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Electrochemistry of rhenium(V) complexes with bidentate-bidentate and tridentate-bidentate schiff base ligands
Polyhedron Vol. 5, No. 12, pp. 1975-1982.1986 Printed in Great Britain
0
0277-5387/86 1986 Pergamon
53.00+.00 Journals Ltd
ELECTROCHEMISTRY OF RHENIUM(V) COMPLEXES WITH BIDENTATGBIDENTATE AND TRIDENTATGBIDENTATE SCHIFF BASE LIGANDS RENATO Dipartimento
di Chimica, Universita di Siena, Pian dei Mantellini 44, 53 100 Siena, Italy GIAN ANTONIO
Dipartimento
SEEBER
MAZZOCCHIN
di Spettroscopia ed Elettrochimica, Universitl di Venezia, Dorsoduro 2137, 30123 Venezia, Italy and
I-JLDERICO MAZZI,* FIORENZO REFOSCO and FRANCFSCO
TISATO
Istituto di Chimica e Tecnologia dei Radioelementi de1 C.N.R., Area della Ricerca, Corso Stati Uniti 4,351OO Padova, Italy (Received 24 February 1986 ; accepted 7 May 1986) Abstract-The
cathodic and anodic behaviour of rhenium(V) complexes, characterized by the Re03+ core, with bidentate and tridentate Schiff base ligands, has been studied in acetonitrile solvent. Cyclic voltammetry and controlled potential coulometry were the main electroanalytical techniques employed to define the electrode processes. Electrolyses were also carried out with the aim to identify the nature of the reduced and oxidized products. In particular, it was possible to isolate and characterize new rhenium(VI) complexes, containing the group Re04+, and the possibility of obtaining stable rhenium(IV) complexes has also been proved.
Our previous electrochemical studies have been devoted to complexes of rhenium in oxidation states ranging from + 1 to +4.‘” Oxidizing or reducing these compounds, containing ligands such as carbon monoxide and different phosphines, we were able to prepare and characterize unusual rhenium compounds, as well as to define the electrode mechanisms through which they are formed. Since rhenium is known to assume a wide range of possible oxidation states (from - 1 to + 7) our attention has now turned toward complexes with the metal in higher oxidation states. In addition to the interest of the chemistry of the complexes of this metal, a further appealing aspect lies in the growing importance of 99mTcin nuclear medicine ; our previous electrochemical studies on complexes of rhenium and technetium’” have allowed us to draw out useful comparisons, as well as to gain, on the basis of the studies on the complexes of the fist metal, *Author to whom correspondence should be addressed.
important suggestions on the properties of those of the latter element. Relatively simple procedures are requested to synthesize rhenium(V) complexes containing the Re03+ core ; this atomic group involves specific interesting chemical properties and the resulting complexes exhibit quite satisfactory stability.“’ We report here electrochemical studies on Schiff base rhenium(V) complexes of the type [ReOCl(BSB)d and [ReO(TSB)(BSB)] where BSB = N-methylsalicylideneiminate or N-phenylsalicylideneiminate and TSB = N-(2-oxidophenyl)salicylideneiminate. Our aim is to elucidate both anodic and cathodic electrode mechanisms, as well as to characterize the final electrolysis products in which Re02+ or Re04+ cores may be present. EXPERIMENTAL Acetonitrile (MeCN) (Fluka-Burdick &Jackson, distilled in glass) was used without further puri-
1976
R. SEEBER
et al.
0
Phsal
Mesa1
OPhsal
Scheme 1.
fication, storing it under nitrogen on 0.3 nm molecular sieves (Union Carbide). NaC104 supporting electrolyte was prepared by neutralizing HC104 with Na2C03 ; then it was twice crystallized from hot water and dried at 110°C. ~Et,]ClO,(TEAP)(C. Erba) supporting electrolyte was dried in a vacuum oven at 50°C and used without further purification. 99.99% pure nitrogen was bubbled through the solutions before the electrochemical tests, in order to remove oxygen. The synthesis of the studied complexes has been described elsewhere.7,8*‘0 In the voltammetric tests the polarizing unit was a PAR 170 Electrochemistry System ; voltammograms were recorded either with a Linseis LY 1800 pen recorder or with a Hewlett-Packard 1223 A storage oscilloscope, depending on the potential scan rate. The stationary working electrode was either a platinum disc (geometrical area of about 5 mm2) or a gold sphere freshly covered with mercury ; they both were surrounded by a platinum spiral counter electrode ; the saturated calomel electrode-reference electrode-was connected with the working electrode compartment through a salt bridge ending in a Luggin capillary. In the controlled potential electrolyses an AMEL model 552 potentiostat with an associated AMEL model 558 coulometer were used ; these tests were carried out in an H-shaped cell with anodic and cathodic compartments separated from each other by a sintered glass disc. A platinum gauze was used as working electrode while the auxiliary electrode was a mercury pool. In the voltammetric as well as in the coulometric experiments, TEAP was used as supporting electrolyte. NaC104 was employed in exhaustive electrolyses performed with the aim to recover the reduction or oxidation products. All electrochemical measurements were carried out at 25 If;:O.l”C. The reference electrode was an
aqueous saturated calomel electrode (SCE) ; however, after performing the voltammetric tests, biscyclopentadienyl iron was added to the solutions in order to refer the calculated E’,,, to Er,,2 of this “internal reference” redox couple. In this way, the measured quantity results are unaffected by variable liquid junction potential at the aqueousorganic solvents interface. In order to isolate the electrolysis products, namely the cathodic reduction products, different procedures were followed for the different compounds. In particular, the solutions after reduction of [ReOCl(BSB)J complexes were dried removing MeCN solvent under reduced pressure and the resulting solids were washed with CHC& to solubilize the rhenium complexes ; after filtering, this last solvent was also removed and the pure solid was analyzed by elemental analysis, IR spectroscopy and magnetic moment measurement. All these recovery procedures were performed carefully avoiding the presence of oxygen and by using deaerated solvents. A similar procedure was followed for the reduction products of [ReO(TSB)(BSB)] ; however, it was observed that decomposition and/or reoxidation of the electrolysis products could be avoided only if the solid obtained after removal of MeCN was treated with a very small amount of a protic solvent, typically ethanol. IR spectra were recorded on a Perkin-Elmer model 580B spectrometer. Magnetic moment measurements were performed with an Oxford Instrument.
RESULTS
AND DISCUSSION
[ReOCl(BSB&J Cathodic behuuiour. Figure 1 reports the voltammetric behaviour of the complex [ReOCl(Phsal)J in acetonitrile solution. A first reversible elec-
Electrochemistry of rhenium(V) complexes
Et-1
1977
CV (SCEII
Fig. 1. Cyclic voltammograms recorded on a 4.9 x lo-’ M [ReOCl(Phsal)& 0.1 M TEAP, and MeCN solution. Platinum working rnicroelectrode; potential scan rate, 0.2 V s-‘. (0) Starting potential.
trode reduction leads to a species which is stable enough on the short voltammetric time scale (the ratio of backward to forward peak currents relative to the peaks system A-B is always quite near to one). Exhaustive coulometric electrolyses at peak A lead to a consumption of 1 mol of electrons per mole of starting compound. Voltammetric curves recorded on the electrolysed solutions show a further reduction peak at potential values quite near those of peak C ; in addition, the reduced complex is oxidizable at an anodic peak, rather low in height, which corresponds to peak D of Fig. 1, but no peak is recorded at the potential of peak B. Oxidizing the reduced solutions at a potential only just less cathodic than those where oxidation of the starting complex occurs, this last can be regenerated with a good yield (cu. 60-70%). The analysis of the reduced solutions reveals the presence of free chloride ions ; accordingly, performing electrolyses using NaClO, as supporting electrolyte, NaCl is precipitated. Moreover, from these solutions the reduced rhenium complex can be recovered and characterized following the procedure described above. The elemental analysis data are as follows (found : C, 52.4 ; H, 3.4 ; N, 4.8. Calc. for CZ6HZ0ReNZ03: C, 52.5 ; H, 3.4 ; N, 4.7%). Two most significant data can be drawn from the IR spectra. The v(Re-Cl) absorption, located at 311 cm-’ in the spectrum of the starting complexes, is now absent, in agreement with the presence of free chloride ions in the reduced solution. On the con-
trary the v(Re=O) is still present, at a wavenumber value quite close to that for the starting complex (960 cm-‘). Magnetic susceptibility measurements on solid samples at room temperature lead to peff.= 1.96 B.M., in agreement with a d3 configuration in a square pyramidal environment, as reported by Hoffman et al.” The compound is soluble in EtOH, CH&, CHC& and MeCN, and insoluble in Et*0 and CsH6. However, it slowly decomposes in chlorinated solvents and no physico-chemical measurements could be performed on these solutions. Accordingly with the reported data the more probable configuration can be the one in Fig. 2. The stereochemical position of the two bidentate Schiff base in the horizontal square pyramidal plane can be deduced from the possible reaction mechanism leading to the product. In fact after the electrode reaction the chlorine atom is the most probable leaving group. The substitution with the phenolic oxygen of the ligand allows the formation of the product without the detaching of the whole ligand and consequently the reproduction of the starting compound both by electrochemical and chemical oxidation.
Fig. 2. Schematic drawing of the rhenium(IV) compound obtained by cathodic reduction of [ReOCl(Phsal),]. l@ = Phsal.
R. SEEBER et al.
1978
On the basis of these findings, the electrode reduction mechanism at peak A can be summarized as follows : [ReOCl(Phsal)d + e- G [ReOCl(Phsal)J-
(1)
[ReOCl(Phsal),]-
(2)
+ [ReO(Phsal)J + Cl-
Since oxidation of [ReO(Phsal)J to the starting compound requires the uptake of a chloride ion in a 1 : 1 ratio, it is reasonable that the regeneration of [ReOCl(Phsal),] can occur only with a non-quantitative yield. The slowness of the chemical reaction following the reduction charge transfer step allows the evaluation of E’,,, for the couple [ReOCl (Phsal)J[ReOCl(Phsal)d-, which results equal to - 1.43 V (referred to E;,2 of the ferricinium ion/ ferrocene couple). As to the electrode process occurring at peak C, it involves the uptake of one further electron, but this reduction is followed by irreversible decomposition reactions leading to products which could not be identified. [ReOCl(Mesal)J exhibits an electrochemical behaviour which is only qualitatively similar to that of the corresponding Phsal complex. In fact, the cathodic reduction of [ReOCl(Mesal)J shows some significant differences ; as to the voltammetric behaviour, the anodic peak directly associated to the first cathodic one does not result, at any potential scan rate, as high as required for a backward to the forward peak-currents-ratio equal to one. In particular, this ratio increases increasing the potential scan rate, reaching unity only for a high enough value of this experimental parameter. Also in this case, in controlled potential coulometries at potentials of the first reduction peak, 1 mol of electrons per mole of starting complex is spent, and chloride ions are present in the solution. The voltammograms of these electrolyzed solutions show that the product can be further reduced corresponding to a peak with (Ep)c N - 1.55 V vs SCE and oxidized at a peak ((E,), N +0.6 V vs SCE) with a small directly associated cathodic peak. Also in this case we have tried to isolate the reduction product following the same procedure, but when CHC& is used it is quickly converted to the starting complex. Nevertheless, since after reduction insoluble NaCl is recovered, also in this case the same reduction mechanism, previously outlined for [ReOCl(Phsal)& can be hypothesized, even if the voltammetric data agree with a faster release of the chloride ion by the electrogenerated anionic species. On the basis of the trend of the backward to forward peak currents ratio at varying the potential scan rate,” a half-life time of about 0.2 s (at 25 +O.lC) can be evaluated for the directly electrogenerated
species. Cyclic voltammograms recorded at high enough potential scan rate, allow us to calculate E;,,, [ReOCl(Mesal)d/[ReOCl(Mesal)~- = - 1.54 V (vs Er,,, of the couple ferricinium ion/ferrocene). As to the reduction and oxidation processes this reduced compound undergoes, while its further reduction, involving the consumption of 1 mol of electrons per mole of starting complex, does not lead to any stable rhenium complex, electrode oxidation at a potential of +0.7 V vs SCE allows one to regenerate the starting compound at a yield higher than 60%. It must be taken into account that the presence in the solution of free chloride ions, which strongly adsorb on the platinum electrode surface, prevents the recording of completely reliable voltammetric curves in the anodic potential region, as well as carrying out satisfactory electrolysis tests at those potentials. As regards the electrode reduction of both the studied [ReOCl(BSB)J complexes, completely similar results are obtained by using a mercury microelectrode, with respect to both shape and location of the cathodic voltammetric peaks, as well as to the trend of the peak currents ratio at varying the potential sweep rate. These facts allow us to conclude that the charge transfer occurs via an outer sphere electrode mechanism.i3 Anodic behaviour. As shown in Fig. 1, [ReOCl(Phsal),] undergoes a reversible electrode oxidation. Controlled potential coulometries reveal it is a one-electron oxidation ; furthermore, the charge transfer is not complicated by any following chemical reaction, neither on the voltammetric nor on the exhaustive electrolysis time scale. In fact, the ratio between backward and forward peak currents is equal to one for any potential scan rate and, even more significantly, cyclic voltammetry after exhaustive oxidation shows the same reversible peaks system (see Fig. 3). Of course, peak F is first recorded in a forward cathodic scan starting at a potential of about + 1.5 V vs SCE and peak E is recorded as an associated peak if the potential scan direction is reversed after traversing peak F. If, on the contrary, the cathodic scan goes on, the peaks systems reported in the cathodic potentials region of Fig. 1 are recorded, suggesting that the starting complex is regenerated at the potentials of peak F. Accordingly, exhaustively reducing, at the potentials of peak F, solutions of [ReOCl(Phsal)d previously oxidized at peak E, the starting compound is regenerated with a yield very near 100%. Quite a similar behaviour is exhibited by the compound [ReOCl(Mesal)J, as regards both voltammetric and coulometric-voltammetric results. These findings allow us to propose a method to prepare via electrochemical procedures stable
Electrochemistry
of rhenium(V) complexes
1979
Fig. 3. Cyclic voltammograms recorded on a 4.9 x low3 M [ReOCl(Phsal)d solution, previously oxidized exhaustively in correspondence to peak E of the curve in Fig. 1. Same experimental conditions as for the voltammograms in Fig. 1. (0) Starting potential.
rhenium(V1) complexes containing the Re04+ core. On the basis of the voltammetric tests the following Ei,, values can be calculated: E’,,,, ~~Cl(Phsal)23’/[Reocl(Phsal)zl= +0.76 V; Ei,z [ReOCl(Mesal);l’/[ReOCl(Mesal)~ = +0.67 V (vs Eip of the couple ferricinium ion/ferrocene). [ReO(TSB)(BSB)] Cathodic behaviour. Figure 4 illustrates the vol-
tammetric behaviour
of [ReO(OPhsal)(Phsal)]
in
acetonitrile solution. It is evident that no anodic response directly associated to the cathodic peak A can be recorded at the potential scan rate employed to draw the figure (0.2 V s-l). Only at sweep rates higher than 2 V s-’ the peak attributable to the oxidation of the species primarily formed at the electrode can be noted, with a relative height progressively increasing at increasing the scan rate. On the basis of the dependence on this last experimental of the parameter of the ratio (ip)b.Ekward/(ip)for~~,~, results of a series of voltammetric measurements
Fig. 4. Cyclic voltammograms recorded on a 4.5 x lo-’ M [ReO(OPhsal)(Phsal)], 0.1 M TEAP, and MeCN solution. Platinum working microelectrode ; potential scan rate, 0.2 V s-r. (0) Starting potential.
1980
R. SEEBER et al.
with different depolarizer concentrations, of chronoamperometric tests and, finally, of the coulometric datum (1 mol of electrons per mole of electrolyzed compound is spent), it is possible to conclude that the electrode reduction consists in an EC mechanism, were a one-electron reversible charge transfer is followed by a first order irreversible chemical reaction. The cyclic voltammetric curves at different potential scan rates allow us to estimate a half-life time of about 25 ms for the short-lived eleo trogenerated species. The E’,,, value of the couple [ReO(OPhsal)(Phsal)]/[ReO(OPhsal)(Phsal)]can be estimated only approximately (N - 1.33 vs E’,,, of the couple ferricinium ion/ferrocene). Voltammetric curves recorded on the exhaustively electrolyzed solutions appear as those shown in Fig. 5. During a sweep starting at the potential of the electrolyses performed, peak B is recorded. By switching the potential after traversing this anodic peak, peak A, attributable to the reduction of the complex [ReO(OPhsal)(Phsal)], is recorded. If, on the contrary, the anodic sweep continues towards more positive potentials, the same peaks (system C/D) recorded on solutions of the starting complex, can be noted. Accordingly, electrolysing the reduced solutions at a potential of 0.0 V vs SCE, the starting complex is quantitatively regenerated through the consumption of 1 mol of electrons per mole of compound.
Solutions of the starting compound were also exhaustively reduced in order to identify the final electrolysis products. As we have already mentioned in the Experimental section, through the usual procedure involving dissolution of the reduction product in CHC13, even if it was carried out in carefully deoxygenated media, we did not manage to avoid that the compound partially decomposed and partially regenerated the starting complex. However, the product was stabilized by treating it with ethanol. Recalling that the voltammetric results suggest that the primary reduction product, [ReO(OPhsal)(Phsal)]-, undergoes a fast chemical reaction leading to a complex that, by oxidation regenerates the starting compound, we can suppose that the chemical reaction consists in the release by the metal centre of one phenolic site of the tridentate ligand, as a consequence of the reduced positive charge on rhenium. On the other hand if, through electrode oxidation, we increase back the rhenium oxidation state, the initial situation is re-established. In this context, the role of the protic solvent in stabilizing the reduction product may consist in the protonation of the free phenolic site. Accordingly, the solubility of the product in MeCN results is noticeably lower after protonation. The elemental analysis data are as follows (found: C, 44.9; H, 2.7; N, 3.9. Calc. for
.!?(-I
1too Fig. 5. Cyclic voltammograms recorded on a 4.5 x lo-’ M [ReO(OPhsal)(Phsal)] solution, previously reduced exhaustively at peak A of the curve in Fig. 4. Same experimental conditions as for the voltammograms in Fig. 4. (0) Starting potential.
Electrochemistry of rhenium(V) complexes
CZ6H2,,ReN204: C, 51.1; H, 3.3 ; N, 4.6%). The compound could not be further purified owing to the continuous transformation back to the starting complex. For this reason the elemental analyses are not in agreement with the formulation but the C: N: H ratio are consistent with the presence of two ligands in the product and presumably the impurities are due to inorganic compounds. IR spectra of the recovered reduction products show a sign&ant lowering of the wavenumber of the v(Re==O) absorption (a shift of about 50 cm-‘). This result is in apparent disagreement with that obtained on the reduction product of [ReOCl(Phsal)J, but a weakening of the Re=O bond can be explained assuming the formation of an intramolecular hydrogen bond. On these bases, and considering, as reported above, only the leaving of the phenolic groups, we propose the formulation reported in Fig. 6 for the product of the [ReO(OPhsal)(Phsal)] reduction. The compound is soluble in EtOH, MeOH and MeCN and insoluble in Et,0 and pentane ; moreover, it easily regenerates the starting complex in CH$I,?, CHCls, C6Hs and acetone. The results obtained with the complex [ReO (OPhsal)(Mesal)] are only qualitatively similar. Apart from the exact values of the potentials at which the different peaks are recorded, Fig. 4 is still useful to discuss the electrochemical behaviour of this compound. The main differences lie in the lower stability of the electrogenerated species [ReOCl (OPhsal)(Mesal)]- (half-life time of about 5 ms), and in the value of Er,,2 for the couple [ReO (OPhsal)(Mesal)]/[ReO(OPhsal)(Mesal)]-, which resulted equal to about - 1.37 V (vs E;,2 of the couple ferricinium ion/ferrocene). Also in this case the reduced solution can be reoxidized at a potential noticeably anodic in respect to the potentials of the cathodic peak (see, qualitatively, peak B in Fig. 5), regenerating quantitatively the starting complex. The recovery of the reduction product has been more difficult in respect to the corresponding Phsal complex, because it exhibits a marked tendency to regenerate the starting compound if only small
OH----O o/~JLZ<~,o N
Fig. 6. Proposed scheme for the rhenium(W) molecule obtained by cathodic reduction of [ReO(OPhsal)(Phsal)], after treatment with ethanol. I@I = Phsal and OH = HOPhsal.
1981
traces of oxidizing agents such as oxygen or water, are present. It was only possible to recover a mixture of starting complex and reduced product. IR spectra of this mixture show a first v(Re=O) band at 924 cm-‘, attributable to the reduction product and a second v(Re=O) band at 957 cm-‘, due to the starting complex. The formulation of this compound can be assumed as reported in Fig. 6 by analogy with the previous one. Also for these tridentate-bidentate compounds, completely similar results are obtained using mercury as the working electrode material. Anodic behaviour. As Fig. 4 shows for the complex [ReO(OPhsal)(Phsal)], both the [ReO(TSB) (BSB)] compounds undergo reversible oxidation. The oxidized species are stable on the voltametric time scale, so much so that backward to forward peak ratios equal unity also at the lowest (50 mV s-‘) potential scan rate used. Controlled potential electrolyses allows one to ascertain that the electrode processes involve a one-electron chargetransfer ; however the oxidized species are not stable at the longer electrolysis times and voltammetric tests after electrolyses do not show the peak system C/D giving any interesting information about the presence in the solution of some particular rhenium complex either. Many attempts to recover the oxidation products have been performed. Unfortunately, even if different procedures have been tried, a brown oil was always obtained and all attempts to obtain crystalline products has failed. On the basis of the voltammetric data it is possible to evaluate E’&, [ReO(OPhsal)(Phsal)]‘/ [ReO(OPhsal)(Phsal)] = +0.57 V, and Er,,2, [ReO (OPhsal)(Mesal)]+/[ReO(OPhsal)(Mesal)] = f0.53 V (vs E;,* of the couple ferricinium ion/ferrocene). As a final consideration, let us observe that the relative values of the E’,,, calculated for the eight redox couples identified result in agreement with the different electron withdrawing effects of methyl and phenyl groups. In fact, both as regard [ReOCl(BSB)d and [ReO(TSB)(BSB)], the resulting E\,* values for neutral/anionic and cationic/ neutral redox couples are more positive for BSB = Phsal than for BSB = Mesal. From the recovery products point of view, the complexes have shown the presence of Re=O 0x0 moiety. This fact is particularly important if it is considered that the Re=O group was found in the same complex with rhenium atom in three different oxidation states : VI, V and IV. To date the principal example of the Re04+ core was given by the complex [ReOCI,]- 14*”and in a few of its substitution products.16 The Re02+ core was never clearly authenticated. In particular the 0x0 oxygen in the fourth oxi-
1982
R. SEEBER et al.
dation state was always considered bound to a hydrogen atom, for example in the [Re(OH)Cl, (PEtlPh)J compound reported by Duatti et ~1.‘~ and the formal oxidation state of oxogroup was - 1. Concerning the [ReO(BSB)J complexes, only phenyl protons can interact with the 0x0 group, while in the case of [ReO(TSB)(BSB)] complexes a phenolic hydrogen can bind Re==O ; however, in both cases 0x0 group is considered in -2 formal oxidation state.
REFERENCES 1. R. Seeber, G. A. Mazzocchin, E. Roncari and U. Mazzi, Transition Met. Chem. 1981,6, 123. 2. E. Roncari, U. Mazzi, R. Seeber and P. Zanello, J. Electroanal. Chem. 1982,132,221. 3. R. Seeber, G. A. Mazzocchin, U. Mazzi, E. Roncari and F. Refosco, Transition Met. Chem. 1984,9, 315. 4. G. A. Mazzoccbin, R. Seeber, U. Mazzi and E. Roncari, Znorg. Chim. Acta 1978,29, 1.
5. G. A. Mazzocchin, R. Seeber, U. Mazzi and E. Roncari, Znorg. Chim. Acta 1978,29, 5. 6. U. Mazzi, E. Roncari, R. Seeber and G. A. Mazzoccbin, Znorg. Chim. Acta 1980,41,95. 7. U. Mazzi, E. Roncari, R. Rossi, V. Bertolasi, 0. Traverso and L. Magon, Transition Met. Chem. 1980, 5, 289. 8. A. Marchi, A. Duatti, R. Rossi, L. Magon, U. Mazzi and A. Pasquetto, Znorg. Chim. Acta 1984,81, 15. 9. U. Mazzi, F. Refosco, G. Bandoli and M. Nicolini, Transition Met. Chem. 1985, 10, 121. 10. U. Mazzi, F. Refosco, F. Tisato, G. Bandoli and M. Nicolini, .Z. Chem. Sot., Dalton Trans. 1986, 1623. 11. R. Hoffman, M. M. L. Chen, M. Elian, A. R. Rossi and D. M. P. Mingos, Znorg. Chem. 1974,13,2666. 12. R. S. Nicholson and I. Shain, Analyt. Chem. 1964, 36,706. 13. V. I. Kravtsov, J. Electroanal. Chem. 1976,69, 125. 14. T. Lis and B. Jezowska-Trzebiatowska, Acta Cryst. 1982, B33,1248. 15. B. J. Brisdon and D. A. Edwards, Znorg. Chem. 1968, 7, 1897. 16. G. Rouschias, Chem. Rev. 1974,74,531. 17. M. Sacerdoti, V. Bertolasi, G. Gilli and A. Duatti, Acta Cry&. 1982, B38, 96.
0
0277-5387/86 1986 Pergamon
53.00+.00 Journals Ltd
ELECTROCHEMISTRY OF RHENIUM(V) COMPLEXES WITH BIDENTATGBIDENTATE AND TRIDENTATGBIDENTATE SCHIFF BASE LIGANDS RENATO Dipartimento
di Chimica, Universita di Siena, Pian dei Mantellini 44, 53 100 Siena, Italy GIAN ANTONIO
Dipartimento
SEEBER
MAZZOCCHIN
di Spettroscopia ed Elettrochimica, Universitl di Venezia, Dorsoduro 2137, 30123 Venezia, Italy and
I-JLDERICO MAZZI,* FIORENZO REFOSCO and FRANCFSCO
TISATO
Istituto di Chimica e Tecnologia dei Radioelementi de1 C.N.R., Area della Ricerca, Corso Stati Uniti 4,351OO Padova, Italy (Received 24 February 1986 ; accepted 7 May 1986) Abstract-The
cathodic and anodic behaviour of rhenium(V) complexes, characterized by the Re03+ core, with bidentate and tridentate Schiff base ligands, has been studied in acetonitrile solvent. Cyclic voltammetry and controlled potential coulometry were the main electroanalytical techniques employed to define the electrode processes. Electrolyses were also carried out with the aim to identify the nature of the reduced and oxidized products. In particular, it was possible to isolate and characterize new rhenium(VI) complexes, containing the group Re04+, and the possibility of obtaining stable rhenium(IV) complexes has also been proved.
Our previous electrochemical studies have been devoted to complexes of rhenium in oxidation states ranging from + 1 to +4.‘” Oxidizing or reducing these compounds, containing ligands such as carbon monoxide and different phosphines, we were able to prepare and characterize unusual rhenium compounds, as well as to define the electrode mechanisms through which they are formed. Since rhenium is known to assume a wide range of possible oxidation states (from - 1 to + 7) our attention has now turned toward complexes with the metal in higher oxidation states. In addition to the interest of the chemistry of the complexes of this metal, a further appealing aspect lies in the growing importance of 99mTcin nuclear medicine ; our previous electrochemical studies on complexes of rhenium and technetium’” have allowed us to draw out useful comparisons, as well as to gain, on the basis of the studies on the complexes of the fist metal, *Author to whom correspondence should be addressed.
important suggestions on the properties of those of the latter element. Relatively simple procedures are requested to synthesize rhenium(V) complexes containing the Re03+ core ; this atomic group involves specific interesting chemical properties and the resulting complexes exhibit quite satisfactory stability.“’ We report here electrochemical studies on Schiff base rhenium(V) complexes of the type [ReOCl(BSB)d and [ReO(TSB)(BSB)] where BSB = N-methylsalicylideneiminate or N-phenylsalicylideneiminate and TSB = N-(2-oxidophenyl)salicylideneiminate. Our aim is to elucidate both anodic and cathodic electrode mechanisms, as well as to characterize the final electrolysis products in which Re02+ or Re04+ cores may be present. EXPERIMENTAL Acetonitrile (MeCN) (Fluka-Burdick &Jackson, distilled in glass) was used without further puri-
1976
R. SEEBER
et al.
0
Phsal
Mesa1
OPhsal
Scheme 1.
fication, storing it under nitrogen on 0.3 nm molecular sieves (Union Carbide). NaC104 supporting electrolyte was prepared by neutralizing HC104 with Na2C03 ; then it was twice crystallized from hot water and dried at 110°C. ~Et,]ClO,(TEAP)(C. Erba) supporting electrolyte was dried in a vacuum oven at 50°C and used without further purification. 99.99% pure nitrogen was bubbled through the solutions before the electrochemical tests, in order to remove oxygen. The synthesis of the studied complexes has been described elsewhere.7,8*‘0 In the voltammetric tests the polarizing unit was a PAR 170 Electrochemistry System ; voltammograms were recorded either with a Linseis LY 1800 pen recorder or with a Hewlett-Packard 1223 A storage oscilloscope, depending on the potential scan rate. The stationary working electrode was either a platinum disc (geometrical area of about 5 mm2) or a gold sphere freshly covered with mercury ; they both were surrounded by a platinum spiral counter electrode ; the saturated calomel electrode-reference electrode-was connected with the working electrode compartment through a salt bridge ending in a Luggin capillary. In the controlled potential electrolyses an AMEL model 552 potentiostat with an associated AMEL model 558 coulometer were used ; these tests were carried out in an H-shaped cell with anodic and cathodic compartments separated from each other by a sintered glass disc. A platinum gauze was used as working electrode while the auxiliary electrode was a mercury pool. In the voltammetric as well as in the coulometric experiments, TEAP was used as supporting electrolyte. NaC104 was employed in exhaustive electrolyses performed with the aim to recover the reduction or oxidation products. All electrochemical measurements were carried out at 25 If;:O.l”C. The reference electrode was an
aqueous saturated calomel electrode (SCE) ; however, after performing the voltammetric tests, biscyclopentadienyl iron was added to the solutions in order to refer the calculated E’,,, to Er,,2 of this “internal reference” redox couple. In this way, the measured quantity results are unaffected by variable liquid junction potential at the aqueousorganic solvents interface. In order to isolate the electrolysis products, namely the cathodic reduction products, different procedures were followed for the different compounds. In particular, the solutions after reduction of [ReOCl(BSB)J complexes were dried removing MeCN solvent under reduced pressure and the resulting solids were washed with CHC& to solubilize the rhenium complexes ; after filtering, this last solvent was also removed and the pure solid was analyzed by elemental analysis, IR spectroscopy and magnetic moment measurement. All these recovery procedures were performed carefully avoiding the presence of oxygen and by using deaerated solvents. A similar procedure was followed for the reduction products of [ReO(TSB)(BSB)] ; however, it was observed that decomposition and/or reoxidation of the electrolysis products could be avoided only if the solid obtained after removal of MeCN was treated with a very small amount of a protic solvent, typically ethanol. IR spectra were recorded on a Perkin-Elmer model 580B spectrometer. Magnetic moment measurements were performed with an Oxford Instrument.
RESULTS
AND DISCUSSION
[ReOCl(BSB&J Cathodic behuuiour. Figure 1 reports the voltammetric behaviour of the complex [ReOCl(Phsal)J in acetonitrile solution. A first reversible elec-
Electrochemistry of rhenium(V) complexes
Et-1
1977
CV (SCEII
Fig. 1. Cyclic voltammograms recorded on a 4.9 x lo-’ M [ReOCl(Phsal)& 0.1 M TEAP, and MeCN solution. Platinum working rnicroelectrode; potential scan rate, 0.2 V s-‘. (0) Starting potential.
trode reduction leads to a species which is stable enough on the short voltammetric time scale (the ratio of backward to forward peak currents relative to the peaks system A-B is always quite near to one). Exhaustive coulometric electrolyses at peak A lead to a consumption of 1 mol of electrons per mole of starting compound. Voltammetric curves recorded on the electrolysed solutions show a further reduction peak at potential values quite near those of peak C ; in addition, the reduced complex is oxidizable at an anodic peak, rather low in height, which corresponds to peak D of Fig. 1, but no peak is recorded at the potential of peak B. Oxidizing the reduced solutions at a potential only just less cathodic than those where oxidation of the starting complex occurs, this last can be regenerated with a good yield (cu. 60-70%). The analysis of the reduced solutions reveals the presence of free chloride ions ; accordingly, performing electrolyses using NaClO, as supporting electrolyte, NaCl is precipitated. Moreover, from these solutions the reduced rhenium complex can be recovered and characterized following the procedure described above. The elemental analysis data are as follows (found : C, 52.4 ; H, 3.4 ; N, 4.8. Calc. for CZ6HZ0ReNZ03: C, 52.5 ; H, 3.4 ; N, 4.7%). Two most significant data can be drawn from the IR spectra. The v(Re-Cl) absorption, located at 311 cm-’ in the spectrum of the starting complexes, is now absent, in agreement with the presence of free chloride ions in the reduced solution. On the con-
trary the v(Re=O) is still present, at a wavenumber value quite close to that for the starting complex (960 cm-‘). Magnetic susceptibility measurements on solid samples at room temperature lead to peff.= 1.96 B.M., in agreement with a d3 configuration in a square pyramidal environment, as reported by Hoffman et al.” The compound is soluble in EtOH, CH&, CHC& and MeCN, and insoluble in Et*0 and CsH6. However, it slowly decomposes in chlorinated solvents and no physico-chemical measurements could be performed on these solutions. Accordingly with the reported data the more probable configuration can be the one in Fig. 2. The stereochemical position of the two bidentate Schiff base in the horizontal square pyramidal plane can be deduced from the possible reaction mechanism leading to the product. In fact after the electrode reaction the chlorine atom is the most probable leaving group. The substitution with the phenolic oxygen of the ligand allows the formation of the product without the detaching of the whole ligand and consequently the reproduction of the starting compound both by electrochemical and chemical oxidation.
Fig. 2. Schematic drawing of the rhenium(IV) compound obtained by cathodic reduction of [ReOCl(Phsal),]. l@ = Phsal.
R. SEEBER et al.
1978
On the basis of these findings, the electrode reduction mechanism at peak A can be summarized as follows : [ReOCl(Phsal)d + e- G [ReOCl(Phsal)J-
(1)
[ReOCl(Phsal),]-
(2)
+ [ReO(Phsal)J + Cl-
Since oxidation of [ReO(Phsal)J to the starting compound requires the uptake of a chloride ion in a 1 : 1 ratio, it is reasonable that the regeneration of [ReOCl(Phsal),] can occur only with a non-quantitative yield. The slowness of the chemical reaction following the reduction charge transfer step allows the evaluation of E’,,, for the couple [ReOCl (Phsal)J[ReOCl(Phsal)d-, which results equal to - 1.43 V (referred to E;,2 of the ferricinium ion/ ferrocene couple). As to the electrode process occurring at peak C, it involves the uptake of one further electron, but this reduction is followed by irreversible decomposition reactions leading to products which could not be identified. [ReOCl(Mesal)J exhibits an electrochemical behaviour which is only qualitatively similar to that of the corresponding Phsal complex. In fact, the cathodic reduction of [ReOCl(Mesal)J shows some significant differences ; as to the voltammetric behaviour, the anodic peak directly associated to the first cathodic one does not result, at any potential scan rate, as high as required for a backward to the forward peak-currents-ratio equal to one. In particular, this ratio increases increasing the potential scan rate, reaching unity only for a high enough value of this experimental parameter. Also in this case, in controlled potential coulometries at potentials of the first reduction peak, 1 mol of electrons per mole of starting complex is spent, and chloride ions are present in the solution. The voltammograms of these electrolyzed solutions show that the product can be further reduced corresponding to a peak with (Ep)c N - 1.55 V vs SCE and oxidized at a peak ((E,), N +0.6 V vs SCE) with a small directly associated cathodic peak. Also in this case we have tried to isolate the reduction product following the same procedure, but when CHC& is used it is quickly converted to the starting complex. Nevertheless, since after reduction insoluble NaCl is recovered, also in this case the same reduction mechanism, previously outlined for [ReOCl(Phsal)& can be hypothesized, even if the voltammetric data agree with a faster release of the chloride ion by the electrogenerated anionic species. On the basis of the trend of the backward to forward peak currents ratio at varying the potential scan rate,” a half-life time of about 0.2 s (at 25 +O.lC) can be evaluated for the directly electrogenerated
species. Cyclic voltammograms recorded at high enough potential scan rate, allow us to calculate E;,,, [ReOCl(Mesal)d/[ReOCl(Mesal)~- = - 1.54 V (vs Er,,, of the couple ferricinium ion/ferrocene). As to the reduction and oxidation processes this reduced compound undergoes, while its further reduction, involving the consumption of 1 mol of electrons per mole of starting complex, does not lead to any stable rhenium complex, electrode oxidation at a potential of +0.7 V vs SCE allows one to regenerate the starting compound at a yield higher than 60%. It must be taken into account that the presence in the solution of free chloride ions, which strongly adsorb on the platinum electrode surface, prevents the recording of completely reliable voltammetric curves in the anodic potential region, as well as carrying out satisfactory electrolysis tests at those potentials. As regards the electrode reduction of both the studied [ReOCl(BSB)J complexes, completely similar results are obtained by using a mercury microelectrode, with respect to both shape and location of the cathodic voltammetric peaks, as well as to the trend of the peak currents ratio at varying the potential sweep rate. These facts allow us to conclude that the charge transfer occurs via an outer sphere electrode mechanism.i3 Anodic behaviour. As shown in Fig. 1, [ReOCl(Phsal),] undergoes a reversible electrode oxidation. Controlled potential coulometries reveal it is a one-electron oxidation ; furthermore, the charge transfer is not complicated by any following chemical reaction, neither on the voltammetric nor on the exhaustive electrolysis time scale. In fact, the ratio between backward and forward peak currents is equal to one for any potential scan rate and, even more significantly, cyclic voltammetry after exhaustive oxidation shows the same reversible peaks system (see Fig. 3). Of course, peak F is first recorded in a forward cathodic scan starting at a potential of about + 1.5 V vs SCE and peak E is recorded as an associated peak if the potential scan direction is reversed after traversing peak F. If, on the contrary, the cathodic scan goes on, the peaks systems reported in the cathodic potentials region of Fig. 1 are recorded, suggesting that the starting complex is regenerated at the potentials of peak F. Accordingly, exhaustively reducing, at the potentials of peak F, solutions of [ReOCl(Phsal)d previously oxidized at peak E, the starting compound is regenerated with a yield very near 100%. Quite a similar behaviour is exhibited by the compound [ReOCl(Mesal)J, as regards both voltammetric and coulometric-voltammetric results. These findings allow us to propose a method to prepare via electrochemical procedures stable
Electrochemistry
of rhenium(V) complexes
1979
Fig. 3. Cyclic voltammograms recorded on a 4.9 x low3 M [ReOCl(Phsal)d solution, previously oxidized exhaustively in correspondence to peak E of the curve in Fig. 1. Same experimental conditions as for the voltammograms in Fig. 1. (0) Starting potential.
rhenium(V1) complexes containing the Re04+ core. On the basis of the voltammetric tests the following Ei,, values can be calculated: E’,,,, ~~Cl(Phsal)23’/[Reocl(Phsal)zl= +0.76 V; Ei,z [ReOCl(Mesal);l’/[ReOCl(Mesal)~ = +0.67 V (vs Eip of the couple ferricinium ion/ferrocene). [ReO(TSB)(BSB)] Cathodic behaviour. Figure 4 illustrates the vol-
tammetric behaviour
of [ReO(OPhsal)(Phsal)]
in
acetonitrile solution. It is evident that no anodic response directly associated to the cathodic peak A can be recorded at the potential scan rate employed to draw the figure (0.2 V s-l). Only at sweep rates higher than 2 V s-’ the peak attributable to the oxidation of the species primarily formed at the electrode can be noted, with a relative height progressively increasing at increasing the scan rate. On the basis of the dependence on this last experimental of the parameter of the ratio (ip)b.Ekward/(ip)for~~,~, results of a series of voltammetric measurements
Fig. 4. Cyclic voltammograms recorded on a 4.5 x lo-’ M [ReO(OPhsal)(Phsal)], 0.1 M TEAP, and MeCN solution. Platinum working microelectrode ; potential scan rate, 0.2 V s-r. (0) Starting potential.
1980
R. SEEBER et al.
with different depolarizer concentrations, of chronoamperometric tests and, finally, of the coulometric datum (1 mol of electrons per mole of electrolyzed compound is spent), it is possible to conclude that the electrode reduction consists in an EC mechanism, were a one-electron reversible charge transfer is followed by a first order irreversible chemical reaction. The cyclic voltammetric curves at different potential scan rates allow us to estimate a half-life time of about 25 ms for the short-lived eleo trogenerated species. The E’,,, value of the couple [ReO(OPhsal)(Phsal)]/[ReO(OPhsal)(Phsal)]can be estimated only approximately (N - 1.33 vs E’,,, of the couple ferricinium ion/ferrocene). Voltammetric curves recorded on the exhaustively electrolyzed solutions appear as those shown in Fig. 5. During a sweep starting at the potential of the electrolyses performed, peak B is recorded. By switching the potential after traversing this anodic peak, peak A, attributable to the reduction of the complex [ReO(OPhsal)(Phsal)], is recorded. If, on the contrary, the anodic sweep continues towards more positive potentials, the same peaks (system C/D) recorded on solutions of the starting complex, can be noted. Accordingly, electrolysing the reduced solutions at a potential of 0.0 V vs SCE, the starting complex is quantitatively regenerated through the consumption of 1 mol of electrons per mole of compound.
Solutions of the starting compound were also exhaustively reduced in order to identify the final electrolysis products. As we have already mentioned in the Experimental section, through the usual procedure involving dissolution of the reduction product in CHC13, even if it was carried out in carefully deoxygenated media, we did not manage to avoid that the compound partially decomposed and partially regenerated the starting complex. However, the product was stabilized by treating it with ethanol. Recalling that the voltammetric results suggest that the primary reduction product, [ReO(OPhsal)(Phsal)]-, undergoes a fast chemical reaction leading to a complex that, by oxidation regenerates the starting compound, we can suppose that the chemical reaction consists in the release by the metal centre of one phenolic site of the tridentate ligand, as a consequence of the reduced positive charge on rhenium. On the other hand if, through electrode oxidation, we increase back the rhenium oxidation state, the initial situation is re-established. In this context, the role of the protic solvent in stabilizing the reduction product may consist in the protonation of the free phenolic site. Accordingly, the solubility of the product in MeCN results is noticeably lower after protonation. The elemental analysis data are as follows (found: C, 44.9; H, 2.7; N, 3.9. Calc. for
.!?(-I
1too Fig. 5. Cyclic voltammograms recorded on a 4.5 x lo-’ M [ReO(OPhsal)(Phsal)] solution, previously reduced exhaustively at peak A of the curve in Fig. 4. Same experimental conditions as for the voltammograms in Fig. 4. (0) Starting potential.
Electrochemistry of rhenium(V) complexes
CZ6H2,,ReN204: C, 51.1; H, 3.3 ; N, 4.6%). The compound could not be further purified owing to the continuous transformation back to the starting complex. For this reason the elemental analyses are not in agreement with the formulation but the C: N: H ratio are consistent with the presence of two ligands in the product and presumably the impurities are due to inorganic compounds. IR spectra of the recovered reduction products show a sign&ant lowering of the wavenumber of the v(Re==O) absorption (a shift of about 50 cm-‘). This result is in apparent disagreement with that obtained on the reduction product of [ReOCl(Phsal)J, but a weakening of the Re=O bond can be explained assuming the formation of an intramolecular hydrogen bond. On these bases, and considering, as reported above, only the leaving of the phenolic groups, we propose the formulation reported in Fig. 6 for the product of the [ReO(OPhsal)(Phsal)] reduction. The compound is soluble in EtOH, MeOH and MeCN and insoluble in Et,0 and pentane ; moreover, it easily regenerates the starting complex in CH$I,?, CHCls, C6Hs and acetone. The results obtained with the complex [ReO (OPhsal)(Mesal)] are only qualitatively similar. Apart from the exact values of the potentials at which the different peaks are recorded, Fig. 4 is still useful to discuss the electrochemical behaviour of this compound. The main differences lie in the lower stability of the electrogenerated species [ReOCl (OPhsal)(Mesal)]- (half-life time of about 5 ms), and in the value of Er,,2 for the couple [ReO (OPhsal)(Mesal)]/[ReO(OPhsal)(Mesal)]-, which resulted equal to about - 1.37 V (vs E;,2 of the couple ferricinium ion/ferrocene). Also in this case the reduced solution can be reoxidized at a potential noticeably anodic in respect to the potentials of the cathodic peak (see, qualitatively, peak B in Fig. 5), regenerating quantitatively the starting complex. The recovery of the reduction product has been more difficult in respect to the corresponding Phsal complex, because it exhibits a marked tendency to regenerate the starting compound if only small
OH----O o/~JLZ<~,o N
Fig. 6. Proposed scheme for the rhenium(W) molecule obtained by cathodic reduction of [ReO(OPhsal)(Phsal)], after treatment with ethanol. I@I = Phsal and OH = HOPhsal.
1981
traces of oxidizing agents such as oxygen or water, are present. It was only possible to recover a mixture of starting complex and reduced product. IR spectra of this mixture show a first v(Re=O) band at 924 cm-‘, attributable to the reduction product and a second v(Re=O) band at 957 cm-‘, due to the starting complex. The formulation of this compound can be assumed as reported in Fig. 6 by analogy with the previous one. Also for these tridentate-bidentate compounds, completely similar results are obtained using mercury as the working electrode material. Anodic behaviour. As Fig. 4 shows for the complex [ReO(OPhsal)(Phsal)], both the [ReO(TSB) (BSB)] compounds undergo reversible oxidation. The oxidized species are stable on the voltametric time scale, so much so that backward to forward peak ratios equal unity also at the lowest (50 mV s-‘) potential scan rate used. Controlled potential electrolyses allows one to ascertain that the electrode processes involve a one-electron chargetransfer ; however the oxidized species are not stable at the longer electrolysis times and voltammetric tests after electrolyses do not show the peak system C/D giving any interesting information about the presence in the solution of some particular rhenium complex either. Many attempts to recover the oxidation products have been performed. Unfortunately, even if different procedures have been tried, a brown oil was always obtained and all attempts to obtain crystalline products has failed. On the basis of the voltammetric data it is possible to evaluate E’&, [ReO(OPhsal)(Phsal)]‘/ [ReO(OPhsal)(Phsal)] = +0.57 V, and Er,,2, [ReO (OPhsal)(Mesal)]+/[ReO(OPhsal)(Mesal)] = f0.53 V (vs E;,* of the couple ferricinium ion/ferrocene). As a final consideration, let us observe that the relative values of the E’,,, calculated for the eight redox couples identified result in agreement with the different electron withdrawing effects of methyl and phenyl groups. In fact, both as regard [ReOCl(BSB)d and [ReO(TSB)(BSB)], the resulting E\,* values for neutral/anionic and cationic/ neutral redox couples are more positive for BSB = Phsal than for BSB = Mesal. From the recovery products point of view, the complexes have shown the presence of Re=O 0x0 moiety. This fact is particularly important if it is considered that the Re=O group was found in the same complex with rhenium atom in three different oxidation states : VI, V and IV. To date the principal example of the Re04+ core was given by the complex [ReOCI,]- 14*”and in a few of its substitution products.16 The Re02+ core was never clearly authenticated. In particular the 0x0 oxygen in the fourth oxi-
1982
R. SEEBER et al.
dation state was always considered bound to a hydrogen atom, for example in the [Re(OH)Cl, (PEtlPh)J compound reported by Duatti et ~1.‘~ and the formal oxidation state of oxogroup was - 1. Concerning the [ReO(BSB)J complexes, only phenyl protons can interact with the 0x0 group, while in the case of [ReO(TSB)(BSB)] complexes a phenolic hydrogen can bind Re==O ; however, in both cases 0x0 group is considered in -2 formal oxidation state.
REFERENCES 1. R. Seeber, G. A. Mazzocchin, E. Roncari and U. Mazzi, Transition Met. Chem. 1981,6, 123. 2. E. Roncari, U. Mazzi, R. Seeber and P. Zanello, J. Electroanal. Chem. 1982,132,221. 3. R. Seeber, G. A. Mazzocchin, U. Mazzi, E. Roncari and F. Refosco, Transition Met. Chem. 1984,9, 315. 4. G. A. Mazzoccbin, R. Seeber, U. Mazzi and E. Roncari, Znorg. Chim. Acta 1978,29, 1.
5. G. A. Mazzocchin, R. Seeber, U. Mazzi and E. Roncari, Znorg. Chim. Acta 1978,29, 5. 6. U. Mazzi, E. Roncari, R. Seeber and G. A. Mazzoccbin, Znorg. Chim. Acta 1980,41,95. 7. U. Mazzi, E. Roncari, R. Rossi, V. Bertolasi, 0. Traverso and L. Magon, Transition Met. Chem. 1980, 5, 289. 8. A. Marchi, A. Duatti, R. Rossi, L. Magon, U. Mazzi and A. Pasquetto, Znorg. Chim. Acta 1984,81, 15. 9. U. Mazzi, F. Refosco, G. Bandoli and M. Nicolini, Transition Met. Chem. 1985, 10, 121. 10. U. Mazzi, F. Refosco, F. Tisato, G. Bandoli and M. Nicolini, .Z. Chem. Sot., Dalton Trans. 1986, 1623. 11. R. Hoffman, M. M. L. Chen, M. Elian, A. R. Rossi and D. M. P. Mingos, Znorg. Chem. 1974,13,2666. 12. R. S. Nicholson and I. Shain, Analyt. Chem. 1964, 36,706. 13. V. I. Kravtsov, J. Electroanal. Chem. 1976,69, 125. 14. T. Lis and B. Jezowska-Trzebiatowska, Acta Cryst. 1982, B33,1248. 15. B. J. Brisdon and D. A. Edwards, Znorg. Chem. 1968, 7, 1897. 16. G. Rouschias, Chem. Rev. 1974,74,531. 17. M. Sacerdoti, V. Bertolasi, G. Gilli and A. Duatti, Acta Cry&. 1982, B38, 96.