er(I I
ation of Thioureas in ~ ~ e ~ o ~ ~ t r ~ ~ consim, Madison, Wis. 53706
The oxidation of a series of thioureas by copper(1l) in acetonitrile has been investigated. I n most cases, a 1:1 stoichiometry is observed. The reaction proceeds with formation of a red complex which was found by electron spin resonance to be a paramagnetic copper (~~)-th~owrea complex which shows no evidence for co~per-n~trogen in-plane bonding.
THIOUREAS ARE OXIDIZED in aqueous solution by several common oxidants, including dichromate, cerium(IV), permanganate, hydrogen peroxide, and bromine. A variety of products are formed, depending on the oxidant and the reaction conditions ( I , 2). Except for a few isolated cases, the effect of solvent on the course of these oxidations has not been investigated, although solvent properties are recognized to be important in determining the direction and extent of organic oxidation processes. Thus, when bromine is added to thiourea in carbon tetrachloride, bromine addition products are produced (3), but in glacial acetic acid either disulfides or benzothiazoles are formed depending on the substituent groups present on the thiourea nitrogens (4). By considering solvent effects, it is possible to obtain data about mechanisms of homogeneous electron transfer processes which are useful in predicting the products of functional group oxidations, in aiding selection of solvent-oxidant combinations which yield desired reaction products in syntheses, and in providing reactions suitable for analytical applications. Copper(1I) perchlorate in acetonitrile has been shown to be an effective oxidant for a variety of compounds, including thiourea (5,6). This paper reports the oxidation of a series of thioureas by copper(I1) in acetonitrile, and considers the nature of the intermediates formed in the oxidation process. EXPERIMENTAL
Acetonitrile was purified by the method described previously (5). The water content, determined by Karl Fischer titration, was in the range of 1 to 5 X 1OW4M. Several experiments were done using anhydrous solutions and conditions. Generally water in moderate concentration had no effect, however, so most of the work could be carried out without special precautions. When needed, anhydrous copper(I1) perchlorate solutions were prepared by dissolving tetrakis(acetonitri1e) copper(I1) perchlorate, prepared from nitrosyl perchlorate and copper metal (3, in anhydrous aceResent address, Department of Chemistry, University of Alabama, University, Ala. 35486. * Present address, Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada.
tonitrile. These solutions were dispensed from automatic burets and protected from the atmosphere by drying tubes containing magnesium perchlorate. Where exclusion of water was not required, solutions were prepared by dissolving hexaquocopper(I1) perchlorate in acetonitrile, filtering the solution to remove a small amount of white precipitate that formed, and storing in a glass stoppered bottle. Thiobenzanilide, thiourea, N,N‘-diphenylthiourea, and tetramethylthiourea were obtained from Eastman Organic Chemicals, Rochester, N. Y . , and the other thioureas from Aldrich Chemical Co., Milwaukee, Wis. Recrystallization of several of the thioureas from various solvents produced little change in either the melting points or in the oxidation equivalent weights, so they were used as received. Potentiometric titrations were performed in a 7 5 4 weighing bottle with a machined Teflon lid containing openings for a platinum foil indicating electrode, a silver-0.01M silver nitrate in acetonitrile reference electrode which made contact with the cell solution through a 0.1M lithium perchlorate in acetonitrile salt bridge via ultrafine glass frits, a buret tip, and a nitrogen inlet. Magnetic stirring was used. For titrations at -40 “C the cell was immersed in a 25x ethanol-75 acetonitrile slurry cooled with liquid nitrogen. ESR spectra were recorded with a Varian Model V-4502-13 spectrometer equipped with 100 kc modulation. Measurements of the g factors were made using 2,2-diphenyl-lpicrylhydrazyl (DPPH), obtained from Bastman, as reference. The acid-base titrations were done in a cell similar to the one described above but with a Beckman 40471 glass electrode in place of the platinum indicating electrode. A Leeds and Northrup Model 7401 pH meter was used for both potential and pX measurements. Visible spectra of the copper(I1)-thiourea intermediates were recorded at -40 “C with a Cary Model 14 recording spectrophotometer equipped with cell holders designed to allow circulation of an acetonitrile-liquid nitrogen slush coolant around the cells. The cell compartment was flushed with dry nitrogen during runs to prevent moisture condensation on the cell surfaces. RESULTS
Potentiometric Titrations. Most of the substituted thioureas studied showed one oxidation equivalent per mole when titrated with copper(I1) in acetonitrile. The reaction products were considered to be the disulfides on the basis of the oxidation stoichiometry. I\JR!2
2 Cu(II>
+2
c =s + 2 Cu(1) +
/
RaN (1) F. Degering, Ed., “An Outline of Organic Nitrogen Compounds,” University Lithoprinters, Ypsilanti, Mich., 1945, pp 459-60. ( 2 ) I. T. Millar and H. D. Springall, “The Organic Chemistry of Nitrogen,” Clarendon Press, Oxford, 1966, pp 430-3. (3) R. W. Sahasrabudhey and R. Singh, J. Indian Chem. Soc., dQ, 499 (1953); 31,628 (1954). (4) T. M. Klatsmani-Gabor and L. Erdey, Acta. Chim. Acad. Sci. Himg., 40, 99 (1964). (5) B. Kratochvil, D. A. Zatko, and R. Markuszewski, ANAL. CHEM., 38, 770 (1966). (6) B. Kratochvil and D. A. Zatko, ibid., 40, 422 (1968). (.
ANALYTICAL CHEMISTRY
e-s-s-c
/ R2N
/
\\
+NR2
For thiourea, the oxidation product was isolated from the titration solution as the hydrochloride salt. Differential thermal analysis showed an exotherm at 76-77 ‘e and an endotherm associated with melting at 172-173 “C. A sample of diformamidine disulfide. 2HC1 prepared by the method of Preisler and Berger (7) gave a corresponding exotherm and (7) P. W. Preisler and L. Berger, J. Amer. Chem. SOC.,69, 322 (1947).
I
I
1.00 Mole Ratio Cu(II)/thiourea
0.50
Figure 1. Potentiometric titration of representative thioureas with 0.1M hydrated copper(I1) in acetonitrile A . N,N'-di-n-butylthiourea; B. N,N'-diphenylthiourea; C. N,-
N'-dimethyl-N'-m-tolylthiourea. All potentials refer to silver0.01M silver nitrate in acetonitrile as reference
endotherm at 75-77 and 173-176 "C. The melting point of diformamidine disulfide. 2HC1 is reported to be 173-175 "C (8, 9). Also, for thiourea and the N,N'-tetramethyl derivative, the expected imine salt bands of the oxidation products were observed in infrared spectra of acetonitrile solutions of thiourea-copper(I1) reaction mixtures and in fluorolube mulls of 1 :1 thiourea-copper(1I) mixtures which had been evaporated to dryness. Solution spectra were run in cells with silver chloride windows because alkali halides are attacked rapidly by copper(1) in acetonitrile. (Even silver chloride cells are fogged to some extent by the reaction mixtures.) A list of the derivatives titrated and the observed equivalence point ratios is given in Table I, and the shapes of some representative titration plots are shown in Figure 1. Considerable sulfur precipitated during the oxidation of several of the aryl-substituted compounds, but the stoichiometry remained near unity. At room temperature N,N'-di-teutbutylthiourea is oxidized further to a mixtureof reaction products, but when the titration is carried out at -40 "C a single one-electron oxidation is observed. The presence of small amounts of water had no observable effect on the reactions other than to decrease the size of the potential break in an amount comparable to that found for the parent thiourea (5). The addition of perchloric acid to solutions of thioureas prior to titration with copper(I1) gave titration curves in which the end points were observed much earlier than 1 :I, and which became more drawn out with increasing acid concentrations. Also, potentials reached equilibrium much more slowly when acid was present. The pKa of thiourea in acetonitrile was determined by titration with perchloric acid in order to evaluate the extent of thiourea protonation in the acid system. A glass electrode calibrated with a buffer mixture of picric acid-tetraethylammonium picrate was used; this mixture has been shown to be a useful reference buffer in acetonitrile (IO, II). A slope of 0.056 V was obtained for a plot of the glass electrode (8) L. C. Leitch, B. E. Baker, and L. Brickman, Can. J. Research, 23B, 139 (1945). (9) M. Busch and K. Schulz, J. Prakt. Chern., 150, 173 (1938). (10) J. F. Coetzee and G. R. Padmanabhan, J. Phys. Chem., 66, 1708 (1962). (11) I. M. Kolthoff and M. K. Chantooni, Jr., J. Amer. Chem. SOC., 87, 4428 (1965).
Table I. Oxidation of Thioureas with Copper(I1) Perchlorate in Acetonitrile Reaction stoichiometry [Cu(II)/ Solution Behavior of Thioureas thiourea] color potentials& Thiourea 1.000 None Stable N-methylthiourea 1.002 None Stable N,N-dimethylthiourea 0.962 None Stable N,N'-diethylthiourea 0.989 None Stable N,N'-diisopropylthiourea 0.995 None Stable N,N-di-n-butylthiourea 0,929 None Stable N,N'-di-n-octylthiourea 0.974 None Stable Tetramethylthiourea 0.977 Redb Stable N,N-diphenylthiourea 0.933 Red-broww Unstabled N,N-di-o-tolylthiourea 1.022 Red-brownc Unstable N,N-di-tert-butylthiourea . . Red-brownc Unstabled N,N-dimethyl-"-0tolylthiourea 0.853 Red-violetb Unstable N;N-dimethyl-N'-mtolylthiourea 0.928 Red-violet6 Unstabled N,N-dimethyl-N'-ptolylthiourea 0.872 Violetb Unstabled a Potentials are listed as stable if system reached equilibrium within 30 seconds after each addition of copper(I1). Equilibration time for systems listed as unstable were on the order of 4-5 minutes. 6 Color persisted through approximately the first half of titration, Color observed only immediately after addition of titrant increments. d Sulfur produced in reaction. ~
potential us. log of the ratio of protonated to nonprotonated thiourea, so the system is nearly reversible. The pKa of thiourea determined in this way was 23.15, or, using a value of 6.3 X 10-33 for the autoprotolysis constant of acetonitrile ( I 2 ) , pK, = 9.0 for protonated thiourea. Nature of the Red Species. During titrations of the thioureas, a momentary red color appeared after each addition of copper(I1) in the region before the equivalence point. The stabilities of these species were assessed qualitatively by carrying out titrations of thioureas with copper(I1) at temperatures just above the freezing point of acetonitrile, -45.7 "C. The intensity of the color increased with copper(I1) addition up to a copper(I1)-thiourea ratio of approximately one to two, then decreased. Some of the red species were found to be stable for hours if kept below -40 "C; at -196 "C (liquid nitrogen b.p.) several appeared stable for days. The order of stability was tetramethyl>N,N-dimethyl-N'o-tolyl>m- and p-tolyl derivatives >N,N'-di-o-tolyl >N,N'diphenyl>N,N-di-tert-butyl>mono- and di-substituted alkyl derivatives. ESR measurements were made on a number of the thiourea-copper mixtures. Colored reaction products of the alkyl derivatives were too unstable to be investigated by the techniques employed in this work; they could be prepared only by reaction at the interface of cold solutions of copper(I1) perchlorate and the corresponding thiourea, and in these cases the large excess of unreacted copper(I1) present swamped out the ESR spectrum of the colored species. Because solutions in pure acetonitrile tended to become polycrystalline upon freezing, producing broadening of the spectra and loss of hyperfine splitting, ethanol-acetonitrile solvent mixtures were used for most of the ESR work. No difference was observed between spectra recorded in predominately ethanol or in pure (12) I. M. Kolthoff and M. K. Chantooni, Jr., J. Phys. Chem., 72,
2270 (1968). VOL. 40, NO. 14, DECEMBER 1968
e
21 21
I AIW k-----4
100 gauss
V
l-----i
- il Y
100 gauss
100 gauss
Figure 2. First derivative ESR spectra of copper (11) derivatives in ethanol at 77 OK
Figure 3. First derivative ESR spectra of copper (11) derivatives in ethanol at 77 “K
A . N,N’-di-tert-butylthiourea; B. jV,N’-di-o-tolyl(hio-
A . N,N-dimethyl-N’-o-tolylthiourea;B. diethyldithio-
Urea
~ 11,b Magnetic l ~ Parameters for cu(II) Complexes of Substituted Thioureas in Ethanol at 77 “K
~
Substituted thiourea
A /ia
gli a
Tetramethylb
2.117 2.117 sym-di-o-tolyl2.118 2.117 sym-diphenyl2.116 2.115 sym-di-teri-butyl2.118 2.117 N,N‘-dimethyl-N’-o-tolyl2.128 2.127 N,N-dimethyl-N’-m-tolyl- 2.123 2.122 N,N-dimethyl-N’-p-tolyl 2.126 2.125 Copper(I1)-bis(diethy12. 090c
dithiocarbamate)
2. 09Sd 2.1218 2.108,f
A1
(gauss)
gl
(gauss)
128 138 140 150 138 150 142 153 122 132 125 133 122 131 162 174 162 177 144 157 152 162
2.031
30
2.033
32
2,032
33
2.025
35
2.032
24
2.030
25
2.031
25
2.028
47
2.035
43
2.040
27
2.023
24
Values for 6 T u and ~ C U respectively. , In acetonitrile. In 60 toluene-40 Z chloroform and in ethanol. d Reference 13, 6 0 Z t o ~ u e n e -chloroform. ~~~ Reference 13, 40 pyridine-60 % chloroform. Reference 14, single crystal.
earbarnate
acetonitrile glasses, though the latter were much more difficult to obtain. First derivative spectra of representative substituted copper(I1)-thiourea complexes are shown in Figures 2 and 3. A spectrum of the complex formed between copper (11) and diethyldithiocarbamate is included in Figure 3 for comparison, since the magnetic and spectral properties of the copper(I1)-thiourea derivatives are very similar to each other and to copper(I1)-bis(diethy1dithiocarbamate) (see Tables I1 and 111). Molar absorptivities in the visible spectra could not be determined because of partial decomposition of the intermediates during their preparation. DISCUSSION
It has been shown by proton magnetic resonance studies that thioureas are protonated in strongly acidic solvents on the sulfur rather than one of the nitrogen atoms. If a sufficiently acidic solvent, such as concentrated sulfuric acid, is used, one of the nitrogen atoms appears to protonate also (15). Titrations of thiourea with perchloric acid in glacial acetic acid indicate a slight inflection at two equivalents of acid per equivalent of thiourea, but the size of the break is not adequate for calculation of a second acid dissociation constant. A Lewis acid such as copper(I1) would be expected also to coordinate preferentially to the more basic sulfur site, as indicated by the greater resistance of thiourea to copper(I1)
a
x
x
f
e
ANALYTICAL CHEMISTRY
(133 H. R. Gersmann and J. D. Swalen, J. Cliem. Phys., 36, 3211 (1962). (14) T. R. Reddy and R. Srinivasan, ibid., 43,1404 (1965). (15) T, Birchall and R. J. Gillespie, Can. 1. Chem., 41, 2642 (1963).
oxidation when the sulfur atom is protonated by the addition of perchloric acid. The similarity of the magnetic properties of the red copper(I1)-thiourea species and copper(I1)-bis-(diethyldithiocarbamate) suggest the structures may be related. In both cases well defined isotopic splitting by the copper-63 and copper-65 isotopes is observed in the ESR spectra; this is not generally seen if copper(I1)-nitrogen bonding is present. Also, no nitrogen hyperfine splitting was detected in any of the copper-thiourea spectra. This fact does not by itself indicate that copper-nitrogen bonding is absent. However, the lack of line broadening, as shown by the appearance of well defined isotopic splitting on the weak downfield peak of 811 suggests that nitrogen interaction with the copper equatorial sites is absent. Axial copper(I1)-nitrogen interaction could of course be occurring, since this weaker interaction would not be expected to affect the shape of the 811 absorption lines and so could not be detected from the ESR spectra. In fact, the magnetic parameters of the copper(I1)-thiourea complexes most closely resemble those of copper(I1)-diethyldithiocarbamate in pyridine-chloroform solvent, where pyridine nitrogen axial interaction with the copper seems quite probable. Further, from the visible spectra, the same electronic transitions appear to be occurring in each. On the basis of these observations we conclude that the red species produced by the action of copper(I1) on thiourea and its derivatives are probably complexes between copper and thiourea involving primarily copper(I1)-sulfur bonding. This
Table 111. Visible Spectra Maxima for Selected Copper(I1)Thiourea Complexes Thioureas Maxima (cm-l) Tetramethylthiourea 19,600, 25,000 N,N-dimethyl-N‘-o-tolylthiourea 19,200, 23,800 N,N-dimethyl-N‘m-tolylthiourea 19,100, 23,4000 N,N-dimethyl-N’-p-tolylthiourea 19,800,24,200a N,N‘-di-o-tolylthiouea 21 ,400, 24,500 Copper(I1)-bis(diethy1dithio19,200, 23, 100b carbamate) 20,000, 23,8000 a Very weak. 6 Reference 13, 60% chloroform-40% toluene solvent. c Reference 13, 6 Oz pyridine-40 chloroform solvent. appears to be the first report of copper(I1)-thiourea complex formation. The complexes are very likely intermediates in the overall oxidation process, although this has not been proven. ACKNOWLEDGMENT
The authors thank Howard Yeager for the IR data, and the G . Frederick Smith Chemical Co. for supplying the copper (11) perchlorate. RECEIVED for review June 21, 1968. Accepted September 20, 1968. Work supported in part by the Research Committee of the Graduate School from funds supplied by the Wisconsin Alumni Research Foundation.
I H~d~oxyc~r~oxyl~c romium(lll) Inhibition of the So T. W. Gilbert Chemistry Department, University of Cincinnati, Cincinnati, Ohio
Leonard Neirman and Paul Klotz Brookhauen National Laboratory, Upton, N . Y.
The solvent extraction of indium(ll1) by thenoyltrifluoroacetone is inhibited by chromium(lll) if certain hydroxycarboxylic acids are also present. t h e experimental results are explicable only on the basis of the formation of a mixed complex containing indium, chromium, and the hydroxyacid. The inhibitory effect was observed only if the acids had the functional group arrangement of malic acid. Such mixed complex formation exists for many different metals and can cause serious interference with separation processes. Spectrophotometric measurements on the chromium, indium, citrate system indicate a complex of stoichiometry 1:f:2. Structural arrangements for the mixed complexes are postulated and discussed.
SEVERAL EXAMPLES of complexes of hydroxycarboxylic acids containing two different metal ions have been reported in the recent literature (1-6). The formation of a mixed complex of cerium(III), chromium(III), and citrate has been shown to repress strongly the solvent extraction of cerium (3). If such behavior is general, it would be of fundamental importance to solvent extraction and ion exchange separations, inasmuch as citric, tartaric, and other hydroxycarboxylic acids are widely used as auxiliary complexing and buffering agents. For this reason, a search was made for other exam-
ples of this behavior. It was found that the extraction of indium(II1) from citrate media of pH 4 was significantly decreased in the presence of iron(”, aluminum(lII), and chromium(II1). Similar experiments using thallium(I), cadmium(II), and lead(I1) showed no appreciable effect. Chromium(II1) gave the most pronounced effect, and the chromium (111)-indium(II1) interaction in the presence of various hydroxy acids was accordingly chosen for further study. EXPERIMENTAL
Apparatus. Continuous spectra were recorded with a Cary Model 14 spectrophotometer, and absorption measurements at single wavelengths with a Beckman Model DU (1) G. L. Booman and W. B. Holbrook, ANAL.CHEM.,31, 10 (1959). (2) H. Flaschka, J. Butcher, and R. Speights, Tuluntu,8,400 (1960). (3) W. W. Schulz, J. E. Mendel, and J. F. Phillips, Jr., J. Inorg. hTucl. Chem., 28, 2399 (1966). (4) A. K. Babko, Pure and Appl. Chem., 10, (4), 557 (1965). (5) T. D. Smith, J. Chem. Soc., 1965,2145. ( 6 ) D. J. Crouse and D. E. Horner in “Solvent Extraction Chemistry of Metals,” H. A. C . McKay, Ed., Macmillan, London, 1965, pp 305-15. VO1. 40, NO. 14, DECEMBER 1968
e
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