.. lene glycol (5) as the solvent rather than 1,2-dimethoxyethane. When ethylene glycol was used, an extremely viscous reaction mixture resulted that was incapable of being stirred effectively in the micro-glassware. Our success rate with the revised preparation is 100%. Our advanced undergraduate inorganic lab is taught in the semester format with two three-hour weekly classes.
The students learn to multi-task to accomplish their lab responsibilities efficiently. We have provided the following suggested format (Table 1) to accomplish the synthesis and characterization of ferrocene and acetylferrocene in two and a half weeks. This format is not provided to the students. They are innovative and are required to submit their own schedules before beginning work. The format allows instructors and teaching assistants to flexibility in the method of ensuring that the students use their time efficiently. ————————————————– —————————— Table 1. Suggested Time Management Schedule Day Program 1 Synthesis of Cp2Fe; teaching assistant to provide cyclopentadiene 2 Sublimation of Cp2Fe; students are given Cp2Fe to perform the acetylation 3 Thin layer and column chromatography of acetylferrocene followed by rotary evaporation; begin characterization of Cp2Fe (melting point, UV-Vis, IR) 4 Characterization of acetylferrocene (melting point, UV-Vis, IR); CAChe modeling 5 Finish characterization including cyclic voltammetry and bulk electrolysis ————————————————– —————————— Crude ferrocene and acetylferrocene were synthesized in 51-79% and 27-58% yield respectively.
An experimental melting point range of 169-171 C was obtained for ferrocene. The reported melting point range is 173-174 C (3). For acetylferrocene, the experimental melting point range was 80-83 C as compared with the reported range of 81-83 C (7). Infrared spectroscopy was performed by the students on ferrocene and acetylferrocene both as a KBr pellet and as a Nujol mull on NaCl plates. The infrared spectra were comparable to those reported for ferrocene (3) and acetylferrocene (8). The main difference between the spectra of ferrocene and acetylferrocene is of course the appearance of a carbonyl stretch at 1736 cm-1 that is present in the acetylferrocene and absent in the ferrocene. Some students also observed a peak at 893 cm-1 that is attributed to the monoacetylferrocene ring.
They did not observe peaks that could be attributed to the 1,2-diacetylferrocene complex at 917 cm-1 or a doublet due to the 1,3-diacetylferrocene complex at 922 and 905 cm-1 (8). The experimental UV-Vis spectra of ferrocene and acetylferrocene were obtained in acetonitrile and Beers law was used to calculate the molar absorptivity. The UV spectrum for ferrocene shows maxima at 330 nm (2 = 52) and 440 nm (2 = 90), and a rising short-wavelength absorption at 225 nm (2 = 5051). This is comparable with the reported spectrum in ethanol (3). The UV spectrum for acetylferrocene shows maxima at 219 nm (2 = 2.2 x 104), 266 nm (2 = 5268) and 320 nm (2 = 1124).
Except for the calculated molar absorptivity of the peak at 219 nm, this is comparable with the reported spectrum in 95% ethanol (8). The students also observed peaks assigned to ferrocene in their acetylferrocene samples. The electrochemistry component of this laboratory was the first time that most students were exposed to cyclic voltammetry and the bulk electrolysis technique. An Amel System 5000 Potentiostat was used for all measurements. For cyclic voltammetry, the electrochemical cell was a 100 mL beaker equipped with a Ag/AgCl reference electrode (student prepared), a BAS (West Lafayette, IN) platinum-disk working electrode (2 mm diameter) and a large (1 cm2) platinum flag counter electrode. After having verified a flat background of tetrabutylammonium hexafluorophosphate (0.01 M) supporting electrolyte in acetonitrile in the range 0.0 to 1.0 V vs.
Ag/AgCl, cyclic voltammograms of ferrocene and acetylferrocene (approximately 3.2 x 10-3 M) were obtained at scan rates of 100 500 mV/sec. A typical cyclic voltammogram of ferrocene showed a reversible oxidation at E1/2 = +0.35 V vs. Ag/AgCl with Ep/2 = 0.057V. A typical cyclic voltammogram of acetylferrocene also showed a reversible oxidation at E1/2 = +0.58 V vs. Ag/AgCl with Ep/2 = 0.044V. Small peaks for ferrocene were also visible in the acetylferrocene cyclic voltammogram.
These results are comparable to the reported E of acetylferrocene at +0.27 V vs. the ferrocene/ferrocenium couple (6). A second new electrochemical component that was recently introduced into this laboratory is the bulk electrolysis of ferrocene to ferrocenium. The electrochemical cell was a 100 mL beaker equipped with an Ag/AgCl reference electrode (student prepared), a BAS (West Lafayette, IN) reticulated vitreous carbon (RVC) working electrode and an extremely large platinum flag counter electrode. After having verified a flat background of tetrabutylammonium hexafluorophosphate (0.01 M) supporting electrolyte in acetonitrile in the range 0.0 to 1.0 V vs. Ag/AgCl, the bulk electrolysis of ferrocene (approximately 7.5 x 10-4 M) was achieved on several occasions.
As expected, a new peak in the UV-Vis was observed at 620 nm and the solution changed color from orange to blue. Unfortunately to date, these experimental conditions are not reproducible. As a supplement to their standard chemical characterization, students used the CAChe molecular modeling program to build a ferrocene molecule in both the eclipsed and staggered conformations and to remove an electron to obtain information about the ferrocenium cation. The results of this modeling were then discussed in relation to their experimental observations. When the students have synthesized and derivatized ferrocene, they have an experimental background for comparison of the unsubstituted ferrocene versus the acetylated ferrocene.
They also have a clear understanding of the potential R groups that are chemically practical. This is especially meaningful if the student has completed organic chemistry and is able to relate the familiar benzene substituents with the ferrocene molecule. We have found that if a student proceeds through the iterative question before understanding the acetylation experiment, they design strange, wondrous and impractical molecules with the aid of the CAChe system. It must be stressed that molecular modeling is only a tool. The input is influenced to a large degree by the understanding of the operator which may be enhanced with guidance from the instructor.
A natural progression at the completion of the two syntheses is the introduction of the iterative question. Students are asked to design a ferrocene with specific properties such as a different colored ferrocene. This question is answered with the aid of CAChe modeling where electronic spectra of the gas phase ferrocene and the substituted ferrocene may be generated by ZINDO (Zerners Intermediate Neglect of Differential Overlap). A more comprehensive iterative project involves both library work and molecular modeling. The students are asked to find the preparation of a substituted ferrocene in the library.
They may also design a synthesis and confirm the synthesis with the aid of library references. They then model the complex and predict its spectroscopic characteristics based upon what they are able to calculate from the molecular model and their knowledge of general chemical trends. Since the students became familiar with cyclic voltammetry, one trend of interest involves the ionization potential of the substituted ferrocenes. One student project involved a comparison of several known substituted ferrocenes (6) and their gas phase models (Figure 1 and Table 2). The gas phase models were used since the expected solvent dependence has not been observed using the CAChe system due to initial limitations with project leader.
The initial calculated ionization potentials were adjusted by subtracting 7.647 eV. This sets the ferrocene/ferrocenium couple at zero as is customary (6). These values and a least squares regression plot were then plotted. In general, a downward trend in the least squares regression is observed with the more easily reduced ferrocenes containing electron withdrawing substituents having positive ionization potentials. Conversely, the more easily oxidized ferrocenes with electron donating substituents are calculated with negative ionization potentials.
Deviations from experimental data may be accounted for since the student was comparing gas phase ferrocene models and acetonitrile ferrocene electrochemistry (6). ————————————————– —————————— Fig. 1. Student CAChe Project ————————————————– —————————— Table 2 Student CAChe Project ————————————————– —————————— Conclusion The incorporation of an iterative question into each of our advanced inorganic undergraduate laboratories has allowed students to plumb the depths of their chemical knowledge and to acquire new tools that improve their use of the scientific method. The students enjoy the high success rate of the ferrocene/acetylferrocene lab. They also appreciate the chance to acquire new synthetic techniques such as the use of Schlenk techniques. In addition, the use of novel instrumental analysis such as electrochemistry is beneficial to their overall undergraduate education.
They seem to thrive on the diverse exposure and the opportunity to stretch themselves. This allows them to become excited about chemistry and like the experiment that they are conducting, they come full circle and view chemistry in a new light as a useful, valuable tool. The addition of the iterative question to a classical laboratory can therefore provide an additional richness to the traditional wet chemistry. ————————————————– —————————— Acknowledgments Research supported by NSF under Grants # DUE-9452023 and DUE-9452131. ————————————————– —————————— Literature Cited 1. Kauffman, George B. J. Chem.
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