Analysis of boron–halogen derivatives of bis(1,2-dicarbollyl)cobalt(III) anions by capillary isotachophoresis

Jorrrnat of Chonmroyapf~~. 2-G ( 19S2) 14-h 1-IS EIsevier Scientific Publishing Company. Amsterdam CHROSI.

-

Printed in The Netherlands

14S50

Note Analysis of boron-halogen derivatives anions by capillary isotachophoresis

of bis(1.Zdicarboilyl

jcobalt(lll)

The system nitrobenzene-cobaltocarborane of the type [(C,B9H, r _nXn)lCO]- H - _ where II = I4 and X = Cl (Br). has excellent extraction properties for mono- and divalent cations (see. e.g.. refs. 1-4). Halogen atoms present in the molecule of this extraction agent (in the positions B-S and B-S’ of the dicarbollyl skeleton) remarkably improve its chemical stability’. High stabilities to radiaton. typical of this group of compounds. make them suitable for the isolation of fission products (r3’Cs and 90Sr) from acidic so1utions3*“. Mixtures of derivatives are formed on chlorination’-’ or brominationS-9 of the substrate[(CIB,H,,),Co]-H-. ((3’). Someofthemcan be obtained in apureform by gel chromatogaphy (Sephades LH-20) using methanol as an eluent’.“. The identity and purity of the isolated compounds is usually evaluated by *rB NMR spectrosco6)v1r_ _The similar physical and chemical properties of DCC {the abbreviation for f(C2B9H, I)2Co]- ) and its halogen derivatives make difficult their chromatographic separation and consequently their analysis. Recently_ an extensive chromatographic investigation of heteroborane anions was published by Plzdk et al.“. Homologous series of haloenated sandwich complexes of the above type (for structural formulae see Fig. 1) were not included in this investigation. However, the retention behaviour of this group of compounds evaluated under similar conditions” implies hardly any separations. The aim of this work is to show that capillary isotachophoresis (ITP) is a powerful technique for the separation of DCC and its halogen derivatives. Because of the low solubihties of cobaltoboranes in water their ITP separations were carried out in water-methanol and water-ethanol mixtures.

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Fi_g. I. Structural formula of the dichioro derixrtke of coba!rocarbor;lne anion (CI,DCC). halogenation of DCC is x follows: 1. position 5: 2. S’: 3. 9: 4. 9’: 5_ 12; 6. 12’.

The ordrr of

EXPERIMENTAL

Apparatzrs An instrument for ITP similar to that developed

by Everaerts et aLI was used. Parts which are in contact with solutions were made of poly(tetrafluoroethylene) (PTFE). A fluorinated ethylene-propylene copolymer (FEP) capillary tube of 0.3 mm I.D. was used. Detection was performed with a conductivity detector’3*‘a. iMaterial to be analysed was delivered with the aid of a six-way valveI also made of PTFE. Stabilized current was supplied by a unit developed by HavaSi”. Reagents

Chemicals used were of pro analysi purity and if necessary were purified by conventional methods’6_ Trichloroacetic and benzoic acids were obtained from Lachema (Brno, Czechoslovakia), N-ethylmorpholine from Fluka (Buchs, Switzerland). Water, methanol and ethanol were doubly distilled before use. The caesium salt of DCC was prepared by Dr. J. PleSek (Institute of Inorganic Chemistry, Czechoslovak Academy of Science, ae.2 near Prague, Czechoslovakia)_ Halogen derivatives of DCC were prepared in our laboratories. The pH values of the leading and terminating electrolytes were measured as A PHM-64 pH meter provided with a G 202 C glass described in the literature’3*“. electrode and a K-401 calomel electrode (all from Radiometer. Copenhagen, Denmark) was used. RESULTS

AND

DISCUSSION

As mentioned above, DCC as well as its halosen derivatives are sparingly soluble in water. Therefore, ITP separations in methanol-water mixtures were studied. Small differences in the effective mobilities of the constituents to be separated were typical for this pair of solvents. Optimization of the separation according to pK

NOTES

146 TABLE

I

OPER4TIOX.4L DERIk-.4T!VES

SYSTEM BY ITP

SUIT_4BLE

Cl,Xc= Trichloroacetate: C,H,COOS-SS (Hoechst. Frankfun;M, G.F.R.).

Solvent

Xnion Coun:er-ion eH+ .%dditi\e-___

FOR THE SEPAR4TION

OF DCC AND

ITS HALOGEN

= benzoate; N-ET&f = N-ethyhnorpholine; hlOW

L.eading rIecr.ro!ne

Terminaring electro~rre

Ethanol-\\ater (S7rl; v. v) C1$ZH2C00XETXI 6.1 0.01 0* _MOW

Ethanol-water (955 v;v) C,H,COON-E-l-M 7.0 -

= Mowiol

Measured as describex! in ret>. 13 and 17. * For the role of the additils see ref. 13.

l l

Fio zA 2. An isctachophoro_w OF zhe separation of chloro derivatiws of DCC. For the operating conditions see Table I. Driving current 10 ~4. 1 = DCC; 2 = ClDCC; 3 = Cl,DCC; 4 = CI,DCC; L = leading anion; T = terminating anion; I = impurity; R = increasing resistance: I = increasing time. Fig_ 3_ .An isotachophorogram of the zparation of some bromo derivatives of DCC. 3r2DCC; 3 = Br,DCC. Other symbols as in Fig. 2.

! = BrDCC: 2 =

NOTES

147

values is ineffective in this instance as these compounds are strong electrolytes in nonaqueous solventslO*‘*_ Therefore; other solvents were considered. An unexpected improvement of the separation was obtained when ethanolwater mixtures were employed_ Differences in the effective mobilities of DCC and its halogen derivatives in this solvent system were sulI?cient for full resolution of available members of the homologous series. Several operating systems are available for ITP separations of these compounds if the ethanol content is sufficiently high (more than 80% v/v). The one used throughout this work is given in Table I. Isotachophorograms of the separation of chloro and bromo derivatives of DCC are given in Figs. 2 and 3, respectively_ The order of migration of the derivatives indicates that their different molecular weights are responsible for the separation (strong electrolytes, same charge type_ very close structural properties). However,

salvation

effects probably

also play a r6le as insufficient

resolution

was found in

methanol-water mixtures. A detailed investigation of the solvation effects for this group of constituents and for different solvents is the subject of current research in this laboratory_ Possible applications of ITP, e.g., for control of purity and stability to radiation, are clear from the above isotachophorograms_ An application to the analytical evaluation of a crude product is given in Fig_ 4. We can see that the dichloro deriva-

Fig. 4. Analytical evaluation of crude CI,DCC. as in Fig. 2.

1 = CIDCC;

Z = Cl?DCC;

3 = CI,DCC.

Other symbols

NOTES

1-a

tive of DCC (CI,DCC) is accompanied by two by-products ClDCC ( z 16%) and CI,DCC ( 2 27 :1;)_ The use of ITP for analysis of metallocarboranes. has several advantages: (1) The analysis time was approsimately 15 min in this investigation. However. modification of the apparatus_ eg., by use of coupled columns”, can decrease this substantially. (2) The reproducilibity of determination is very good (1-2 % of the amount to be determined is typical). (3) Only small amounts of material are necessary for a complete analysis. which can be of seat importance when radioactive material is involved. (3) The solvent in which the sample is dissolved was found to be of minor importance in this application’*_ REFERESCES 1 2 7 1 5 6 7 b 9 10 11 12 13 14 15 i6 17 1s i9 10

J. F. \I. J. i. P. i.

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