In the Laboratory
Determining the Quantum Efficiency for Activation W of an Organometallic Photoinitiator for Cationic Polymerization
An Experiment for the Physical or Inorganic Chemistry Laboratory David M. Hayes,* Maura Mahar, R. Chris Schnabel, and Paras Shah Department of Chemistry, Union College, Schenectady, NY 12308; *
[email protected] Alistair J. Lees and Vladimir Jakubek Department of Chemistry, Binghamton University, Binghamton, NY 13902-6000
We wish to present a new experiment in mechanistic organometallic photochemistry that can be used in either the physical, inorganic, or advanced unified laboratory. This experiment involves measuring the photochemical efficiency (quantum yield) for arene displacement from a mixed-sandwich compound of iron, [(η 5 -C 5 H 5 )Fe(η 6 -isopropylbenzene)](PF 6 ) (Scheme I). Arene displacement is the dominant photochemical reaction of mixed Cp–M–arene complexes in polar organic solvents when M = Fe, Ru, and Os. If M = Fe, then II (Scheme I) is unstable at room temperature and will react further to form Cp2Fe, Fe2+, and arene. Moreover, if suitable ligands, L, are present, then II will react to form stable complexes of the type CpML3+. This laboratory introduces students to the experimental and data analysis methods of quantitative organometallic photochemistry. Quantitative methods in photochemistry are treated at this college as part of our coverage of chemical kinetics in the first course in physical chemistry, which is required of all chemistry and biochemistry majors. This experiment could as well be included in an advanced inorganic or unified laboratory. Although there have been a number of experiments reported in this Journal and elsewhere on the photochemistry of organometallic compounds, most treat the synthesis and structural characterization of reaction intermediates and products (1–10), a few describe kinetic measurements (11–14), and only one includes the measurement of quantum yields (15). Complex I is of practical interest because it can be used as a photoinitiator for the polymerization of epoxides, styrene, acrylates, dicyanate esters, pyrrole, and dioxolenes, which have applications in coatings, microlithography, and the manufacture of printed circuit boards. Park and Schuster (16) using flash photolysis and Kutal et al. (17) with electrospray mass spectrometry have examined the early stages of
the photoinitiation of polymerization of epoxides by CpFe(arene) complexes, identifying complex II as the reactive species that actually initiates polymerization. This experiment focuses on studying the quantum yield for its formation. Quantum yield measurements under varying conditions of solvent, irradiation wavelength, and counterion have been used by others to elucidate the mechanism of formation of CpFeS3+ (18–22). In this experiment, which can be easily completed in one 3–4 hour laboratory period, our students measure the quantum yield under just one set of conditions: solvent = acetonitrile, irradiation wavelength = 458 nm, and counterion = PF6− . The quantum yield is measured by trapping the reaction intermediate II with the well-known transition metal ligand 1,10–phenanthroline, III:
This species does not react with I in the dark (thermally) but is a potent ligand with respect to the photochemically generated intermediate, displacing not only the arene, but also the Cp− group on iron to produce the well known octahedrally coordinated complex cation Fe(phen)32+ (λmax = 510 nm, molar absorptivity at 510 nm in acetonitrile = 1.03 × 104 L mol᎑1 cm᎑1). Experimental
Preparation of Solutions Complex I, is not commercially available and must be synthesized. We used the procedure of Doggweiler and Desobry (23), which is presented in the Supplementary
Scheme I. Arene displacement from a mixed-sandwich compound of iron, [(η5–C5H5)Fe(η6–isopropylbenzene)](PF6), I.
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In the Laboratory
Materials.W Although not air-sensitive, solid I is somewhat light-sensitive and must be stored in the dark. It can, however, be exposed briefly to ambient light, such as would occur during weighing. The solutions are extremely light sensitive, however, and must be prepared and handled in the dark under red-light conditions. In our laboratory, several 25 W photographic darkroom lamps are used. Solutions of I are stable for at least a few days and their photochemistry is unaffected by air.
Figure 1. Instrumental setup used for photolyzing the solutions. Each solution is contained in a standard 1 cm pathlength quartz cuvette covered with a rubber septum to reduce solvent evaporation.
Description of the Photolysis System The instrumental setup used for the photolyses is shown in Figure 1. Either a laser or the light from a filtered arc lamp can be used as the irradiation source. We used an argon–krypton laser (Coherent) because it was already available in our laboratory. For us a convenient wavelength to work at is 458 nm but any wavelength in the visible spectrum will do; the range 430–530 nm gives the greatest accuracy because the absorption band of I is weak beyond 530 nm. Our beam was adjusted to give a power of about 1 mW incident on the cell containing the solution of I and 1,10-phenanthroline. Since our laser could not be operated at such low powers, it was run at a higher power and the beam then attenuated using a neutral density filter, as shown in Figure 1. The incident light intensity was measured using a calibrated laser power meter (Coherent), rather than by solution actinometry. The rated error in the power meter is ±4%. It is essential for good results that the solutions be stirred thoroughly while being photolyzed so that the concentrations are uniform throughout. To this end, the cell holder (Agilent) is mounted directly on a magnetic stirrer that is run at its highest speed.
Data Collection The progress of the photochemical reaction was monitored by following the formation of the orange product Fe(phen)32+ using a diode-array UV–vis spectrophotometer (HP 8452A). The instrument should be set to report absorbance data in tabular form at both the product monitoring wavelength (510 nm) and the irradiation wavelength (458 nm). A sample spectrum is shown in Figure 2 and the corresponding numerical data are presented in Table 1. Hazards
Figure 2. UV–vis time-resolved spectra of an acetonitrile solution of 0.00108M complex I and 0.051 M 1,10-phenanthroline undergoing photolysis at 458 nm with an incident laser beam intensity of 1.16 mW.
Table 1. Numerical Data Corresponding to the Overlaid Spectra in Figure 2 Time/s
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Absorbance 458 nm
510 nm
000
0.075
0.034
060
0.157
0.136
120
0.215
0.212
210
0.298
0.319
300
0.370
0.412
390
0.435
0.496
480
0.495
0.572
600
0.567
0.661
750
0.649
0.762
930
0.740
0.869
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The three substances used in this experiment are acetonitrile, 1,10-phenanthroline, and [CpFe(isopropylbenzene]PF6. Acetonitrile is a common laboratory solvent that is flammable and harmful if swallowed, absorbed through the skin, or inhaled. 1,10-phenanthroline, a solid, is toxic if swallowed. [CpFe(isopropylbenzene)]PF6, also a solid, is harmful if swallowed. We recommend that students wear appropriate gloves when doing this experiment and prepare solutions in a fume hood. Data Analysis The data analysis follows the procedure of Lees (24). Because there is a significant inner filter effect from the absorption of light by the product Fe(phen)32+ at the irradiation wavelength (458 nm), we may not assume in calculating the quantum yield that all the absorbed photons are absorbed by the reactant. Our data analysis method takes account of this complication and is based on the following equation, t
AP, t − AP, ∞ ln AP, 0 − AP, ∞
= −α 0
1 − 10 − Airr Airr
dt
(1)
which is derived in the Supplementary MaterialW and closely follows the original presentation by Lees. Equation 1 relates
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In the Laboratory
the increasing absorbance of the product AP,t measured at 510 nm to the changing total absorbance Airr of the solution at the irradiation wavelength. The integral is evaluated by calculating the area under a plot of the integrand versus time from t = 0 to t = t, for ever increasing times. Therefore, the data analysis requires the construction of two plots, the one just mentioned followed by a plot of the natural logarithm versus the integral according to eq 1. The result of the second plot is a straight line of slope ᎑α, (2)
α = φ I0 a R,irr b
where φ is the quantum yield, I0 is the incident light intensity of the laser beam in photons per second per liter of solution, aR,irr is the molar absorptivity of the reactant at the irradiation wavelength, and b is the path length of the cell (1 cm). φ can be calculated since α is found from the slope of the plot of eq 1, I0 is calculated from the power meter measurement, and aR,irr is 61 cm᎑1 mol᎑1 L. The data presented in Figure 2 and Table 1 give a quantum yield for arene ring displacement of 0.74. The calculations can be done using an Excel spreadsheet, with the integral in eq 1 approximated using the formula for a trapezoid. AP, ∞ is not directly measured by following the reaction to completion, but rather by calculation using the molar absorptivity for Fe(phen)32+ at 510 nm (vide supra) and assuming its concentration at infinite time is the same as the initial concentration of I. At the initial reactant concentration used in this experiment, AP,∞ is too large to be measured directly. Conclusions There are very few experiments available for undergraduates that explore the efficiency of photochemical reactions. This experiment is reliable and can fit into an inorganic, physical, or advanced unified laboratory. The students are introduced to a wide range of new laboratory skills and instrumentation. They learn how to prepare and handle solutions of light-sensitive compounds, as well as operate a laser or other light source suitable for photochemical studies. Of further advantage is this lab’s flexibility; it can easily be done in one 3–4 hour laboratory period, or may be extended into a multiweek research project. Organometallic compounds have become well know, and this experiment introduces the students to this intriguing class of compounds. Acknowledgment The authors would like to thank the Kresge Foundation for funding, which supported the purchase of our argon– krypton ion laser.
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W
Supplemental Material
Instructions for students and notes for the instructor, including an appendix deriving and discussing eq 1, are available in this issue of JCE Online. Literature Cited 1. Cortes-Figueroa, J. E. J. Chem. Educ. 2003, 80, 799. 2. McNeese, T. J.; Ezbiansky, K. A. J. Chem. Educ. 1996, 73, 548. 3. Flair, M.; Fletcher, T. R. J. Chem. Educ. 1995, 72, 753. 4. Boyd, D. C.; Johnson, B. J.; Mann, K. R. J. Chem. Educ. 1992, 69, A315. 5. Manuta, D. M.; Lees, A. J. J. Chem. Educ. 1987, 64, 637. 6. Calabro, D. C.; Lichtenberger, D. L. J. Chem. Educ. 1982, 59, 686. 7. Post, E. W. J. Chem. Educ. 1980, 57, 819. 8. Ernhoffer, R.; Kovacs, D.; Subak, E., Jr.; Shepherd, R. E. J. Chem. Educ. 1978, 55, 610. 9. Aravamudan, G.; Gopalakrishnan, J.; Udupa, M. R. J. Chem. Educ. 1974, 51, 129. 10. Kantrowitz, E. R. J. Chem. Educ. 1974, 51, 202. 11. Bengali, A. A.; Charlton, S. B. J. Chem. Educ. 2000, 77, 1348. 12. Baker, A. D.; Casadevall, A.; Gafney, H. D.; Gellender, M. J. Chem. Educ. 1980, 57, 314. 13. Kantrowitz, E. R. J. Chem. Educ. 1974, 51, 202. 14. Zare, R. N.; Spencer, B. H.; Springer, D. W.; Jacobson, M. P. Laser Experiments for Beginners; University Science Books: Sausalito, CA, 1995. 15. Ernhoffer, R.; Kovacs, D.; Subak, E., Jr.; Shepherd, R. E. J. Chem. Educ. 1978, 55, 610. 16. Park, K. M.; Schuster, G. B. J. Organomet. Chem. 1991, 402, 355. 17. Turner, C. A.; Ding, W.; Amster, I. J.; Kutal, C. Coord. Chem. Rev. 2002, 229, 9. 18. Gill, T. P.; Mann, K. R. Inorg. Chem. 1980, 19, 3007. 19. Gill, T. P.; Mann, K. R. Inorg. Chem. 1983, 22, 1986. 20. Schrenk, J. L.; Palazzotto, M. C.; Mann, K. R. Inorg. Chem. 1983, 22, 4047. 21. Chrisope, D. R.; Park, K. M.; Schuster, G. B. J. Am. Chem. Soc. 1989, 111, 6195. 22. Jakubek, V.; Lees, A. J. Inorg. Chem. 2000, 39, 5779. 23. Doggweiler, H. O.; Desobry, V. (Ciba-Geigy A.-G., Switz.). Process for the Preparation of Iron Arene Complexes. Eur. Pat. Appl. 270,490, 1988. 24. Lees, A. J. Anal. Chem. 1996, 68, 226.
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