In the Laboratory
Simple Preparation and NMR Analysis of mer and fac Isomers of Tris(1,1,1-trifluoro-2,4-pentanedionato)cobalt(III)
W
An Experiment for the Inorganic Chemistry Laboratory Ashley W. Jensen and Brian A. O’Brien* Department of Chemistry, Gustavus Adolphus College, Saint Peter, MN 56082; *
[email protected] Preparation of homoleptic 2,4-pentanedionato (acac) complexes of metals is a straightforward and common inorganic chemistry experiment (1). The insight that students gain from stereochemical study of the products is, however, minimal owing to the high symmetries of the complexes. We sought to improve the pedagogical value of the experiment through use of an unsymmetrical ligand that would provide mer and fac isomers (see structures), thus increasing the interest from the standpoint of stereochemical analysis. The placement of the experiment in our intermediate inorganic course is designed to expand on organic stereochemical concepts, and to supplement 1H and 13C NMR with 19F NMR analysis. F 3C F 3C H3C
F3C CH3
O O O
Co
O O
O H3C mer-Co(tfa)3
H3C F 3C
CF3
CH3
O O O
Co
O O
O
CF3
H3C fac-Co(tfa)3
1,1,1-Trifluoro-2,4-pentanedione suits this purpose well, since it is commercially available and affords students an opportunity to obtain 1H, 13C, and 19F NMR spectra from the two products of the reaction. Cobalt(III) was chosen as the metal because it is of relatively low toxicity, kinetically inert, and diamagnetic in its β-diketonate complexes, and the Co(II) precursor, hydrated Co(II) carbonate, is inexpensive. Attempts were made to synthesize Co(tfa)3 by a procedure used to prepare Co(acac)3, in which 10% hydrogen peroxide is added to a heated mixture of hydrated cobalt(II) carbonate and excess 2,4-pentanedione (1a). When 1,1,1-trifluoro-2,4pentanedione was substituted for 2,4-pentanedione, only trace amounts of Co(tfa)3 were formed; use of higher temperatures or 30% hydrogen peroxide did not improve the yield. The two published preparations of Co(tfa)3 include reaction of 1,1,1-trifluoro-2,4-pentanedione with Na3Co(CO3)3⭈3H2O (2a) and a preparation requiring 90% hydrogen peroxide (2b). We have found that a simple modification of the 10% hydrogen peroxide procedure allows direct, rapid preparation of Co(tfa)3 from hydrated cobalt(II) carbonate. The use of wet tert-butyl alcohol as the solvent in the initial part of the procedure produces a solution of Co(II), which can then be oxidized to Co(III) by 10% hydrogen peroxide. Yields are somewhat lower than those from the most commonly used literature procedure (2a) (6–10% vs 26%), but our procedure constitutes a significant improvement in terms of time and safety. Isolation of the Co(tfa)3 is simple, and the amount of 954
Co(tfa)3 obtained from 0.5 g of hydrated cobalt(II) carbonate is more than sufficient for chromatographic and spectroscopic analysis. Separations of the mer and fac isomers of Co(tfa)3 have been reported through TLC on silica gel (3), alumina (3c), or polyacrylonitrile (4), and by column chromatography on silica gel (5) or alumina (2a, 6 ). Separation has also been done by GC (7, 2b) or HPLC (6, 8). We chose TLC (silica gel) as our separation method for simplicity and to reduce the amounts of solvent and adsorbent that would otherwise be required. Toluene was found to be the most effective eluting solvent. Pooled TLC samples from a class of ca. 40 students provide enough of each isomer for 1H and 19F NMR analysis of both mer- and fac-Co(tfa)3 and for acquisition of the 13C spectrum of the more abundant mer isomer. The TLC samples are run as bands streaked on glass- or plastic-backed TLC plates; the collective samples are extracted with acetone. The inequivalences of the various sets of NMR-active nuclei in mer-Co(tfa)3 are not obvious to most students without consideration of models or drawings of the two isomers, and consideration of the structures gives the students practice in thinking about possible consequences of octahedral stereochemistry. NMR analysis of the fac and mer isomers of Co(tfa)3, especially if results obtained from NMR analysis of Co(acac) 3 are included for comparison, clearly reveals the differences in symmetry of the two isomers of Co(tfa)3 and of Co(acac)3 (2a, 6, 9) and in addition illustrates a solvent shift in the 19F spectrum of mer-Co(tfa)3. The three methyl groups of fac-Co(tfa) 3 are NMRequivalent, since they are related by a threefold rotation axis (as are the three methine hydrogens, the three trifluoromethyl groups, and each of the two sets of carbonyl groups). The 1 H and 19F NMR spectra are thus singlets; the 13C spectrum is similarly straightforward, and in addition reveals one- and two-bond couplings of 13C with 19F. The corresponding atomic groups of mer-Co(tfa)3, in contrast, are not related through symmetry. Thus, the three methyl groups, the three methine hydrogens, the three trifluoromethyl groups, and each member of each of the two sets of carbonyl groups are expected to be NMR-inequivalent. In CDCl3 solution, the 1H spectrum shows this to be the case for both methine and methyl groups. However, in the 13 C and 19F spectra in CDCl3, two of the methine carbons, two of the methyl carbons, and two of the trifluoromethyl groups (19F) give rise to coincident signals, whereas the 13C signals of the inequivalent carbonyls are resolved. Changing the solvent to acetone-d6 allows all three 19F signals to be observed, but does not provide resolution of the coincident methine and methyl signals.
Journal of Chemical Education • Vol. 78 No. 7 July 2001 • JChemEd.chem.wisc.edu
In the Laboratory
The experiment described here has excellent potential for modification or interfacing with other analytical techniques. Numerous chromatographic techniques for separating mer and fac isomers are available, and it would be of interest to include preparation of other sets of isomers or adaptation of the experiment to include GC–MS analysis.
2.
3.
Hazards This procedure should be carried out in a well-ventilated area such as a fume hood. The 10% hydrogen peroxide is corrosive to tissues. Goggles should be worn throughout the procedure, and appropriate gloves should be worn when handling the hydrogen peroxide solution. The stock 10% hydrogen peroxide should be stored in a vented or loosely capped plastic bottle.
5. 6. 7.
Acknowledgments We thank the students who performed the experiment for data used in refinement of the procedures, and Gretchen E. Hofmeister for useful discussions. We also thank the National Science Foundation (DUE-9352027, Instrumentation and Laboratory Improvement) and The Camille and Henry Dreyfus Foundation for awards that were used for partial funding of our Varian Gemini 300-MHz NMR spectrometer. W
4.
8.
Supplemental Material
Supplemental material, including notes for the instructor, experimental background and instructions for students, NMR spectra and chemical shift tabulations, and IR spectra, are available in this issue of JCE Online. 9.
Literature Cited 1. See for examples (a) Shalhoub, G. M. J. Chem. Educ. 1980, 57, 525–526. (b) Glidewell, C. In Inorganic Experiments; Wollins, J. D., Ed.; VCH: New York, 1994; pp 116–126. (c)
Szafran, Z.; Pike, R. M.; Singh, M. M. Microscale Inorganic Chemistry; Wiley: New York, 1991; pp 224–229. (a) Fay, R. C.; Piper, T. S. J. Am. Chem. Soc. 1963, 85, 500–504. (b) Veening, H.; Bachman, W. E.; Wilkinson, D. M. J. Gas Chromatogr. 1967, 5, 248–250. (a) Tesic, Z. Lj.; Janjic, T. J.; Vukovic, G. N.; Celap, M. B. J. Chromatogr. 1988, 456, 346–350. (b) Yamazaki, M.; Igarashi, R.; Suzuki, T. Bunseki Kagaku 1983, 32, 234–240, as referenced in Chem. Abstr. 1982, 99, 63400. (c) Yamazaki, M.; Igarashi, R.; Ichinoki, S. Bunseki Kagaku 1982, 31, 702–707, as referenced in Chem. Abstr. 1982, 98, 10035. (d) Johnson, A.; Everett, G. W. Jr. Inorg. Chem. 1973, 12, 2801–2805. Janjic, T. J.; Milojkovic, D. M.; Tesic, Z. L.; Celap, M. B. J. Planar Chromatogr. 1990, 3, 495–499. Ferrandi, G.; Grutsch, P. A.; Kutal, C. Inorg. Chim. Acta 1982, 59, 249–250. Cardwell, T. J.; Lorman, T. H. J. Chromatogr. 1989, 479, 181–188. Dilli, S.; Patsalides, E. J. Chromatogr. 1979, 176, 305–318. Fujinaga, T.; Kuwamoto, T.; Murai, S.; Sugiura, K. Mem. Fac. Sci., Kyoto Univ., Ser. Phys., Astrophys., Geophys. Chem. 1974, 34, 309–319, as referenced in Chem. Abstr. 1974, 83, 107678. Guiochon, G.; Pommier, C. Gas Chromatography in Organics and Organometallics; Ann Arbor Science Publishers: Ann Arbor, MI, 1973; Chapter 8. Moshier, R. W.; Sievers, R. E. Gas Chromatography of Metal Chelates; Pergamon: New York, 1965. Tsukahara, S.; Saitoh, K.; Suzuki, N. J. Chromatogr. 1991, 547, 225–237. Timerbaev, A. R.; Petrushkin, O. M. Anal. Chim. Acta 1984, 159, 229–244. Kirkman, E. M.; Cheng, Z.; Uden, P. C.; Stratton, W. J.; Henderson, D. E. J. Chromatogr. 1984, 317, 569–578. Yamazaki, M.; Ichinoki, S.; Igarishi, R. Bunseki Kagaku 1983, 32, 605–610, as referenced in Chem. Abstr. 1983, 100, 16840. Uden, P. C.; Bigley, I. E.; Walters, F. H. Anal. Chim. Acta 1978, 100, 555-561. Tollinche, C. A.; Risby, T. H. J. Chromatogr. Science 1978, 16, 448-454. For an extensive discussion of NMR analysis of metal complexes of 2,4-pentanedionates and related compounds, see: Mehrotra, R. C.; Bohra, R.; Gaur, D. P. Metal β-Diketonates and Allied Derivatives; Academic: New York, 1978; pp 76–109. For a discussion of NMR analysis of bromination products of Co(tfa)3, Cr(tfa)3, and Rh(tfa)3, see ref 6.
JChemEd.chem.wisc.edu • Vol. 78 No. 7 July 2001 • Journal of Chemical Education
955
