An Experiment on Isomerism in Metal-Amino Acid Complexes Preparation and Characterization of Cobalt(lll) Complexes Containing N-Bonded Monodentate. O-Bonded Monodentate and N,O-Chelated Glycine Ligands R. G r a e m e Harrison a n d Kevin 6. Nolan' University of Surrey, Guildford, GU2 5XH, Surrey, England Although t h e existence of isomerism is a notable and widespread feature of coordination chemistry, the number and varietv of undermaduate e x ~ e r i m e n t sdoes not match t h e deserved prominence given t o this topic in lecture courses. Indeed, of t h e manv e x ~ e r i m e n t on s isomerism vuhlished in THIS JOURNAL oniy a f e w relate t o complex chemistry ( I ) . T h e reasons for this scarcity probably result from one or a combination of the following factors-the difficulties frequently encountered in preparing sets of isomers, t h e prohibitive expense sometimes involved (e.g., isomers containing precious metals) or the difficulties in distinguishing between the isomers prepared, using available physical methods. I n this paper we describe convenient syntheses of the cohalt(II1) complexes, 1-111, which illustrate three possible
treated dropwise over a 30 rnin period with 96% ethanol (75 ml). If the ethanol is added more rapidly, an oilmay beformed; however, if this occurs, vigorous stirring of the mixture for about 30 min causes precipitation. The resulting suspension is stirred at room temperature for 15 mi", suction filtered, washed with 96%ethanol (30 ml), ether (20 ml) and air dried. The yield of orange colored product should be about 1 g. The product obtained analyses satisfactorilp for [Co(en)zNH~CHzCOz]C1~~2HzO (Required: C, 20.0; H, 6.7; N, 19.4%. Found: C, 20.5; H, 6.3; N, 19.6%). 11. cis-Chloro(glycine)bis( 1,2diaminoethane)cobalf(II~ Chloride This complex is prepared by acid hydrolysis of the ester group in the analogous glycine ethyl ester complex (6) viz.,
1 1
i
+ CzHsOH cis- [CO(~~)~(NH~CH~CO~H)CI]~+ c~~-[CO(~~)Z(NH~CHZCO~CZH~)CI]C~Z. The preparation of this complex uses Meisenheimer's reaction (7,8),viz., trans-[Co(en)zClzlc RNHz cis-[Co(en)z(RNHz)C112++ CI-. To an aqueous solution of elvcine ethvl ester hvdrochloride 12.3 e. 5 ml)..in a 150-ml beaker. ~. . ... iridded 9670ethnnol t:((lmll. Trans- .Cnten)~CI~]CI (Rg) isaddcd 1,. rhissolution,and theru~pmshmir treated drupwse wth stwring !use a magnetic stirrer and a large magnetic follower) with diethylamine (1.6 ml). The resultingmixture isstirred vigorously for 5 min during which time it foms into a thick paste and its cplor changes to mauve. After the addition of more 96%ethanol(30ml), the mixture is stirred for a further 10 min. The mauve precipitate is collected by suction filtration. washed with 96%ethanol 130 mll. then ether 130 mli. and
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modes of bonding of glycine t o a metal ion (the complex cations I1 and 111 are linkaaelaeometric isomers). We also describe convenient spect~ophotometricand potentiometric methods to distinguish among t h e products. I and I1 are readily isolated as chloride salts. Unfortunately, the chloride salt of 111is extremely hygroscopic and difficult to manipulate. T h i s is also t h e case for other salts of I11containing the most simple anions. However, the perchlorate salt of 111 is isolated readilv and in view of the notential hazards of such salts lalthough perchlorate salts o~cobalt(111)complexes seem relativelv safe). . . the exneriment is desiened so t h a t each student deals with a small quantity of complex, and n o heating is invol~ed.~.~ T h e starting materials required for this experiment are glycine, glycine ethyl ester hydrochloride, and trans[Co(en)zCb]Cl(en = 1,2-diaminoethane). T h e preparation of this complex is commonly included in undergraduate practical courses 12.3). . . . Material from this exneriment. if available.. mav. be emphyed usefully for the preparations descril~edherein. Alternatively students may be supplied with this material which could be prepared in advance without mu( h difficulty. T h e method of Hailar 14) is a recommended larre - scale preparation.
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The complex has an intense C==O hand in its infrared spectrum (nujol mull) at 1740 cm-I. Although the product is sufficiently pure to proceed to the next s t e ~it. mav. if desired, be convenientlv recrvstallized as follows. The comolex and . at room temoerature . 15 . ... is dissol'ved in water (2dm11 .~ the resulting iolutim filfered. ('onrktrnted hydrochloric acid tL in11 is added to the filtrate ;and thp solu~imcwhd in an icr bath. 'The crystalline complex thus d,taind i i rsahed wuh 96coethanol(20 mi,, ether (20 ml) and air dried. eis-[Co(en)z(NHzCHzCOfl)C1]Clz. To a solution of the complex prepared above, cis-ICo(en)~(NH~CHzC0~C2HrlClp.H20 (1 a) in - ~. water (15 mi) is added concentrated hidrochloric acid (15 mil.- he resultine solution is heated on a wate; bath for 30 min so that the temperature of the a h r w n is mamtainrd in the range 70 :So(', Iexcesiiw heating lends w the omnar-culured rl~elatedglycinatc cumplrx, while inadequate heatmg results i n incwnplrte ester hy-
e)
~
~
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Preparation of Complexes I. Glycinatobis( 1,2diarninoethane)cobalf(llljChloride This rumplpx is prepared by a mud~ficntionuf prcv~ouslydeswihrd methuds r5.61. To a solution of glycinr (0.G y~ In dtlutr sod~urnh y droxide (1 moldm-'. 11,ml) isadded tranq-K'oren. UJICI 12 at.'i'hr resulting maroon solution is heated on a st&m bath G t h occ&onai stirrine for 10 rnin durine which time its color chances thraueh wine red t15, I1 is a monobasic acid with pK, value less than 3 (6)while I11 is a weak monobasic acid having a pK, of about 8.5 (9). The complexes (25 cm3, 0.01 rnol dm-3) may be titrated with standard sodium hydroxide solution (0.05 rnol dm-3). Complex I1 consumes base around pH 3 and shows a well defined inflection in a d o t of DH versus volume of base added. Complex 111, on the-other hand, consumes base around pH 8.5 but the inflection after neutralization is not well defined, due to the onset of base hydrolysis (in which alkali is also consumed) at pH >9 (9). The pK, values for I1 and 111 are 3.1 and 8.5, respectively, at 25°C. If allowance is made for the hydrogen ion concentration (15).the corrected OK, value of I1 is 2.8. If time does permit ; complete acidibaie titration approx-
1056
Journal of Chemical Education
imate pK, values may be obtained by measuring the pH of half-neutralized solutions of the complexes, e.g., a 0.01 rnol dm-3 solution of I1 in 0.005 rnol dm-3 sodium hydroxide has a pH of 3.1, while a similar solution of III has a pH of 8.5 (such a solution of I will of course have a pH of 12). The whole exueriment can be comuleted comfortablv bv an individual in 6-hr lnhoratory peri(G or alternatively it can he divided uu convenientlv into two 3-hr sessions, one for the preparative a o r k and one for measurement and analysis of spectra.
a
Acknowledgment
We thank the Teaching and Learning Methods Committee, University of Surrey for the award of a vacation studentship to RGH. Literature Cited
See. for eramole. Lamars, G. A,, and Rir. C. J., J. CHBM. EDUC.,56,211 11979): Hohmen, W. H.,J. CHBM: EDUC., 51.553 (1974) (2) Foust, R. D. and Ford, P.C., J. CHEMEDUC., 47,165 (1970). (3) Haworth. D. T. and Elsen, K. M., J. CHEM. EDUC..50.m (1973). (4) Bsi1ar.J. C.,lnorg. Synlh.2.222 (1946). (5) Lin, C. T.,and Doughs, B. E., Inorg. Chem..3,1356 (19M). (6) Alexander, M.D.. and Busch, D. H., Inorg. Chem., 5,1590 (1966). (7) Mei8enheimer.J.. A n m k n , 438.217 (1924). (8) House. D. A., Coord. Chem Rsus.. 23,223 (1977)p. 266. (9) Nolan,K.B.andSoudi,A.A,, J Chem.Soe.,DoIton,1419(1979). (10) Silverstein, R. M., and Bader, G. C., '%peetmmetrio Identification ofOrgmie Compounds,"2nd Ed.. Wiley andsons,New York, 1967.p. 97. (11) S e for example Nakamoto, K., and Mecarthy, P . J . "Spedraropyand Shudureof MFtsl Chelate Compounds,). Wiley and Sons, New York, 1968, pp. 257-259. I.l .l
(12) cotton, F. A., and Wilkinaon, G., "Advanced Inorganic Chemistri," 3rd Ed.. Intarscience. New York, 1972. pp. -5. (13) Rigg, J. M, and Sherwin, E.,J Inorg. Nvcl Chem., 27,653 (1965). (14) See ref. 181,Table 15, and refemnees listed in TsMes 17 and 18. (15) Albert. A and Sergeant, E. P.."The Deteminationof IoDiVation ConsfsnVI," Chapand Hall, London, 1971.
