Electrochemical investigation of palladium complexes with organic sulphides and their use in extraction differential pulse polarography

Talrmta,Vol. 34, No. 1, pp. 219-222, 1987 Rinled in Great Britain

~3~91~/87 $3.00+ 0.00 hrgamon Journals Ltd

ELECTROCHEMICAL I~ESTIGATION OF PALLADIUM COMPLEXES WITH ORGANIC SULPHIDES AND THEIR USE IN EXTRACTION DIFFERENTIAL PULSE POLAROGRAPHY H. C. BUDNIKOV* Faculty of Chemistry, V.I. Ul’yanov-Lenin State University, Lenina 18, Kazan, 420008, USSR V. N. MAYSTRENKO and Yu. I. MURINOV Faculty of Chemistry, Bashkirian State University, Frunze 32, Ufa, USSR (Received 15 October 1985. Accepted 1 February 1986) ~~-~~yl-, amino-, keto- and keto~nos~p~de complexes of ~a~~~1) are shown to undergo one- or two-step reduction in a mixed acetonitrilectoluene solvent containing Bu,NClO~ at O.lM concentration. The half-wave potentials of the complexes show a certain dependence on the ligand structure, a positive shift of the potentials being caused by an increase in the x-acceptor ability of the ligands. The limiting currents are proportional to the concentration of the complex, according to the IlkoviE equation. Fast-scan differential pulse polarography was applied to the determination of palladium(H) in the organic phase after extraction of its complex with dihexylsulphide.

There are a number of papers and reviews dealing with the co-ordination chemistry of ligands and chelates containing the thio-ether group in their structure.’ These organic compounds are of some importance on account of their biological activity and also their usefulness as extraction reagents for the concentration and determination of metal ions.2 The analytical application of organic extractants for the separation and determination of metals, in particular palladium, in the presence of the other platinum metals, is of special interest. This paper reports on the electrochemical reduction of palladium(I1) complexes with organic sulphides, with composition PdL,Cl,, where m = 1,

L = C&HUSC2H4~

0)

C~~COCH(CH~)CH~SC*H,~ C&I&‘CH(CH,SC&,,)Z m = 2,

L = C,H,COC2H,SC,H,,

(II) (III) W)

WI,,XS

(v)

G%)tS

Ivr)

in an acetonitrile-toluene mixture at the dropping mercury electrode, and discusses the possibility of determining palladium(I1) by fast-scan differential pulse polarography (FSDPP) with dihexylsulphide (DHS) as an extractant. The general principles of extraction ~laro~aphy used in the investigation have been described in a review.3

*Author to whom correspondence T.A.L. w-0

should be addressed.

EXPERIMENTAL Apparatus The electrochemical experiments were done with a PU-1 polarograph (USSR) for recording d.c. and a.c. polarograms, a dropping mercury electrode (d.m.e.) with m = 1.96 m&ec at zero potential and controlled drop time t = 0.21 sec. Cyclic and FSDPP voltamperograms were obtained with a PA-3 polarograph and a stationary mercury drop electrode of type SMDE- 1 (Czechoslovakia). All voltammetric measurements were made with a three-electrode cell at 25 & 0.1” with Ag/O. 1M AgNO, in acetonitrile as a reference electrode, and a platinum win: as an auxiliary electrode. Reagents The organic sulphides and palladium complexes were prepared as described in the literatureaM Their purity was checked by infrared spectroscopy and elemental analysis. Organic solvents were purified by the methods recommended in the literature’ and were kept over 4A molecular sieves. Tetrabutylammoni~ perchlorate (0.W) was used as background electrolyte. The solutions were deoxygenated by a stream of argon. The purity of the electrolyte was checked by recording blank polarograms with the d.m.e. The base-line for the solvent mixture was run as a check on electrochemical purity. Solutions Measurements were made on a 2:3 v/v mixture of the extracting and ionizing solvents: O.lSM Bu,NClO, in acetonitrile as ionizing solvent and 0.05M DHS in toluene as the extractant. Stock solutions of organic sulphides were prepared by accurate direct weighing. The buffer solutions for the extraction were prepared from reagents of high purity. Extraction procedure A 5-ml portion of 0.05M DHS in toluene and a 25-ml portion of 10-s-10-7M aqueous solution of palladium(H) in 1M hydrochloric acid were shaken together in a separating funnel for IO min. After separation of the phases by

H. C. BUDNIKOV et al.

220

centrifuging, a 2-ml portion of the extract was transferred to a poiarographic cell and 3 ml of 0.15M Bu,NClO, in acetonitrile were introduced. The FSDPP voltamoeroaram . was recorded after deaeration of the solution. RESULTS AND

1 (6) 4 z3

5 k

DISCUSSION

4 v

.z-2

3 2

1

Palladium(I1) complexes with the ligands mentioned above show one reduction step in acetonitrile at the d.m.e. in the concentration range 1O-5-5 x 10eSM (Fig. la), but those. with the ligands &IV, containing the keto-group, give a second reduction wave at more negative potentials, from - 2.1 to -2.3 V. The experimental data indicate that this step arises from reduction of free ligand released during reduction of the complexed metal ion at less negative potentials. A number of electrochemical characteristics of some palladium(II) complexes are given in Table 1. Microcoulometric measurements indicate that two electrons react with one molecule of palladium(I1) complex to give elemental palladium. Cyclic voltamperograms show a pair of cathodic ($,J and anodic (i,) peaks corresponding to the d.c. waves. The peak current ratio (i,/i,) tends towards unity as the scan-rate is increased within the range 0.01-0.5 V/XX. Decrease of this ratio with decrease in scan-rate is thought to be due to degradation of the two-electron reduction product within the period from one scan to the next. Alternating current voltamperograms show one symmetric peak. Both the half-width of the a.c. peak (4648 mV) and the slope of the plot of Ep us. log (ih - i)/i (Table 1) indicate that the electrode process is close to being completely reversible. The reduction scheme may be as follows: [pdL,$lJ

+ 2e - e [PdL,,,Cl$- + Pd + m L + 2Cl-

Comparison of the half-wave potentials for the reversible reduction of Pd(I1) complexes (Table 1) and PdCl, (Q = -0.105 V) shows the contribution of the metallic atomic orbitals to the localized valency molecular orbital energy to be essential. Therefore the fast degradation of the negatively charged product of the two-electron reduction, [PdL,ClJ2-, produces Pd and free ligand, this being noticeable at slow scan-rate. The reduction may also proceed by a one-electron transfer and simultaneous cleavage of a metal-ligand bond, owing to the relatively low atomic orbital energy of the central ion. The second

1 0

I

I

-E(V)

3

-EW I

Fig. 1. a-Voltamperograms (d.c., curves l-3; cyclic, curve 4) for palladium(I1) complexes at 5 x IO-‘M concentration in O.lM Bu,NClO, solution in acetonitrile, at the d.m.e.: l-l&and II; 2-ligand V; 3 and kligand I. bVoltamperograms (d.c., curves l-4, cyclic, curve 5) for the complex with ligand I in O.lM Bu,NClO, solution in acetonitrile at the d.m.e. Concentrations: l-5 x 10e5M; 2-l x 10F4M; 3-1.5 x 10e4M; 4 and 5-3 x 10e4M.

electron transfer occurs at potentials close to the E;,, values for the complexes, depending on the strength of the metal-ligand bond. Examples of analogous reduction mechanisms of metal complexes with organic ligands have been reviewed.’ If the concentration of the complexes is higher than 5 x lO-sM, new waves appear at more negative potentials (E,,2 from -0.58 to -0.67 V) on the voltamperograms, and the sum of the wave-heights corresponds to a two-electron reduction (Fig. lb). The cyclic and a.c. voltamperograms both contain peaks in the potential range under discussion. The parameters of the peaks show the electrode process to be not fully reversible. Increasing the complex concentration increases the half-width of the a.c. peaks to 78-82 mV and decreases the current ratio i,,Jipc on the cyclic curves to 0.17-0.23. A second scan of the cyclic voltamperogram at the stationary mercury electrode shows only the second peak. Figure 2a illustrates the dependence of the limiting currents of the complex with ligand V on its concentration. When the concentration is increased the diffusion current of the first wave reaches a limit when C > 5 x lo-‘M. The height of the second wave is also dependent on the concentration, and the sum of the wave-heights is a linear function of concentration. The limiting current of the first step increases linearly with the height of the mercury column and

Table 1. Electrochemical data for the reduction of palladium(I1) complexes at the d.m.e. in O.lM Bu,NClO, solution in acetonitrile; palladium concentration 5 x lo-‘M Ligand

-E,,r, V

i,, PA

AE/A log (i,,,,,- i)/i, IPIV

I II III IV V VI

0.255 0.270 0.360 0.225 0.205 0.200

0.36 0.24 0.30 0.20 0.32 0.30

35 35 37 33 31 32

*Scan-rate 0.5 V/set.

12

$J,’ 0.92 0.87 0.84 0.95 0.97 0.98

A log i@log 0.54 0.57 0.58 0.52 0.48 0.49

V

Palladium complexes

221

Table 2. Determination of palladium in solutions with lOOO-foldexcess of Pt, Rh, Ru and Ir; O.lM Bu,NClO, solution in 2:3 toluene-acetonitrile mixture (extraction reagent 0.05M DHS in toluene)

Fig. 2. a-The relationship between the limiting current of the reduction steps of the complex with ligand V, and the concentration; l-the first step; 2-the second step. bPolarograms of the palladium complex with ligand VI in 0.1M Bu,NClO, solution in acetonitrile at the d.m.e. at various temperatures: l-15”; 2-25”; 3-35”; 4-45’. with temperature, at any concentration in the range l-5 x 10_5M. The temperature coefficient is about 6-8% per degree, a value characteristic for reduction of adsorbed species. When the temperature reaches 45” the second waves for the complexes with ligands V and VI practically vanish (Fig. 2b) and only one step is recorded on the voltamperograms. The appearance of the second waves at high concentration indicates that the first reduction step is inhibited by the adsorbed film of the reduction product. The first wave corresponds to a process at the clean electrode surface and the second to one at the surface covered with electrolysis products. Adsorption of reduction products of Pd(I1) complexes at the mercury electrode has been reported before. ‘g9Interestingly, PdCl, has been shown to undergo a two-step reduction identical to the one described for the complexes. The reversibility of the first reduction and its occurrence at the free electrode surface indicates that the & values should be close to the standard reduction potentials (E,). Hence, the shifts in & may be considered to be a measure of the ligand effect on the electron transfer. The E,,, values for the complexes studied become more negative, for the following sequence of ligands: V, VI > IV > I > II > III. The data are in accordance with the idea that the properties of organic sulphides 2.5 r

Taken, K?lml 0.063 0.128 0.250 0.500

Found, /J&?glml 0.059 * 0.0035 0.123 f 0.004 0.240 + 0.006 0.479 f 0.010

**Confidence limits (n = 5, p = 0.95). as n-acceptors affect the reduction potentials of the complexes. lo Strengthening the n-acceptor properties of the ligand shifts the reduction potential of the palladium(I1) complexes in general. However this relationship is not too clear: it is possible that the first three complexes are in c&form, and the others in trans-configuration. A linear relation between the limiting current and the concentration of the complexes (for concentrations below 5 x lo-‘M) was also found with the 3: 2 acetonitrile-toluene mixture, which is of practical importance for the extraction-polarographic determination of Pd(I1) in the presence of other platinum group metals. The usefulness of this method of analysis is based on the selectivity of the extraction of palladium dialkylsulphides from hydrochloric acid solution.2 To lower the limit of detection the FSDPP method of determination was used. The relation between Ai,, and the concentration of palladium(I1) in the organic phase is linear over the range 10-5-10-7M. Figure 3 shows the differential pulse voltamperogram of a 10e6M Pd(I1) solution in the 3:2 acetonitril+toluene mixture in the presence of DHS. The limit of detection is 0.01 pg/ml. To select the optimum conditions for the determination of palladium the influence of different factors on the limiting current of the extract was investigated. Artificial mixtures were used to show that Ai,, is practically constant for extraction from 1-4M hydrochloric acid. Extraction times greater than 10 min did not further improve the extraction efficiency, which was >98%. The optimum extraction conditions are listed in the experimental section. Results for the determination of palladium in the presence of lOOO-fold excesses of platinum, rhodium, ruthenium and iridium are summarized in Table 2.

REFERENCES

Fig. 3. The differential pulse polarogram of 10e6M palladium(III comDlex in 0.02M DHSIO.IM Bu,NClO. solution in toluen&a&onitrile mixture’(2: 3), inGal poG.ntial +0.3 V, pulse magnitude -50 mV, scan-rate 20 mV/sec.

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H. C.

BUDNIKOVet al.

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