Voltammetric behaviour of transition metal complexes with extended π systems schiff base ligands

J. Electroanal. Chem., 121 (1981) 301--309 Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

301

VOLTAMMETRIC BEHAVIOUR OF TRANSITION METAL COMPLEXES WITH EXTENDED r SYSTEMS SCHIFF BASE LIGANDS N,N'-ETHYLENEBIS(MONOTHIOACETYLACETONIMINATO)COPPER(II) COMPLEX

A. CINQUANTINI, R. CINI, R. SEEBER and P. ZANELLO Istituto di Chimica Gener'ale dell'Universitd di Siena, Piano dei Mantellini 44, 53100 Siena (Italy) (Received 18th June 1980; in revised form 29th September 1980)

ABSTRACT The cathodic and anodic behaviour of the complex formed by Cu(II) and the extended 7r-system ligand N,N'-ethylenebis(monothioacetylacetonimine),[Cu {(sacac)2en)], was investigated in acetonitrile solvent at platinum and mercury electrodes by cyclic voltammetry, chronoamperometry and controlled potential coulometry. The first reduction process gives rise to the corresponding Cu(I) complex, stable in the electrolysis solution, the one at more negative potentials leads to unstable species. The oxidation products are a cationic compound with z = 2, from which the starting ligand can be regenerated electrolytically, and free Cu(I) or Cu(II) ions, depending on the working potential.

INTRODUCTION

Ligands which can give rise to complexes characterized by a r-electrons delocalization extended along the ligand as well as the metal centre have been shown to be able to stabilize first-row transition metals in unusual oxidation states; the resulting complexes are of great interest from a redox viewpoint [1--3]. In this connection the electrochemical properties of the complexes formed by copper with ligands of this class [4,5] appear to be promising ones, also in view of the role played by copper ion in some biochemical processes [6--13]. As part of a systematic study on the voltammetric properties of complexes of transition metal ions with extended r-systems ligands [ 14], the present paper deals with the electrochemical behaviour of the N,N'-ethylenebis(monothioacetylacetoniminato)Cu(II) complex, [Cu((sacac)2en}], both at platinum and mercury electrodes, in acetonitrile solvent. EXPERIMENTAL

Reagent grade acetonitrile (MeCN) (C. Erba) was twice distilled from phosphorus pentoxide and then from calcium hydride; the purified product was 0022-0728/81/0000--0000/$02.50, © 1981, Elsevier Sequoia S.A.

302

stored on 0.3 nm molecular sieves (Union Carbide) under nitrogen atmosphere. Other chemicals and reagents were obtained as described elsewhere [14]. The [Cu((sacac)2en)] complex was synthesized, purified and characterized according to literature methods [ 15]. Pure CuC104 acetonitrile solutions were prepared b y anodizing an electrolytic copper foil (Alfa Products) at a b o u t --0.2 V vs. SCE, in the presence either of NaC104 or of (C2Hs)4NC104 (TEAP); the titre as well as the oxidation state of the metal ion in the colourless solution obtained were checked b y weighing the metal deposit formed b y controlled potential coulometric reduction. By electro-oxidation of these acetonitrile solutions with a platinum gauze polarized at a b o u t +1.2 V vs. SCE, bleu-green solutions of Cu(C104)2 were obtained in a quantitative yield. The apparatus employed and the experimental conditions have already been described [14]. The visible spectrophotometric measurements on the reduction products were carried o u t b y transferring the solutions from the electrolysis cell into 1 cm thick quartz cells in a carefully deoxygenated atmosphere. The oxidation products were characterized both b y visible spectrophotometric tests, directly performed on the electrolysed solutions, and b y infrared (IR) spectra, recorded b y KBr pellets technique on the solid obtained as follows: the MeCN solvent was removed under reduced pressure, the residue was treated with anhydrous dichloromethane solvent and the solution obtained was dried under vacuum. Recovery of H2 ((sacac)2en} from the solution was carried o u t b y evaporating the MeCN solvent and then shaking the residual solid with anhydrous benzene. Unless otherwise specified, all potential values are referred to a saturated aqueous calomel electrode. RESULTS AND DISCUSSION

Anodic behaviour

Figure 1 shows the cyclic voltammetric anodic responses recorded at a platinum microelectrode on a [Cu ((sacac)2en~], MeCN solution with 0.1 M TEAP as supporting electrolyte. Two peaks, A and B, are evident; a cathodic peak, C, is directly associated with the first peak, A, and no other cathodic response is detectable until the reduction of the starting [Cu ((sacac)2en}] c o m p o u n d occurs (see below). In order to define the nature of the first anodic process, cyclic voltammetric tests were performed at potential scan rates ranging from 0.05 to 100 V s -1. The voltammograms obtained showed features typical of an uncomplicated reversible charge transfer up to a scan rate of a b o u t 10 V s-l; at higher values a slight degree of irreversibility became evident. The one-electron character of the involved charge transfer was established b y comparison with the response of a solution of bis(cyclopentadienyl)Fe(II); the E~n value of the [Cu{(sacac)2en}]*/[Cu((sacac)2en}] couple was --0.11 V with respect to that of the ferricinium/ferrocene couple. Controlled potential coulometric tests performed at +0.5 V (first anodic process) led to a consumption of three electrons per molecule of the starting compound. In Fig. 2a a cyclic voltammogram recorded on this electrolysed solution

303

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A

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Fig. 1. Cyclic v o l t a m m e t r i c responses relative to a 2.5 × 1 0 - 3 M [Cu((sacac)2en}], 0.1 M T E A P acetonitrile solution. A n o d i c scan; p l a t i n u m working m i c r o e l e c t r o d e , starting potential 0.00 V; scan rate 0.2 V s -1.

is shown. By comparison with the voltammetric picture exhibited by a CuC104, MeCN solution (Fig. 2b), the D--F peaks system must be ascribed to the metal electrodeposition, with the associated anodic dissolution of deposited metal

a

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Fig. 2. (a) Cyclic v o l t a m m e t r i c responses relative to a 4.5 x 1 0 - 3 M [Cu ((sacac):en}], 0.1 M T E A P acetonitrile solution, exhaustively electrolysed at +0.40 V. Starting potential 0.00 V; platinum working m i c r o e l e c t r o d e ; scan rate 0.2 V s -1. (b) Cyclic v o l t a m m e t r i c responses relative to a 3.0 X 10 -3 M CuC104, 0.1 M T E A P acetonitrile solution. Platinum working m i c r o e l e c t r o d e ; scan rate 0.2 V s -]. (c) Cyclic v o l t a m m e t r i c responses relative to a 4.6 X 10 - 3 M [ Cu ((sacac)2 en} ], 0.1 M T E A P acetonitrile solution, exhaustively electrolysed at +1.50 V Starting potential +1.50 V; scan rate 0.2 V s -1 ; platinum working m i c r o e l e c t r o d e ; c a t h o d i c scan. (d) Cyclic v o l t a m m e t r i c responses relative to a 3.1 x 10 -3 M Cu(ClO4)2, 0.1 M T E A P acetonitrile solution. Platinum working m i c r o e l e c t r o d e ; starting potential +1.50 V; scan rate 0.2 V s -1 ; c a t h o d i c scan.

304

film; in addition, the G--H peaks system should be attributed to the Cu(II)/ Cu(I) redox couple. As expected, coulometric tests carried o u t at potentials corresponding to peak D revealed that one electron per molecule of the starting [Cu ((sacac)2en)] was involved and that the quantitative electrodeposition of copper metal occurred. After removing the solvent and the supporting electrolyte, conductometric tests on nitromethane solutions of the residual solid revealed the presence of a 2 : 1 electrolyte type. Moreover, coulometric tests performed at the potential of peak E after the electrodeposition of copper, revealed the consumption of t w o electrons per molecule of starting c o m p o u n d ; the colourless solution turned orange yellow. The yellow solid recovered was identified as H2 ((sacac)2en) b y infrared spectroscopy, elemental analysis and melting-point tests. These results indicate that the one-electron oxidized complex, still stable in the voltammetric time scale, decomposes slowly, leading to species which are, in their turn, oxidizable at the working potential. These species from the overall three-electron oxidation process proved to be the oxidized form o f the ligand, [(sacac)2en] 5+, and the free Cu(I) ion, the thermodynamically stable oxidation state of the free copper ion at the potential values at which the starting solution was oxidized. If the electrolysis was performed at potentials more anodic than +1.0 V, b o t h before and b e y o n d the second anodic peak, four electrons per molecule of starting c o m p o u n d were used and the resulting solution exhibited the voltammetric picture reported in Fig. 2c. By comparison with the cyclic voltammetric response of a pure Cu(C104)2, MeCN solution (Fig. 2d), and on the basis of tests similar to those described above, the presence in solution of free Cu(II) ions and of [(sacac)2en] 2+ was checked. Accordingly, the solutions containing Cu(I) and [ (sacac)2en] 5+ were transparent in the visible region, whereas those resulting from the four-electron oxidation exhibited a weak, broad absorption band with a maximum at 750 nm, all coincident with that of a pure Cu(C104)2, MeCN solution. Furthermore, IR spectra obtained from b o t h three- and four-electron oxidation products were coincident with those recorded on the recovered species after the quantitative electrodeposition of metal copper, so confirming that in the oxidized solutions copper ions are free and that in b o t h cases the same ligand moiety is formed. A broad intense band centred at 1100 cm -1 showed the presence of perchlorate ions: two sharp bands at 1540 and 1470 cm -1 were attributed to C'"--C and C'"--C stretching vibrations respectively; a broad band centred at 800 cm -1 could be assigned to a combination of C'~S and C--CH3 stretching vibrations. In comparison with IR spectra obtained from pure [Cu((sacac)2en)] and H2((sacac)2en) [15], all the absorption frequencies undergo a noticeable shift towards lower wave numbers in agreement with the presence of [(sacac)2en] 2÷ species in the oxidized solutions. Regarding the electrode process occurring in correspondence to peak B, the voltammetric tests performed at different potential scan rates (from 0.05 to 100 V s -~) revealed no directly associated cathodic response; moreover, the peak potential value shifted anodically to an extent of 40 + 5 mV per tenfold increase in the potential scan rate, and the (Ep - - E p n ) quantity was constant and equal to 65 + 5 mV. Furthermore, in the scan rate range where the first

305 anodic process appears reversible in character, the ratio between the peak current of the more anodic response and that of the less anodic one was constant and equal to 1.92 + 0.03. All these data are consistent with an irreversible twoelectron charge transfer; they allow a value of 0.75/2 to be calculated for the "overall" charge-transfer coefficient (a); hence the slowest charge-transfer step has to be the first one, characterized b y a charge-transfer coefficient of 0.75 [16,17]. Coulometric and products analysis data indicate that the [Cu{(sacac)2en}] 3+ species primarily formed at the electrode surface undergoes a slow chemical reaction and a further one-electron oxidation leading to Cu(II) ions and [(sacac)2en] 2÷. The data reported above can be summarized b y the reaction pathway in Scheme 1. The cathodic peak C directly associated with the first anodic peak, peak A

peak B

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H2L L = (sacac)2en ( t h e h y d r o g e n a t o m d o n o r in t h e process at p e a k E is t h o u g h t t o b e t h e M e C N solvent, in a g r e e m e n t w i t h several e x a m p l e s f o u n d in l i t e r a t u r e . T h e r e a c t i o n s h o u l d o c c u r w i t h a n i n t e r m e d i a t e c a t i o n radical L .+) SCHEME 1

A, was well detectable in the voltammetric tests in which the potential scan direction was reversed after traversing the second anodic peak. This cathodic response must be attributed to the reduction of the [ Cu ((sacac)2en} ] ÷ species which arises b y oxidation o f the neutral complex during the back scan after traversing the potential region corresponding to the second anodic process. The ratio between the heights of this cathodic response and the corresponding anodic one was almost constant (0.62 + 0.07) at different scan rates, provided the switching potential b e y o n d the peak B was kept constant. In order to verify the correctness of a similar assumption and to check the value of the chargetransfer coefficient previously evaluated, the experimental responses have been compared with the theoretical ones obtained b y means of the digital simula-

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Fig. 3. C o m p a r i s o n b e t w e e n s i m u l a t e d r e s p o n s e (full line) a n d e x p e r i m e n t a l values (*) for t h e a n o d i c o x i d a t i o n o f [Cu {(sacac)2en}]. Scan rate 0.50 V s -1 ; c u r r e n t in m A . F o r simplicity t h e E ° value of t h e r e d o x c o u p l e involved in t h e m o r e a n o d i c process was a s s u m e d equal to El~ 2 o f t h e c o u p l e [Cu {(sacac)2en}]+/[Cu {(sacac)2en}] a n d a k s value o f 3.13 X 10 -20 c m s -1 was e m p l o y e d in t h e s i m u l a t i o n .

tion technique following the finite difference explicit method [18], employing a non-uniform space-time grid discretization [ 19], which enables the simulation of such responses, occurring in a wide potential range, to be performed with a noticeable saving in the computation time. By employing an a value of 0.75/2, the simulated responses overlap the experimental responses very well for every potential scan rate. Figure 3 shows the comparison between simulated and experimental responses at a potential scan rate of 0.5 V s -1.

Cathodic behaviour Figure 4 shows the cyclic voltammetric curve recorded with a mercury electrode on a 0.1 M TEAP, MeCN solution of [Cu {(sacac)2en} ] by scanning the potential initially in the cathodic direction. Two reduction peaks (I and L) are observed, with an anodic peak (M) directly associated to the first one. By using a platinum microelectrode, only the first catho-anodic process appeared because of the solvent discharge; b y comparison with the peak current values measured for the oxidation of bis(cyclopentadienyl)Fe(II), the one-electron character of the redox process responsible for the I--M peaks system could be deduced in the voltammetric time scale. The same conclusion was gained from chronoamperometric tests. Controlled potential coulometric tests carried o u t with both platinum and mercury electrodes at --1.4 V (first cathodic process) confirmed that one electron per molecule of starting complex was spent. The voltammetric curves obtained at a mercury electrode b y varying the potential scan rate in the range 0.05--100 V s -1 showed the features typical of an uncomplicated one-electron reversible process, up to the highest explored scan rates; in fact, (Ep)c was constant at the value of --1.10 V, the (Ep:2 --Ep)c quantity was always equal to 57 + 3 mV, the (Ep)a -- (Ep)c value was 58 + 3

307 -100

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Fig. 4. Cyclic v o l t a m m e t r i c p i c t u r e relative t o a 2.4 × 10 -3 M [Cu {(sacac)2en}], 0.1 M T E A P a c e t o n i t r i l e s o l u t i o n C a t h o d i c scan; m e r c u r y w o r k i n g m i c r o e l e c t r o d e , scan rate 0.2 V s -1 ; s t a r t i n g p o t e n t i a l 0.00 V.

mV, the ratio ( i p ) a / ( i z J c was equal to 1 and the (ip)¢/v ~/2 parameter (v = potential scan rate) was constant. An Ex/2 value equal to --1.49 V with respect to that of the ferricinium/ferrocene couple could be evaluated. The same catho-anodic process appeared non-reversible in character at every scan rate when a platinum electrode was used. The typical parameters of the responses, (Ep)¢, ( E p n - - E p ) c , ( E , ) a - - (Ep)c and (ip)a/(ip)~ assumed the following values: --1.145 V, 70 mV, 140 mV and 0.9 at 0.2 V s-*;--1.215 V, 78 mV, 325 mV and 0.8 at 10 V s-l; --1.250 V, 80 mV, 415 mV and 0.75 at 50 V s -*. These data, as well as all the current--potential responses, were coincident with those obtained b y digital simulation of an uncomplicated one-electron process, when an a value of 0.6 and a ks value of 6.18 × 10 -3 cm s -1 were assumed. The E ° value of the redox couple was calculated as the mean value between (Ep)¢ and (Ep)a in the response recorded at 0.05 V s-~; a similar assumption seems reasonable as the a value is n o t very different from 0.5 [20]. The difference in the degree of reversibility of the redox process depending on the electrode material is high enough to suggest the occurrence of an innersphere type, heterogeneous charge-transfer mechanism [ 21 ]. During the electrolysis at peak I the solution, initially red-brown, became progressively deep green in colour; the presence in the solution of the reduced form of the redox couple involved in the I--M system of Fig. 4 was checked by cyclic voltammetry. Ultraviolet-visible spectrophotometric measurements performed on the electrolysed solution showed an absorption band at 550 nm; in the parent compound two bands, located at 480 and 730 nm, assigned to d - d transitions [ 13 ], appeared. It seems reasonable that the absorption band at 550 nm of the reduced c o m p o u n d should be attributed to a charge-transfer transition [13]. The ESR spectra recorded.at different stages of electrolyses showed a progressive decrease of the multiplet with a g value of 2.19, due to the metal centre in a d 9 configuration in the starting compound. No signal was detectable at fields typical of unpaired electrons in organic moieties. These findings allow us to conclude that the extra electron is localized on the metal centre; t h e y are

308 in agreement with the conclusions drawn on the reduction product of [Ni((sacac)2en}] [14] and with Schrauzer's theory on metal complexes with "oddeven" ligands [22]. Regarding the reduction process at the more negative potential shown in voltammograms performed with a mercury microelectrode, it must be noted that at varying potential scan rate the ratio between the peak current values relative to peaks L and I remained unchanged and equal to 2.9 + 0.2. The difference between half-peak and peak potentials was always equal to 28 + 5 mV, and the peak potential shifted towards cathodic values at an extent of a b o u t 17 + 5 mV for a tenfold increase in the scan rate. No associated anodic response could be detected, even at the highest scan rates. Similar features are typical of a two-electron irreversible charge transfer; from the value of the "overall" charge-transfer coefficient directly evaluable from the voltammetric data, it can be deduced that the slowest charge-transfer step is the second one, characterized by a charge-transfer coefficient equal to 0.70 [16,17]. Controlled potential coulometric tests performed b y polarizing the working electrode directly at --2.60 V (second cathodic process) led to the following results: if the electrolysis current was switched off after the consumption of one electron per molecule of starting c o m p o u n d , the presence in the solution of the species [Cu {(sacac)2en}]- alone was detected in an amount of about 90% in respect to that of the parent c o m p o u n d ; on starting the electrolysis again, the current fell to a low value corresponding to an overall consumption of three electrons per molecule of the starting c o m p o u n d ; however the electrolytic current never decreased to the value of background current, indicating that a decomposition of the produced c o m p o u n d was occurring and led to species in their turn reducible at that potential value. The ESR spectra recorded during the electrolysis at peak L on solutions previously exhaustively electrolysed at peak I showed no signal at fields proper both for paramagnetic metals and for organic radicals. This fact agrees with the voltammetric datum, suggesting that the [Cu ((sacac)2en} ] 3- species arises at the potentials of peak L. Hence the reduction processes of [Cu ({sacac)2en)] consist of a first uncomplicated one-electron charge transfer (peak I), followed b y a two-electron process (peak L). In addition, a chemical reaction has to be coupled to the more cathodic charge transfer when the parent c o m p o u n d is still present in the solution, as shown b y the results obtained when the reduction carried out directly at the second peak was interrupted after the consumption of one electron per molecule of starting compound. A reaction accounting for these data is [Cu ((sacac)2en~] 3- + 2 [Cu ((sacac)2en}] -~ 3 [Cu ((sacac)2en)] Any attempt to isolate the products from the exhaustively electrolysed solution failed owing to their extremely high reactivity. REFERENCES 1 2 3 4 5

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