In the Laboratory
An Experiment for the Inorganic Chemistry Laboratory
W
The Sunlight-Induced Photosynthesis of (η2-C60)M(CO)5 Complexes (M = Mo, W) José E. Cortés-Figueroa Department of Chemistry, University of Puerto Rico, P. O. Box 9019, Mayagüez, Puerto Rico, 00681;
[email protected] Fullerenes (1) and their transition metal carbonyl complexes offer the opportunity to teach concepts of carbonyl– transition metal backbonding (2) and chemical applications of group theory (3). Buckminsterfullerene (C60), the most stable and abundant of the fullerene allotropes, is the most symmetrical of the known molecules (4). It has sixty identical carbons that are bonded to each other in a truncated icosahedral (soccer-ball) shape. A carbon atom occupies each vertex. The structure of C60 contains twenty hexagonal rings and twelve pentagonal rings. Two distinct carbon–carbon bonds are observed. One type of C⫺C bond separates two hexagons (6:6 junction) and the other type of C⫺C separates a hexagon from a pentagon (6:5 junction; ref 5). The carbons at 6:6 junctions behave as olefinic units (5) forming transition metal complexes with η = 2 hapticity (6, 7). Because of the highly symmetrical structure of C60, only four of its 174 vibrational modes are infrared active (8). These active bands are observed at 1400 cm᎑1, 1180 cm᎑1, 580 cm᎑1, and 510 cm᎑1 (8). The experiment described in this paper is the sunlight-induced photoreactions of M(CO)6 with C60 in hexane producing (η2-C60)M(CO)5 (M = Mo, W). Experiment
Hazards
The complexes are prepared from M(CO)6 (Aldrich) and C60 (Aldrich) in hexane (eq 1). The photosynthesis of (η2C60)W(CO)5 from W(CO)6 and C60 in hexane using a medium-pressure mercury lamp is described in the Journal of Coordination Chemistry (7). However, for safety reasons, as well as economics, the use of sunlight as the radiation source is suggested for the photoreaction (9).
M(CO)6 + C60
stretching region (νCO; 2100 cm᎑1–1800 cm᎑1) of the sample is measured. The reaction progress is assessed from the increase of the νCO band intensities of (η2-C60)M(CO)5 and the decrease of the band intensity of M(CO)6. For example, for M = W these bands are located at 2085 cm᎑1, 1996 cm᎑1, and 1973 cm᎑1 for (η2-C 60)M(CO)5 and 1983 cm ᎑1 for W(CO)6 (7). Nitrogen is injected into the reaction mixture using a syringe and long needle each time that a sample is withdrawn to ensure a positive pressure within the reaction vessel. The reaction time is approximately 2 h when the reaction is performed at noon in summer at the latitude of Puerto Rico (ca 18.5⬚ N). After the reaction is complete, as judged by no further change in the CO vibrations, the reaction volume is reduced to induce precipitation. Further precipitation is induced by placing the reaction mixture in a freezer. Upon filtration a reddish-brown solid is obtained. A typical yield is in the vicinity of 80–90%. The infrared spectrum of the complex(es) should be measured in carbon disulfide where the product solubility is very high. The complex(es) can also be identified by 13C NMR. The spectrum of (η2-C60)W(CO)5 in a 50:50 v兾v mixture of toluened6兾CS2 displays two signals at 180 ppm and 143 ppm (7).
hν or sunlight
Students should work in a fume hood to prepare reaction mixtures. Protective eyewear should be worn. All metal carbonyl complexes are regarded as toxic because they liberate carbon monoxide upon decomposition or exchange reactions. Carbon disulfide is an extremely volatile, malodorous, and toxic liquid. Discussion
(η2-C60)M(CO)5
(1)
In a typical experiment equimolar amounts (ca. 0.06 mmol) of M(CO)6 and C60 are dissolved in 15 mL of nitrogen-purged, dry n-hexane in a 25-mL two-necked roundbottom flask equipped with a magnetic stir bar. The flask had been fitted with a rubber septum on one neck and a condenser on the other neck. A rubber balloon had been attached to the top of the condenser. The solution is reddish-purple. Nitrogen is forced through the rubber septum using a stainless steel needle until the balloon inflates. Students must be careful not to rupture the balloon by forcing too much nitrogen into the reaction vessel. The reaction mixture is then exposed to sunlight under constant stirring. Samples of approximately 0.2 mL are withdrawn from the reaction mixture using a syringe with a long needle at 15-min intervals. The infrared spectrum in the carbonyl
This activity is designed for upper-division undergraduate students or first-year graduate students. The use of lowcost equipment makes it attractive for large groups where cost is an important consideration. The reaction is carried out under nitrogen, promoting students skills of working under inert conditions. This experiment offers the opportunity to integrate inorganic chemistry laboratory with key concepts of symmetry, group theory, and backbonding. For example, the student can be instructed to predict the number of CO vibrations that should be observed in the infrared spectra of these LM(CO)5 complexes. Two excellent papers for this purpose have been published in this Journal (10). Since the pentacarbonyl fragment has a C4v symmetry, three active νCO vibrations (A1, A1, E) are expected. In addition, comparison of the A1 νCO band of (η2-C60)M(CO)5 complexes with other L-pentacarbonyl complexes (L = phosphines) will enable the students to explore the concept of π-acidity of C60 and the
JChemEd.chem.wisc.edu • Vol. 80 No. 7 July 2003 • Journal of Chemical Education
799
In the Laboratory
relative π-acidity of C60 with respect to phosphines such as triphenyl phosphine. For example, the A1 νCO band of (η2C60)W(CO)5 is 10 cm᎑1 higher in energy (2085 cm᎑1 vs 2075 cm᎑1) than the corresponding band for (η1-PPh3)M(CO)5 (7), suggesting that C60 is a better π-acceptor than PPh3. The students should be encouraged to link the concept of C60 πacidity with the electronic structure of C60, which contains a set of three low-lying LUMOs (11) capable of accepting electron density from a transition metal. In summary, this is an inexpensive learning activity that promotes laboratory skills, chemical applications of group theory to predict or explain the νCO spectrum of the inorganic moiety of metal carbonyl complexes, and the understanding of the concept of π-acidity and its relation to the electronic structure of the coordinated ligand and the metal. Acknowledgments The teaching ideas presented in this paper come from working on the project Synthesis, Reactions, Mechanisms, and Electrochemistry of Fullerene–Transition Metal Complexes (SRMEFTMC). The author gratefully acknowledges the financial support to the SRMEFTMC project by The National Science Foundation (grant CHE-0102167) and by The Donors of the Petroleum Research Fund, administered by the American Chemical Society (grant ACS-PRF 36623-B3). W
Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online. Literature Cited 1. For interesting readings about fullerenes and its chemistry see: Kadish, K. M.; Ruoff, R. S. Fullerenes; Wiley & Sons: New
800
2. 3. 4. 5.
6.
7. 8.
9. 10.
11.
York, 2000; Curl, R. F.; Smalley, R. E. Scientific American 1991, 54 (Oct), 32 (and references therein). Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; Wiley & Sons: New York, 1988. Carter, R. L. Molecular Symmetry and Group Theory; Wiley & Sons: New York, 1998. Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162. Bürgi, H. B.; Blanc, E.; Schwarzenbach, D.; Lu, Y.; Kappes, M. M.; Ibers, J. A. Angew. Chem., Int. Ed. Engl. 1992, 31, 640; Satpathy, S. Chem. Phys. Lett. 1986, 130, 545. Bach, A. L.; Hao, L.; Olmstead, M. M. Angew. Chem., Intl. Ed. Engl. 1996, 35, 188; Fagan, P. J.; Calabrese, J. C.; Malone, B. Science 1991, 252, 1160; Chemega, A. N.; Green, M. L. H.; Haggitt, J.; Stephens, H. H. J. Chem. Soc., Dalton Trans. 1988, 755; Park, J. T.; Song, H.; Cho, M. K.; Lee, J. H.; Suh, I. H. Organometallics 1998, 17, 227; Balch, A.; Olmstead, M. M. Chem. Rev. 1998, 98, 2123 (and references therein). Rivera-Rivera, L.; Colón-Padilla, F., Del Toro-Novalés, A.; Cortés-Figueroa, J. E. J. Coord. Chem. 2001, 54, 143. Krätschmer, W.; Fostipoulos, K.; Huffman, D. R. Chem. Phys. Lett. 1990, 170, 167; Wu, Z. U.; Jellski, D. A.; George, T. F. Chem. Phys. Lett. 1987, 137, 291; Hare, J. P.; Dennies, T. S.; Kroto, H. W.; Taylor, R.; Allaf, A. W.; Balm, S,; Walton, D. R. N. J. Chem. Soc., Chem. Commun. 1991, 412; Weeks, D. E.; Harter, W. G. Chem. Phys. Lett. 1998, 144, 366. Roy, S.; Sarkar, S. Proc. Indian Acad. Sci. (Chem. Sci.) 1996, 108, 59. Darensbourg, M. Y.; Darensbourg, D. J. J. Chem. Educ. 1970, 47, 33; Darensbourg, D. J.; Darensbourg, M. Y. J. Chem. Educ. 1974, 51, 787. Xie, Q.; Pérez-Cordero, E.; Echegoyen, L. J. Am. Chem. Soc. 1992, 114, 3978; Haddon, R. C.; Brus, L. E.; Raghavachari, K. Chem. Phys. Lett. 1986, 125, 249.
Journal of Chemical Education • Vol. 80 No. 7 July 2003 • JChemEd.chem.wisc.edu
