Silvia Peressini

Electrochemical reduction of dioxygen in the presence of 4,6-dimethyl-2-thiopyrimidine in DMF

Abstract The electrochemistry of dioxygen reduction in the presence of a model molecule of biological significance, 4,6-dimethyl-2-thiopyrimidine (LH) was studied in dimethylformamide at a glassy carbon (GC) electrode. The first, mono-electronic, reduction step took place under kinetic control of charge transfer or of the following chemical reactions, or both, depending on the time scale of the experiment, leading to a complicated pattern of events. A father–son reaction step and other cross protonation reactions could explain the detailed results obtained from the electrochemical investigation. The involvement of LH in the dioxygen reduction path was indicated by the presence of a new cathodic peak in cyclic voltammetry, which did not pertain to the voltammetric behavior of either species, and was shifted by ca. 70 mV from that due to the O 2 /O 2 − reduction in the absence of LH. The experimental results could be consistently interpreted by assigning the new peak to the O 2 /O 2 − coupled with a fast protonation of O 2 − by LH, and explaining the amount of the displacement by the rate of the protonation reaction.

Electrocatalytic dioxygen reduction in the presence of a rhodoxime

Polarographic and voltammetric evidences are reported for the production of intermediates superoxo [Rh]OO and hydroperoxo [Rh]OOH derivatives at the electrode surface, in the reduction of dioxygen in the presence of rhodium bis(dimethylglyoximato) chloro triphenylphosphine, [ClRh(III)(DMG)2(PPh3)] (I), in the absence and in the presence of protons in non-aqueous solvents. In the latter case the electrocatalytic reduction of dioxygen is observed. From the trends of the concentrations of dioxygen vs the rhodium complexes, a mechanism for the electrochemical process is proposed. The molar ratio of dioxygen to the rhodium complex when the latter is completely consumed for the formation of the adduct, as compared with the 1:1 stoichiometric ratio determined by the Hill plot, together with the influence of the electrode material, suggests that the reduction of dioxygen involves adsorbed species.

Para-substituted diphenylborylated organocobaloximes: effects of substituents on conformation and redox properties.

Abstract Some new derivatives of organocobaloximes containing para -substituted diphenylboryl groups, RCo(DH) 2− n (DB( p- XPh) 2 ) n L (R=alkyl or aryl group, L= N -MeIm, Py or H 2 O, X=OCH 3 , CH 3 or Cl, n =1 or 2) have been synthesized. The X-ray structures and the 1 H-NMR spectra are compared with those of the corresponding RCo(DH) 2− n (DBPh 2 ) n L and RCo(DBF 2 ) 2 L complexes. The insertion of X groups in the phenyl rings does not significantly affect the equatorial CoN distances, whereas the CoPy distances increase slightly in the order (DB( p- OCH 3 Ph) 2 ) 2 p- ClPh) 2 ) 2 2 ) 2 . 1 H-NMR spectra suggest that the conformational distribution in solution is similar to that observed in the corresponding BPh 2 derivatives. Electrochemical studies on the corresponding MeCo(DB( p- XPh) 2 ) 2 H 2 O compounds show a mono-electron Co(III)/Co(II) transfer reaction followed by two parallel reactions: (a) mono-electron Co(II)/Co(I) transfer; (b) homolytic dissociation of the CoC bond with the formation of Co(I) species, the relative rates of the two processes being dependent on X. As the electron-withdrawing power of the equatorial ligand increases, the reduction potentials associated with both Co(III)/Co(II) and Co(II)/Co(I) processes shift towards more positive values, indicating a decrease of electron density on the Co atom. The effects are comparable with those observed by changing the axial ligands.

Redox thermodynamics of cytochrome c in mixed water–organic solvent solutions

Abstract Bovine heart cytochrome c was studied through cyclic voltammetry in mixed water–organic solvent solutions under different conditions of temperature and the thermodynamic properties Δ S ° rc and Δ H ° rc calculated by the dependence of E ° by temperature. The effect of the organic fraction of the solvent on the E ° values of the native cyt c was found to be determined mainly by the decrease in dielectric constant of the medium. Specific interactions on the protein surface do not seem to play a remarkable role. The thermodynamic properties changes induced by the organic fraction have been interpreted tentatively in terms of solvation properties of cytochrome c and structural features of the protein environment.

Immobilized unfolded cytochrome c acts as a catalyst for dioxygen reduction

Unfolding turns immobilized cytochrome c into a His-His ligated form endowed with catalytic activity towards O(2), which is absent in the native protein. Dioxygen could be used by naturally occurring unfolded cytochrome c as a substrate for the production of partially reduced oxygen species (PROS) contributing to the cell oxidative stress.

Electrochemistry of dioxygen activation in the presence of adenine

Abstract The electrochemical behavior of dioxygen in the presence of adenine in dimethylformamide solution on the Hg electrode is studied. Evidence for the activation of O 2 is given by the shift of 150 mV towards more positive values of the one-electron reduction, accompanied by the autooxidation of adenine, while the adsorption of O 2 is shown by the characteristics of a polarographic pre-wave and voltammetric peak. The activation of O 2 is attributed to the interaction with adenine co-adsorbed on the Hg electrode.

Electrochemistry of 2-thio- and 2-oxo-pyrimidines in dimethyl formamide in the presence of dioxygen

Abstract Electrocatalysis of dioxygen reduction to superoxide ion in dimethylformamide is afforded by a reversible interaction with thiol or hydroxo forms of pyrimidine which takes place only on the mercury surface electrode even in the absence of transition metal ions. The interaction is revealed by a polarographic wave and by a voltammetric peak which can be recorded only in the presence of dioxygen in the pyrimidines solution. The polarographic wave and voltammetric peak are attributed to the reduction of the oxygenated pyrimidine. In the range of concentration where the polarographic limiting currents due to the reduction of the oxygenated and unoxygenated pyrimidine are of the same order of magnitude, the stoichiometric ratio dioxygen/pyrimidine is determined to be 1. The electrode process probably involves the formation of a complex of mercury with the thiol or the hydroxo form of the pyrimidine on the electrode surface. Adsorption or coadsorption of the reactants seems to be involved.

Substituent Effects in the Reduction Behaviour of Thio- and Oxopyrimidines in Non-Aqueous Solvents

The electrochemical reduction of a series of thio- and oxopyrimidine derivatives has been investigated in organic solvents on mercury electrodes. In all cases the electrochemical process gave a dimeric species as the major product. The overall reduction mechanism is the same for oxo and thio derivatives, and is found to be dependent only on the nature of the ring nitrogen substituent. A ‘father–son reaction’ is observed when hydrogen is bound to the ring nitrogen atom: the radical anion obtained from the first electron transfer draws out the nitrogen proton of a non-reduced molecule and this then dimerizes. In the presence of a protonating agent as well as for the N-substituted derivatives, the ‘father–son reaction’ is not observed. Theoretical calculations have been performed to gain insight into the proposed mechanisms: the LUMO energy and the vertical electron affinity show a linear correlation with the reduction potentials. Analysis of the theoretical parameters has allowed step-by-step determination of the electrochemical reduction process. The manner in which solvent properties influence electrochemical behaviour has been examined, and the role of the acceptor number (AN) has been discussed.

Organometallic cobalt(III) complexes with tridentate imino-oximic ligands: structural, spectroscopic and electrochemical properties

A series of binuclear complexes of the type [(μ-OH)(RCoIIIL)2]+ were L is the tridentate 2-(2-pyridylethyl)imino-3-butanone oximato ligand and R = Me, Et, CH2CF3, CH2Cl or Cy have been prepared and characterised. Some of their physico-chemical properties, namely the Co–R bond lengths, the νCo–Me Raman frequencies and the E1/2 half-wave potentials of the one-electron reduction process have been determined. Comparison of these properties with those of the metalloorganic derivatives containing tetradentate ligands, such as cobaloximes, Costa model derivatives and cobalamins, suggests that in the present compounds the Co–C bond strength and the electrochemical behaviour are more influenced by electronic than by steric factors.