Bipyridyl cyclic voltammetry

Systems which fulfil these conditions are tris(2,2 -bipyridyl)rhodium complexes [63] and, more effectively, substituted or unsubstituted (2,2 -bipyridyl) (pentamethylcyclopentadienyl)-rhodium complexes [64], Electrochemical reduction of these complexes at potentials between — 680 mV and — 840 mV vs SCE leads to the formation of rhodium hydride complexes. Strong catalytic effects observed in cyclic voltammetry and preparative electrolyses are... 

A molecule will rarely take up or release a large number of electrons without bond disruptions. An exceptional example involves the reduction of an Fe(III) tris-bipyridyl complex, in which six electrons are added stepwise to the complex. The chemical reversibility of the processes may be followed by cyclic voltammetry (Fig. 23.11). 

One of the first reports on the quasireversible electrochemistry of redox proteins appeared in 1977 when Eddowes and Hill demonstrated (10) cyclic voltammetry of horse heart cytochrome c at a gold electrode in the presence of 4,4 -bipyridyl (Bipy) in solution. In the voltammo-grams (Fig. 1), the peak-to-peak separations were close to 60 mV and the faradaic currents varied linearly with (scan rate), indicating a quasireversible one-electron transfer process controlled by linear diffusion of redox species to the electrode surface. The midpoint potential... 

The theme of photosensitizing semiconductor electrodes introduced in Section 57.3.2.5(iii) may be developed with an example from ruthenium—bipyridyl chemistry. The sequence (40) is well known. The effectiveness of the photosensitization should be increased by the covalent attachment of the tris(bipyridyl)ruthenium(II) entity to the semiconductor surface, for example to Sn02. This has been achieved using the versatile halosilane chemistry shown in equation (41). The coimter anion was PFg . Cyclic voltammetry showed that the behaviour of the sjretems Sn02/aqueous [Ru(bipy)3] " and Sn02(Alm)/electroly te were very similar but with a -1-0.05 V shift in E°. The coated electrode gives a photocurrent with a red shift of 10 nm which is twice as large as for the non-coated electrode. Unfortunately the current falls off due to promotion of the hydrolysis of the Aim. 

In the reduction with 2,2 -bipyridyl, redox reactions are absent and the limiting stage is the transfer of the 2nd electron, [AEo being —60 to —70 mV. Reduction of the nickel(II) complex with PPhs, ( PrO)3P, PhP(OBu)2, or (PhO)3P is limited by the transfer of the 1st electron and is accompanied by comproportionation (PPh3, AEo= 90 mV) and disproportionation reactions (phosphites, AEq< 0). The redox properties of some transition metal-cinnamonitrile cyclo-phosphazene derivatives, e.g., 107-109 have been stuoied by cyclic voltammetry (CV) and controlled potential electrolysis (CPE) in aprotic media. 

As discussed before in the case of nucleic acids the authors have also considered the incidence of the interfacial conformation of the hemoproteins on the appearance of the SERRS signals from the chromophores. Although under their Raman conditions no protein vibration can be observed, the possibility of heme loss or protein denatura-tion are envisaged to explain a direct interaction of the heme chromophores with the electrode surface in the case of the adsorl Mb. extensive denaturation of Cytc at the electrode appears unlikely to the authors on the basis of the close correspondence of the surface and solution spectra. Furthermore, the sluggish electron transfer kinetics measured by cyclic voltammetry in the case of Cytc is also an argument in favour of some structural hindrance for the accessibility to the heme chromophore in the adsorbed state of Cytc. This electrochemical aspect of the behaviour of Cytc has very recently incited Cotton et al. and Tanigushi et al. to modify the silver and gold electrode surface in order to accelerate the electron transfer. The authors show that in the presence of 4,4-bipyridine bis (4-pyridyl)disulfide and purine an enhancement of the quasi-reversible redox process is possible. The SERRS spectroscopy has also permitted the characterization of the surface of the modified silver electrode. It has teen thus shown, that in presence of both pyridine derivates the direct adsorption of the heme chromophore is not detected while in presence of purine a coadsorption of Cytc and purine occurs In the case of the Ag-bipyridyl modified electrode the cyclicvoltammetric and SERRS data indicate that the bipyridyl forms an Ag(I) complex on Ag electrodes with the appropriate redox potential to mediate electron transfer between the electrode and cytochrome c. 

Exotic calix[4]arene monotopic (23) and ditopic (24) anion receptors containing one and two ruthenium(II) bipyridyl moieties have been prepared (Schemes 5 and 6) and shown by NMR and cyclic voltammetry to bind and electrochemically recognise halide, dihydrogen phosphate and hydrogen sulphate anions. 

Eddowes and Hill found [48,49] that essentially reversible cyclic voltammetry of horse mitochondrial cytochrome c could be achieved with a Au electrode onto which was adsorbed, from the same solution, the reagent 4,4 -bipyridyl. The result is shown in Fig. 4. The criteria described by Nicholson and Shain [50] for a one-electron process controlled by linear diffusion of species to a planar electrode surface are met very closely indeed. The value of E°, given by (Ep -f- Epa)/2, was 255 mV, in good agreement with values determined by potentiometry. It could be argued that free, reduced 4,4 -bipyridyl played no part in the mechanism, since its reduction potential is much lower than that of cytochrome c. It was proposed [7] that the organic adsorbate allowed electron transfer to occur directly by providing, at the electrode surface, functionalities with which the protein could interact specifically and reversibly and thereupon donate or accept electrons rapidly. It was thus termed a promoter as opposed to a mediator, in which the latter is considered to convey electrons in bulk solution. 

Fig. 4. Cyclic voltammetry of horse cytochrome c (approx. 0.4 mM) in NaC104 (0.1 M), phosphate buffer (0.02 M) at pH 7, in the presence of 0.01 M 4,4 -bipyridyl. Scan rate (a) 20 mVs (6) 50 mVs (c) lOOmVs . Inset shows dependence of peak current upon the square root of the scan rate. From Ref. 49, redrawn with kind permission...

Fig. 23. Coupling the cyclic voltammetry of horse cytochrome c (as promoted at a Au electrode in the presence of 4,4 -bipyridyl) to reduction of Oj by Pseudomonas aeruginosa cytochrome cd via a sequence of protein-protein electron-transfer reactions. Aerobic solutions contained 0.1 M NaC104, 0.02 M phosphate, pH 7.0. Scan rate 1 mVs . a) horse cytochrome c (0.44 mM) alone, b) after an addition, of cytochrome cd to 6 pM. c) after a further addition, of azurin to 0.25 pM. Redrawn from Ref. 198, with kind permission...

---