In the Laboratory
[60]Fullerene Displacement from (Dihapto-Buckminster-Fullerene) Pentacarbonyl Tungsten(0)
W
An Experiment for the Inorganic Chemistry Laboratory, Part II José E. Cortés-Figueroa* Organometallic Chemistry Research Laboratory, Department of Chemistry, University of Puerto Rico, Mayagüez, PR 00681-9019; *
[email protected] Deborah A. Moore-Russo Department of Learning and Instruction, Graduate School of Education, State University of New York at Buffalo, Buffalo, NY 14260-1000
The sunlight-induced photosynthesis and characterization of (dihapto-[60]fullerene) pentacarbonyl tungsten(0), (η2-C60 )W(CO)5 , combine for an educational activity that enhances students’ laboratory skills by allowing them to predict and explain the carbonyl stretching region, νCO, in the IR spectrum of the metal carbonyl complexes using chemical applications of group theory and symmetry (1). Here we propose the kinetics of the ligand–C60 exchange reactions on (η2-C60 )W(CO)5 as a laboratory experience that promotes student understanding of kinetics, mechanisms, temperature dependence of rate constant values, and graphical interpretations of chemical behavior. This laboratory is a hands-on experience that provides students the opportunity to actively apply the concept of activation parameters and its use in the mechanistic interpretation of a chemical reaction. In the proposed laboratory, students learn how to estimate the W⫺C60 bond enthalpy or the W⫺C60 bond energy from the experimentally determined enthalpy of activation and activation energy, respectively. Why use buckminsterfullerene? Buckminsterfullerene ([60]fullerene or C60) as a ligand is an interesting molecule. It is the most stable and abundant of the fullerene allotropes and is the most symmetrical of the known molecules (2, 3). It has sixty identical carbon atoms, which are bonded to each other in a truncated icosahedral (soccer-ball) shape where a carbon atom occupies each vertex. The structure of C60 contains twenty hexagonal rings and twelve pentagonal rings (4). Although all the carbon atoms in C60 are chemically equiva-
lent, it contains two distinct carbon–carbon (C⫺C) bonds. One type of C⫺C bond separates two hexagonal rings (6:6 junction) and the other type separates a hexagonal ring from a pentagonal ring (6:5 junction) (5, 6). The carbons at 6:6 junctions behave as electronically deficient olefin units. These olefin units have the capacity to bind transition metals to form complexes with η = 2 hapticity (7 –11). The “splayed out” LUMOs on the C60 rigid surface may explain the preferred η2 mode of coordination and the fact that higher hapticities are rare (12–14). The C⫺C “double” bonds at the 6:6 junctions on the curved surface of C60 are strained (15). The release of this tension upon forming a C60–metal bond may explain the reactivity of C60. The C60–metal bond enthalpy is in the range of 60 to 105 kJ兾mol depending on the metal (16–18). The sunlight-induced preparation of (η2-C60 )W(CO)5 has been reported in this Journal (1) and its C60 displacement reactions have been reported elsewhere (17). We propose some kinetics experiments on the C60 displacement from (η2-C60 )W(CO)5 as an educational activity for the inorganic chemistry laboratory. The Lewis bases triphenyl phosphine, PPh3 , and triethyl phosphite, P(OEt)3, displace C60 from (η2-C60 )W(CO)5 (17). Since it may be easier to work with PPh3 than with P(OEt)3, the experiments proposed here employ PPh3:
(η2-C60)W(CO)5 + PPh3
(η1-PPh3)W(CO)5 + C60 (1)
The reported mechanism is shown in Figure 1 (17). The rate law, assuming that the intermediate species, W(CO)5 is at steady-state concentration, is
(
) ( )5
d η2 - C 60 W CO − Figure 1. Proposed mechanism for the C60 displacement from (η2-C60 )W(CO)5 by L, a Lewis base. The involvement of the solvent is not shown. If displacement of C60 from (η2-C60 )W(CO)5 is solvent-assisted the proposed intermediate species should be (solvent)W(CO)5.
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dt
=
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k1 k2 PPh 3
(η2 - C60 ) W (CO)5
k−1 C 60 + k2 PPh 3
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(2)
In the Laboratory
Equation 2 can be simplified when the experimental conditions [PPh3] >> [(η2-C60 )W(CO)5 ] and [PPh3] >> [C60 ] are met:
(
) ( )5
d η2 - C 60 W CO −
dt = kobsdd
kobsd =
( η - C 60 ) W (CO)5
(3)
2
k1 k2 PPh 3 k−1 C 60 + k2 PPh 3
(4)
Since [PPh3] >> [C60 ], eq 4 can be approximated to
Hazards (5)
kobsd ≈ k1
425 nm using a UV–vis spectrophotometer. The reactions are studied in benzene under flooding conditions where the concentrations of PPh3 are approximately two hundred times the concentration of (η2-C60 )W(CO)5 and the concentration of C60 is negligible. Under these conditions plots of ln(At − A∞ ) versus time (At = absorbance at a given time t, A∞ = absorbance at time infinity) are linear to more than three halflives. The pseudo-first-order rate constant values are determined from the slopes of these plots. The instructor can choose instead nonlinear curve fitting methods, described in this Journal (20–30), to estimate the rate constant values. The pseudo-first-order rate constant values are determined at three different temperatures for a specific [PPh3 ]. By plotting ln(kobsd 兾T ) versus (1兾T ) the student can determine the activation parameters (∆H ‡, ∆S ‡).
• use a graphing calculator or a suitable software to determine the pseudo-first-order rate constant values (kobsd ≈ k1),
Students should use a fume hood to prepare reaction mixtures. All metal carbonyl complexes are regarded as toxic because they may liberate carbon monoxide upon decomposition or exchange reactions. Triphenylphosphine is toxic by inhalation; may cause sensitization by skin contact; may cause long-term adverse effects in the aquatic environment; is a neurologic hazard; and is an irritant. [60]Fullerene is irritating to eyes and respiratory system. Benzene is highly flammable; may cause cancer; may cause heritable genetic damage; and is irritating to eyes and skin.
• demonstrate that kobsd values are [PPh3]-independent as predicted by eq 5,
Discussion
Objectives of the Experiment In this experiment the students are expected to • conduct kinetics runs under flooding conditions of the reaction in eq 1,
• determine kobsd values at three temperatures (ca. 317 K, 327 K, 337 K), • use a graphing calculator or a suitable software to determine the values of the activation parameters (∆H ‡, ∆S ‡), • use the criterion of ∆S ‡ to decide whether the C60 displacement, step governed by k1, is solvent-assisted, • explain why the ∆H ‡ value may be used as an estimate of the W–C60 bond enthalpy, and • propose other experiments to study the proposed mechanism.
Experiment
Preparation of (η2-C60)W(CO)5 The complex (η2-C60 )W(CO)5 can be prepared following the instructions given in the Supplemental MaterialW of part 1 of this experiment (1). The complex can also be prepared using a medium-pressure mercury lamp (19). Since a minute amount of (η2-C60)W(CO)5 is required for the experiment, the instructor can prepare the complex or may choose to include the synthesis as part of the laboratory activities. Kinetics Runs under Flooding Conditions The rate of disappearance of (η2-C60 )W(CO)5 is monitored by observing the decrease of the absorbance values at
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This activity is designed for upper-division undergraduate students or first-year graduate students. The complex (η2-C60 )W(CO)5 is air sensitive and slowly decomposes by oxidation. Therefore, the reactions are carried out under nitrogen, providing students valuable experience that should enhance their skills for working under inert conditions. This experience offers the opportunity to integrate inorganic chemistry laboratory with key concepts of chemical kinetics and bonding. For example, instructors can guide students to propose a mechanism or mechanisms for the ligand exchange using as criteria the observation that the rate constant values are independent of [L]. If the experiment includes determination of rate constant values at various temperatures the instructor should encourage the students to use the criterion of the entropy of activation to support or disprove the proposed mechanism. For example, a positive entropy of activation should support a dissociative displacement of C60 from (η2-C60 )W(CO)5. Once the students have used entropy of activation as the criterion to decide whether the displacement of C60 is associative or dissociative, they should ponder whether the enthalpy of activation (∆H ‡) value can be used to estimate the C60-W bond enthalpy. Because the solvent may affect the transition-state stability, the goodness of the W–C60 bond enthalpy estimate may depend on the degree of solvent involvement in the step governed by k1 (31–34). Once the students estimate the C60-W bond enthalpy, they should compare the es-
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In the Laboratory 44.1 °C 54.1 °C 64.1 °C
0
ln(A t − A ⬁)
−1
−2
−3
−4
−5
−6 0
1000
2000
3000
4000
5000
6000
t/s Figure 2. Plots of ln(At − A∞) vs time for the C60 displacement by PPh3 from (η2-C60 )W(CO)5 in benzene.
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ln(k/T )
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ene, respectively, and positive (but small) for the reactions in benzene and in carbon disulfide (CS2 ) (17, 31). An excellent discussion on the subject is available in the literature (17, 18). The instructor may ask the student where it is appropriate to use ∆H ‡ to estimate the C60–W bond enthalpy (32– 34) and compare the estimated values with the values reported elsewhere (17, 18). The role of the solvent on the ligand exchange reactions of closely related metal-carbonyl complexes has been reported extensively elsewhere (35–38). For example, the difference between the gas-phase W–CO bond-dissociation enthalpy (192 ± 12 kJ兾mol) (33) and the W–CO bond dissociation in decalin (167 ± 7 kJ兾mol) (34) has been ascribed to a transition-state (TS) stabilization by decalin. This TS stabilization may be a result of a partial C–H–W agostic interaction. The corresponding entropy of activation for W–CO bond dissociation in decalin is negative as expected for a process where two reacting species (the solvent and the complex) become partners in a single TS. A positive entropy of activation for C60 displacement from (η2-C60 )W(CO)5 in benzene may suggests a negligible assistance by the solvent in the W– C60 bond breaking.
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Summary
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This is a modular activity where the instructor can decide the level of complexity of the experiment. Different parts of the experiment can be assigned to various subgroups and then their findings shared. This learning activity promotes laboratory skills; understanding the relation between kinetics and mechanisms; the role of solvents in ligand exchange reactions; the use of activation parameters to confirm or disprove a mechanism; and the relation between enthalpy of activation and the thermodynamic bond enthalpy.
−13.5 −14.0
−14.5 −15.0 2.95
3.00
3.05
1 T
3.10
3.15
3.20
(10ⴚ3 Kⴚ1)
Acknowledgments
Figure 3. Plot of ln(k / T ) vs (1/ T ) for the C60 displacement by PPh3 from (η2-C60 )W(CO)5 in benzene at 44.1 °C, 54.1 °C, and 64.1 °C in benzene. Calculated activation parameters: ∆H‡ = 103 ± 1 kJ/mol and ∆S ‡ = 12 ± 2 J/K mol.
timated values with the values reported elsewhere (17, 18). For a good discussion on this subject the instructor should consult these references (17, 18). Figures 2 and 3 show a plot of ln(At − A∞) versus time and ln(kobsd 兾T ) versus (1兾T ), respectively, of actual results obtained by undergraduate students in our laboratory. The information used to construct these graphs is provided in the Supplemental Material.W A longer version of this activity is presented in the Supplemental Material,W where the students probe the mechanism in more detail. In the longer version the students have the opportunity to confirm the chemical species involved using IR spectroscopy in the carbonyl stretching frequency νCO. Also in the longer version the students have the opportunity to determine the kobsd values in various solvents. They should find that the entropies of activation are negative and close to zero for the reactions in chlorobenzene and in tolu-
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The teaching ideas presented in this article come from working on the project Synthesis, Reactions, Mechanisms, and Electrochemistry of Fullerene-Transition Metal Complexes (SRMEFTMC). JECF gratefully acknowledges the financial support to the SRMEFTMC project by The Donors of the Petroleum Research Fund, administered by the American Chemical Society (grant ACS-PRF-41267-B3). Helpful comments and suggestions by reviewers and Editor of this Journal are gratefully acknowledged. W
Supplemental Material
Instructions for the students and notes for the instructor are available in this issue of JCE Online. Literature Cited 1. Cortés-Figueroa, J. E. J. Chem. Educ. 2003, 80, 799–800. 2. For interesting readings about fullerenes and its chemistry see: Kadish, K. M.; Ruoff, R. S. Fullerenes; John Wiley and Sons: New York, 2000. 3. Curl, R. F.; Smalley, R. E. Sci. Am. 1991, October, 32 (and references therein).
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In the Laboratory 4. Kroto, H. W.; Heath, J. R.; O’ Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162. 5. Bürgi, H. B.; Blanc, E.; Schwarzenbach, D.; Lu, Y.; Kappes, M. M.; Ibers, J. A. Angew. Chem., Int. Ed. Engl. 1992, 31, 640. 6. Satpathy, S. Chem. Phys. Lett. 1986, 130, 545. 7. Bach, A. L.; Hao, L.; Olmstead, M. M. Angew. Chem., Intl. Ed. Engl. 1996, 35, 188. 8. Fagan, P. J.; Calabrese, J. C.; Malone, B. Science 1991, 252, 1160. 9. Chernega, A. N.; Green, M. L. H.; Haggitt, J.; Stephens, H. H. J. Chem. Soc. Dalton Trans. 1998, 5, 755–768. 10. Park, J. T.; Song, H.; Cho, M. K.; Lee, J. H.; Suh, I. H. Organometallics 1998, 17, 227. 11. Balch, A.; Olmstead, M. M. Chem. Rev. 1998, 98, 2123 (and references therein). 12. Rogers, J. R.; Marynick, D. S. Chem. Phys. Lett. 1993, 205, 97. 13. Haddon R. C. J. Comp. Chem. 1998, 19, 139. 14. Jemmis, E. D.; Manoharan, M. Curr. Sci. 1999, 76, 1122. 15. Haddon, R. C. Science 1993, 261, 1545. 16. Nunzi, F.; Sgamellotti, A.; Re, N.; Floriani, C. Organometallics 2000, 19, 1628. 17. Rivera-Rivera, L. A.; Colon-Padilla, F. D.; Ocasio-Delgado, Y.; Martinez-Rivera, J.; Mercado-Feliciano, S.; Ramos, C. M.; Cortes-Figueroa, J. E. Inorg. Reac. Mech. 2002, 4, 49. 18. Rivera-Rivera, L. A.; Crespo-Román, G. C.; Acevedo-Acevedo, D.; Ocasio-Delgado, Y.; Cortés-Figueroa, J. E. Inorg. Chim. Acta 2004, 357, 881–887. 19. Rivera, L.; Colón-Padilla, F.; Del Toro-Novalés, A.; CortésFigueroa, J. E. J. Coord. Chem. 2001, 54, 143. 20. Vera, L. R.; Ortega, P. A.; Guzmán M. J. Chem. Educ. 2004, 81, 159.
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21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.
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Denton, P. J. Chem. Educ. 2000, 77, 1524. Lo, G. L. J. Chem. Educ. 2000, 77, 532. Levie, R. J. Chem. Educ. 1999, 76, 1594. McNaught, I. J. J. Chem. Educ. 1999, 76, 1457. Vitz, E. J. Chem. Educ. 1998, 75, 1661. Harris, D. C. J. Chem. Educ. 1998, 75, 119. Zielinski, T. J.; Allendoefer, R. D. J. Chem. Educ. 1997, 74, 1001. Bunting, J. W. J. Chem. Educ. 1988, 65, 839. Bisby, R. H.; Emrys, W. J. Chem. Educ. 1986, 63, 990. Becsey, J. C.; Berke, L.; Callan, J, R. J. Chem. Educ. 1968, 45, 728. A concern expressed by one of the reviewers is that the transmission coefficient may vary from solvent to solvent, rendering activation entropy comparisons meaningless. The quantity lies between 0 and 1. The inclusion of this coefficient in the Eyring equation comes from data that suggest that transition states of some gas-phase reactions are reflected back to reactants. However, this observation is ignored (or taken as unity) when the Eyring equation is used in reactions in solution (Espenson, J. H. Chemical Kinetics and Reaction Mechanisms, 2nd ed.; McGraw-Hill: New York, 1995; p 171.) Lewis, K. E.; Golden, D. M.; Smith, G. P. J. Am. Chem. Soc. 1984, 106, 3905. Morse, J.; Parker, G.; Burkey, T. J. Organometallics 1989, 8, 2471. Graham, G. R.; Angelici, R. J. Inorg. Chem. 1967, 6, 2082. Simon, J. D.; Xie, X. J. Phys Chem. 1989, 93, 291. Xie, X.; Simon, J. D. J. Am. Chem. Soc. 1990, 112, 1130. Zhang, S.; Dobson, G. R.; Zang, V.; Bajaj, V. C.: van Eldik, R. Inorg. Chem. 1990, 29, 3477. Zhang, S.; Dobson, G. R. Polyhedron 1990, 9, 2511.
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