonyl is thermally unstable and is difficult to handle (8), which often renders thermal reactions inconvenient for the synthesis of Ru(CO)4L compounds. However, advantage can be taken of the efficient photochemical metal–metal bond cleavage in Ru3(CO)12 to prepare Ru(CO)4(η2-alkene) compounds; when a sufficiently labile alkene is used, the latter can serve as [Ru(CO)4] transfer reagents to generate different Ru(CO)4L compounds (9, 10). In contrast, thermal reactions of Ru3(CO)12 typically yield ligand-substituted trinuclear clusters (11). From a pedagogic perspective, the students are introduced to the notion that it is not always a metal–carbonyl bond that cleaves in photochemical reactions, but that larger metal clusters can be induced to fragment with useful results. Although Ru3(CO)12 is relatively expensive,1 the experiments that will be described can be carried out on a scale of only a few mg兾student, with the compounds characterized in situ by IR spectroscopy rather than being isolated. This exposes the students to a common microscale method used in research laboratories when working with reagents that are either very
costly or are difficult to prepare in large quantities. We have found that our students appreciate being exposed to the (somewhat novel) prospect of carrying out a photochemical synthesis and being able to learn a great deal about the chemistry of a system without necessarily needing to submit an isolated compound at the conclusion of the experiment. Experimental Background The photochemical fragmentation of Ru3(CO)12 can take place under a variety of conditions, ranging from filtered high-intensity light (9, 10) to sunlight or conventional fluorescent lighting (12), rendering the experiment quite flexible in terms of the sophistication of equipment required. The UV–vis spectrum of Ru3(CO)12 reveals a single band at the short wavelength extreme of the visible spectrum, centered at 390 nm (Figure 1). This band has been assigned to a σ → σ* transition in the Ru3 framework (13), and allows one to understand why a metal–metal bond is cleaved upon photochemical excitation rather than the more common rupture of a M⫺CO bond. The inclusion of excess ethylene, methyl acrylate (CH2⫽CHCO2CH3; MA) or dimethyl fumarate (transCH3CO2CH⫽CHCO2CH3; DMFU) in the illuminated reaction vessel produces Ru(CO)4(η2-alkene) compounds of varying thermal stability:
Ru3(CO)12 + 3RHC = CHR′
hν
3Ru(CO)4(η2-RHC = CHR′ )
(1)
stability order R, R′: H, H < H, CO2CH3 (MA) < CO2CH3, CO2CH3 (DMFU) The above stability order can be demonstrated by the subsequent reaction of Ru(CO)4(η2-C2H4) (1) with either MA or DMFU under mild conditions to generate the corresponding Ru(CO)4(η2-MA) (2) and Ru(CO)4(η2-DMFU) (3) compounds: Ru(CO)4(η2-C2H4) + MA 1
Ru(CO)4(η2-MA) + C2H4
(2)
2
Ru(CO)4(η2-C2H4) + DMFU 1
Ru(CO)4(η2-DMFU) + C2H4
Figure 1. UV–vis spectrum of Ru 3 (CO) 12 (CH 2 Cl 2 solution, 1 × 10᎑4 M).
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Similarly, the MA ligand in 2 can be displaced by the more π-acidic DMFU: Ru(CO)4(η2-MA) + DMFU 2
Ru(CO)4(η2-DMFU) + MA
(4)
3
Once formed, the DMFU complex 3 is thermally stable and can be isolated as a white, crystalline solid at room temperature.2 In contrast, solutions of Ru(CO)4(η2-C2H4) deposit Ru3(CO)12 upon removal of a C2H4 atmosphere (12), and Ru(CO)4(η2-MA) readily decomposes to Ru3(CO)12 in the absence of excess methyl acrylate (9, 10); compounds 1 and 2 are therefore best studied in situ by infrared spectroscopy. The use of compounds 1 and 2 as [Ru(CO)4] transfer reagents can be further demonstrated by reaction with triphenylphosphine, PPh3 (Ph = C6H5): Ru(CO)4(η2-H2C = CHR) + PPh3
0 °C
r.t.
Ru(CO)4(PPh3 ) + H2C = CHR (5) 4
Ru(CO)4(η2-H2C = CHR): R = H ( 1), CO2CH3 (2)
Careful control of the reaction conditions produces Ru(CO)4(PPh3) in moderate yield, but is invariably accompanied by small quantities of Ru(CO)3(PPh3)2 as a byproduct. Experimental Procedure All chemicals are reagent grade and are used without further purification. Ru3(CO)12 can be purchased or (more economically) prepared by reductive carbonylation of RuCl3⭈xH2O, either in a high-pressure autoclave (14) or by an ambient pressure route (15).1 Compounds 1–4 are moderately air-stable in solution and can be handled as such in air for short periods of time (e.g., to record IR spectra), but rudimentary Schlenk techniques should be used to prevent unwanted decomposition over the course of an experiment. As solids, compounds 3 and 4 should be stored under nitrogen and refrigerated for long-term storage.
Photochemical Preparation of Ru(CO)4(η2-alkene) Generally, a large excess of alkene is added to a stirred, nitrogen-purged hexane solution of Ru3(CO)12, and the yellow solution is irradiated until it is colorless or until the IR spectrum indicates consumption of Ru3(CO)12. Three photochemical techniques can be used. Methods 1 and 2 are suitable for preparative-scale reactions, up to 0.2 g Ru3(CO)12, while method 3 is more appropriate for an in situ study monitored by IR spectroscopy. Method 1 A high-pressure mercury lamp is used to irradiate the solution, either in an immersion well apparatus or by placing the lamp assembly next to a Schlenk tube containing the reaction mixture. This method is essentially as reported in ref 10. A cutoff filter (λ > 370 nm) is required to produce
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Ru(CO)4(η 2-MA) (2) cleanly, but Ru(CO)4(η 2-C2H4) (1) can be prepared using standard Pyrex glass (λ > 300 nm) as a filter, often in as little as 15–30 minutes depending on the scale of the reaction. Method 2 A homemade light box is used; this device has six domestic fluorescent tubes and a cooling fan and is designed to sit directly over a stirrer–hot plate. Photochemical reactions leading to 1 and 2 can be completed in less than 90 minutes. Method 3 Septum-sealed Erlenmeyer flasks are left under standard fluorescent lighting (e.g., in an aluminum foil-lined fume hood) until the reaction mixtures become colorless. The solutions for this approach should be approximately 2 × 10᎑3 M in Ru(CO) 4 (η 2 -alkene), which translates to ∼0.5 mg Ru3(CO)12 per 1 mL of hexane. Formation of compound 1 is usually complete in 1 day compared to 2–3 days for 2 or 3; once formed, the solutions are stable for up to 1 week in sealed flasks (and the photochemistry can thus be conveniently carried out between consecutive lab periods). Sunlight should work equally well as an irradiation source if conditions permit. Given the small volumes required for solution IR spectroscopy, this method could be carried out on a scale of only a few mg Ru3(CO)12 if suitable microscale equipment is available. When methods 1 or 2 are used to prepare Ru(CO)4(η2C2H4) (1), a septum-sealed Schlenk tube roughly three times the solution volume should be used and the vessel should be freeze–thaw degassed before introducing ethylene gas by needle (to slightly greater than atmospheric pressure, as judged by gentle “bulging” of the septum); for method 3, the solution should be saturated with C2H4 by purging through a needle, but need not be left under pressure. The direct photochemical synthesis of Ru(CO)4(η2-DMFU) (3) is not suggested if isolation of the compound is desired, as it is virtually impossible to separate the excess DMFU from the desired compound 3. For the isolation of compound 3 as a solid, it is best to follow route (b), described below.
Alkene Ligand Exchange in Ru(CO)4(η2-alkene) For qualitative reactions, an excess of alkene is added to a stirred hexane solution of the Ru(CO)4(η2-alkene) compound prepared in (a). The reaction is monitored by IR spectroscopy until the starting Ru(CO)4(η2-alkene) compound is consumed. The reactions described in eqs 2 and 3 can be completed more quickly if a gentle nitrogen purge is used to remove the ethylene atmosphere after the MA or DMFU has been added. The reaction described in eq 4 proceeds more quickly when a warm water bath is placed beneath the flask. If the preparation of 3 is carried out at room temperature over a longer timeframe, it is best to dissolve the DMFU in a minimum of dichloromethane before adding it to the hexane solution of 1 or 2. Good yields of 3 (uncontaminated by excess DMFU) are realized only when a slight excess of DMFU is added at the outset and the reaction is allowed to proceed at room temperature until completion; crystalline material is obtained if the resulting hexane solution is then concentrated to about one tenth the original volume and stored in a freezer (preferably for 1 week).
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In the Laboratory
Preparation of Ru(CO)4(PPh3) (4) A slight excess (no more than 1.1 equivalents) of PPh3 dissolved in a minimum volume of dichloromethane is added to a hexane solution of 1 or 2 at ice temperature. This reaction is best carried out by allowing the reaction to slowly warm from 0 ⬚C to room temperature, as this tends to minimize the formation of Ru(CO)3(PPh3)2 as a byproduct. It is often convenient to allow an ice-water bath to slowly thaw overnight and have the student concentrate the solution to about one tenth the original volume the following morning. Moderate yields (often no better than 50%) of Ru(CO)4(PPh3) (4) can be attained if the compound is left to slowly crystallize; rapid precipitation tends to form an extremely fine solid that is difficult to isolate. Hazards As with most metal carbonyls, the ruthenium compounds encountered in this experiment have the potential to be absorbed through the skin and should be handled with due care, wearing gloves wherever possible. Hexane and methyl acrylate are flammable liquids, and methyl acrylate is also a lachrymator. Triphenylphosphine is harmful if swallowed or if its dust is inhaled and is an irritant. Ethylene is a highly flammable gas that forms explosive mixtures with air and must only be used in a well-ventilated fume hood. Care must be taken when pressurizing closed reaction vessels with ethylene gas; use of a safety screen is strongly recommended and this operation should only be carried out under the close supervision of trained teaching staff. Appropriate measures should be taken to avoid the accidental over-pressurizing of sealed flasks during purge cycles (i.e., use of vented mineral oil bubblers). If high intensity lamps are used for the photochemical reactions, protective eyewear must be worn and the lamp should be screened from view (e.g., with aluminum foil).
situ approach that is commonly practiced in research laboratories when investigating the chemistry of a system for the first time. Once prepared, compounds 1–4 are all easily studied by IR spectroscopy in solution; the number and relative intensity of the νCO bands provide all information needed to carry out a detailed study of the compounds’ structure and electronic properties. Depending on the level of theory presented in the course, the students can propose reasonable structures on the basis of simple pattern matching to representative spectra (16) or, at more advanced levels, apply group theory to the analysis (9). Once the compounds’ structures are appreciated, the Dewar–Chatt–Duncanson (DCD) model (17) can be applied to gain an understanding of the interdependent relationship of the metal–carbonyl and metal–alkene bonding. In this case, the effect of introducing electron-withdrawing groups on the alkene (i.e., increasing the alkene’s π-acidity) in a stepwise fashion can be readily seen in the shift to higher frequency of the νCO bands in the IR spectra (Figure 2). The trend deduced from the IR data mirrors the relative stabilities of compounds 1–3. In the case of the MA and
Results and Discussion The photochemical preparation and subsequent ligand exchange reactions of Ru(CO)4(η2-alkene) compounds provides a novel experiment for upper-level inorganic chemistry laboratory courses and demonstrates many of the basic topics discussed in introductory organometallic chemistry. From an experimental perspective, the chemistry of the system is quite flexible and should be adaptable to most lab situations. We have successfully introduced small groups of upper-level students to some of the most advanced techniques possible in photochemistry (method 1), while also developing ways for a small, advanced lab to carry out the same chemistry with far less costly equipment (method 2). Furthermore, we have developed an approach that allows a relatively large class (∼75 students) to experience a photochemical reaction without the need for special equipment that is not normally found in an upper-level chemistry lab (method 3). The experiment can also be adapted to a variety of time requirements, either a combination of two 3-hour lab periods (methods 1 and 2) or two consecutive lab periods of about 45 minutes and 1–2 hours, respectively (method 3). The experiment is suited to the usual synthetic focus of many inorganic chemistry laboratories, and also can be used to demonstrate a microscale in www.JCE.DivCHED.org
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Figure 2. FT–IR spectra of compounds 1-3 (hexane solution, 2 × 10᎑3 M): (A) Ru(CO)4(η2-C2H4) (1); (B) Ru(CO)4(η2-MA) (2) and excess MA (䉬); and (C) Ru(CO)4(η2-DMFU) (3), excess MA (䉬) and excess DMFU (䊉). The spectra were recorded from the “one pot” reaction sequence (method 3) beginning with the photochemical preparation of 1 from Ru3(CO)12, followed by addition of 10 equivalents of MA to form 2 and then by addition of 5 equivalents of DMFU to form 3.
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DMFU compounds, it is also possible for the student to relate the shift of the ester carbonyl group of the coordinated alkene to lower frequency to the DCD model and to recognize the need to identify the IR active bands of any excess reagents (free MA and DMFU in this case) when interpreting solution IR spectra of reaction mixtures. Finally, a comparison of the structures of the Ru(CO) 4 (η 2 -alkene) compounds 1–3 with that of the phosphine-substituted derivative 4 illustrates the different coordination site preferences of the largely π-acceptor alkenes compared with the more σdonor phosphine (18). The experiment described above was designed to provide a system in which the changing electronic properties of the alkene ligands could be easily connected to the resulting spectroscopic data and thermal stability of the Ru(CO)4(η2alkene) compounds. However, the Ru(CO)4(η2-alkene) system is also suited to serve as a starting point for a mini-project or other open-ended experiment. A larger range of alkenes could be studied, both with respect to the photochemistry (9) or thermal ligand exchange reactions (9, 10). An undergraduate experiment has already been described in which M(CO)5(η2-C60) compounds are formed under mild sunlight irradiation (3) and there is no reason why Ru(CO)4(η2-C60) could not be prepared to extend its scope (19, 20). The latter synthesis could be carried out by first preparing the important synthon Ru(CO) 5 using an extension of the photochemical techniques described for Ru(CO)4(η2-C2H4) (12, 19), or it could be achieved by reacting Ru(CO)4(η2MA) with C60 (20). Acknowledgments We would like to thank Josef Takats (University of Alberta) for his early work on the adaptation of the immersion-well photochemical synthesis of Ru(CO)4(η2-MA) and its subsequent thermal reactions with DMFU and PPh3 to the undergraduate laboratory and for donating the necessary equipment for the advanced version of this experiment. The helpful suggestions of the many graduate teaching assistants over the years is also gratefully acknowledged, as are the efforts of the numerous undergraduate students who have successfully completed the experiment at the University of Alberta and the University of Victoria. W
Supplemental Material
Detailed experimental descriptions and additional instructor notes are available in this issue of JCE Online. Notes 1. The current price of triruthenium dodecacarbonyl is ∼$70 CDN兾g compared to ∼$30 CDN兾g for ruthenium(III) chloride hydrate (Aldrich Chemical Catalog 2005–2006). Reductive carbonylation by the route suggested in ref 14 invariably proceeds in good yield (> 75% based on Ru) whereas, in our students’ hands,
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the preparation described in ref 15 can be less reliable, particularly if the student faces time constraints in the laboratory period. 2. If suitable equipment exists for manipulating compounds under nitrogen at low temperature, Ru(CO)4(η2-MA) can also be crystallized and isolated as a solid at temperatures below ᎑30 ⬚C (9, 10).
Literature Cited 1. Recent examples include (a) Hughes, L. A. Chem. Educator 2004, 9, 105–107. (b) Bengali, A. A.; Mooney, K. E. J. Chem. Educ. 2003, 80, 1044–1047. (c) Overby, J. S.; Rieter, W. J. Chem. Educator 2003, 8, 367–370. (d) Ardon, M.; Hayes, P. D.; Hogarth, G. J. Chem. Educ. 2002, 79, 1249–1251. (e) Davis, C. M.; Klein, M. F. J. Chem. Educ. 2001, 78, 952–953. (f) Inorganic Experiments, 2nd ed.; Woolins, J. D., Ed.; Wiley–VCH: Weinheim, Germany, 2003; Experiments 3.14, 3.30, 4.4, 4.6, 4.8, and 4.32. 2. Geoffroy, G. L. J. Chem. Educ. 1983, 60, 861–866. 3. Cortés–Figueroa, J. E. J. Chem. Educ. 2003, 80, 799–800. 4. Bengali, A. A.; Charlton, S. B. J. Chem. Educ. 2000, 77, 1348– 1351. 5. McNeese, T. J.; Ezbiansky, K. A. J. Chem. Educ. 1996, 73, 548–550. 6. Manuta, D. M.; Lees, A. J. J. Chem. Educ. 1987, 64, 637– 638. 7. Post, E. W. J. Chem. Educ. 1980, 57, 819–822. 8. Dictionary of Organometallic Compounds, 2nd ed.; Chapman and Hall: New York, 1995; p 3433. 9. Grevels, F.–W.; Reuvers, J. G. A.; Takats, J. J. Am. Chem. Soc. 1981, 103, 4069–4073. 10. Grevels, F.–W.; Reuvers, J. G. A.; Takats, J. Inorg. Synth. 1986, 24, 176–180. 11. Bruce, M. I. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, 1982; Vol. 4, pp 844–887. 12. Johnson, B. F. G.; Lewis, J.; Twigg, M.V. J. Organomet. Chem. 1974, 67, C75–C76. 13. Tyler, D. R.; Levenson, R. A.; Gray, H. B. J. Am. Chem. Soc. 1978, 100, 7888–7893. 14. Bruce, M. I.; Jensen, C. M.; Jones, N. L. Inorg. Synth. 1989, 26, 259–261. 15. Lavigne, G.; Saccavini, C.; Chauvin, R. In Inorganic Experiments, 2nd ed.; Woolins, J. D., Ed.; Wiley–VCH: Weinheim, Germany, 2003; pp 347–351. 16. For example, see Shriver, D. F.; Atkins, P.; Langford, C. H. Inorganic Chemistry, 2nd ed.; W. H. Freeman: New York, 1994; pp 672–673. 17. (a) Chatt, J.; Duncanson, L. A. J. Chem. Soc. 1953, 2939– 2947. (b) Dewar, M. J. S. J. Am. Chem. Soc. 1979, 101, 783– 791 and references therein. (c) For an example in an educational textbook, see Housecroft, C. E.; Sharpe, A. G. Inorganic Chemistry, 2nd ed.; Pearson Education: Harlow, United Kingdom, 2005; pp 701, 704. 18. Rossi, A. R.; Hoffmann, R. Inorg. Chem. 1975, 14, 365–374. 19. Rasinkangas, M.; Pakkanen, T. T.; Pakkanen, T. A. J. Organomet. Chem. 1994, 476, C6–C8. 20. Chernega, A. N.; Green, M. L. H.; Haggitt, J.; Stephens, A. H. H. J. Chem. Soc., Dalton Trans. 1998, 755–767.
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