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California Association of Chemistry Teachers
George B. Kauffman and Lloyd T. Takahashi Fresno State College
Resolving Optically-Active Coordination Compounds
Fres lo, California
An experiment for the inorganic laboratory
Although a laboratory study of polarimetry is now included in most instrumental analysis and physical chemistry courses, the optically-active compounds investigated are invariably naturally occurring organic substances-usually sugars. Unfortunately, time requirements dictate that resolution, the process of separating an optically-active material from a racemic mixture, be by-passed. A few students may be exposed to this important technique in the organic laboratory. Despite the historical and current importance of the resolution of inorganic complexes, the topic is almost completely neglected in inorganic laboratory manuals. The authors hope that this experiment, simplified from the work of Dwyer et al. (I, ?2), mill help complement the generally excellent coverace of the subject in inorganic texts (3-6). ~asteur'sracemic modification method, which is the commonest procedure to the resolution of ionic complexes, uses an optically-active resolving agent to convert the racemic mixture into diastereoisomers which are then separated by fractional crystallization. Pasteur's technique was employed by Werner (7) to demonstrate conclusively the octahedral configuration of cohalt(III) which is the basis of his coordination theory (8-10). such resolution is still widely used todav in structure-proof (3).
or the trisoxalatocohaltate(III) ion (M = CoJ+, AA = CZ042-)is almost ideal for student use, since, in each case, one antipode forms an insoluble precipitate with the resolving agent while the other diastereoisomer is quite soluble. The need for tedious fractional crystallization is thus eliminated, and separation is accomplished with a single precipitation. The tris(1,lO-phenanthroline)nickel(II) ion is prepared in racemic form: and is resolved by precipitating the D-antipode with potassium D-antimony1tartrate:
-
+
++
2 j n ~ [ ~ i ( o - ~ h e n ) ; ] ~2l!I< ~ ) o-[(SbO)C4H4OeI .l/?H1OI 6H.O o-INi(o-phen)~l-~-[(SbO)C~H~O~l~~ 7H,O 1 2KCI ~r[Ni(o-phen)aICl~.
+
The bantipode is precipitated as the perchlorate 3hydrate. The diastereoisomer is dissolved in NaOH, . 2nd the D-antipode is separated as the sparingly perchlorate 3-hydrate. Stable complex ions, once resolved, may themselves be used to resolve other complex ions of opposite charge. The btris(1,lO-phenanthroline)nickel(II) ~erchlorate resolved above is used to precipitate the D-antipode of the racemic trisoxalatocobaltate(II1) ion:
The Experiment
The resolution of trisbidentate complexes into their optical ant,ipodes
has been extensively used as a diagnostic tool in coordination chemistry. The resolution of the tris (1,lOphenanthroline)nickel(II) ion (M = Ni2+, AA =
Treatment of the diastereoisomer with XI and H2O2 precipitates L-tris (1,lO-phenanthroline)nickel(II) triiodide. The optical antipodes of potassium trisoxalatocobaltak(II1) are precipitated from the filtrates with ethanol. Since they racemize so rapidly, this resolution challenges the student with a severe test of the technique which he has developed in the relatively easy resolution of the tris(1,lO-phenanthro1ine)nickel(11) ion. The rotatory power of a dissolved substance depends upon the thickness of the layer traversed by the light, the wavelength of the monochromatic light, the concentration of the solution, the nature of the solvent, and the temperature (11, 12). Rotation is usually reported as specific rotation, Volume 39, Number
9, September 1962
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where t = temperature (OC), A = wavelength of light (usually Na-D line or green Hg line, 5461A), a = observed rotation in degrees (+ if clockwise, - if counterclockwise), d = lengt,h of polarimeter tube (dm), and c = concentration of solute (g/ml soln.). Procedure
The water used for washing precipitates should be iced. Resolution of the [Ni(o-phen)3I2+Ion. A solution of DL- [Ni(~-phen)~] CIz is prepared by stirring 3.0 g (0.015 mol) of l,l0-phenanthroline 1-hydrate into a solution of 1.2 g (0.005 mol) of NiC12.6HzO in 40 ml of water. To 4 ml of this deep burgundy solution 1 ml of 1 M NaCIOa (7.02 g NaC1O4.Hz0/50ml soln) isadded dropwise with stirring. The precipitated D,L-perchlorate2-hydrate is collected by suction filtration, washed, and air-dried. A sample may be dissolved in a 50% v/v acetone-water mixture and tested in t,he polarimeter for absence of optical activity. To the remaining burgundy solution which has been cooled to 15" a similarly-cooled solution of 4.5 g (0.0135 moll of KD [(SbO)CnH~Os].'/zHzO (tartar emetic) in 90 ml of water is added slowly with stirring whereupon the tan, crystalline diastereoisomer precipitates. The mixture is cooled rapidly to 5' and filtered; im,mediately the L-antipode is precipitated from the filtrate (A) with NaCIOa as described two paragraphs below. The washed diastereoisomer (ca. 2.7 g) is purified by dissolving in 27 ml of 0.1 M NaOH, filtering, and reprecipitating by dropwise addition of HC2H3O3(ca. 1 ml) until the solution is slightly acidic.. The yield of washed and air-dried diastereoisomer is ca. 2.6 g or 89%. A 0.440% solution in 0.10 M NaOH in a 2-dm tube gives aD2"5 = +7.50°, from which [aID?8J = +915" ( [ ~ ] 0 = ' ~ +950°(1)). The diastereoisomer is dissolved in a minimum volume (ca. 70 ml) of 0.05 M NaOH and filtered if necessary. Four and one-half milliliters of 1.0 M NaClO, is added dropwise, and the pink precipitate is collected, washed, and dissalved in a minimum volume (ca. 80 ml) of warm (5540') 30% v/v acetone-water mixture. The solution is cooled to 25', and the perchlorate is precipitated by dropwise addition of 9 ml of 1.0 M NaC104. The washed D-perchlorate is purified by dissolving i t in a minimum volume (ca. 70 ml) of warm (55-60') 30% v/v acetone-water mixture, cooling the solution to 25', and reprecipitating it by dropwise addition of 3.6 ml of 1.0 M NaClOa. The yield of washed and air-dried d-perchlorate 3-hydrate is 0.786 g or 40.6%. A 0.4084% solution in 50% v/v acetone-water mixture in a 2-dm tube gives a D 2 5 = +12.0Bo, from which [ a ] D z 5 = +1476" ( [ a ] = ~ ~+1463'(1)). ~ Four and one-half milliliters of 1.0 M NaCIOa (auoid excess) is added dropwise to the pink filtrate (A), and the pink crystalline precipitate is collected, washed, and dissolved in a minimum volume (ca. 90 ml) of warm (55-60') 30% v/v acetone-water mixture. The solution is cooled to 25', and the L-antipode is reprecipitated by dropwise addit,ion of 14 ml of 1.0 M NaC104. The yield of washed and air-dried L-perchlorate 3-hydrate is 1.04 g or 53.4%. A 0.3960% solution in 50% v/v 482
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Journol of Chemicol Education
= acetone-water mixture in a 2 4 m tube gives aDZS -11.74', from which [aIDz5= -1482' ([aID15 = -1463' (1)). Resolution, of the [Co(C20&l3- Ion. Since the optical antipodes of K3 [Co(C;Op)~]racemize rapidly in solution, all operations should be carried out as quickly as possible using iced solutions and iced apparatus. Students should work in pairs. One gram (0.00117 mol) of L-tris(1,lO-phenanthroline)nickel(II) perchlorate 3-hydrate (prepared above) is dissolved in 16mlof a 50% v / v acetone-water mixture. The pink solution is cooled in an ice bath and added to an iced solution of 0.740 g (0.00150 mol) of freshly prepared potassium DL-trisoxalatocobaltate(II1)3-hydrate (13, 14) and 1 g of KC2H302in 12 ml. of water. The green diastereoisorner precipitate is collected and treated as described in the following paragraph. The L-antipode is zmmediateiy precipitated from the filtrate by slow addition with stirring of 100 ml of iced ethanol. The green precipitate is collected, washed with iced absolute ethanol or methanol, air-dried, and dried in z~aeuoover HrS04in a light-protected desiccator. The yield is cs. 0.26 g or 70%. A 0.01745% aqueous solution in a 2-dm tube gives a D S = -l.285°, from which [alp5 = -3680' ([a]l10 = -4050' (%)). The high optical density of even dilute solutions makes observation of the field diicult. The half-shade angle control on the Rudolph high-precision polarimeter should be opened to the maximum extent. The green diastereoisomer from the preceding paragraph is washed with 20 ml of iced ethanol, air-dried, suspended in 16 ml of ice water, and 2 ml of 3y0 HzOZ, 4 drops of HC2H30z,and 1 g of XI are added. The olive mixture is stirred in an ice bath for 2 min and then filtered, and then the D-antipode is precipitated by addition, with stirring, of 100 ml of iced ethanol to the filtrate. The product is collected, washed, and dried in the same manner as the L-antipode. The yield is ca. 0.30 g or 81%. A 0.01608% aqueous solution in a 2dm tube gives aD5= +1.232", from which [a],5 = 4-3830' ([aIDz0= +4050° (2)). Recovery of 1,lO-Phenanthrolim. The insoluble L[Ni(o-phen)l](I& may be converted to the corresponding iodide by treating a cold ethanolic suspension with SOz or NazSO,. The iodide may then be used exactly as the perchlorate in the resolution of more K,iC~(czO&l. Alternatively, lJ0-phenanthroline can be recovered from the nickel complex by boiling 1 g of either the perchlorate or the triiodide in 250 ml of 3 M KCN for I min, cooling the light orange solution, decanting to remove any undissolved material, and shaking the solution in a separatory funnel for 2 min with 250 ml of benzene. The aqueous phase is extracted again with 125 ml of benzene, the benzene extracts are combined and evaporated to dryness, and the crude l,l0-phenanthroline recrystallized (with seeding) in 60-80% yield (m.p., 116.5-117.5°) from a minimum amount (ca. 25 ml) of boiling water by cooling in an ice bath. Additional Projects. Students may analyze their products for nickel by heating a sample with H,SOr until all fuming has ceased, dissolving the residue in H2SOI, diluting, making the sdution ammoniacal, and electrolyzing. Cobalt may be similarly determined, but heating is unnecessary.
Racemisation, the decrease in optical activity with time, may be studied and the results compared with literature values (16-17). Student results appear in the figure, which clearly shows that the two complexes racemize by entirely different mechanisms. Students may also determine the order of these reactions; and if they study the racemizations a t several temperatures, they may calculate heats of activation. Rotatory dispersion, the change in optical rotation with change in
valuable advice, our thanks go to Dr. William M. Miller and Dr. Dale C. Burtner of Fresno State College and Dr. Stanley Kirschner of Wayne State University. Literature Cited
( I ) DWYER, F. P., AND GYARFAS, E . C., J . P m . Roy. Soc. N.S. Wales, 83, 232 (1949). A. M., J . Phys. Chem., 60, (2) DWYER,F. P., AND SARGESON, 12:Xl .-. - I195R> - ..- - ,, (3) WILKINS,R. G., A N D WILLIAM,M.J . G., "The Isomerism of Complex Compounds" in "'Modern Coordination Chemistry," LEWIS,J., A K D WILKINR,R. G., edits., Interscience Publishers, Inc., New York, 1960, chap. 3. (41 o . and PEARSOS.R. G.. "Mechanisms of Inor, , B ~ s o ~ F.. gsnic~eactions,"John l&y a"d Sons, Inc., New York, 1958, chap. 5. (5) Bamr.o, F., "Stereochemistry of Hexacovslent Atoms" in "The Chemistry of the Coordination Compounds," BAILAR,J . C., edit., Reinhold Publishing Carp., New York, 1956, chap. 8. (6) ~ I A R T E LA. L ,E., A N D CALYIS,M., "Chemistry of the Metal Chelate Compounds," Prentice-Hall, Inc., New York, 1952. (7) WERNER, A., Rw. deut. chen~.GPS.,47, 3087 (1914). G. B., J. CHEM.EDUC.,36, 521 (1959). (8) KAUFFMAN, U. B., Chymia, 6, 180 (1960). (9) KAUFFMAN, G. B., A N D PINXELL,R. P., Inorganic Syntheses, (10) KAUFFMAN, 6, 176 (1960). H. H., MERRITT,L. L., A N D DEAN,J. A,, "Instru(1 1) WILLARD, mental Methods of Analwis." 3rd ed.. D. Van Nostrand Co., Inc., Princeton, N. j., 1'958, chap: 12. (12) HELLER,W., A N D FITTS,D. D., "Polarimetry" in "Physical Methods of Organic Chemistry," WEISSBERGER,A,, edit., 3rd ed., Interscience Publishers, Inc., New York, 1960, Vol. I, Part 3, chap. 33. (13) BAKAR,J. C.. A N D JONES,E. M., Inorganic Syntheses, 1, 35 (1939). (14) PALMER,W G., "Experimental Inorganic Chemistry," Cambridge University Press, Cambridge, Eng., 1954, p. \
550 -~~
Student result.: 0,D-and L-[Ni(o-phen)rl(ClOd)z; 0. K~L-ICO(C~O~)J.
wavelength, may be investigated by using photographic filters and a white light source or the spectrophotometric polarimeter described by Kirsehner et al. (18). Both the nickel and cobalt antipodes exhibit anomalous rotatory dispersion (1, 17, 19). The decomposition of K3[C~(C204)3] by heat or light may also he studied (20-23). Acknowledgment
We gratefully acknowledge the assistance of Prof. Francis P. Dwyer of the Australian National University for helpful discussions and the G. Frederick Smith Chemical Company for a generous supply of 1,10phenanthroline. We are also indebted to Mr. James Hma-san Tsai of Fresno State College for experimental assistance with rarcmieation studies and analyses. For
(15) DAVIES,N. R., AND DWYER,F. P., Trans. Faraday Soe., 48, 244 (1952); 49, 180 (1953). (16) KRISHNAMURTY, K. V., A K D HARRIS,G. M., Chem. Rews., 61, 213 (1961). (17) BEESE,N. W. D., AND JOHNSON, C. H., Trans. Faraday Sm., 31, 1632 (1935); JOHNSON, C. H., AND MEAD,A,, Tmns. Faraday Sac., 29, 625 (1933); 31, 1621 (1938); BUSHRA, E., AND JOHPSOS,C. H., J . Chem. Soe., 1939, 1937
(18) ALRINAK,M. J., BHATNAOAR, D. C., KIRSCHNER, S., A K D S o m ~ s s a A. , J., "The Effects of Optically' Inactive I0118 on the Rotatory Dispersion of Asymmetric Complex Ions" in "Advances in the Chemistry of the Coordination Compounds," KIRSCHNER, S., editor Macmillan Co., New York, 1961, p. 151; Can. J . Chem., 39, 2360 (1961). ( 1 9 ) KUHN,W., AND BEIN, J.; Z . phgsik. Chem., B24,335 (1934); Z. anorg. Chem., 216, 331 (1934); KUHN, FV., AND R~METSCH, R., Heh. chin^. Acta, 27, 1080 (1944). (20) JAEGER, F. M., AND BERGER,G.. Rec. lmu. chim., 40, 163 ( 1 2 ) ; Proc. Acad. Sci. .4msterdam. 23,84 (1920). (21) KRANIG,J., Ann. chim., 11, 44 (1929). (22) M u ~ ~ u m s c 1. u ,G., Bzd. soc. jtiinfe Clu,i, 8, 193 (1935). T.B., AND URI, N.,Pmc. Roy. Sot. (London), (23) COPESTAKE, A228, 252 (1956).
NBS Issues Supplement to Kinetics Tables The tables contained in the new Monograph 34 supplement NBS circular 610, published in 1951 and Supplement 1, published in 1956. These two publications comprised a critically evaluated compilation of the then wsilable numerical data on rates and rate constants of homogeneous chemical reactions, emphasizing experimental facts. Monograph 34 contains information pertaining t o substitution, exchange, and elimination reaction types. The "Tables of Chemical Kinetics, Homogeneous Reactions (Supplementary Tables)," Nztional Bureau of Standards Monograph 34, 459 pages, $2.75, may be ordered from the Superintendent of Documents, U. S. Government Printing Office, Washington 25, D. C. Volume 39, Number 9, September 1962
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