The mercury-crown complexes and their interaction with quinone

I45

J. Electroanal. Chem. 290 (1990) 145-153 Blsevier Sequoia S.A., Lausanne

The mercury-crown complexes and their interaction with quinone L. PospiSii

l

and P. Papoff

Istituto di Chimica Analitica Strumentale, (Received

Via Risorgimento

35, 56126 Pisa (Italy)

31 January 1990)

ABSTRACT The anodic dissolution of a mercury electrode in 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile proceeds in the presence of dibenxo-24-crown-8 (DB24C8) in two waves, which were ascribed to the formation of complexes of Hg(I1) and Hg(I). Both the ligand and the complexes are adsorbed at the electrode. Much weaker wmplexation was observed in the case of dibenxols-crownd. The complex of Hg(I)-DB24C8 formed in situ reduces beruoquinone catalytically at potentials about 300 mV more positive.

INTRODUCTION

Cyclic polyether ligands, crowns, have found a variety of applications as ionselective complexing agents, solubilizers of reagents in non-polar media, or as models of natural ion-carriers. In our previous communications we reported the change of reaction pathways of crown-bound viologen resulting in catalytic activity towards the reduction of oxygen [l]. The present work deals with the interaction of benzoquinone with ions of mercury bound in the cavity of crown ether. Quinone-type redox couples are of biological importance, and hence an interaction of this type may have some relevance to the mechanistic pathways involved in the poisoning of heavy metals. It was reported that the ligand exchange reaction of organomercury compounds and the formation of adducts are fast enough to permit the extraction of a toxic metal from cell or tissues [2] and hence the designing of a “metal regulator” which could initially carry the ions of the essential metal (like K+ or Na+) and then exchange them in cells for toxic ions. In this respect, the adducts of quinones and mercury were investigated by ESR [3], and also ion-pairs of quinone radicals with

Permanent address: The J. Heyrovsky Institute of Physical Chemistry DolejSkova 3,18223 Prague, Czechoslovakia. l

0022-0728/9O/SO3.50

0 1990 - Elsevier Sequoia S.A.

and Electrochemistry,

CSAV,

146

an alkali-metal crown complex were detected [4-61. The later work provided evidence for a new type of ion-pair involving two quinone molecules and one crown complex. The redox reactions of ions of mercury with quinones are also used for selective synthetic reactions [7-91, and hence the electrochemical investigation of the mercury-crown complexes could enlarge the application field in this sense as well. In the present work, we have chosen acetonitrile as the solvent in order to avoid the complicated protonation equilibria of quinone and its reduction products [lo-131. We will describe first the complexation of mercury by crown ligands in acetonitrile and then the interaction with p-benzoquinone. EXPERIMENTAL

The electrochemical instrumentation consisted of a Polarographic Analyzer model 174 (PAR, U.S.A.) and an X-Y recorder model 7044A (Hewlett-Packard, U.S.A.). A dropping mercury electrode with a flow rate of 1.3 mg/s and a drop time of 3.1 s was used as the working electrode. The auxiliary electrode was a spiral of Pt wire. The reference electrode was a silver/silver chloride electrode in aqueous 1 M lithium chloride separated from the sample solution inside the cell by a non-aqueous salt bridge of the same composition as the base electrolyte. The half-wave potential of the ferrocene/ferricinium couple with respect to our reference electrode was +0.440 V. The indifferent electrolyte used in this work was tetrabutylammonium hexafluorophosphate (Fluka), which was recrystallized twice from hot ethanol prior to use. 1,4-Benzoquinone (Merck) was purified by sublimation in vacuum. Acetonitrile (Carlo Erba) was dried in some experiments by passing it over a column of molecular sieves, 0.3 nm (Merck). The following chemicals were used as received: mercurous nitrate (Merck), mercuric nitrate (Merck), dibenzo-1%crown-6, dibenzo-24-crown-8 (Fluka). Oxygen was removed from the solutions by passing a stream of nitrogen saturated with vapours of the solvent. RESULTS

AND

The formation

DISCUSSION

of crown complexes with mercury cations

The anodic oxidation of mercury electrodes is influenced strongly by the presence of ions forming either mercury complexes or insoluble precipitates on the interface. These phenomena were observed in aqueous solutions as early as the 1930s [14,15] and later in many other systems [16-221. The electro-oxidation of the mercury electrode in acetonitrile and 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF,) proceeds in accordance with the behaviour found in similar non-aqueous and non-complexing media [19-211: the dissolution starts at +0.6 V and the reduction of Hg*+ ions is a two-electron reversible process with E,,, = +0.600 V (for 0.1 mM Hg(NO,),). The reduction of Hg,(NO,), on Pt proceeds at +0.47 V [20], whereas here on the Hg electrode the Hg(1) ions are reduced at the same potential as Hg(I1) ions [20], probably owing to rapid disproportionation. In the

147

presence of ions forming complexes with mercury, the anodic current due to the dissolution of the mercury electrode is shifted to more negative potentials and is controlled by the diffusion of the complexing ions. In the case of halides [19-211, two separated anodic waves were observed and interpreted as due to the formation of mercury-halide complexes with one to four ligands, depending on the concentration of halide in the bulk. A similar electrochemical behaviour to that of halides was observed in the presence of dibenzo-24-crown-8 (DB24C8) in acetonitrile. The anodic dissolution of mercury in the presence of DB24C8 starts at more negative potentials and two well-developed anodic waves are observed (see Fig. 1). Although our system resembles the behaviour of halides, the mechanism in the present case is very likely to be different. The more positive wave (wave I) has a half-wave potential at +0.225 V which is independent of the concentration of DB24C8, whereas for a similar wave of mercury dissolution in the presence of halides the potential is shifted by 33 mV per decade of halide concentration. For the more negative anodic wave in the presence of DB24C8 (wave II), the half-wave potential is shifted to negative potentials with increasing concentration of DB24C8 (Fig. 2) and the amount of this shift is 30 mV/decade in the range from 1 to 40 mM DB24CB. The upper limit of the concentration is given by the solubility of the crown in acetonitrile. The anodic limiting currents due to the complex formation in the present case cause a slight deviation from the linear dependence on the concentration which is very likely to be caused by its limited solubility. From the properties of the half-wave potentials we

o-

I

1

I

$A

I

I

40.5

0

I

E/V

-0.5

Fig. 1. Anodic dc polarogramsof DB24C8+0.1 M TBAPF, in acetonitrile at various concentrations of DB24C8: (1) 3 mM; (2) 6 mM; (3) 12 mM; (4) 15.5 mM; (5) 19 mM,

148

can interpret the behaviour in Fig. 1 as due to the subsequent formation of complexes of two different compositions. As was shown both theoretically and experimentally, the subsequent formation of mercury complexes during the anodic dissolution proceeds according to Hg + Hg*++ 2 eHg*++p

(1)

L + [Hg(L),]*+

where L denotes the ligand and p is the number of ligands [21]. Then the shift of the half-wave potential of the complexed form is given by E 1,2 = E” + (R7’/2F)

ln(2P-‘/pK[L]P-‘)

(2)

where E o is the standard redox potential of free mercury ions and K is the stability constant of the complex. The difference in the diffusion coefficients was neglected in eqn. (2). It is known that the shift of the anodic dissolution of mercury is concentration-dependent only if at least two ligands are bound in the complex ( p a 2). In accordance with this general behaviour of mercury ions, we can interpret the anodic wave I as due to the formation of [Hg(DB24C8)]*+ complex with a metal-to-ligand ratio of 1: 1, because the half-wave potential is independent of the concentration of ligand. The anodic wave II shifts by 30 mV per decade of ligand concentration, which corresponds to p = 2 and hence two ligands form the complex. It is rather unexpected that two crown ligands surround one metal centre because the more usual type of complexation by crowns is with the metal ion inside the polyether cavity. Another explanation could be based on the consideration that wave II is due to Hgi+ complexation which binds two crowns. The ratio of wave I to wave II is about 1: 2 for almost all the concentrations of ligand. The stability constants for complexes of Hg(I1) and Hg(1) can be evaluated from waves I and II,

I

-8O-

-3

-L

W1~lDB24C8lIrnM)

Fig. 2. Dependence of the half-wave potential of the anodic wave II from Fig. 1 on the concentration DB24C8.

of

149

Fig. 3. DC polarograms of DB24C8+0.37 mM Hg(N4), +O.l M TBAPF, in acetonitrile concentrations of DB24C8: (1) 0 mM; (2) 1 mM; (3) 2.6 mM; (4) 5.8 mM; (5) 11.1 mM.

at various

respectively, by applying eqn. (2). The values obtained are pK, = 12.5 and pK,, = 24.5, respectively. Further support for the interpretation of the anodic waves in terms of mercury ion complexation was obtained from experiments in which the free mercuric ions were added to the bulk of the solution (Fig. 3). By stepwise addition of DB24C8 to a solution of Hg(NO,), in acetonitrile the cathodic wave of the reduction of Hg(I1) is changed gradually into an anodic-cathodic wave. Originally the one-step process of Hg(I1) reduction consists of two separate waves: the first one is at potentials of wave I and the second one corresponds to wave II observed in solutions of DB24C8 in the absence of ions of mercury in the bulk. The complexation of Hg(1) and Hg(I1) ions by DB24C8 ligand is similar to the formation of complexes of mercury with diaza-polyoxa-macrobicyclic ligands [22,23], like cryptate (222). The cage-like ligand is also size-selective. Our previous study [22] indicated the difference in the complexation of Hg(I1) in the bulk and the complexation of Hg(1) formed in situ at the electrode surface. The differences in the reduction potentials of cryptate (222) complexes of Hg(I1) and Hg(1) were only about 100 mV, whereas the difference in the present case is considerably larger, almost 600 mV for DB24C8. A similar experiment performed with dibenzo-l&crown-6 (DB18C6) shows that the cyclic crown ether with a smaller ring size forms a considerably weaker complex (Fig. 4). The complex formation of DB18C6 is almost at the end of the detection limit and only careful evaluation of the shift of the wave position indicates a shift of 30 mV/decade. The estimated value of the stability constant is pK = 3.8 f 0.3. This gives a value of K larger than the stability constant reported for unsubstituted 18C6 by Rodriguez et al. [24], who found K = 263.

Fig. 4. DC polarograms of DBlK6+0.37 mM Hg(N4), +O.l concentrations of DB18C6: (1) 0 mM; (2) 1 mM; (3) 2 mM.

M TBAPF,

in acetonitrile

at various

The mechanism of complexation of metal ions by crown ligands is currently being investigated and many problems still remain unresolved. Two distinct ratsdetermining steps in the complexation reaction were considered: (i) Chock’s mechanism [25] is based on the assumption of a pre-association change of the crown structure prior to the formation of a complex. The comparative study of Rodriguez et al. [24] seems to exclude such a step for the case of non-spherical cations like Pb2’ and Hg 2+, because these cations are coordinated more rapidly than cations with the structure of an inert gas (like Sr2+ or Ba’+). (ii) The J3igen and Winkler [26] model assumes slow desolvation of cations prior to their complexation inside the crown cavity to be the rate-determining step. This model was supported by Rebek et al. [27] on the basis of experiments in which very slow complexation was observed for the case of organomercury compounds and Hg(CN), (month to hours, respectively) complexed to DB18C6. However, a larger crown (2OC7) reacts in under 1 s. Also, the dibenzo-substituents increase the rate by a factor of 60. The experiments performed by us did not indicate any time effects. The complex formation reported in the present study is consistent with a mechanism including anodic dissolution of the mercury electrode followed by fast complexation by dibenzo-24-crown-8 without any energetic barrier involving the conformational changes of the crown ring.

151

The interaction of the quinone redox couple with the Hg-crown The reduction two one-electron Q + e-+

Q’-+

of p-benzoquinone (abbreviated steps in acetonitrile: e-+

complex

Q) is well known

and proceeds

in

Q*-

In this study we will discuss only the first, reversible reduction, which is strongly dependent on the presence of the crown-Hg complex formed at the electrode by the anodic dissolution process described in the previous section. Already a very small concentration of Q (in the range from 10 to 100 PM) in the solution containing DB24C8 changes the anodic wave of Hg-crown formation to an anodic-cathodic wave (Fig. 5). The reduction wave of quinone at - 0.4 V is not observed at all in this concentration range, although the reduction of quinone is clearly detectable at these concentrations in the absence of the crown (see the bottom part of Fig. 5). The normal reversible wave of Q develops at higher concentrations of Q and maintains all of its features.

f---jg

0

E/V

-1

Fig. 5. DC polarograms of 15.5 mM DB24C8 +O.l M TBAPF, in acetonitrile at various concentrations of p-benzoquinone: (1) 50 PM; 92) 100 CM; (3) 200 PM; (4) 300 CM. Curves 1’. 2’, 3’ and 4’ at the bottom show the reduction of benaoquinone at the same concentrations as above but in the absence of the crown. Anodic curves of DB24C8 in the absence of quinone are shown for the crown concentrations: (a) 0; (b) 7 mM; (c) 15.5 mM.

152

The appearance of the anodic-cathodic wave in the presence of DB24C8 which is more positive than the reduction of quinone indicates the formation of a complex adduct of the reduced form of quinone Q’- with the mercury-crown complex. The shift of quinone reduction towards positive potentials confirms that there is complexation of the reduced form of quinone with complexed mercurous ions. The formation of ion-pairs of organic radicals with metallic cations is well known in the literature. Also, the ion-pair formation of quinone radical and alkali-metal cations has been described by non-electrochemical techniques [4-61, which solved the questions of possible cation migration between the two oxygen atoms in the quinone anion radical and the mobility of equilibria on the ESR time-scale. The ESR experiments with mixtures of Q + Q’- even postulated the formation of triple-ions involving two quinone species and two alkali-metal cations which was strongly dependent on the solvation properties of the cations. The experimental conditions in the present study exclude the possibility of quinone-quinone radical interaction because no reduction wave of free quinone is observed in the concentration range from 10 to 100 PM (see Fig. 5). The appearance of an anodic-cathodic wave at 0 V in the presence of quinone indicates the catalytic reduction of quinone at that potential accompanied by re-oxidation of the Hg(I)-crown complex. The inverse reaction, the oxidation of hydroquinone in water by Hg(1) or Hg(I1) ions, has been reported as a facile step [7,8,28,29] and was suggested for analytical determination, or for selective preparative purposes. This would imply that the reduction of quinone is difficult, even though the results in aqueous solvent mixtures may not have a straightforward parallel in the non-aqueous medium used in this study. Since there is a concentration limit at about [Q] = 0.1 mM beyond which no further increase of the anodiccathodic wave at 0 V is observed, there must be kinetic control of the reaction between the mercury-crown complex and quinone. The adsorption of mercurycrown complex was detected by ac polarography and therefore it is quite plausible to consider that only the adsorbed complex can participate in the catalytic cycle involving the reduction of quinone. The reaction mechanism possible according to the available data can be written as /

Hg( II)-crown

electrode +e- fast I

Hg(I)-crown

slow

J

Q’-

k

L

\

[Hg+crown-Q’-]

Q

I-

The rate constant can be evaluated under pseudo-monomolecular conditions using the Kouteckjr theory [20]. The resulting value was estimated to be k = 108.8 mol-’ 1 s-i.

153 CONCLUSIONS The anodic dissolution of mercury in the presence of two crown ligands proceeds with the formation of complexes. The smaller DBlK6 forms a rather weak complex, whereas DB24C8 exerts much stronger complexing properties. In analogy to the complexation of mercury ions by cryptate (222), the two anodic waves were interpreted as due to the formation of complexes with Hg(I1) and Hg(1) ions. The complex of Hg(I)-DB24C8 formed in situ is capable of reducing benzoquinone at potentials more positive than its normal redox potential. ACKNOWLEDGEMENTS

This study was performed under a research agreement between the Istituto di Chimica Analitica Strumentale C.N.R. and the Czechoslovak Academy of Sciences. The authors are indebted to Professor A.A. Vlcek for valuable discussions and to F. Bocci for technical assistance. Financial support from P.F. Chim. Fine is gratefully acknowledged. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

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