In the Laboratory
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Copper(III) Dithiocarbamates An Undergraduate Experimental Project with Unexpected Challenges Luis I. Victoriano Facultad de Ciencias Químicas, Universidad de Concepción, P.O. Box 160-C, Concepción, Chile;
[email protected] In the classroom and laboratory, coordination chemistry competes poorly against catalysis or macromolecular chemistry in the comparative ability to motivate undergraduate students. In our students’ view, coordination compounds are “predictable” or “unchallenging” when compared to the bewildering array of possibilities offered by the other branches of chemistry. Of course, these are just words of a polite youngster to mean “boring”. We endeavor to use examples from our own current research to present the students with examples of experiments that are not quite predictable. A short account of one such system has been published (1). We describe here the products resulting from the reaction of copper(II) chloride and the ligand N, N, N ′, N ′-tetraethylthiuram disulfide. This project experiment for undergraduate students involves the complete characterization of the reaction product and an explanation of the course followed by the reaction. This project has been successfully incorporated into a program for selected senior B.Sc. students. It is challenging because its results are unpredictable from the point of view of the students’ chemical knowledge, but understandable using this same knowledge. A satisfactory answer to the general problem outlined rests on the correct interpretation of the data supporting the structure involved and also on a global understanding of the related chemistry. Additional advantages are inexpensive, readily available starting materials and labware and a challenging system to handle (since the product degrades eventually, so students must hone their experimental skills to accomplish the synthesis in a reasonably short period of time, before the onset of decomposition). After the initial synthesis is performed, approximately six hours of laboratory work is required. It is desirable that the students have previous knowledge of elementary analytical chemistry, interpretation of molecular spectroscopy (particularly IR), and a standard undergraduate introductory course on inorganic chemistry. The minimum necessary experimental skills involve familiarity with vacuum filtration and titrations, although an alternative is to make extensive use of Schlenk and cannulation techniques. Preparation of the Complex A solution of 0.297 g (1.00 mmol) of the organic disulfide in 20 mL of tetrahydrofuran is added dropwise to a magnetically stirred slurry of 0.341 g (2.00 mmol) of cupric chloride dihydrate in 20 mL of THF, cooled with a slush of ice–salt. After the addition is completed, the solid product is filtered from the dark red solution and washed with two 5-mL portions of diethyl ether. The solid product is pumped dry to yield 0.50 g of crude product. Elementary analyses indicate a purity of better than 98% for this material. Small amounts (0.10 g) may be recrystallized from 10 mL of fresh THF by
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addition of 2.5 mL of diethyl ether. If care is taken to layer the ether above the THF, shiny small crystals can be observed after two hours. This crystalline material turns amorphous when the mother liquor is removed. Amorphous dark red solid, yield 78%, mp 132 °C dec. Anal. Calcd for C5H10 NS2CuCl2: C, 21.2; H, 3.6; Cu, 22.5; Cl, 25.1. Found: C, 21.0; H, 3.5; Cu, 22.4, Cl, 24.8. IR (KBr) ∼νmax (cm᎑1): 2999 (w), 2934 (m), 2875 (m), 1604 (vs), 1460 (s), 1400 (w), 1364 (w), 1289 (m), 1193 (m), 1150 (w), 1100 (w), 1086 (w), 1071 (w), 1000m, 788 (w). IR (Nujol): 400 (m). Λ M (THF, 0.01 mol/L): 13 mho-L/mol. µEFF: 0.0 µB. This method has been used to obtain the full series of compounds X2CuS2CNR2 (X = Cl and Br; R = Me, Et, and i-Pr). The Br2CuS2CN(i-Pr)2 derivative has been characterized by single-crystal X-ray diffraction. Hazards Diethyl ether and tetrahydrofuran are highly flammable liquids and may contain peroxides. When exposed to vigorous heating they may decompose explosively. Use under a wellventilated hood and never evaporate to dryness. Ingestion of tetraethylthiuram disulfide (Antabuse) may cause nausea and vomiting, especially prior to consumption of an alcoholic beverage. The properties of the product have not been fully investigated, but it may be expected to be a strong oxidizing agent. Never evacuate a large closed vessel such as a desiccator. When drying the product use a small round-bottomed flask connected to a vacuum system through a stopcock. The Structure of the Product Metal and halide analyses, as shown under the protocol for the product allow the determination of the atomic ratios of these constituents. The ratio found is 1:2 Cu:Cl. The defect of percentages hints at an overall composition CuCl2L1/2. This conclusion is corroborated by microanalyses of the compound, which give the correct percentages of S, C, and H for the formula proposed. Speculation on this piece of data may lead to the possible dimeric formulation Cu2Cl4L, where two copper(II) centers are bridged by the sulfur ligand. At this point, the laboratory instructor may wish to point out that there is no previous example of a thiuram disulfide complexing a transition metal (2) and that the overwhelming majority of reactions similar to the present one lead to scission of the S–S bond in the disulfide with concomitant formation of a dithiocarbamate ligand. Room-temperature magnetic measurements give the first hint of trouble. The diamagnetic nature of the sample rules out formulations involving Cu(II). One possibility is the presence of univalent copper. It is conceivable that curious students
Journal of Chemical Education • Vol. 79 No. 10 October 2002 • JChemEd.chem.wisc.edu
In the Laboratory
might be aware of the reducing properties of tetraalkylthiuram monosulfides as reported earlier (1) and thus by analogy might come up with an oxidation process similar to R2N S
S
S R2N
S
S 2+
S
+ 2 e− NR2
R2N
involving the disulfide. The solution conductivity measurements put this hypothesis in the wrong, since it is not possible to reconcile the proposed formulation with the nonelectrolyte character of solutions of the unknown in THF. The impasse is satisfactorily settled by analysis of the IR spectrum of the sample. Tetraethylthiuram disulfide is characterized by absorptions at 1500, 971, and 823 cm᎑1, assigned respectively to R2C–N, C=S, and C–S stretches (3), and the divalent copper dithiocarbamate shows absorptions at similar wavenumber values (Table 1). In the product, the first absorption is found at 1604, and the absence of bands between 800 and 1000 cm᎑1 is more reminiscent of the spectrum of a chelated dialkyldithiocarbamate than of the parent disulfide. Thus the nature of the product is a consequence of the reductive cleavage of the original disulfide to dithiocarbamate and is not due to oxidative cyclic cation formation, as was the case for the monosulfide (1). A vast amount of literature is available in support of the oxidative properties of thiuram disulfides (2). The course of the reaction must be represented by 2CuCl2 + R2NC(S)S–SC(S)NR2 → 2Cl2CuIIIS2CNR2 The unusual trivalent state for copper would explain the abnormally high R2N–C stretching frequency found. Metal dithiocarbamates normally show this feature in the range 1490–1530 cm᎑1. A high oxidation state on the metal causes a drain of electron density from the ligand, which tends to enhance the contribution of the polar canonical form R 2 N + =C(S ᎑)2 to the total resonance hybrid. In conclusion, the equation above explains all of the features present in the characterization of the product. Table 1 presents a comparison of the properties of the product with those of the complex bis-(diethyldithiocarbamato)copper(II). Concluding Remarks The discussion above raises the issue of unusual Cu(III). The instructor may wish to point out that most copper(III) complexes reported so far are diamagnetic and consistent with a low-spin d8 electronic configuration for the metal (4 ). A lively discussion could ensue, centered on factors affecting the existence of compounds in “usual” oxidation states versus their “unusual” counterparts. Zero-valent metal carbonyls could be contrasted to high-valent metal 1,1-dithiolates such as the one presented here. A further question that the students should formulate and attempt to answer is: what additional experiments, whether chemical or physical, support or disprove the structure proposed? The interpretation of photoelectron spectra has proved of little value, since Cu(I), Cu(II), and Cu(III) states display relatively similar values of binding energies (5). In addition, cyclic voltammetry shows a reversible one-electron oxidation wave
Table 1. Comparison of Properties of Cu(II) and Cu(III) Diethyldithiocarbamates ∼ ᎑1 ∼ ᎑1 ∼ ᎑1 / ν mp/°C µEFF /µB Compound C–N cm νC=S /cm νC–S /cm Cl2Cudtc
1604
1000
400
dec a
0–0.2
Cu(dtc) 2
1508
1998
355
193
2.73
material begins to darken at 70 °C and is fully decomposed by 108 °C. aRed
assignable to a CuIII/CuIV process. The reduction side is characterized by a quasi-reversible one-electron wave attributable to the couple CuIII/CuII. This piece of data supports but does not prove the issue of the “unusual” oxidation state. Visible spectra for solutions of the product in THF are featureless between 800 and 250 mm (12,500 to 40,000 cm᎑1), which is consistent with a square planar d8 species, similar to that observed for isoelectronic Ni(II) systems whose spectra show very weak bands in this region (ε = 60 [4 ]). Student Activities Students prepare and isolate the complex, and analyze the substance (6 ) for copper (iodometrically) and chloride (Volhard) content. These simple and reliable analytical methods are not subjected to interference in the matrix considered, provided the analyte has been destroyed with concentrated nitric acid. The students also measure the magnetic susceptibility (Gouy method), the IR spectrum (KBr disk), and the solution conductivity (THF solution). Microanalytical data for the compound are provided by the instructor. Over the past three years, 3 of a total of 20 students have found values of µEFF as high as 0.5 µB. This seems to be caused by contamination with copper(II) diethyldithiocarbamate, which is the degradation product of the Cu(III) compound. All other students found values between 0.0 and 0.1 µB. Acknowledgment Support was available from FONDECYT 1990494. W
Supplemental Material
Instructions for students and notes for the instructor are available in this issue of JCE Online. Literature Cited 1. Victoriano, L. I.; Carbacho, H.; Parraguez, L. J. Chem. Educ. 1998, 75, 1295. 2. Victoriano, L. I. Coord. Chem. Rev. 2000, 196, 383. 3. Willemse, J.; Steggerda, J. J. Chem. Commun. 1969, 1123. Beurskens, P. T.; Bosman, W. P.; Cras, J. A. J. Cryst. Mol. Struct. 1972, 2, 183. Bond, A. M.; Colton, R.; D’Agostino, A; Harvey, J.; Traeger J. C. Inorg. Chem. 1993, 32, 3952. 4. Cotton, F. A.; Wilkinson G. Advanced Inorganic Chemistry; Wiley: New York, 1999; p 872. 5. Victoriano, L. I.; Cortés, H. B.; Yuseff, M. I. S.; Fuentealba, L. C. J. Coord. Chem. 1996, 39, 241. 6. Vogel A. I. A Textbook of Quantitative Inorganic Analysis; Longman, Green: London, 1951.
JChemEd.chem.wisc.edu • Vol. 79 No. 10 October 2002 • Journal of Chemical Education
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