Porphyrins bound to Ru(bpy)2 clusters: electrocatalysis of sulfite

www.elsevier.nl/locate/ica Inorganica Chimica Acta 312 (2001) 53 – 66

Porphyrins bound to Ru(bpy)2 clusters: electrocatalysis of sulfite N. Rea a, B. Loock b, D. Lexa a,* a

Laboratoire de Bioe´nerge´tique et Inge´nierie des Prote´ines, UPR 9036 CNRS, 31 Chemin J. Aiguier, 13402 Marseille Cedex 20, France b Laboratoire de Chimie Bioorganique, UMR 176 CNRS-Institut Curie, Bat 112, Centre Uni6ersitaire, 91405 Orsay, France Received 11 May 2000; accepted 12 September 2000

Abstract Mono-, di-, tri- and tetra-ruthenated porphyrins derived from phenyl/4-pyridyl mesosubstituted porphyrin and a trans di-ruthenated dipyridyl octaethylporphyrin have been synthesised. Coated on carbon electrodes, they have been tested as a sensor for sulfite using the oxidation wave of the Ru(II)/Ru(III) couples in hydroalcoholic solutions. At least two peripheral Ru are necessary to trigger the catalysis off. No major influence of the central metallic ions has been detected, but the Co (II) is coordinated by the sulfite. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Ru(bpy)2 clusters; Electrocatalysis; Porphyrins; Ruthenated porphyrins

1. Introduction Sulfite is widely employed as an antioxidant and inhibitor of bacterial and yeast growth in the food and beverage industries and paper production [1]. Sulfur dioxide emitted by chemical industries also plays an important role in air pollution and leads to acidic rain [2]. Determination of its concentration needs rapid and precise methods and in recent years various indirect methods have been developed, such as iodometric, bromometric, amperometric, enzymatic methods and fluorescence detection [3]. Direct electrochemical oxidation of sulfite on a carbon electrode in an acidic solution is characterised by an irreversible two-electron wave leading to sulfate or other S(VI) oxygenated derivatives [4]. Several studies have shown that oxidation of sulfite is complicated and strongly dependent on the electrode material. However, the results are not very reproducible because the oxidised compounds gave adsorption phenomena on the electrode and the waves are not well defined. It is thus interesting to try to catalyse this oxidation process to improve the reproducibility. * Corresponding author. Tel.: + 33-4-9116 4404; fax: + 33-4-9116 4578. E-mail address: [email protected] (D. Lexa).

In recent years porphyrin-modified electrodes, and especially polymetallic porphyrins involved in multielectron transfer catalysis, have been employed with increasing interest in analytical and bioanalytical chemistry [5]. Polymetallated porphyrins obtained by attaching four Ru(bpy)2Cl complexes (bpy= 2,2%-bipyridine) to the peripheral pyridyl residues of various meso-tetra (4-pyridyl) porphyrinates have been reported recently to act as efficient catalysts in multi-electron transfer reactions [6]. Depending on the central metallic ion inserted in the porphyrin ring, Zn, Co, Ni or Fe, these complexes are able to enhance either the catalysis of the reduction of O2 [7] and CO2 or the oxidation of ascorbic acid, NO or sulfite [6]. In this work we report a study of the electrocatalytic activity of such polymetallated porphyrins in the oxidation of sulfite in acidic aqueous media with the aim of determining which complex is the most efficient [8] and how many electrons are required in such oxidation of sulfite. The purpose is to use such compounds (as they are very slightly soluble in water) adsorbed as a sensor of sulfite on a carbon electrode in wine-model solutions. We first studied several metallo-tetra-ruthenated pyridyl porphyrins in organic solution N,N%-dimethylformamide (DMF) and acetonitrile (ACN). Then they were studied coated on carbon electrodes in aqueous media in order to elucidate the role of the central

0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 0 ) 0 0 3 1 9 - 4

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metallic ion in the catalytic oxidation of sulfite. With the aim of elucidating the influence of the number of Ru necessary for leading to a good catalysis we studied the mono-, di-, tri-ruthenated pyridyl – phenyl porphyrins. Finally, we investigated the influence of substitution on the porphyrin ring in studying the 5,15-(4-pyridyl) octaethylporphyrin and its cobalt complex (Fig. 1).

2. Experimental section

2.1. Material All chemical reagents were analytical grade. Sodium sulfate and sodium sulfite were purchased from Prolabo; citric and tartaric acids were from Sigma; malic acid, nickel (II) acetate tetrahydrate 99%, cobalt(II)

Fig. 1. Molecular structures of the polymetallic porphyrins.

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acetate tetrahydrate, lithium trifluoromethanesulfonate (LiTFMS) and 5,10,15,20-tetra(4-pyridyl) 21H,23Hporphine (P) were from Aldrich; cis-dichlorobis (2,2%bipyridine) ruthenium(II) dihydrate (Ru(bpy)2Cl2) was from Strem Chemicals; ACN was from Merck; DMF was from Fluka.

2.2. Porphyrins synthesis 2.2.1. Phenyl/4 -pyridyl meso-substituted porphyrins The phenyl/4-pyridyl meso-substituted porphyrins studied in this report were prepared by the mixed aldehyde procedure according to literature methods [9]. The isomers separation was made by preparative column chromatography on silica gel. The compounds were characterised by their 1H NMR spectra, which were identical to those reported [9c,d]. The crude mixture of porphyrins (8.32 g) prepared from benzaldehyde (8.90 g, 0.084 mol), 4-pyridine-carboxaldehyde (26.32 g, 0.246 mol) and pyrrole (22.71 g, 0.339 mol) were dissolved in chloroform (250 ml); silica gel (80 g) was added and the solvent was evaporated. The adsorbed porphyrins were placed on the top of a silica gel column (5×30 cm) in methylene chloride. Elution with the same solvent gave TPP; a mixture of methylene chloride –diethyl ether (1:1, v/v) followed by a mixture of methylene chloride – acetone (1:2, v/v) gave P2Py2Ptrans; elution with acetone gave P2Py2P-cis; finally, a mixture of methylene chloride – methanol (100:10, v/v) gave PPy3P followed by TPyP with a mixture of methylene chloride –methanol (100:25, v/v). All the fractions were crystallised from methylene chloride – methanol mixtures, affording TPP: 0.21 g, (0.4%); P3PyP: 0.908 g, (1.7%); P2Py2P-trans: 0.395 g, (0.7%); P2Py2P-cis: 1.262 g, (2.4%); PPy3P: 2.07 g, (1.7%) TPyP: 185 g, (3.5%). The NMR results are gathered in the supplementary material (Table 4). 2.2.2. 5,15 -(4 -Pyridyl) -octaethylporphyrin 5,15-(4-Pyridyl)-octaethylporphyrin was prepared by the same method described for the synthesis of 5,15-(onitrophenyl)-octamethylporphyrin [9c]. A solution of (3,4,3%,4%-tetraethyl-2,2%-dipyrryl)methane (1.29 g, 5 mmol), 4-pyridine-carboxaldehyde (0.53 g, 5 mmol), and p-toluenesulfonic acid (0.24 g, 1.25 mmol) in methanol (50 ml) was stirred overnight under argon. Pyridine (1 ml, 12 mmol) was added, followed by a solution of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ, 4.54 g, 1.20 mmol) in THF (10 ml). The resulting solution was stirred for 1 h at room temperature then evaporated to dryness. The residual solid, treated with 10% aqueous solution of NaOH to remove the hydroquinone, was then taken up in methylene chloride. The organic solution was washed with 10% aqueous solution of NaOH (×3), water, dried over Na2SO4 and then evaporated to dryness. The residue

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was chromatographed on silica (Macherey –Nagel, silica gel 60, 70–230 mesh, 3× 20 cm) eluted with methylene chloride –diethyl ether mixtures (100:5 to 100:20, v/v). Crystallisation from a methylene chloride – methanol solution afforded purple crystals of the title compound (0.664 g, 43%). umax (m mmol l − 1 cm − 1) (CH2Cl2): 409 (194.6), 509 (15.3), 543 (6.0), 576 (6.8), 628 (1.9). l(CDCl3) NMR 1 H (CDCl3), l(ppm): 10.26 (2H, s, 10, 20-H); 8.97 (4H, d, J= 5.7 Hz, a-Pyr); 8.21 (4H, d, J= 5.7 Hz, b-Pyr); 4.00 (8H, q, J= 7.4 Hz, 2, 8, 12, 18-CH2 CH3); 2.82 (8H, q, J= 7.4 Hz, 3, 7, 13, 17-CH2CH3); 4.00 (8H, q, J= 7.4 Hz, 2, 8, 12, 18-CH2CH3); 2.82 (8H, q, J = 7.4 Hz, 3, 7, 13, 17-CH2 CH3); 1.85 (12H, t, J=7.4 Hz, 2, 8, 12, 18-CH2CH3 ); 1.21 (12H, t, J= 7.4 Hz, 3, 7, 13, 17-CH2CH3 ); − 2.10 (2H, s, NH).

2.3. Fixation of the Ru(bpy)2Cl2 by the peripheral pyridines The ruthenated porphyrins were prepared as described in the literature [6a]. The reactions were followed and the purity checked by optical absorption spectroscopy.

2.4. Insertion of metallic ions Insertion of Co(II), Ni(II), Fe(III), Zn(II) was made after the complexation by the pyridyl moieties with Ru(bpy)2Cl2 using the acetate method [9h]. The chloride solid product was converted to the trifluoromethanesulfonate (TFMS) salt following the published procedure [6].

2.5. Apparatus and procedures The UV –vis spectra were obtained on a Cary – Varian model 5E spectrophotometer. Proton NMR spectra were recorded on deuterochloroform samples on a 200 MHz Bruker AC200. Cyclic voltammetry (CV) experiments were carried out using a Princeton Applied Research Corp (PAR) potentiostat/galvanostat model 263A in conjunction with a PAR 175 Universal Programmer. An EGG 379 digital coulometer and an EGG 363 potentiostat were used for setting the preparative electrolyses and coulometry.

2.6. Cells and electrodes CV was conducted in an inactinic all-glass three-electrode cell. In the case of cyclic voltammetry in solution, the working electrodes were home-made either with a glassy carbon (GC) cylindrical rod from Tokai Carbon Co. (grade GC-20; |= 0.07 cm2) or with an edge plane pyrolytic graphite rod from Advanced Ceramics Corpo-

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ration (| =0.196 cm2) included in a polymeric resin. The GC electrode (GCE) was polished consecutively with 6.0, 3.0, 1.0 mm diamond pastes until a mirror finish was obtained. The polished electrode was cleaned in an ultrasonic water bath for 5 min to remove the diamond and carbon particles. The edge plane pyrolytic graphite electrode (EPGE) was polished on an emery disk (P400) then sonicated for 5 min and dried with hot air. A Pt wire served as the auxiliary electrode and a saturated calomel electrode (SCE, Tacussel) isolated from the main solution by a medium-porosity ceramic frit was used as reference electrode. Constant-potential coulometry was conducted on a 5 ml DMF solution on a large gold or graphite gauze electrode (|=7.4 cm2) in anaerobic condition, the auxiliary electrode being separated from the main solution by a glass frit. Spectroelectrochemistry was conducted on a Pt minigrid as working electrode in an OTTLE cell with an optical path length of 0.5 mm [10].

2.7. Procedures for the preparation of the coated electrodes For the CV experiments the same polished electrodes (EPGE or GCE) were immersed for 15 min in 1 mM porphyrin DMF solution containing the supporting electrolyte (0.25 M LiTFMS). Then after being dried with a paper cloth it was plunged in the aqueous wine-model solution. For the coulometric measurements during long-term electrocatalysis in the same aqueous solution a reticulated GCE was used. Its preparation as a coated electrode was the same as described before, except it was not polished. It was allowed to dry slowly under argon flushing. Its exact area is not known.

2.8. Wine-model solutions The CV experiments and some long-term electrocatalytic electrolysis were carried out using an aqueous buffered solution at pH 3.4. This wine-model solution contains distilled water, ethyl alcohol (12%), tartaric acid (5× 10 − 2 M), adjusted to pH 3.4 with NaOH. The supporting electrolyte was LiTFMS (0.25 M).

As described before, in ACN at 25°C, the tetrametallated porphyrins M2 + P[PyRu(bpy)2Cl]4(TFMS)4 (M2 + = Co, Ni, Zn) in oxidation exhibit a four-electronswave oxidation at 0.74 V versus SCE which involves the oxidation of the four Ru(II) to Ru(III), characterising a system containing identical, non-interacting redox groups [6,11]. Even with CV at 0°C the four-electronswave keeps the same pattern, DEp = 85 mV (DEp = Epa − Epc; Epa anodic peak, Epc cathodic peak). The standard potentials are not dependent on the working electrode material. Nevertheless, for Co and Ni complexes this potential value of the four-electrons-wave is 50 mV more negative than the oxidation potential of Ru(II)/Ru(III) in the free [Ru(bpy)2PyCl] (E°=0.79 V) measured in ACN [12b,c], which means that the p electrons of the porphyrin ring influence through the linked-pyridine complexation is about this value. In DMF, compared with the results found in ACN, a 100 mV positive shift is observed for the same metalloporphyrins, whereas this value is only 80 mV for the ruthenated porphyrin free base [6c], Table 1. We used voltammetry as a purity test. In the case of the metalloporphyrins we did not observe the oxidation of free Ru(bpy)2Cl2 itself, which should occur at 0.35 V, proof that no free Ru species remain in the solution [12a,b] except for the free base porphyrin, which was more difficult to purify. For the Co porphyrin we could detect a one-electron-wave oxidation around 0.36 V involving the oxidation of Co(II) in Co(III). This wave was, in several cases, ill defined, as already noted by Araki et al. [6d]. Contrary to expectation, no change in the formal potentials of the Ru(II)/Ru(III) according to the nature of the central metallic ion was observed, whatever the solvent. Usually, as the nature of the central metallic ion has an influence on the potential oxidation or reduction waves of the porphyrin ring, this was not expected. This fact has already been mentioned in previous studies [5,6]. Table 1 Formal potentials and electron number of the oxidation waves of M2+P[PyRu]4 in DMF solutions. The formal potentials E F are determined as the middle potential between the anodic and cathodic peaks of the oxidation waves. DEp(mV)=Epa−Epc; Epa anodic peak, Epc cathodic peak

3. Results

Electron number

3.1. CV and coulometry in non-aqueous solutions

Ru(II)/Ru(III) E F (V versus SCE)

M2+/M3+ Ru(II)/Ru(III) 2+

3.1.1. Studies on M P[PyRu]4: influence of the central metallic ion (M 2 + =Co, Ni, Zn) CV was carried out either on EPGEs or GCEs in organic solvents such as ACN or DMF as previously described [6].

CoP(PyRu)4 NiP(PyRu)4 ZnP(PyRu)4 H2P(PyRu)4

1

4 4 8 (?)

0.84 0.84 0.83 0.83

(70) (70) (73) (80)

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3.1.1.1. Determination of the number of electrons by coulometry. Successive quantitative controlled-potential coulometric electrolysis were set at a potential about 100 mV higher than the E F corresponding to each oxidation wave. The faradic results correspond to the number of electrons which are supposed to be involved for the oxidation of the cobalt first and for the oxidation of the four rutheniums second, except for the free base ruthenated porphyrin, Table 1. In this case, the free [Ru(bpy)2Cl2] detected by its oxidation wave in CV (described in the Section 3.1.1) present in the solution must interact. The coulombic measurements have been started after the oxidation of the Ru of the impurity but we could not avoid the oxidation of the labile Cl− or an other impurity at the potential set for oxidation of the Ru attached to the porphyrin ring (0.95 V versus SCE). 3.1.2. Role of the number of Ru(bpy)2Cl2 bound to the porphyrin ring 3.1.2.1. Studies of CoP[PyRu(bpy)2Cl]nPhenyl4 − n(TFMS)n. With the aim of investigating the role of the number of rutheniums complexed by the porphyrin via the pyridine moiety on the catalytic process we studied porphyrins derived from tetraphenyl porphyrin, Fig. 1. The number of phenyl groups substituted by pyridine groups vary from one to four. This leads to five new porphyrins, as for n =2 there are two isomers (cis and trans) isolated. Then we conducted the study on the cobalt porphyrin series in DMF solution. For n = 2 we studied only the cis isomer. 3.1.2.2. CV. Fig. 2 shows the oxidation voltammograms of the four porphyrins in DMF. Two oxidation waves

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can be observed: the first features the oxidation of Co(II) to Co(III) and the second (around 0.84 V) features the oxidation of Ru(II) to Ru(III). According to the number of rutheniums anchored by the pyridine groups on the porphyrin ring, the current of this second wave increases while the current of the first wave keeps roughly the same value, in spite of the splitting of this wave in some cases (dash –dot and dash –dot –dot curves) and the difference between their diffusion coefficients. This splitting must be due to a different axial complexation of cobalt. In the previous studies on CoP(PyRu)4 the wave observed at the foot of the oxidation wave of the Ru was assumed to be an adsorption wave [6g]. But in our case, when the sweep rate is increased its intensity does not vary simply with 6. The number of electrons involved in the oxidation on the second wave for each porphyrin has been measured by coulometry after a previous oxidation of Co(II) to Co(III). Formal potentials on GCEs and coulometric results are gathered in Table 2. The electron number measured by coulometry for the CoPPh3(PyRu) is too high, as there is only one Ru. When considering the plain curve in Fig. 2, an explanation for the increase of coulombs could be the participation of the oxidation wave of the porphyrin ring; this appeared to be shifted to less positive values in this case, while it is not for the other compounds. Nevertheless, for a complex with the same porphyrin ring, Mn PPh3(PyRu), the first oxidation E 1 of the porphyrin ring has been found at 1.56 V versus a normal hydrogen electrode (NHE) in ACN by other authors [13]. Moreover, for ZnP(PyRu)4 the first oxidation peak of the porphyrin ring has been found in DMF at 1.57 V versus NHE [6b,c].

Table 2 Formal potentials in DMF, DEp and electron number measured by coulometry successively on the oxidation waves of Co(II)/Co(III) and Ru(II)/Ru(III) of Co(II)porphyrins Porphyrin Co(II)/Co(III) E F (V versus SCE) Electron number

Fig. 2. Cyclic voltammograms of Co(II) porphyrins 10 − 3 M in DMF+ 0.25 M LiTFMS on GCE; scan rate, 50 mV s − 1. ( — — ) blank current; ( — — ) Co(II)PPh3[PyRu]; (– – –) Co(II)PPh2[PyRu]2; ( — · — ) Co(II)PPh[PyRu)3; (— · · —) Co(II)P[PyRu]4.

PPh3(PyRu) PPh2(PyRu)2 PPh(PyRu)3 P(PyRu)4

0.37

0.37

0.37

0.37

1

1

1

1

0.84

0.84

0.84

Ru(II)/Ru(III) E F (V versus 0.82 SCE) DEp (mV) 70 Electron 2 number

74 2.5

80 3.5

70 4

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some free Ru(bpy)2Cl2 traces difficult to eliminate by recrystallisation, as for the free base H2P[PyRu]4.

3.2. Spectroelectrochemistry With the aim of verifying the attribution of the oxidation waves to each redox state of the metallic ions, we carried out spectroelectrochemical studies at each potential corresponding to the various electron steps for each porphyrin in reduction and in oxidation. We also tested the reversibilities after each oxidation or reduction step.

Fig. 3. Cyclic voltammograms of porphyrins 10 − 3 M in DMF+ 0.25 M LiTFMS on EPGE; scan rate, 50 mV s − 1: (— —) blank current; ( – – – ) Co(II)PPh2[PyRu]2; ( — · — ) Co(II)OEP[PyRu]2.

3.2.1. M 2 + P[PyRu]4 Spectroelectrochemical studies of CoP[PyRu]4, NiP[PyRu]4, ZnP[PyRu]4, and H2P[PyRu]4 on a Pt grid in DMF corroborate the redox states localisation already attributed, corresponding to each oxidation wave [6d]. They are reported in the supplementary material (Fig. 10 and Tables 5 and 6). 3.2.2. CoP[PyRu(bpy)2Cl]nPhenyl4 − n According to the number of pyridines substituted on the porphyrin ring able to link the Ru(bpy)2Cl, the maximum of the 295 nm band featuring the absorption of the Ru(bpy)2Cl increases for the starting materials in the oxidation states Co(II) –Ru(II)n. The other bands relative to the porphyrin transitions are hardly changed. When the Ru(II) is oxidised the extinction molecular coefficients of the two new bands (296 and 318 nm) decreased with the number of rutheniums, and a similar enhancement of the Co(III) Soret band observed for the tetra-ruthenated porphyrin is found.

Fig. 4. Spectroelectrochemistry of Co(II)OEP[PyRu]2 10 − 4 M in DMF+ 0.25 M LiTFMS; ( — — ) −0.5 V; (– – –) + 0.5 V; (— · — ) + 0.95 V.

3.1.3. Role of the substitutions on the porphyrin ring; studies of 5,15 -(4 -pyridyl) -octaethylporphyrins: H2, CoOEP[PyRu]2 In Fig. 3, the dash – dot curve features the oxidation of CoOEP[PyRu]2. Compared with the dashed curve featuring the redox behaviour of CoPPh2(PyRu)2, the oxidation wave of the two rutheniums is irreversible and the first oxidation wave involving the Co is shifted a more negative value. The CV of H2OEP[PyRu]2 at the same scan rate is also characterised by an irreversible oxidation wave of the two rutheniums at the same potential observed for the CoOEP[PyRu]2. Nevertheless, a small oxidation wave at 0.4 V is observed that features the oxidation of

3.2.3. CoOEPPy2[Ru(bpy)2]2 -trans First, a comparison of the molecular extinction coefficients of this porphyrin with those relative to Co(II)PPhe2(PyRu)2-cis concerning the specific absorption bands of the Ru shows that they are very similar. This means that the same number of Ru is fixed and that the interactions between the Ru themselves and the porphyrin ring are not strong, even if one is a trans isomer and the other a cis isomer. (Table 5, supplementary material) Nevertheless, an important difference is revealed by spectroelectrochemistry. After the oxidation at the state of Ru(III) the reversibility is not found again. This is described in Fig. 4: starting at − 0.5 V versus SCE we obtained the plain line curve typical of the Co(II)OEP[PyRu(II)]2; then, after oxidation at 0.5 V, the Co(II) is oxidised to Co(III) (dashed curve). When the potential is fixed at 0.95 V the 294 nm band characteristic of Ru(II) is split into two bands at 296 and 313 nm, showing the oxidation to Ru(III), but the Soret

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band of the porphyrin corresponding to Co(III) decreases as if the porphyrin were to be oxidised at its turn (dash –dot curve). The reversibility is lost after this last oxidation step, showing a chemical attack of the porphyrin ring.

Fig. 5. (a) Cyclic voltammograms of Co(II)PPh2[PyRu]2 10 − 3 M in DMF+ 0.25 M LiTFMS with increasing sulfite concentrations on EPGE; scan rate, 50 mV s − 1: (— —) absence of SO2; (– – –) SO2 = 10 − 3 M; ( — · — ) 5 × 10 − 3 M; ( — · · — ) 10 − 2 M. (b) Cyclic voltammograms of Co(II)P[PyRu]4 10 − 3 M in DMF+ 0.25 M LiTFMS with increasing sulfite concentrations on EPGE; scan rate, 50 mV s − 1: ( — — ) absence of SO2; (— —) SO2 = 10 − 3 M; ( – – – ) 5× 10 − 3 M; ( — · · — ) 10 − 2 M.

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3.3. Reaction with sulfite in DMF solution Na2SO3 was prepared in acidic aqueous solution and concentrated aliquots were added to the DMF solution, Fig. 5. CV of Co(II)PPhe2(PyRu)2 in the presence of sulfite aliquots in DMF solution had shown interaction of sulfite with the oxidation wave of the cobalt. The wave featuring the oxidation of Co(II) is first shifted to + 0.65 V with a concomitant increase of intensity according to the concentration of sulfite. When more sulfite is added, the Ru(II)/Ru(III) wave increases and loses its reversibility at its turn (Fig. 5(a) dash –dot –dot curve). The same reaction is observed with the other Co porphyrins derived from TPP (see Fig. 5(b), involving Co(II)P(PyRu)4) except for the mono-ruthenated porphyrin, which has given no reproducible results. We followed by spectroelectrochemistry the reaction of sulfite on Co(II)P(PyRu)4. This is shown on Fig. 6(a). The plain curve features the spectrum of the Co(II) –Ru(II)4 complex; then the sulfite (1 mM) is added, leading to the dashed curve showing a bathochromic shift of the Soret band. When the potential is set to + 0.95 V the Ru(II) is not oxidised to Ru(III), as shown by the absorption band which is not split into the usual bands, 300 and 316 nm (dash –dot curve). Moreover, the intensity of the Soret band of the porphyrin is not enhanced, as was noticed when Ru(II) is oxidised in the absence of sulfite. These facts show that the sulfite reduces the Ru(III) produced at the electrode and regenerates the Ru(II) and the Co(III) is changed. We followed the complexation of Co(II) by sulfite by ESR measurements. First we recorded the spectrum in the absence of sulfite and oxygen at low temperature (T= 100 K). Then we added aliquots of the acidic solution of sulfite. The two ESR spectra are shown in Fig. 6(b). The hyperfine constants and the g are different, especially the g [14]. When sulfite is added to the Ni2 + P[PyRu]4 in a DMF solution, in CV no other wave appears at the foot of the ruthenium wave as the nickel has no oxidation wave and, maybe, is not complexed by the sulfite.

3.4. CV on porphyrin-coated electrodes (wine-model solution)

Fig. 6. (a) Spectroelectrochemistry of CoTPyPRu4 10 − 4 M in methanol+ 0.25 M LiTFMS in presence of SO2: (— —) absence of SO2 at − 0.5 V; ( – – –) SO2 = 10 − 3 M at − 0.5 V; (— · — ) + 0.95 V. (b) ESR spectrum (9.4 GHz, 100 K) of Co(II)P[PyRu]4 10 − 3 M in DMF– water (80:20) +0.25 M LiTFMS: (— —) absence of SO2; (– – –) SO2 = 2.5× 10 − 3 M.

3.4.1. M 2 + P[PyRu]4 When the four porphyrins are coated on EPGEs and then plunged into the pH 3.4 buffered aqueous solution, the oxidation wave of the four Ru(II)/Ru(III) is observed at 0.72 V and is reversible, Fig. 7. We thus observed a 120 mV negative shift going from DMF to an aqueous medium. The peak intensities vary as a function of 6 as expected for adsorbed compounds. Only for CoP(PyRu)4 did we observe an ill-defined

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from the DMF to the acidic aqueous solution. The oxidation wave of the cobalt is found around 0.25 V. In the case of the corresponding free base the twoelectrons oxidation wave of Ru has a DEp larger than 70 mV, as if the two one-electron waves were not exactly superposed with a negative potential shift showing more interaction between the two Ru [15,16].

Fig. 7. Cyclic voltammograms on EPGE modified by cobalt porphyrins in wine model solution + 0.25 M LiTFMS; scan rate, 50 mV s − 1: ( — — ) Co(II)PPh3[PyRu]; (– – –) Co(II)PPh2[PyRu]2; ( — · — ) Co(II)PPh[PyRu]3; ( — · · — ) Co(II)P[PyRu]4. Table 3 Interfacial concentrations Y of the various porphyrins adsorbed on GCE and EPGE electrodes estimated by coulometry [16] assuming that all the ruthenium sites in the film are electrochemically active 1010Y (mol cm−2) GCE

EPGE

H2P(PyRu)4 NiP(PyRu)4 CoP(PyRu)4

0.60 0.65 0.56

0.53 0.50 0.5

CoPPhe(PyRu)3 CoPPhe2(PyRu)2 CoPPhe3(PyRu)

0.54 0.68 0.73

0.51 0.74 0.43

oxidation wave involving the oxidation of Co(II) at 0.27 V (100 mV negatively shifted compared with what occurs in DMF solution), the height of which is moreor-less four times smaller than that observed for the four Ru(II). For the others porphyrins even with the Ni(II)P no other oxidation wave was detected before the four Ru(II) oxidation wave.

3.4.2. CoP[PyRu(bpy)2Cl]nPhenyl4 − n (Fig. 7) Fig. 7 shows the voltammogram of the four porphyrin-coated EPGEs. The Co(II)/Co(III) wave can be guessed as being between 0.2 and 0.4 V. For the Ru(II)/ Ru(III) wave the anodic and cathodic peak potentials are almost at the same potential. DEpa −Epc =50 mV when 6 =50 mV s − 1. The intensity is dependent on the number of rutheniums per mole and the number of adsorbed moles of porphyrin Y at the surface electrode. 3.4.3. CoOEPPy2[Ru(bpy)2]2 When it is coated either on GCEs or EPGEs the oxidation wave of the two Ru is more reversible than the diffusion-controlled wave found in DMF and a 100 mV negative potential shift is observed when going

3.4.3.1. Determination of Y. The adsorption procedure described in Section 2 was the same for every complex. The initial concentration of the porphyrin solution was 10 − 3 M in DMF before coating the electrodes. The Y for each porphyrin has been calculated by taking the average of three experiments. They are gathered in Table 3. The interfacial concentrations do not vary meaningfully from the GCE to the EPGE and are of the order of 5× 10 − 11 mol cm − 2 for each porphyrin. 3.5. Catalysis of the oxidation of the sulfite on coated electrodes In aqueous solutions at 25°C, zero ionic strength, dissolved SO2 is best regarded as SO2·H2O and ionises to HSO3− and SO32 − - with pKa values of 1.89 and 7.18. These equilibria are sensitive for the ionic strength. There is another equilibrium involving HSO3− which leads to the formation of metabisulfite with a stability constant [1h] of 7× 10 − 2 M − 1: 2HSO3− = S2O52 − + H2O The degree of association increases with the concentration of HSO3− and the ionic strength [1f,g]. In contrast, in hydroalcoholic solution the pKa values are modified and increased according to the ethanol content in the solution [1f]. In our case, at pH 3.5, in a solution with 12% ethanol, the HSO3− should be the predominant form. The studies of the catalytic activity of each porphyrin in the presence of increasing sulfite concentrations were performed on the oxidation wave of the rutheniums.

3.5.1. M 2 + P[PyRu]4 Fig. 8 shows the CV (6= 50 mV s − 1) obtained with the GCE coated with the Zn(II) porphyrin in the presence of three increasing concentrations of sulfite. The four-electrons oxidation wave intensity increases slightly, while the corresponding reduction wave loses its reversibility smoothly. Nevertheless, the direct irreversible oxidation wave of the sulfite is still observed at a more positive potential around 0.98 V at a weak sulfite concentration 5× 10 − 5 M, a phenomenon that indicates the catalysis is very poor. Nevertheless, the direct oxidation of sulfite on a GCE is not observed with the electrode coated with the free base, Ni(II)P(PyRu)4 or Co(II)P(PyRu)4, except at great sulfite concentrations. In the three cases we ob-

N. Rea et al. / Inorganica Chimica Acta 312 (2001) 53–66

Fig. 8. Cyclic voltammograms in a wine model solution + 0.25 M LiTFMS (pH 3.4) on a Zn(II)P[PyRu]4 10 − 3 M coated GCE; scan rate, 50 mV s − 1: ( — — ) blank current; (— —) in absence of SO2; ( – – – ) SO2 = 5 ×10 − 5 M; ( — · — ) 10 − 4 M; ( — · · — ) 7× 10 − 4 M.

served that the intensity of the four Ru(II)/Ru(III) oxidation wave increases according to the concentration of the sulfite, while concomitantly the corresponding reduction wave becomes irreversible, as is to be expected for a catalytic process. Fig. 9 concerns the same studies performed on the CoP[PyRu]4 coated EPGE. Comparatively with the coated GCE at similar concentrations of sulfite the

61

direct sulfite oxidation at 0.98 V is not more observed, sign of a more efficient catalysis. The intensity increases of the wave are proportional to the concentration of sulfite, as shown in Fig. 9 (inset), and are more enhanced on the EPGE than on the GCE. Similar behaviour was obtained with the Zn(II)P[PyRu]4 and the H2P[PyRu]4. As the Zn(II)P[PyRu]4 was studied in another aqueous solution (citric acid, sodium phosphate buffer pH 3.4), we will not discuss this porphyrin any further, except to say that it was working pretty well coated on EPGE. Nevertheless, in the case of the cobalt porphyrins the oxidation wave of Co(II) to Co(III) at 0.27 V is still ill-defined when the porphyrins are adsorbed, as shown in Fig. 7. As the rate of the reaction seems to be controlled by the diffusion of sulfite and is not very high, the best sensitivity is obtained at a low sweep rate, so we chose 6 =50 mV s − 1 for the following studies.

3.5.2. CoPPh2[PyRu]2 and CoOEP(PyRu)2 For these two porphyrins the Co(II)/Co(III) wave is well identified. Its intensity height is half that of the Ru(II)/Ru(III) wave (2e−). Nevertheless, in the presence of small quantities of sulfite their redox behaviours are different. For CoPPh2(PyRu)2, after the addition of sulfite the Co(II)/Co(III) wave is shifted to a positive value until reaching the foot of the oxidation wave of the ruthenium, similar to what has been observed in DMF solution (Fig. 5(a)). Then, after adding more sulfite, the two waves merge and the Ru(III)/Ru(II)

Fig. 9. Cyclic voltammograms in a wine model solution + 0.25 M LiTFMS (pH 3.4) on a Co(II)P[PyRu]4 10 − 3 M coated EPGE in presence of increasing sulfite concentrations; scan rate, 50 mV s − 1: (— — ) in absence of SO2; ( – – – ) SO2 =10 − 4 M; ( — · — ) 5×10 − 4 M; ( — · · — ) 10 − 3 M; (······) 2 × 10 − 3 M. Inset: plot of peak current versus sulfite concentration.

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Scheme 1. Standard plots of peak current versus sulfite concentration on EPGE in model wine solution+ 0.25 M LiTFMS; scan rate, 50 mV s − 1. (1) Co(II)P[PyRu]4; (2) Ni(II)P[PyRu]4; (3) H2P[PyRu]4; (4) Co(II)PPh2[PyRu]2; (5) Co(II)OEP[PyRu]2.

oxidation wave increases and becomes irreversible. For CoOEP(PyRu)2, only a 100 mV shift of the cobalt oxidation wave is observed and the increase of the Ru(II)/Ru(III) wave with concentration of sulfite is smaller. The calibration curves obtained on coated EPGEs involving some porphyrins are gathered in Scheme 1. The calibration curve obtained with CoPPh(PyRu)3 is not presented in Scheme 1 to improve the clarity. It has the same pattern and is between the curves obtained for four Ru and two Ru. As expected, the best sensitivity is observed for M2 + P[PyRu]4. We can see that at low sulfite concentration the curves obtained for the M2 + P[PyRu]4 are quite superposed, whereas at higher concentrations the curves seem to reach a plateau. The same phenomenon is observed for the curves obtained for the porphyrin with two Ru. Explanations for this could be the presence of an oxidised species other than sulfate at high concentration of sulfite, a light adsorption of the oxidation product at the surface electrode, or a light dissolution of the catalyst during the experiment. Another cause could be the formation of S2O52 − decreasing the concentration of sulfite, but this reaction was expected to be negligible according to the stability constant [1h] at the sulfite concentration used. In their work, Araki and co-workers [6] did not mention this difference in behaviour, but they were using multifilm coated electrodes and the calibration curves are given for concentrations of sulfite inferior or equal to 10 − 3 M.

3.5.2.1. Long-term electrolysis. Using a reticulated GCE as a working electrode, two kinds of electrolysis were conducted: the first was in a homogeneous solution using a solution of porphyrin dissolved in DMF added to the wine-model solution at the 10 − 6 M concentration; the second was conducted on the same working electrode that had been modified by plunging it in an

ACN solution of porphyrin at a concentration of 10 − 3 M and allowing it to dry under argon flushing. ACN was used because it is a more volatile solvent than DMF. This coated electrode was then immersed in the wine-model solution. The two experiments were carried out with CoP[PyRu]4 and CoPPh2(PyRu)2. In both kinds of electrolysis, the starting and remaining concentration of sulfite (after 1 h of electrolysis) were measured with the modified EPGE by CV. So we can calculate the sulfite concentrations that were oxidised during the electrolyses. Then, knowing the number of coulombs that were consumed, we found that SO3H− is oxidised in a two-electrons process in both experiments for each porphyrin. First of all we carried a 1 h electrolysis of the blank solvent. In the heterogeneous catalysis experiments the quantity of porphyrin that was adsorbed at the working electrode surface was difficult to evaluate without a great error, so we could not calculate the turnover.

4. Discussion From all the results obtained in these studies it appears that the catalysis is driven by the oxidation of ruthenium complexed by the pyridines of the porphyrin ring, as has been demonstrated previously [6] on Co and for NiP[PyRu]4. We have shown that the driving force, which is represented by the redox potential of the ruthenium, is dependent on the porphyrin ring but it is not influenced by the nature of the central metallic ion studied (Table 1), whatever the solvent is. This potential is not changed when one, two or three pyridine groups are replaced by phenyl groups in the meso position of the ring, a fact that means the electron attracting power for these two aromatic groups is very similar. However, the pyridines must be maintained strictly perpendicular to the porphyrin ring by the complexation with the [Ru(bpy)2]Cl. This is certainly the reason why for the M2 + P[PyRu]4 there is no interaction between the four Ru and only one single fourelectrons oxidation wave is observed. Nevertheless, the increasing number of [Ru(bpy)2]Cl anchored to the ring ought to give rise to increasing constraints on the porphyrin ring, especially in the asymmetric substitution. The role of the porphyrin ring is not evident in the electron uptake, but it is reflected indirectly in spectroelectrochemistry. Why is there a systematic enhancement of the Soret band when Ru(II) is oxidised to Ru(III), whatever the central metallic ion, and even with the free base porphyrin? Nevertheless, as was shown in previous work [17], when the substitutions are made on the pyrrole moieties their effects are more efficient on the redox potentials. The presence of eight ethyl groups of the pyrroles of

N. Rea et al. / Inorganica Chimica Acta 312 (2001) 53–66

CoOEPPy2[Ru(bpy)2]2 shifts the Ru(II)/Ru(III) E F positively (30 mV), but it has a negative effect on the localisation of the first oxidation wave of the porphyrin ring. For this compound this oxidation wave occurs at quite the same potential as those of the rutheniums. The disadvantage is an oxidation of the porphyrin ring during long-term electrolysis, which induces an irreversible chemical modification of the catalyst (maybe an hydroxylation of the ring). This is reflected in the efficiency of the catalysis of this porphyrin, which is smaller than the one corresponding to CoPPy2[Ru(bpy)2]2 (Scheme 1). The coulometric measurements on heterogeneous and homogeneous catalytic electrolyses have shown that the sulfite is oxidised in a two-electrons process, so the number of ruthenium atoms is a determining factor in the catalytic mechanism. The CV experiments corroborate these results, as we have found that in the studies on the Co complexes the catalysis does not work at all with the porphyrin made up with only one ruthenium attached to the ring. At least two ruthenium atoms are required at the periphery of the porphyrin. This was also noticed in the study of the catalytic reduction of O2 by similar bimetallic porphyrins, which involves a fourelectron reduction process through the Co [7b,c,g]. But in this case the cobalt was directly involved in the oxygen fixation and its reduction. Moreover, this catalysis does not work without cobalt and needs almost two ruthenium atoms attached to the porphyrin. In the study of the oxidation of sulfite by CoPPh2(PyRu)2, the Co seems to play a role, but it is difficult to define. The cobalt is more accessible to the sulfite. We have shown that the cobalt(II) is surely complexed by the sulfite and that Co(III) is reduced by sulfite, as its reduction wave is difficult to observe in CV. ESR spectra at low temperature (100 K) have shown the coordination of sulfite by Co(II). In the same way, at room temperature, spectroelectrochemical studies in the UV –vis range on Co(II)P[PyRu]4 have shown unambiguously the complexation of Co(II) by the sulfite (Fig. 6(a): dashed line). Moreover, when the electrolysis potential is kept at +0.95 V the Ru(III) characteristic bands are not observed, which means that the Ru(III) made at the electrode is reduced by the sulfite and the Ru(II) is regenerated. But what happens with Co(III)? To test more deeply the role played by the cobalt we have added some imidazole to the solution of porphyrin in DMF and we have used the same procedure described before for making the modified electrode. We expected imidazole to complex the cobalt in axial positions, and not the ruthenium, and to prevent the coordination of the sulfite by the Co(II) during the sulfite electrocatalysis. Then this new electrode was used to establish a calibration line in the same conditions as those without imidazole. The slope of the calibration line decreases, showing that the catalytic process is

63

slowed down. Nevertheless, the catalysis is still working but certainly with only the ruthenium oxidation, as has been shown with Zn-, Ni-, and H2-P[PyRu]4. The role of the Co seems not determining.

4.1. Why a multielectron process? The electrocatalysis on these modified carbon electrodes is working in aqueous solution and requires almost two electrons. It has been shown in the mechanism of sulfite oxidase that the oxidation of sulfite into sulfate also involves a two-electrons process, the electrons being given in the last oxidation step through the oxidation of the Mo(IV) to Mo(VI) with the concomitant leaving of an oxo group linked to the molybdenum [18]. In the case of the [Ru(II)(bpy)2Cl]2 attached to the porphyrin ring the Cl− is labile and may be replaced by H2O. This could explain the 100 mV negative potential shift observed for the oxidation wave of Ru when going in our experiments from DMF to aqueous solution [12b,c]. On the assumption that this is what occurs, we envisage that the catalysis could be involved in the mechanism of oxidation through a water molecule giving an oxo- or an OH-group to the sulfite, like in the biological oxidation with sulfite oxidase. Identification of the exact coordination of Ru would be very interesting for understanding the oxidation mechanism, and a study of this catalysis in a strictly non-aqueous solvent with tetraalkylammonium sulfite is in our future plans. Our conclusion now is that, among the porphyrins studied, the free base, Ni P[PyRu]4 and even CoP[PyRu]4 are the best catalysts for the oxidation of sulfite on coated carbon electrodes for several reasons. First, the sensitivity is proportional to a four-electrons wave, which means more intensity for the calibration curve (Scheme 1). There is less distortion of the porphyrin ring. They are easy to synthesise and to purify. For the choice of the carbon electrode, the EPGE, which is more hydrophilic than the GCE, is more reliable. Moreover, among the porphyrins, NiP[PyRu]4 is the most suitable as it is inert toward the sulfite. Nevertheless, for voltammetric studies in wine solution, the major disadvantage is the positive potential of the attached Ru, which will give rise to the prediction that every molecule more oxidisable such as polyphenols will disturb oxidisable molecules, especially the polyphenols [8], will disturb the voltammogram and, as a consequence, prevent an accurate measure of free sulfite in such a medium.

5. Supplementary material

5.1. NMR The NMR results are presented in Table 4.

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N. Rea et al. / Inorganica Chimica Acta 312 (2001) 53–66

5.2. Spectroelectrochemistry With the aim of verifying the attribution of the oxidation waves to each redox state of the metallic ions we carried out the spectroelectrochemical studies at each potential corresponding to the various electron steps for each porphyrin in reduction and in oxidation. We also tested the reversibilities after each oxidation or reduction step.

5.3. M 2 + P[PyRu]4 A spectroelectrochemical study of CoP[PyRu]4 on a Pt grid in DMF (Fig. 10) corroborates the redox states localisation already attributed, corresponding to each oxidation wave [6d]. At − 0.5 V versus SCE the plain curve features the spectrum of Co(II); when the potential is fixed at 0.65 V we obtain the oxidation to Co(III), the dashed curve, with isosbestic points at 357, 427, 502, and 550 nm. Then at 0.95 V the oxidation of the four Ru(II), the dash – dot curve, is obtained with isosbestic points at 302, 336, 427, 442 nm. It is interesting to note that the Soret band of the Co(III)P is enhanced by the oxidation of the four Ru(II) despite the decrease of an absorption band (u= 470 nm) corresponding to Ru(II) just at the foot of the Soret band of the porphyrin. During the oxidation of the four Ru(II) at 0.95 V, the band at 290 nm relative to the specific optical absorption of the four Ru(II)(bpy)2Cl decreases and one other band at 316 nm appears. All these oxidation steps are reversible and the spectrum of the starting complex is retraced step by step. In reduction, at −1.2 to − 1.4 V the Co(II) complex is reduced in a one-electron

process to Co(I) (dash –dot –dot curve) which corresponds to its known spectrum. After reduction at − 1.8 V, the spectrum obtained (curve not drawn on the figure), corresponds to the obtaining of multiple spectra and the reduction of the eight bipyridyl ligands. The 290 nm absorption band specific to these ligands decreases and the isosbestic points at 280, 302, 367, and 520 nm moved. This last reduction step is irreversible and seems to lead to the disassembling of the porphyrin pyridine groups with the Ru(bpy)2Cl. The same behaviour was obtained for each Co porphyrin. In the case of the Ni(II)P(PyRu)4, electrolysis at 0.95 V leads to a decrease of the 295 nm band featuring the absorption of the Ru(II)(bpy)2Cl2 and the increase of the same band observed at 316 nm with Co(III)P(PyRu(III)4, with isosbestic points and the enhanced Soret band of the porphyrin. The intensities of the two visible bands of the porphyrin decrease slightly but are not shifted in wavelength, showing that Ni stays in the same redox state. For the reduction step at −0.95 V (versus SCE), the Soret band and the visible bands of the porphyrin decrease and other bands appear, leading to Ni(I)P(PyRu)4, as was already observed previously with Ni(II)TPP [6c,e], whereas the 295 nm band is unchanged. Reduction at − 1.8 V leads to the disassembling of the complex with Ru, as occurred with the corresponding cobalt complex. With Zn(II)P(PyRu)4 the oxidation at 0.95 V leads to the same spectrum pattern [6b] obtained with Ni(II)P(PyRu)4 and Co(II)P(PyRu)4 (Table 5) For these three metalloporphyrins, after the oxidation of the Ru(II) the Soret band is enhanced and the visible bands decrease. In the case of the free base ruthenated porphyrin H2P(PyRu)4 the oxidation spectrum observed at 0.95 V did not show any attack of the porphyrin ring by Cl−, which might be in contradistinction with the coulometric results (8e−). The original porphyrin spectrum was recovered after reduction [6a]. The explanation could be a rapid dimerisation of the Cl− (provided by the free Ru(bpy)2Cl2 impurity which appears in the CV) as they are produced at the electrode far from the porphyrin ring and are no longer able to attack it. These results are gathered in Table 6.

Acknowledgements

Fig. 10. Spectroelectrochemistry of Co(II)P[PyRu]4 10 − 4 M in DMF+ 0.25 M LiTFMS; (— —) − 0.5 V; (– – –) + 0.65 V; ( — · — ) + 0.95 V; ( — · · —) − 1.6 V.

We thank A. Fournel for his assistance in the ESR measurements, and B. Daoudi and S. Terrillon for their technical assistance in chemistry. This work was funded by a grant given to N. Rea by the CIVB (Conseil Interprofessionel des Vins de Bordeaux).

H pyrrole

8.90 (d, 4H) 8.85 (s, 4H) 8.86 (s, 8H)

8.85 (s, 4H) 8.86 (s, 2H) 8.83 (s, 2H) 8.79 (d, 4H) 8.81 (d, 2H)

8.78 (d, 2H) 8.80 (d, 2H)

9.02 9.04 9.03 9.05 9.06

(dd, (dd, (dd, (dd, (dd,

2H) 4H) 4H) 6H) 8H)

5.7

436 434 294 296 318

Ru(II)(bpy)2

Ru(III)(bpy)2

b

Corresponds to the Co(III)Ru(III) Soret band. Corresponds to Co(III)Ru(II) Soret band. c Irreversible process: oxidation of the porphyrin ring.

a

14.1 16 15.7

418

Co(II)

Co(III)a Co(III)b

4.3 4.8

6.7 7.6

297 317

296

436 434

418

365 429

7.9 6.2

10.8

16.9 15.9

14.8

7.1 8.2

m×104 (M−1 s−1)

u (nm)

u (nm)

m×104 (M−1 s−1)

PPh2(PyRu)2

PPh3PyRu

364 427

Co(I)

Metal ion oxidation state

299 316

296

437 435

419

369 432

u (nm)

9.4 8.6

15.8

18.1 15.8

18

7 8.7

m×104 (M−1 s−1)

PPh(PyRu)3

(dd, (dd, (dd, (dd, (dd,

2H) 4H) 4H) 6H) 8H)

300 316

296

437 434

421

370 433

u (nm)

P(PyRu)4

8.15 8.16 8.16 8.15 8.15

10 9.3

17.3

17 15.7

13.4

6.4 7.9

m×104 (M−1 s−1)

8.21 (m, 2H)

8.18 (m, 6H) 8.21 (m, 4H)

o-H

a-H

b-H

Phenyl

Pyridyl

Table 5 Optical data for the CoP[PyRu(bpy)2Cl]n Phenyl4−n and CoOEP(PyRu)2 according to the oxidation state of the metal ions

TPP PyPh3P 8.88 (d, 2H) cis-Py2Ph2P 8.89 (d, 2H) trans-Py2Ph2P Py3PhP 8.92 (d, 2H) TPyP

Table 4 NMR data

296 313

296

438 437

410

u (nm)

2.84 2.85 2.85 2.89 2.93

(s, (s, (s, (s, (s,

7.8 5.9

10.7

2.0c 7.7

6.5

m×104 (M−1 s−1)

OEP(PyRu)2

7.78 (m, 3H)

7.76 (m, 9H) 7.78 (m, 6H)

m,p-H

NH

br) br) br) br) br)

N. Rea et al. / Inorganica Chimica Acta 312 (2001) 53–66 65

N. Rea et al. / Inorganica Chimica Acta 312 (2001) 53–66

66

Table 6 Optical data of H2-, Zn-, Ni-P(PyRu)4 according to the oxidation state of Ru ZnP(PyRu)4

Ru(II)(bpy)2

NiP(PyRu)4

u (nm)

m×104 (M−1 s−1)

u (nm)

m×104 (M−1 s−1)

u (nm)

m×104 (M−1 s−1)

297

12.4

296

13.6

296

21.4

297 316

10.1 8.7

301 316

12.1 11.3

420

20.2

420

28.5

422

22.5

423

32.5

Ru(III)(bpy)2 SoretRu(II)

H2P(PyRu)4

429

18

SoretRu(III)

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