Stereoisomerism in Coordination Chemistry A Laboratory Experiment for Undergraduate Students Maria F e Gargallo', Laura Lechuga, and M. Carmen Puerta University of Cadiz, Spain Francisco Gonzilez-Vfichez and Rosario Vilaplana University of Sevilla, Spain Coordination chemistry is presented to our students, after two obligatory courses in inorganic chemistry, as an optional third one. The eminently experimental character of the subject offers a number of possibilities for laboratory, and the challenge is to choose a few experiments relative to the most relevant asoects of todav's coordination chemistrv. ~ris-(2,3'-butanediamine)cobalt(111)[ ~ 0 ( 2 , 3 - b & ~ +is an interestine" comnound for stereochemical studies: its stereoisomers have been separated and studied in recent years (13). The ligand 2,3-butanediamine (bn) exists into two isomeric forms: meso (ms-hn) and racemic ( f h n ) (Fin. 1). Both of them form five-membered chelate rings uponcoordination and give rise to a number of stereoisomers for each complex. [ C o ( r n ~ - b n ) ~presents ]~+ two isomers: facial and meridional (Fig. 2). Because of the relations hi^ between molecular confieuration and ring conformation,'there are four possible p a k of enantiomers for ICo(fbn)13+ desienated as the lell. lelv ob. lel 0h2. and oh3 isomers ( ~ i 3). ~ .?he isomers can b e easil; separated by ion-exchange chromatography and identified mainly by 13CNMR.
fac C3
rner C1
Figure 2. Facial and meridional Isomers of [Cqms-bnh13+(A form)
Alms The general aims of the experiment are: (1) Make the students hecome acquainted with stereochemical eon-
cepts and several techniques used in this field. (2) Incorporate in the learning some of the recent research tasks. The immediate aim of the experiment is the isolation and characterization of the stereoisomers of a compound. Background The students know already the basic stereochemical concents. Thev are ~ r o v i d e dwith several kev references (2.4-n foi backgroundand complementary stuiy. General Method The work is carried out in groups of two or three students. Usually each group works on the isomers of a single compound and writes a report including theoretical background. method, results, and conclusions. A final general session is dedicated to share their experiences. They are specially invited to communicate how interested they were in this experiment and any conclusions about it.
Preparation of the Complexes and Chromatographic Se~aration A aolution of the appropriate 2,:l-butanediaminedihydrochloridr salt 13.22g, 20 mmd, in ;hl mL uf H.0 isadded ro a slurry of sodium rrls rnrhonatoo colmltate Ill), tia.lC'otCO,1;I3H:Ot2.18g. lj mmolJ and charcoal in20mL of H.0, and thr mixture issrirrcd and heated ar 60 "C fm 2 h. Fullouiny the rrmowl ot the rhmeanl hy filtmtlon. the orange iulutions are chromatugmphed on SP Sephadcx ('-2;
1018
Journal of Chemical Education
Id3
lelZob
lel ob2
Ob3
Figure 3. Isomers of [Co(*bn)#+ (A form). cation exchange resin by use of a 2.7- X 250-cm glass column for the tris meso compound and a 2.7- X 50-cm glass column for the tris racemic compound. The sample is pipetted carefully onto the drained surface at the top of the column in fractions of 3 mL. The isomers are eluted with aqueous 0.15 M NanSOn at a flow rate of 1 mL/min. The elution of the tris meso compound gives two orange hands, the first corresponding to the meridional isomer and the second to the facial ( I ) . Three well-defined bands, and a very faint fourth one, are obtained upon elution of the tris racerniceompound, assigned as the ]el3,l e l ~oh, lel ob2,and obs isomers (3).The relative isomer abundances are determined from the elution curves plotted . , ,A (Tris meso compound: A,., from the spectral absorbances at 475 nm, em., 220. Tris racemic compound:, . ,A 465 nm, em,, 200.) These relative abundances are: meso compound: mer 64.590, fac Presented at the 30th International Congress of Pure & Applied Chernlstw Manchester. U.K.. Sent. 1985. , IIUPAC). . ' A~thorto whom correspondence she-Id be aodressed. Tnm experlment does not lncluoe separat on of optlcal isomers. See footnote 9 n ref 2 for a omuss on of the Isomer nomenclature
Figure 4. '%NMR spectra of [Calms-bnh13+isomers 35.5%, racemic compound: leb 50.2%,lelnob 33.07%;lel ohn 12.6596, and ob34.08%. The addition of methanol to the concentrated chromatographic fractions allows the removal of sodium sulfate. The methanolic solutions are evaporated to dryness under air stream to give the solid isomers as sulfate salts. (Yields: meso compound 85%,racemic compound as%.)
isomers of the racemie compound are quite similar, though, as might be expected, the spectral hands found for the isomers of higher symmetry (Ddlels and ohs are sharper than those determined for the isomers leln oband lel ibz, of lower symmetry (Cz)
13C NMR Spectra In the preparation of the NMR samples, the sulfate salts (60 mg) were dissolved in 0.5 mL of D20and placed in a 5-mm sample tuhe. MerSi was employed as an external standard in acoaxial 1-mmtuhe. Natural abundance 13CNMR spectra were recorded at -35 'Con a Varian FT-ROA using a 2000-Hz sweep width, 0.5 s acquisition time, proton decoupling, and no pulse delay. Approximately 50,000' acquisitions were taken for each spectrum. The facial isomer shows two methyl and two methine carbon resonances, as expected for a compound with symmetry Cs, due to the axial and equatorial conformation of methyl groups. The additional peaks observed in the spectrum of the meridional isomer result from its lower symmetry CI (Fig. 4). The four isomers of [Co(ihn)3I3+show the resonances: lels and obs, one for methyl and one for methine groups; I& oh and lel oh?, three formethyl and three for methinegoups (Fig. 5).
Concluslon This experiment is quite complete, because the students become acquainted with a number of techniques, such as synthesis, chromatography, IR, UV-vis, and I3C NMR spectroscopy. It is a very interesting practical complement to the theoretical studies on stereochemistry. T h e use of molecular models allows the student to visualize more easily and clearly the stereochemical concepts. T h e results obtained by t h e different techniques are in eood ameement with biblioeravhic data. It eives didactic Galue & this experiment; itavoids mistakes a n d surprises and allows the students t o follow it convenientlv. As a teaching aid the experiment has proved t o be very successful in stimulating interest both in stereochemistry and laboratory work.
]~+ Figure 5. '=C NMR spectra of [ C ~ ( i b n ) ~isomers.
Attainment of Welldeveloped Crystals Solid bromide salts are prepared by ion exchange with QAE Sephadex A-25 resin in the bromide form. The concentration of the solution permits the isolation of good crystals with Br as counterion. IR Spectra The soeetra were collected on a PYE-UNICAM Model SP3-300 spectrophotometer using KBr pellets. The IR spectra of fac and mer isomers as bromide salts are indistinguisable. IR spectra of the
6. Gargallo, M. F.:Tap.imtt.R. E.:Dueder. E . N . lnorg. Chem. 1984,23.91&922. 7. Ymhikawa, Y.: Yama%aki,K. Coord. Cham. Reu. 1979,28.20&229.
Volume 65
Number 11 November 1988
1019
