The Direct Electrochemical Synthesis of

An Example of the Use of Anodic Oxidation in the Preparation of Metal Ion Complexes

Colln Oldham1 and Dennls G. Tuck2 University of Windsor, Windsor. Ontario, Canada N9B 3P4

Althoueh electrochemistrv is an imnortant Dart of under- - graduate lecture courses, experiments designed to familiarize students with this area of chemistrv are freauentlv absent from the associated laboratory. 1; general; much of the teaching of electrochemistry, particularly in introductory courses, concentrates on the equilibrium aspects of the subject, which are conveniently related through electrode potentials to elementary thermodynamics. The student may then he unaware of the many applications of electrochemistry in the laboratory and on the industrial scale, and topics such as electrodeposition, electrowinning, and electrochemical refining often receive only passing mention.

Another neglected topic is the use of electrochemistry in svnthesis. which is unfortunate since the electrochemical technique is one of the sunplest and must direct methodsof achieving oxidation or reduction. Electrons can he removed from or added LO a system without the complications often otherwise associated with the presenre of redox reagents, and there are several useful examples of rhe use of electrochemiral oxidnrion or reduction in organic iynthesis. The numher of analognus experiments involving the preparation uf inorganic or organometallic compounds is more limited, despite the large amount of information a\.ailat~le from physiro-chemical studies of the hehavior of inurganir and organometullic species at electrodes (1.2). One ohviuus diftirulrv is that exneriments in aqueous solution may result in hydrolysis rea'etions in- volving either starting materials or nroducts.

In addition to those electrochemkal syntheses which in- volve inert electrodes, there are svstems in which one or both of the electrodes serve not on l i as a source of, or sink for, electrons, but also under~o reaction with species mesent in solution, or grnerated in solution as the t~lrcirolysisproreeds. Such reactions are said to inwlve sacrificial electn~des, and the metal electrode may now serve as the starting point of a synthesis. This is important, since there has been much in-

terest in recent vears in the use of metals as reazents. A ~ a r t " from classical examples like the Grigard reaction, recent ex- periments in which the vaporization of a metal is followed by reaction with a suitable ligand have considerably extended the svnthetic a~~l ica t ion of elemental metals (3.4). In reneral such-vapor syntheses involve fairly c&n&icatkd ap- paratus, whereas the electrochemical oxidation or reduction bf a metal generally can be carried out a t room temperature with unsophisticated equipment. A variety of inorganic and organometallic compounds have now heen prepared by this method (5) , and the suggested industrial applications include the manufacture of tetraethyl lead directly from the metal.

The present experiment presents a simple method for the direct synthesis of a phosphonium salt ofthe tetrachloroco- haltate(I1) ion, COCI~~- . The source of electrical energy is a readilv available 6 V lantern hatterv. Anart from this. the " . apparatus required is that generally available in undergrad- uate laboratories. The cohalt metal, which serves as the anode of a cell, is oxidized in situ, thus allowing the student to ob- serve that electrical energy derived from the chemical reac- tions in a dry cell can he used to drive other reactions, in this case the synthesis of a transition metal complex. The salt which is ~roduced has some interestina features. since the cation is the unusual triphenylphosphGnium species, which mav be studied hv vibrational s~ectrosconv. and the anion is a ciassical tetrahedral cobalt ~ I I ) speciks.'The electrolyte phase in the cell consists of acetonitrile containing triphen- ylphosphine and a few milliliters of concentrated aqueous

' Permanent address: Depanment of Chemislry, University of Lan- caster. Lancaster, England Saooatcal leave from the Un'vers ty of Lancaster and a travel grant from the MinnaJames-Heineman-Stiflung of Hanover are both gratefully acknowledged.

Author to whom correspondence should be addressed.

420 Journal of Chemical Education

hydrochloric acid. Recently, we h'sve shown that solutions of similar composition find gen- eral application in the prepa- ration of anionic complexes of transition and main group metals (6). One particular ad- vantage is that the presence of the hydrochloric acid results in a high conductivity, which in its turn allows the use of a cheap low voltage battery.

Procedure

Apparatus The electrical power is supplied

by a6VLanterndry battery (e.g., EverReadP No. 731). A simple voltmeterlammeter is the only other piece of equipment re- quired.

The experiment is most simply performed in a 100-mL tall-form beaker fitted with a rubber stopper through which are inserted elec- trical leads to the electrodes, and two glass tubes. One tube serves to permit escape of hydrogen gas generated at the cathode, and this arrangement allows input and exit of nitrogen gas should one wish to do experiments in an inert atmo- where. The leads need to be

Simple cell for electrochemical synthesis of [(CBH&PH]~[COCIII and related experiments.

chemically resistant and mechanically strong, and 1 mm diam. plat- inum wire is adequate, although some saving can be effected by using a cohalt rodanode inserted into the cell through the rubber stopper. ~~ ~~ - . . A ewnd plarinum wire, mod usrfully wmnd mlo a mmll rcd, serves as the cathude of the cell. Other and more elalrmatr rrll- ran be used at the CUSI of losmg the rimpliriry nnd nhusinrss of the arrangemen1 shown in Figure 1.

Experimental Conditions Requirements: a strip of cobalt, --3 X 1 X 0.1 em

30 mL acetonitrile (Reaeent made) 2 mL cone. aqueous hydrochloric acid 0.5 g triphenylphosphine

The weighed cobalt foil suspended on a platinum wire lead forms the anode of the cell; a small hole drilled through the foil makes for easy operation. The solution phase can be made up directly into the 100-mL beaker, and the rubber stopper carrying the electrodes and gas inputleait tubes fitted in place. With the conditions described, completion of the electrical circuit typically gives a current of 35-50 mA for an applied voltage of 5 V. The time at which the circuit is closed should be noted. As the electrochemical reactions proceed, hydrogen gas is evolved at the cathode, and a deep blue coloration forms at, and streams away from, the cobalt anode. The current should be read from time to time during the electrolysis to ensure that a constant value is being maintained, since such constancy is assumed in the calculation of the current efficiency.

It is convenient to run the electrochemical oxidation for ~ 9 0 min., corres~ondine to the Dassaee of ~ 2 4 0 Coulomb. The time at which the ce i is disconnectei shouid he noted. The cell then contains a deeo ~ ~~ ~ ~~ ~ ~ ~ ~ ~

Irlur solutim, indi~aling the iormarion in sin, ot'rhe O,CllL- nnim. Thr solvent ran he conwnicnrly removed in vacuo, when the crys- tnllme depp blue [~CsHsrcPlI]r[C~~C14] prerplrnres; thu is wnrhed

a i t h dirrhyl ether to remove any triphrnylphosphine. The cobalt anudr i, washed wlth acetonitrile, dried, and weighed, so that the quantity of metal diisulvtd ran be determined.

Results and Discussion

In a typical experiment, 70 mg of cobalt (1.2 mmol) was removed from the anode, and 0.8 g of product (1.1 mmol) was isolated. Yields of 9070, based on weight of metal dissolved, have been obtained routinelv hv this orocedure.

The current efficiency (EF~, defined as the number of moles of metal dissolved per Faradav of electricitv passed, aives valuable information on the oxidation it the anode. The weight loss gives the moles of cohalt dissolved, and the number of Faradays is given by It /F (I = constant current, in A; t = time of electrolysis, in sec; F = Faraday constant = 96480 Coulomb). With the apparatus and conditions specified, EF is 0.50 f 0.01 mol F-l; in our experience, any significant deviation from this value is usually the result of current variation during the electrolysis. The value of EF implies that the overall stoichiometry of anode process is

which is followed by formation of the complex

CoClz + 2c1- * CoCL2- (2)

The triphenylphosphonium cation is obviously formed by

(C6Hd3P + HCI = (CGHE)~PH+ + Cl- (3)

and of course hydrogen evolution a t the cathode is the result of the discharge of hydrogen ions.

Extenslons

The triphenylphosphonium cation has been the subject of a number of preparative and spectroscopic investigations. A recent paper (6) in which an X-ray determination of the structure was reoorted also noted some imoortant asoects of the vibrational specrrum; "(P-H, at 2360 & - I is difiicult to h e w e in the infrared, but it is readily detected in the Raman spertrum, and t w P-H bending modes are observed at 1112 and 722 cm- ' in the infrared (KHr disc). The paramagnetic tetrachlorocobalute(111 anion is n well-characterized example ot a tetrahrdriil~l- rvmplex 17), and can serve as the focal point of discussion of electronic spectroscopy and structure: The compound is air-stable, a t least over some hours, and soluble in acetonitrile, so that spectral and other information can be obtained easily.

Two other interestine discussion noints involve a cornoar- ison of the chemical an2 electroche&ical reactions of m&ls with acid, and the effect of prolonged electrolysis which eventually favors the formation of the neutral adduct [(C6H5)3P]&C12 by depleting the hydrogen ion concentra- tion. Experimental variations of metal or mineral acid offer challenges for the thoughtful student.

Literature Cited

il l Lehmkuhl. H . . " h i c El&x&emistrv." M. M. Bsizer IEdiforI. MareolIkkkar. N.Y.. . . .. . .. . . 1973. pi . 6214%

(2) Settineri, W. I. and McKeeuer. L. D. "Technique af Elenrmrganic Synfheaia," N. L. Weinberg (Editor) Wilay~Interseirnee, New York, PeriI1,1915, pp. 397598.

(3) Timms. P. L., Ad". lnorg Radioehem., 14.121 11972). (4) Ozin, G. A, and Van der Vort, A.. Prog Inorg. Chem.. l9.105 (19'75). 15) Tuck, D. G..PVreAppl. Chem..5l,ZW511979l. (61 Khan. M., Oidham. C., Taylor. M. Land Tuck, D. G., lnorg. Nucl. Chem. Lett., 16,469

il9Mi

(7) Cotton. F. A. and Wilkimn, G., "Advaneod Inolgsnie Chemistry: Wiley-lntemicna. New York, 3rd Ed., pp. 880-882.

Volume 59 Number 5 May 1982 421