Indian Journal of ChemistryVol. 21A, October 1982, pp. 958-960

Polarographic Studies of Cobalt (III) Complexes withBiguanide & Substituted Biguanides

BANANI DATTA, KANAKLATA SINGH· & D SENDepartment of Chemistry, Indian Institute of Technology, Kharagpur 721 302

Received 3 November 1981; recised and accepted II August 1982

The polarographic reduction of Co(III) chloride complexes with biguanide and substituted biguanides has been studied inaqueous 0.1 M K CI and 0.01 :;~gelatin. The complexes are reduced in two steps (one-electron and two-electron respectively) atthe d.m.e. The process is irreversible for all the complexes except for the simple biguanide complex. The E 1/2 values. overall rateconstants and activation energies of the electrode processes etc .. have been found to be related to the average Dq values of thecomplexes. Variation ofthese parameters with the size and nature of the substituent on the parent ligand has also been studied.

In our previous papers t.2 we have reported thepolarographic behaviour of Cu(II) complexes withbiguanides and N' -amidinoisoureas. These ligands,due to their strong n-basic character, produce ringcurrents in the chelate ring? and stabilize higheroxidation states of the metal atoms". As biguanides arenot known to stabilize lower oxidation states of metals,the reduction of Co(IlI) complexes with these ligandswas considered to proceed in two steps:Co(III)c:.,Co(II).3.i Co(O). This behaviour is unlikethat of some M-L a-bonded complexes of Co(1I1)which undergo three-step reduction" via COIl)oxidation stage". The present paper relates thepolarographic behaviour of Co(III) complexes ofbiguanide and its alkyl or aryl derivatives with differentspectral parameters. The gradual change in thesevalues with the change in the size of the substituent isexpected to be due to decreased delocalization in therespective ligands.

Materials and MethodsFor polarographic studies. LP7 Polarograph with

EZ7 line recorder was used. The polarograms wererecorded using a dropping mercury electrode (m = 2.40rng/sec and t = 3.7 see in open circuit in 0.1 M KCl and0.01 '1~ gelatin solution) and saturated calomelelectrode (S.C.E.) connected through agar-saturatedKCl bridge with the test solutions. All thepolarographic studies were carried out using aToshniwal constant temperature bath. The con-ductance measurements and electronic spectra wererecorded on Systronics conductivity bridge type 303and Cary spectrophotometer model 170 respectively.

The complexes were prepared and purified byrecrystallization from water according to knownmethods 7. Stock solutions were prepared inconductivity water and test solutions were madeoxygen-free by passing purified nitrogen gas for 8-

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10 min. The experimental procedures were the same asdescri bed elsewhere 1.2.

Results and DiscussionThe polarograms of Co(I1IHris(1igand) complexes

showed two waves in aqueous as well as in non-aqueous (EtOH +0.2 M LiCl) media. The first wavegave a fairly large plateau while the second showedonly a very small plateau beyond which current wenton rising probably due to hydrogen evolution. Unlikethe behaviour of many Co(lIl) complexes", this two-step reduction of biguanide complexes led us to believethat Co(IIIJ-tris(biguanide) and related complexes donot get reduced via Co(l) state. Both the waves werediffusion-controlled as revealed by linear plots of id vs

rr=':concentration and id vs V he!! and the second waveinvolved double the number of electrons as comparedto those transferred in the first wave. In the case ofsubstituted biguanide complexes, the plots of log i/(id- i) vs E were linear in all the cases with slopes muchhigher than 2.303 RT/ F and 2.303 RT /2F for the firstand second steps respectively showing irreversibility ofthe electrode process. For the Co(III)-tris(biguanide),these slopes were, however, 0.059 and 0.037respectively suggesting its reduction to be reversible orquasi-reversible, as found in the case of nickel(II)-bis(biguanide) complex". For the two step process, thetransfer coefficients 'Y.' and 'Y." are 0.91 and 1respectively. For all the other complexes 'Y. values werefound to be around 0.8 or less (Table 1). Thus,excepting CorIl l-tristbiguanide}, all the othercomplexes are reduced in the following manner:

Co(III) ~ Co(ll) ~ Co(O)

The reduction of [Co(BigHhP + may be representedas:

Co (III) ~ coun ~ Co(O)

DATTA et al.: POLAROGRAPHY OF Co(lII)-BIGUANIDE COMPLEXES

--_._--_._------Table I-Polarographic and Spectral Parameters of Cobalt(lIl) Complexes of Biguanide and Biguanide Derivatives

Complex -E,/2(V) :r LogK,(cm sec-l)at Activation energy(kcal) at Dq(cm-I)

(VsSCE)I II I II -0.6V 0-1.1 V -0.6V -I.IV

Co(lll)-tris(biguanide) 0.89 1.28 0.91 1.00 22380Co(III)-tris(methylbiguanide) 0.85 1.24 0.87 0.67 -687 -7.41 9.80 14.40 22350Co(lll)-tris(ethylbiguanide) 0.83 1.21 0.82 0.62 -5.42 -6.04 9.60 12.80 22310Co(IIIHris(dimethylbiguanide) 0.78 1.18 0.71 0.64 -4.61 -4.19 7.00 8.40 22290Coli II Hris(d iet hylbiguanide) 0.73 1.14 0.46 0.29 -4.27 -3.83 3.80 5.60 22080Co(I I I)-tris(phenylbiguanide) 0.77 1.16 0.64 0.59 -5.37 -5.52 8.00 12.00 22300Co(111)-Iris( methylguanylu rea) 0.113 1.23 0.80 0.65 -6.24 -7.37 8.80 12.20 22150Co( I II)-tris( ethylguan yl urea) 0.75 1.15 0.78 0.62 -5.28 -6.18 8.60 10.40 21850

K.IKb for Co(BigH)~+ ICo(PhBigH)~+ =0.7 (I); 0.7 (potentiometric); 1.8 (II), 2.7 (potentiometric)

.----.---~--.------------.----

Table 2-Polarographic Characteristics of Cobalt (III) Complexes of Biguanide and Biguanide Derivatives

Complex o, x 106 lii,.did/dt dE,/2Idt QD Qe(cmisec " ') (deg ""] (V deg-I) (kcal) (kcal) at

II I II I II -0.6V -1.1 V

Co(lll)-tris(methylbiguanide) 6.38 1.40 0.90 0.13 0.40 4.00 5.92 9.14 14.72Co( I I1)-tris( et hylbiguan ide) 5.82 1.32 0.94 0.15 0.15 3.81 5.68 9.22 12.84Co(III)-tris(dimethylbiguanidc) 5.62 1.40 L07 0.30 0.22 2.46 4.21 3.08 4.45Co(III)-tris(diethylbiguanide) 5.48 1.54 1.40 0.23 0.28 2.48 4.23 6.73 8.99Co(III)-tris(phenylbiguanide) 7.08 U6 1.00 0.50 0.50 2.19 4.85 7.24 12.43Co(lll)-tris(methylguanylurea) 6.02 1.30 0.85 0.11 0.35 2.40 5.90 7.20 11.44Co( III)-t ris(et hylguan ylu rea) 5.47 1.30 0.90 0.20 0.42 2.00 5.10 6.43 9.80

. -------- ------- -----

The above dissimilarity in electrode processes ofsimple and substituted biguanide complexes had alsobeen observed for Ni(I1) complexes",

Approximate values of diffusion coefficients ofvarious complexes were obtained from conductancemeasurements at infinite dilution, using the equation".

D= RT .Ie""nF2

The number of electrons involved in the reductionprocess was obtained from the Ilkovic equation 9 usingthe above-mentioned D values. The integral values of n'

and n" for both the steps were finally used to evaluatethe accurate values of diffusion coefficients D', and Dj'which were then used for the calculation of reactionrate constants K~ and K~' for the first and secondreduction steps (Table I) according to the equation:

2.303RT 9 jtE1/Z = E, + log 0.8 K, -D--nF j

It must be mentioned here that as Co(II)-bis (ligand)complexes are not very stable, we had measured theconductance values for Co(III)-bis (ligand) complexesin order to get the Dj values for Cofl l) complexes.

From the polarograms at different temperatures, theactivation energies were calculated (Table 1) usinglinear plots of log i/(id - i) vs I IT and log id vs liT (ref.10). Activation energies thus obtained were compara-ble to those obtained from log K, vs liT plots (Table 2).

The relative stability constant K uI K ~ I for[Co(BigHhl1 + with respect to [Co(PhBigHh]3+ wasfound to be (1.7 which is exactly the value obtained frompotentiometric method for the first wave (Table t,footnote). Polarograms of Co(lll)-tris(\igand) com-plexes have been recorded at different pH-values byadding HCI or KOH solutions in appropriate dilutionto the test solutions. Higher pH-value (6.8) of thesolution has no significant effect. on the nature ofpolarograms except that the E1/2 becomes morenegative owing to the higher stabilities of thecomplexes in alkaline solution. The more negative half-wa ve potential may also be due to the formation ofCo(Bigh3HzO or some other intermediate species inthe course of its formation, which may be representedas,

[Co(BigHh]3+ ow [Co(BigHh(Big)]2+->

-> Co(BigH)(Big)zOH -> Co(Bigh.3HzO

In the acidic pH-range, the E1/2 of the second waveremained practically constant as in the neutral solutionwhile that of the first wave shifted towards morepositive potentials (Table 3). This may be due to theconversion of the tris-(Iigand) complex into its biscounterpart according to the following equation,

[Co(BigHhJ3+ + H + +2HzO->[Co(BigH)z(HzO}z] + BigH{

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INDIAN 1. CHEM .. VOL. 21A, OCTOBER 1982

Table 3-- Half Wave Potentials of DifTerent Complexes of Co(lII) in Various Supporting Electrolytes

Complex -E',~,-E';~ -E/, -e;', -£',:, -E',:, -£','; -E/,'Co(III)-tris(biguanide) 0.800 1.270 0.895 1.315 0.870 1.305 0.846 1.295Co(III)-tris(methylbiguanide) 0.770 1.240 0.850 1.280 0.825 1.270 0.805 1.240Co(III)-tris(ethylbiguanide) 0.740 1.200 0.840 1.225 0.845 1.250 0.785 1.220Co(III)-tris(phenylbiguanide) 0.688 1.150 0.790 1.195 0.800 L205 0.720 1.135con II )-bis(biguanide) 0.800 1.26

• in 0.1 M KCI +0.01 ':;, gelatin at pH 4.5: t in 0.01 M KCI +0.01"'0 gelatin at pH 8.5, 1: in 0.2 M LiCI+0.OO5 0 II gelatin, tt in 1 M KCI+ 0.0 I'~"gelatin

This was verified by recording the polarograms of[Co(BigH}z(H20}z] CI2 under similar experimentalconditions as for tris-Iligand l complex. This complexgave two waves at -O.gO V and -1.26 V respectivelywhich are same as those of tris-tligand) complex in acidmedia (pH 3-5)(Table 3). Thus, the reduction process ofthis series of complexes in neutral and acid media mayhe represented as:

. ]3 + e -(a) [Co(BlgHb,-- -------.Eu = -0.g9 V

[C B· H ]2 +. ze :o( Ig}z + BlgH-: .. =-- -- ') •. C0(0)

£1'2 - -I.~X-> Co(amalgam)

(h) [Co(BigHh]3+ : [Co(BigH)J.l+

+ BigH - ~ .. ·_[Co(BigHlzr +EI!2 = -o.x V

2e--- ..--=--=-j----.- Co(O)->Co(amalgam)£1/2- L6V

The final product of polarographic reduction wasfound to be metal amalgam as obtained by controlled-potential reduction of I x 10-'\ M Cof l l l l-tris (ligand)solution in 0.1 M KCI and 0.01 n" gelatin at -1.35 Vfor 5-6 hr.

£1'2 values of the twin waves of Cotl l lj-bis-higuanide complex are also reported by De et al. 12 inaqueous solutions of 0.1 M KCI and 0.002 0" gelatin.

The electronic spectra of cobalt tl l l l-tris (ligand)complexes showed two maxima around 480 and350 nm due to the transitions I A I~ -> I Tig and I A 19

-> I T2g respectively. From these I.max values, theaverage splitting energy DlJ was calculated. Thethermodynamic and kinetic parameters as determinedfrom polarographic studies are listed in Tables I and 2.

The magnitudes of all the parameters, viz. E 1'2'

transfer coefficient (x), activation energy, averagesplitting energy and negative rate constant valuedecrease with increasing size of the substituent in theligand. This is because of the decreased ring currentdue to the substitution. The different parameters listedin the Tables 1-3 have been obtained by considering therising portion of the polarographic waves for which thetransfer coefficients have been calculated. The

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activation energies of reduction process of thecomplexes with substituted ligands obtained polarog-raphically, follow the same trend as the average DlJvalues determined spectrophotornetrically. Howeverthe simple cobaltrl l l l-tris (biguanide) complex isreduced reversibly at the d.rn.c.

The electronic arrangement for a db octahedralCo(lll) ion in strong field may be represented as f~g. c~;the two {'y orbitals tL, and d,' _ I' orbitals are vacantwith Iigands in .v, rand:: directions. In the case ofsimple biguanide complex. in contrast to thesubstituted biguanides, no steric factor is involvedmaking the electrode process relatively fast almostreversible or quasi-reversible. However, in the lattercase. for the proper orientation of complex ion towardselectrode surface some extra energy and time arcrequired which render the reaction slower andirreversible (Eu and K, values are given in Table I).This is because of decreased basic character of liganddue to substitution.

In the second step the respective cobalt(lI)complexes will have d" square-planar electronicarrangement: d;= . tI~: . d~1 . tll, . dO , and direct. . . =- \:2 _ rl

transfer of a pair of clcctrones from electrode to thevacant d,' I' leads to the reversible reduction of cobaltbis (biguanide) complex to cobalt amalgam. In thecases of his (substituted biguarnide) complexes, againpresence of substituents leads to difficulty in directelectron transfer.

ReferencesI Dalla B. Singh. Kanaklata & Sen D. Electrochim Acta, 25 (1980)

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:2 Dalla B & Sen D, l ndian J Chern. 19A (1980) 669.3 Saha C R. Guha S & Sen D,) ('hem Soc (D), (1975) 1701.

4 Duna R L & De, D,) l ndiun ('helll SOL 46 (1969) 69.5 Vlcck A A, Z Phvs Cilelll, (1956) 143.6 Giacomo, Costa.,' Am chern Soc. 17 (1970) 2870.7 Ray P. Chern Re<'. 61 (1981) 313.8 Meites L. Polarographic techniques (interscience, New York),

1955, 55.9 I1kovic D. CollI! Czeck Chern Commun, 6 \1934) 498.

10 Vleck A A, Progress in Inorg Chern, S (1963) 797.II Lane J Thomas, Adl'ances ill polarographr (Pergamon Press,

London), 1960. 797.12 De A K & Saha P K.llllli<lll) Chell!. 16A (197X) 1042.