The cyclic voltammetry of some sulphonated transition metal phthalocyanines in dimethylsulphoxide and in water

161

J. Electroanal. Chem., 271 (1989) 161-172 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

The cyclic voltammetry of some sulphonated transition metal phthalocyanines in dimethylsulphoxide and in water John T.S. Irvine and Brian R. Eggins * Department of Chemistry, Universiry of Ulster al Jordansrown, Newtownabbey, Co. Antrim BT37 OQB (Northern Ireland)

James Grimshaw Department

of Chemurry,

The Queen’s University of Belfast, Belfrrrr BT9 SAG (Northern Ireland)

(Received 6 March 1989; in revised form 9 June 1989)

ABSTRACT The aqueous voltammetry of Co, Ni, Cu and Zn sulphonated metallophthalocyamnes has been examined and compared to the voltammetry in DMSO. In DMSO, Ni, Cu and Zn phthalocyanines exhibit two quasi-reversible, phthalocyanine hgand based, one-electron transfers. Two reduction waves were also observed for Co phthalocyanine. However, the first reduction wave appeared to involve both a metal based electron transfer and a change in the coordmatlon of the metal. The aqueous voltammetry of the cobalt phthalocyanine was similar to its non-aqueous behaviour. The other three phthalocyanines did not show the quasi-reversible behaviour in aqueous solution. Reversibility was much diminished and the forms of the waves changed greatly. indicating the involvement of protons in the redox processes.

INTRODUCTION

Metallophthalocyanines have been used as photocatalysts or electrocatalysts in the reduction of methyl viologen [1,2], the reduction of oxygen [3-51, the oxidation of water [5] and the reduction of carbon dioxide [6,7]. In this latter connection, carbon dioxide is reduced in many aqueous solutions to formic acid [8]. In aprotic solvents, carbon monoxide [9] or oxalic acid [9] are formed. But in aqueous solutions containing tetraalkylammonium salts, reduction to oxalic acid followed by further reduction to glyoxylic acid is observed [lo]. With these possibilities in mind it was desirable to have a water soluble electrocatalyst and hence the sulphonated metal phthalocyanines were used. The results of the interaction of carbon dioxide with metal phthalocyanines have been reported separately [ll]. Solubility of the phthalocyanines has been improved either by sulphonation of phthalocyanine itself or by methylation of the phthalocyanine derived from pyridine-3,4-dicarboxylic acid. 0022-0728/89/$03.50

0 1989 Elsevier Sequoia S.A.

162

The redox behaviour of metallophthalocyanines has been the subject of much investigation. Rollmann and Iwarnoto [12] reported the half-wave potentials on polarography and at a rotating platinum electrode for a number of tetrasulphonated examples in DMSO and values have also been reported in dimethylformamide [13]. Lever and his co-workers have reported on the cyclic voltarnmetry of a number of transitional metal phthalocyanines in aprotic solvents [14-171 and analysed the behaviour of iron(I1) [14] and manganese(II1) [16] derivatives at fast sweep rates. Little has been reported on the aqueous voltammetry of phthalocyanines except for the adsorption on silver using resonance Raman and cyclic voltammetry techniques In this study, the cyclic voltammetry of four sulphonated phthalocyanines Ni, Cu, Zn) has been investigated in both aqueous and dimethylsulphoxide tions.

(Co, solu-

EXPERIMENTAL

The tetrasulphonated phthalocyanines TSCoPc, TSNiPc, and TSZnPc were prepared using a method modified from that of Weber and Busch [19]. Disulphonated copper phthalocyanine, DSCuPc, was obtained as the dye Direct Blue (CI 74180) and was purified by two recrystallizations from a water + ethanol mixture (1: 2). The purity of these materials was checked by their visible spectra [12] and by the absence of carbonyl bands in the infrared spectrum. Structures of these materials are summarised in Scheme 1. Deionized water and Spectrosol grade DMSO, dried over 4A molecular sieves, were used in the voltammetric experiments. Tetramethylammonium chloride (0.1 M) was the supporting electrolyte in water and tetraethylammonium perchlorate (0.1 M) in DMSO. Solutions were purged with nitrogen prior to each voltammetric measurement.

N

,

AN

N

-23 -

\

/

R

Scheme 1. Metallophthalocyanines used in this work. TSCoPc: M = Co(H), R = SO; Ni(II), R = SO; ; TSZnPc: M = Zn(II), R = SO; ; DSCuPc: M = Cu(II), R = H.

; TSNiPc: M =

163

Cyclic voltammograms were performed using a Bioanalytical cyclic voltammetry instrument CVIA. A three-electrode system with a saturated calomel reference electrode and a mercury pool counter electrode was used. The working electrode was either a hanging mercury drop or an inlaid platinum disc (0.2 cm2). Voltammograms were recorded on a Bryans Southern X-Y recorder (29000) and potentials monitored using a Sinclair Multimeter (DM 450). The baseline for measurements from the cyclic voltammetric curves was estimated by inspection. This results in errors in E, and i, of kO.01 V and *lo%;, respectively. Additional errors arose in the measurement of the surface area (A) of the hanging mercury drop (*5%). The uncertainties in values of &,/log u were generally less than 0.01. RESULTS

AND

DISCUSSION

Non-aqueous voltammetry Typical voltammograms for each of the phthalocyanines are shown in Figs. l-5. The corresponding polarographic waves of the Co, Ni, and Cu phthalocyanines are described in the literature [12]. Two one-electron reduction waves were observed for the Ni, Cu, and Zn phthalocyanines, one in the region -0.4 to -0.9 V and the second in the region - 1.1 to - 1.4 V. A comparison between our data and those in the literature is given in Table 1. These waves show reversible behaviour at slow sweep rates on cyclic voltammetry at both mercury and platinum but the heterogeneous electron transfer rate is relatively slow so that the cathodic to anodic peak separation varies with sweep rate. The similarities in reduction potential between these phthalocyanines indicate that reduction involves the phthalocyanine ligand rather than being metal centred. The variation of E,, with sweep rate changed from the reversible case to the charge transfer rate determining case within the-sweep rates studied. A typical result

Fig. 1. Cyclic voltammogram

of TSCoPc

(1.4X10K4

M) in DMSO

on Pt with u = 0.04

164

-1.6

-1.2

Fig. 2. Cyclic voltammogram

of TSCoPc

(2.0

0

-0.4

-0.6

x

10e4 M) in DMSO

on Hg with u = 0.1 V s-‘.

is shown in Fig. 6. Values were calculated for the diffusion coefficients (II) associated with these waves using the Randles--SevCik equation. Then the standard heterogeneous rate constant was calculated from the sweep rate which gave a peak separation of 0.1 V. The data are collected in Table 2 and relevant equations are appended there. The values of the diffusion coefficients obtained for these phthalocyanines are low, though this is consistent with the large size of the molecules. The high values of the diffusion coefficients for the cobalt species may be related to the fact that they are 4-coordinated, whereas the others are 6-coordinated and include coordinated solvent molecules, compounded by errors in the process of measuring the voltammograms. The rate constants for these electrode processes are intermediate in the range of quasi-reversibility as specified by Heinze [20].

E/V

VSSCE

-1.2 I

I

-0.6 I

I

I

-0.4 I

1 -

-

Fig. 3. Cyclic voltammogram

of TSNiPc

(2 X lo-’

M) in DMSO

20

-20

on Pt with u = 0.1 V s-l.

165 TABLE 1 Cyclic voltammetry and polarography of metal phthalocyanines in DMSO Half-wave potentials/V

Substrate:

vs. aqueous SCE Cyclic voltammetry

Polatography [12] RPE

DME

Hg

Pt + 0.38 0.048 -0.42 0.144 -1.35 0.067

TSCoPc4 1 AC a 2 hC a 3 ACa

- 0.547 0.065 - 1.346 0.065

- 1.355 0.065

- 0.40 0.060 -1.30 0.084

TSNiPc41 AC a 2 ACa 3 AC a

- 0.672 0.075 - 1.165 0.082 - 1.933 0.061

- 0.682 0.078 - 1.171 0.084 -1.925 0.060

- 0.67 0.030 -1.14 0.034 -1.72 0.062

-0.67 0.041 -1.13 0.037

DSCuPc41 AC a 2 AC a 3 AC’

- 0.721 0.065 -1.111 0.072 - 1.895 0.062

-0.735 0.074 -1.113 0.072 - 1.88 0.070

- 0.80 0.062 -1.20 0.042 -1.90 0.078

- 0.79 0.065 -1.18 0.066

-0.80 0.062 -1.26 0.050

- 0.83 0.055 - 1.30 0.089

+ 0.455 0.084

TSZnPc41 ACa 2 AC a

a AC =

E,,,

- El14 (polarography);

AC = %?,/a

log o (CYCI~C vokmunetry)

TABLE 2 Current functions, diffusion coefficients and electron transfer rate constants for metal phthrdocyanines on platinum in DMSO Substrate TSCoPc TSNiPc DSCuPc TSZnPc

XP 1 2 1 2 1 2 1 2

B

1.00 1.10 0.94 0.55 0.40 0.30 0.51 0.55

AC’

all

10” D/ m2 s-’

ii/v

0.144 0.067 0.041 0.037 0.065 0.066 0.055 0.089

0.21 0.44 0.72 0.80 0.45 0.45 0.54 0.33

5.5 3.1 1.39 0.43 0.4 0.22 0.55 1.00

0.005 0.079 0.038 0.020 0.056 0.042 0.105 0.091

a xp = i,/Ac’/* = 2.98~ lO’n(oln) ‘/* D “2; hence D when an is known. b AC = aE,,/a log o =0.0295/m; hence an. ’ k, = 6.28 ( DC)‘j2 (5 = value of u for which E, = 0.1 V).

s- ’

lo6 k,,’ ms-” 3.29 9.8 4.56 1.84 2.97 1.91 4.77 5.99

166 E/V

Fig. 4. Cyclic volt-ogram

vsSCE

of DSCuPc (2 x 1O-3 M) in DMSO on Pt with v = 0.04 V s-l.

Additional irreversible reduction waves were observed at - 1.8 V for Ni and Cu phthalocyanines at a mercury electrode where the background current is less than for platinum. The cyclic voltammetry of TSCoPc was very different from that of the other phthalocyanines studied. Two reduction waves were observed on sweeping to

E/V

Fig. 5. Cyclic volt-ogram

vsSCE

of TSZnPc (3.2 X 1O-4 M) in DMSO on Pt with u = 0.1 V s-’

167

-l.o~-----

E/V VSSCE

I

I -1

-2

log

0 (V/

VS -9

Fig. 6. Peak potentials vs. log u for DSCuPc on Pt in DMSO.

negative potentials with the corresponding anodic waves on the reverse sweep. For the first of these two waves, the cathodic and anodic peaks were separated by large potentials on Hg, and Pt (Figs. 1 and 2). The midpoint of the anodic and cathodic peaks was -0.42 V. Peak potentials also showed large shifts with change of sweep rate. This is indicative of a change in the number of axial ligands attached to the

I 4Q0

I

500

I

I

600

700

X Inm

Fig. 7. Absorption spectrum of reduced TSCoPc in DMSO.

168

E/v VSSCE -1.0

-1.4

Fig. 8. Cyclic voltammogram

-0.6

-0.2

of TSCoPc (10W3 M) in water at pH 6.1 on Hg with u = 0.1 V s-l.

central metal atom after electron transfer, which is metal centred and involves Co(II)/Co(I). A related change in the coordination sphere of iron phthalocyanine upon reduction has been demonstrated by Lever and Wiltshire [14]. The absorption spectrum of the reduction product from TSCoPc in this potential region was almost identical in water and in DMSO (Fig. 7) so we can conclude this product has no axial ligands as was previously suggested by Lever and Minor [15].

E/V vr -1.4

-1.0

Fig. 9. Cyclic voltammogram

SCE

-0.6

-0.2

of TSCoPc (10m3 M) in water at pH 4.7 on Hg with o = 0.1 V s-l.

169 E/V

VBSCE

-1.4 I

-1.0

I

I

-0.6

I

I

I

@A

--10

--20

Fig. 10. Cyclic voltammogram

of TSNiPc

(2~10~~

M) in water

at pH 7.2 on Hg with u = 0.1 V s-‘.

The second reduction wave for TSCoPc showed the expected variation of peak potentials for a slow electron transfer process. This is probably ligand based and does not involve Co(O) species. The anodic peak height decreased at slow sweep rates, indicating slow decomposition of the reduced species. An oxidation wave was observed on sweeping in the positive direction at Pt or glassy carbon, with a corresponding cathodic peak on the reverse sweep. This system was centred at +0.38 V and had the characteristic response to changes in sweep rate of a slow electron transfer process. It is a metal centred electron exchange involving co(II)/co(III).

E/V

I

vrSCE

-1.4

-1.2

-1.0

-0.6

I

I

I

I

Fig. Il. Cyclic voltammogram

of DSCuPc

I

(2X10e3

M) in water

at pH 7.3 on Hg with u = 0.2 V s-l.

170 E/V -1.0

VSSCE -0.6

Fig. 12. Cyclic voltammogram

-0.4

-0.6

I

I

I

I

of TSZnPc (4X10e4

-2

M) in water at pH 7.3 on Hg with u = 0.1 V s-l.

Aqueous voltammetry The cyclic voltammetry of these four phthalocyanines was investigated at the hanging mercury drop electrode in neutral aqueous solution (Figs. 8-12). Data are collected in Table 3. The voltammetry of TSCoPc was similar to its non-aqueous behaviour with two quasi-reversible one-electron waves, both with a follow up chemical reaction as indicated by the low iJi, ratio (0.15). The first wave may be due to Co(II)/Co(I) but the second wave is more likely to involve the phthalocyanine ring Co(I)Pc’-/Co(I)Pc6-. A cathodic pre-peak observed in acid solution may be due to reduction of a protonated phthalocyanine to form a metal hydride as proposed by Fischer and Eisenberg [21] for another cobalt tetra-aza macrocycle. Reduction waves of Cu, Ni and Zn phthalocyanines were irreversible and must involve addition of both electrons and protons to the phthalocyanine system, despite their being independent of pH. The results were generally quite different from those in non-aqueous solutions.

TABLE 3 Voltammetry

of metal phthalocyanines

Substrate TSCoPc TSNiPc DSCuPc TSZnPc

1 2 1 2 2 1 2

in water a

E&V vs. SCE

XP

- 0.85 - 1.35 - 1.34 - 1.10 - 1.25 -0.85 -1.00

4 4 7 6

a.b Experimentally derived parameters ’ Assumed value for (in.

6 7

ACb

tin

n

10” D/ m* s-l

0.079 0.053 0 0.035 0.032 0.043 0.049

0.37 0.56 0.5 c 0.84

1 1 2 2 Ads 1 1

4.9 3.2 2.15 1.21

as in Table 2.

0.69 0.60

5.9 9.2

171

TSNiPc showed a single irreversible two-electron wave, Ni(II)Pc4-/Ni(II)Pc6-, though prepeaks were observed which increased with decreasing pH and which could involve metal hydride formation as with cobalt. TSCuPc showed two cathodic peaks, the second of these showing the characteristics for strong adsorption of reactant as indicated by the wave shape and the constant value of 0.15 V peak separation over the scan range studied. The main peak at - 1.1 V would appear to be a two-electron reversible wave (E,/log I’= 0.030 V) due to the couple Cu(II)Pc2~/Cu(II)Pc4with following irreversible chemical reaction. TSZnPc showed two quasi-reversible one-electron cathodic peaks which changed relative heights with the scan rate. The corresponding anodic wave was one composite quasi-reversible two-electron wave. The two redox processes involved are Zn(II)Pc4-/Zn(II)P& and Zn(II)Pc’-/Zn(II)Pc6-. The ratio i,/(ib + i,‘) = 1 but the ratio i,/iz decreased with scan rate from 2 to 1 over the range 0.02 to 0.2 V s-t CONCLUSION

The nature of the redox reactions involving phthalocyanine ligand based electron transfers is influenced strongly by the proticity of the solvent. In DMSO these reactions are quasi-reversible, one-electron charge transfers. In aqueous solution these reactions are much more irreversible, peak potentials and shapes are different and more than one electron may be transferred, indicating the involvement of protons. The nature of the first reduction wave for Co phthalocyanine, which involves a metal based electron transfer, is much less influenced by the proticity of the solvent. REFERENCES 1 A. Harriman, G. Porter and M.C. Richoux, J. Chem. Sot. Faraday Trans. 2, 77 (1981) 1175. 2 A.B.P. Lever, S. Licoccia, B.S. Ramaswamy, S.A. Kandil and D.B. Stynes, Inorg. Cbim. Acta, 51 (1981) 169. 3 G.L. Elizarova, L.G. Matvienko, N.B. Lozhkina and V.N. Parmon, React. Kinet. Catal. Lett., 16 (1981) 285. 4 A.K. Shukla, C. Paliteiro, R. Manoharan, A. Hamnett and J.B. Goodenough, J. Appl. Electrochem., 19 (1989) 105. 5 K.I. Zamaraev and V.N. Parmon, Energy Resources through Photochemistry and Catalysis, Academic Press, New York, 1983, p. 123. 6 S. Meshitsuka. M. Ichikawa and K. Tamaru, J. Chem. Sot. Chem. Commun., (1974) 158. 7 K. Hiratsuka, K. Takahashi, H. Sasaki and S. Toshima, Chem. Lett., (1979) 305. 8 K.S. Udupa, G.S. Subramanian and H.V.K. Udupa, Electrochim. Acta, 16 (1971) 1593. 9 J.C. Gressin, D. Michelet, L. NadJo and J.M. Savtant, Nouv. J. Chim., 3 (1979) 403. 10 B.R. Eggins, E.M. Brown, E.A. O’Neill and J. Grimshaw, Tetrahedron Lett., 29 (1988) 945. 11 B.R. Eggins, J.T.S. Irvine and J. Grimshaw, J. Electroanal. Chem.. 266 (1989) 125. 12 L.D. Rollmann and R.T. Iwamoto. J. Am. Chem. Sot., 90 (1968) 1455. 13 D.W. Clack, N.S. Hush and I.S. Woolsey, Inorg. Chim. Acta, 19 (1976) 129. 14 A.B.P. Lever and J.P. Wiltshire, Inorg. Chem., 17 (1978) 1145. 15 A.B.P. Lever and P.C. Minor, Adv. Mol. Relat. Int. Proc., 20 (1980) 2550.

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A.B.P. Lever, P.C. Minor and J.P. Wiltshire, Inorg. Chem., 20 (1981) 2550. A.B.P. Lever and P.C. Minor, Inorg. Chem., 20 (1981) 4015. B. Sink-Glavaski, S. Zecevic and E. Yeager. J. ElectroanaI. Chem.. 150 (1983) 469. J.H. Weber and D.H. Busch, Inorg. Chem., 4 (1965) 472. J. Heinze, Angew. Chem. Int. Ed. Engl., 23 (1984) 831. B. Fisher and R. Eisenberg, J. Am. Chem. Sot., 102 (1980) 7361.