In the Laboratory
Tuning the Optical and Electrochemical Properties of M(CO)4(phenanthroline) Compounds via Substituents on the Phenanthroline Ligand: An Upper-Division Inorganic Laboratory Anthony W. Brown and Anthony L. Diaz* Department of Chemistry, Central Washington University, Ellensburg, Washington 98926 *
[email protected] The synthesis and characterization of coordination compounds is generally regarded as an essential aspect of the training of undergraduate chemistry majors. Several textbooks for the student laboratory have been published (1, 2), and numerous articles on the subject have appeared in this Journal. More recently, a trend has developed toward inorganic laboratory coursework that is more project or research oriented (3-6). We have adopted this type of approach by couching the experiment in the context of the development of materials for dye-sensitized solar cells. Efficient dye-sensitized solar cells, first reported by O'Regan and Gratzel (7), have the potential to compete with traditional solid-state solar cell technology. The cell design requires a dye compound that absorbs sunlight and then hands off the excited electron to TiO2 particles where further electron transport takes place. It is an interesting modern application of coordination chemistry, in that the dye is composed of a transition-metal complex with optical and electrochemical properties that are carefully tuned via the choice of ligands. An overview of this technology (8), as well as a student lab in which dye-sensitized cells are prepared from berry extract (9), have appeared previously in this Journal. Compounds produced for useful dye-sensitized solar cells are unreasonably complex for a student laboratory, so we utilize a simpler set of model compounds that can be prepared and characterized in a single six-hour lab period. Alternatively, the synthesis and analysis could be split between two three-hour periods. The synthesis is essentially that reported by Manuta and Lees (10) and Stiddard (11) for similar compounds. The analysis focuses on observing the effect of the ligands on the optical and electrochemical properties of the complex, and is analogous in many respects to studies of dye compounds for solar cells (12-14). Infrared (IR) spectroscopy is used to study the degree of π backbonding to the CO ligands, UV-vis is used to study the energy of the metal-to-ligand charge-transfer transition, and cyclic voltammetry (CV) is used to study the redox properties of the complex. The research question the students are posed is, “Can the optical and electrochemical properties of these compounds be tuned by modifying the relative electron-donating character of the ligand?” We initially performed this laboratory using only Cr, and have since switched to Mo for reasons discussed in the supporting information. If time and resources permit, it would be instructive to divide the synthesis among student groups not just by ligand, but by metal as well.
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Figure 1. Phenanthroline ligands used in the reaction with Cr(CO)6 or Mo(CO)6.
Experimental Procedure Synthesis Ideally, each student would prepare a single compound and share their product with other students for analysis. Laboratory space at our institution does not allow for this arrangement, and students instead work in groups of two. We have found that this still provides a good learning experience and also gives students an opportunity to work collaboratively. The starting metal hexacarbonyl and phenanthroline ligands (Figure 1) are commercially available. Synthesis of the metal compounds is achieved via reflux under nitrogen of the desired phenanthroline ligand with the corresponding metal hexacarbonyl. Details are provided in the supporting information. It should be noted that other synthetic routes to similar materials have appeared previously in this Journal (15, 16). Analysis Details on how IR, UV-vis, and CV measurements are performed, along with representative spectra, are included in the supporting information. When CO binds to metals, there is a bonding interaction of the metal d orbitals with π* antibonding orbitals of CO, a phenomenon known as π backbonding (17, 18). The result of this interaction is a reduction of the effective bond order between carbon and oxygen. This is readily observed using IR spectroscopy, as the weakening of the CO bond results in lower observed CO stretching frequencies. IR results for all compounds are summarized in Table 1 (wherein phenanthroline is abbreviated phen). Four IR active stretching modes are expected given the C2v symmetry of these complexes (19), which appear in the region between 1750 and 2050 cm-1. From the data in Table 1 it can be seen that as the phenanthroline ligand becomes more electron donating (5-chloro f unsubstituted f tetramethyl), the degree of electron donation through the metal to the π* orbitals of CO increases, resulting in a decrease of the
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In the Laboratory Table 1. Observed CO Stretching Modes in the IR Spectra for the Compounds of This Study stretching frequencies/cm-1 compound Cr(CO)4(5-chloro-1,10-phen)
2010
1909
1885
Cr(CO)4(1,10-phen)
2009
1906
1883
1830
Cr(CO)4(3,4,7,8-tetramethyl-1,10-phen)
2007
1901
1875
1823
Mo(CO)4(5-chloro-1,10-phen)
2016
1912
1883
1834
Mo(CO)4(1,10-phen)
2015
1909
1880
1831
Mo(CO)4(3,4,7,8-tetramethyl-1,10-phen)
2013
1905
1873
1822
complexes is not reversible, regardless of phenanthroline ligand, so values reported in Table 3 are simply for the one-electron oxidation of the complex. For Cr the oxidation is reversible, and formal oxidation potentials are estimated from the average of the peaks in the forward and reverse scans (20). If both Cr and Mo complexes are to be studied, it is more appropriate to compare the observed potential of the oxidation under identical instrumental conditions. For both metals, as the phenanthroline ligand becomes more electron donating, it becomes easier to remove an electron from the complex, and the oxidation potential shifts to lower values.
Table 2. UV-Vis Absorption Data for the Compounds of This Study compound
λmax/nm
color
Cr(CO)4(5-chloro-1,10-phen)
maroon
528
Cr(CO)4(1,10-phen)
orange-red
509 466
Cr(CO)4(3,4,7,8-tetramethyl-1,10-phen)
coral
Mo(CO)4(5-chloro-1,10-phen)
crimson
498
Mo(CO)4(1,10-phen)
orange-red
485
Mo(CO)4(3,4,7,8-tetramethyl-1,10-phen) bright orange
447
Student Report
Table 3. CV Data for the Compounds of This Study compound
Students prepare a formal report for this experiment using American Chemical Society guidelines. They report on the trends observed for each type of measurement, and explain these trends based on the relative electron-donating character of the ligands. If laboratory space and class time permit, the experiment may be expanded to include synthesis of compounds using both Cr and Mo metal centers.
oxidation potential/mV
Cr(CO)4(5-chloro-1,10-phen)
463
Cr(CO)4(1,10-phen)
449
Cr(CO)4(3,4,7,8-tetramethyl-1,10-phen)
411
compound
observed oxidation/mV
Mo(CO)4(5-chloro-1,10-phen)
663
Mo(CO)4(1,10-phen)
650
Mo(CO)4(3,4,7,8-tetramethyl-1,10-phen)
608
Hazards
CO stretching energies along this series. It should also be noted that Mo exhibits a lesser degree of backbonding than Cr, an effect explained by the fact that the larger 5d orbitals of Mo do not overlap as effectively with the π* orbitals of CO (19). UV-vis absorption data on all compounds are summarized in Table 2. The broad absorption in the visible region has previously been assigned to a metal-to-ligand charge transfer (MLCT) transition in related compounds (10). The molar absorptivity, ε, of these transitions is on the order of 5500 L mol-1 cm-1. As the phenanthroline ligand becomes more electron donating, MLCT becomes less favorable, as evidenced by a shift in the absorption to higher energy (shorter observed λmax). For a given phenanthroline ligand, the energy of the MLCT is greater for Mo than Cr, consistent with data reported for M(CO)4bipy compounds (10). Interestingly, the energies of the MLCT transition of related group 6B transition-metal complexes are ordered Mo > W > Cr (10). Compounds with W have not been investigated for use in this experiment. The oxidation potential of a complex is an important factor to consider in the design of materials for solar cells. An effective dye must also exhibit a reversible oxidation so that the electroncycling process can be repeated during cell operation. Both of these properties can be studied using CV (20). CV data on all compounds are summarized in Table 3. Oxidation of the Mo 976
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The compounds of Mo and Cr are toxic, and chloroform is a known carcinogen. Toluene and pentane are flammable organic solvents. Acetonitrile is hazardous in the case of eye and skin contact and inhalation and injection. The phenanthroline ligands are graded as only moderate health and contact hazards. Students wear goggles and nitrile gloves throughout the synthesis and analysis, and work in a fume hood when preparing solutions for analysis. Metal carbonyls should be weighed by difference in a closed container. Student Response By applying a variety of instrumental methods to the characterization of these complexes, students gain valuable experience in using different pieces of analytical information to attain a more complete description of a chemical system. Students who have done this lab comment that they enjoy the quick pace (an illusion created by the use of several different instruments!), and they like that they must use the information to “figure out what's going on”. Students are also generally not accustomed to studying transition-metal complexes in which the charge on the central metal atom is 0. For many students conducting this experiment, it is also their first experience using a gas manifold. Conclusions A research-based experiment designed for an upper-division integrated physical or inorganic laboratory course was described.
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In the Laboratory
The synthesis and data collection can be achieved in a single sixhour session. By changing the relative electron-donating or electron-withdrawing character of substituted phenanthroline ligands on complexes of the type M(CO)4phen, trends in oxidation potential, the energy of metal-to-ligand charge transfer, and the degree of π backbonding to CO are observed. In addition, the effect of changing the metal from Cr to Mo may be examined.
8. 9. 10. 11. 12.
13.
Acknowledgment The authors thank John Bullock at Bennington College and Tim Sorey at Central Washington University for helpful discussions. Literature Cited 1. Jolly, W. L. The Synthesis and Characterization of Inorganic Compounds; Waveland Press: Prospect Heights, IL, 1991. 2. Inorganic Experiments, 2nd ed.; Woollins, J. D., Ed.; Wiley-VCH: Weinheim, 2003. 3. Hunter, A. D.; Bianconi, L. J.; DiMuzio, S. J.; Braho, D. L. J. Chem. Educ. 1998, 75, 891–893. 4. McDonagh, A. M.; Deeble, G. J.; Hurst, S.; Cifuentes, M. P.; Humphrey, M. G. J. Chem. Educ. 2001, 78, 232–235. 5. Vallarino, L. M.; Polo, D. L.; Esperdy, K. J. Chem. Educ. 2001, 78, 228–232. 6. Blyth, K. M.; Mullings, L. R.; Phillips, D. N.; Pritchard, D.; van Bronswijk, W. J. Chem. Educ. 2005, 82, 1667–1670. 7. O'Regan, B.; Gratzel, M. Nature 1991, 353, 737–740.
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Meyer, G. J. J. Chem. Educ. 1997, 74, 652–657. Gratzel, M.; Smestad, G. P. J. Chem. Educ. 1998, 75, 752–757. Manuta, D. M.; Lees, A. J. Inorg. Chem. 1983, 22, 3825–3828. Stiddard, M. H. B. J. Chem. Soc. 1962, 4712–4715. Kuang, D.; Ito, S.; Wenger, B.; Klein, C.; Moser, J. E.; HumphryBaker, R.; Zakeeruddin, S. M.; Gratzel, M. J. Am. Chem. Soc. 2006, 128, 4146–4154. Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gratzel, M. J. Am. Chem. Soc. 2001, 123, 1613–1624. Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Muller, E.; Liska, P.; Vlachopoulos, N.; Gratzel, M. J. Am. Chem. Soc. 1991, 115, 6382–6390. Manuta, D. M.; Lees, A. J. J. Chem. Educ. 1987, 64, 637–638. Ardon, M.; Hayes, P. D.; Hogarth, G. J. Chem. Educ. 2002, 79, 1249–1251. Housecroft, C. E.; Sharpe, A. G. Inorganic Chemistry, 3rd ed.; Pearson Prentice Hall: Essex, U.K., 2008; pp 109-110. Montgomery, C. D. J. Chem. Educ. 2007, 84, 102–105. Miessler, G. L.; Tarr, D. A. Inorganic Chemistry, 3rd ed.; Pearson Prentice Hall: Upper Saddle River, NJ, 2004; pp 467-470. Kissinger, P. T.; Heineman, W. R. J. Chem. Educ. 1983, 60, 702–706.
Supporting Information Available Notes for the instructor; student handout. This material is available via the Internet at http://pubs.acs.org.
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