ELSEVIER Journal of Electroanalytical Chemistry 397 (1995) 205-210 Electrochemistry of a tetraruthenated cobalt porphyrin and its use in modified electrodes as sensors of reducing analytes Koiti Araki, Lkio Angnes, Carla M.N. Azevedo, Henrique E. Toma lnstitu~ de Quimica, Uniuersidade de SZo Paula, Caixa Postal 26.077. CEP-05599-970, Siio Pauln (SP), Bruzil Received 16 February 1995; in revised form 31 May 1995 Abstract The properties and analytical applications of a new polymetallic species containing four [Ru(bipy),Cl]+ (bipy = 2,2’-bipyridine) groups coordinated to the pyridine residues of the meso-tetra(pyridyl)porphynate cobalt(III) complex are reported. The cyclic voltammo- grams of the tetraruthenated cobalt porphyrin, in DMF solution, exhibit an intense reversible wave at E,,, = 0.93 V associated with the Run’/” redox couple, and two irreversible cathodic waves at I?~ = - 1.37 and - 1.63 V, involving the stepwise reduction of the bipyridine ligands in each peripheral ruthenium complex. Three successive reductions of the cobalt porphyrin center can be detected in the - 0.5 to - 0.7 V range, using spectroelectrochemical techniques. Stable, homogeneous films of the polymetallic porphyrin can be generated by dip-coating on to electrode surfaces, exhibiting pronounced electrochemical activity. The modified electrodes have been successfully employed for analytical purposes, using conventional and flow injection methods, showing excellent performance in the determination of reducing analytes, such as nitrite and sulphite ions, even at ppb levels. Keywords: Porphyrins; Tetraruthenated cobalt porphyrin; Electrodes; Sensors; Reducing analytes 1. Introduction The chemistry of polymetallic porphyrins and other supermolecules has attracted the attention of many re- search groups interested in molecular devices [l-4] and multi-electron transfer catalysis [5,6]. In our laboratory, we have been employing coordination compounds as building blocks for the design of multi-bridged supermolecules. Using this approach, [Ru”/“‘(edta)], [Ru” “ “ (NH~)~], [Fe”/“‘(CN),], [Ru “/“‘(bipy),Cl] and related complexes have been attached to bridging groups in TPyP (meso-te- trapyridylporphyrin) [7- 121, [Ru(bpz)a12+ (bpz = bipyrazine) [ 13- 151 and ruthenium-acetate trigonal clus- ters [ 16-181, leading to a new generation of symmetric supermolecules. Along these lines, we reported recently on the electro- chemical, spectroelectrochemical and luminescence proper- ties of TPyP [9] and Zn-TPyP [lo] modified with four [Ru(bipy)CI] groups, as well as of the nickel derivative [ 111, emphasizing its pronounced electrocatalytic activity towards CO, reduction and the rectifying properties of the corresponding films [ 121. Here we describe the electro- chemical properties of a new tetraruthenated cobalt por- phyrin (Fig. I), and the behaviour of modified electrodes 0022-0728/95/$09.50 0 1995 Elsevier Science S.A. All rights reserved S.SDf 0022-0728(95)04193-l containing multi-layered films of the supermolecular species. It should be noted that porphyrin-modified electrodes have been employed, with increasing interest, in analytical and bioanalytical chemistry [19]. In this work, the investi- gation of the modified electrodes containing tetraruthen- ated cobalt porphyrin was directed to the detection of reducing analytes, particularly those which are not suitable for conventional electroanalytical monitoring, such as the nitrite and sulphite ions, because of their poor electrochem- ical activity on bare electrodes. 2. Experimental The [Co’nTPyP(Ru” (bipy),Cl],l(TFMS), .4H,O com- plex (TFMS- = triflate or trifluoromethanesulphonate ion) was prepared by the reaction of CoTPyP with [Ru(bipy),Cl,], in a molar ratio of 1 :4.1, as previously reported for related complexes [9- 121. Elemental analysis for C,,,H,,N2,0,,!3,F’,C1,Ru,Co: calcd. C, 46.2; H, 3.1; N, 10.3; found: C, 45.6; H, 2.9; N, 10.2%. Spectroelectrochemicai and cyclic voltammetric mea- surements were carried out as described previously [9- 121.