In the Laboratory
Advanced Chemistry Classroom and Laboratory
Metal Complexes of Trifluoropentanedione
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An Experiment for the General Chemistry Laboratory Robert C. Sadoski School of Physical and Life Sciences, Arkansas Tech University, Russellville, AR 72801 David Shipp and Bill Durham* Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701; *
[email protected] Almost all current general chemistry textbooks include at least one chapter that describes the chemistry of transitionmetal complexes. To provide some further insight into the lecture material, many general chemistry laboratory classes incorporate exercises dealing with transition-metal complexes. Unfortunately, it is difficult to provide a meaningful experience in the laboratory because of the limited knowledge of the students at this stage of their education. At the freshman level, transition-metal chemistry is primarily descriptive. The emphasis in the lecture is typically centered on coordination numbers, the structures of some representative examples, the relative stabilities of different complexes, and nomenclature. Ideally, a laboratory exercise should focus on as many of these topics as possible. The experiment described here provides a brief survey of the reactions of most of the first-row transition elements and addresses the first three topics. This is done by combining an efficient synthetic reaction for the preparation of a series of trifluoropentanedione complexes (1) with the power of mass spectrometry to examine the molecular composition of metal complexes. The structural aspects are addressed by examining the isomers generated by the reaction of Cr(III), which can be separated by gas chromatography (2). The experiments are centered on the reactions of 1,1,1trifluoro-2,4-pentanedione with metal salts. The reactions are simple, fast, and safe. Students can readily produce and characterize three or more examples each in a single 3-hour laboratory session. If the data are pooled, the class can examine the complexes produced by most of the first-row transition metals. Experience during the last two years has shown that the students readily grasp the concept of mass spectroscopy and have little difficulty interpreting the data. Experimental Procedure 1,1,1-Trifluoro-2,4-pentanedione (tfac) reacts with the metals ions Cr(III), Mn(II), Fe(III), Co(II), Ni(II), and Cu(II) within a few minutes in the mixed solvent methanol/aqueous acetate buffer. Most of the products precipitate immediately and can be recovered by filtration, washed with water, and air-dried. The yields range from 40 to 60% and the reactions can be performed with 150 mg of the hydrated metal nitrates or less. To avoid the potential hazards associated with handling tfac and to capitalize on the small quantities required to produce a manageable yield, the reagent is provided to the students in a methanolic solution. Typically, the metal nitrate is
dissolved in 1.5 mL of 50% aqueous sodium acetate solution, mixed with 0.5 mL of 16% tfac in methanol, heated to boiling, and cooled. The tfac complexes are extremely stable and in some cases can be sublimed. In the case of Cr(tfac)3, the complex is sufficiently stable to pass through a gas chromatograph. Since the ligand is not symmetrical, two isomers should be formed if the complexes form in the octahedral geometry. Thus, we can loosely follow the logic used by Alfred Werner to conclude that the complexes are in fact octahedral if the two isomers are detected. A GC–MS analysis of the sample reveals two products with identical mass spectra. Under the conditions described in the supplemental material,W a complete GC analysis requires about 14 minutes. We usually divide the class into manageable groups of about 6 students and have each group make one GC–MS run while the remainder of the class prepares the other metal complexes. While the run is in progress the details of the instrument are explained with the help of a set of quadrupoles and an electron multiplier removed from an older mass spectrometer (3). Mass spectra of the complexes formed by all of the metals can be obtained by using a direct insertion probe. The mass spectrum of each sample can be obtained in 2–3 minutes if the students load the sample tubes themselves. Hazards All solutions are potentially toxic if ingested or inhaled. Methanol can be absorbed through the skin and, in sufficient quantities, can damage the optic nerves. Avoid inhaling fumes from the reagent solutions and perform reactions in a hood. Results and Discussion Complexes of Cr(III), Mn(II), Fe(III), Co(II), Ni(II), and Cu(II) were prepared by students in 40–60% yields in less than 15 minutes each. More than 100 mg of each complex was obtained, which is more than adequate for numerous methods of characterization yet not enough to be a significant disposal problem. Visible and IR spectra, for example, could be recorded. The mass spectrum of each metal complex contains a strong parent ion and an isotope pattern dominated by the isotopes of the metal. The isotope pattern provides an excellent opportunity to reiterate the fact that the atomic weights found in the periodic tables are weighted averages of the isotopic masses. The students readily gained a fundamental under-
JChemEd.chem.wisc.edu • Vol. 78 No. 5 May 2001 • Journal of Chemical Education
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In the Laboratory
standing of the mass spectrometry experiment and were able to recognize and understand some of the common fragmentation patterns. GC–MS of the Cr(tfac)3 revealed two isomers with identical molecular masses consistent with the placement of the asymmetric ligands in an octahedral arrangement around the metal ions. Although no attempt was made to separate the isomers of the square planar complexes, their existence was addressed in the laboratory discussion. Pooling of the data revealed that M3+ metals form complexes with three ligands and the M2+ metals form complexes with two ligands. In some cases, the M2+ complexes contain solvent molecules coordinated to the metals and these can be detected by color changes that accompany heating under vacuum. This was particularly evident with Cu(tfac)2, which undergoes a subtle color change from green to gray before it sublimes when heated under a moderate vacuum. Each of the complexes has a characteristic color, and collecting all the samples to a central display area was a very dramatic demonstration of this notable feature of transition metal complexes.
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This laboratory has been offered four times in various forms over the last two years. The students always prepare the complexes in good yield and no problems have arisen with the reagents employed. A recent poll of the students indicated that they enjoyed the experience and especially appreciated the opportunity to use the mass spectrometer. W
Supplemental Material
The complete description of this experiment, a laboratory handout, and notes to instructors are available in this issue of JCE Online. Literature Cited 1. Fay, R. C.; Piper, T. S. J. Am. Chem. Soc. 1963, 85, 500. 2. Sievers, R. E. In Coordination Chemistry; Kirschner, S., Ed.; Plenum: New York, 1960; p 270. 3. Henchman, M.; Steel, C. J. Chem. Educ. 1998, 75, 1042– 1049.
Journal of Chemical Education • Vol. 78 No. 5 May 2001 • JChemEd.chem.wisc.edu
