A. B. P. Lever

Spectroscopic and Electrochemical Studies of the Transition Metal Tetrasulfonated Phthalocyanines. Part 5. Voltammetric Studies of Adsorbed Tetrasulfonated Phthalocyanines (MTsPc) in Aqueous Solutions.

Abstract Cyclic voltammetric measurements of adsorbed Fe- and Co-tetrasulfonated phthalocyanine (TsPc) and Co-phthalocyanine (Pc) have been carried out on ordinary pyrolytic graphite and silver electrodes at different solution pH ranging from 1 to 13. Many voltammetric peaks were found to be pH dependent with a slope of −59 mV/ unit pH. In some instances this dependence was observed in alkaline or acid solutions only. The influence of oxygen has been also examined.

Monomeric and Polymeric Tetra-aminophthalocyanatocobalt(II) Modified Electrodes: Electrocatalytic Reduction of Oxygen

The monomeric and polymeric tetra-aminophthalocyane to, cobalt(II) species adsorbed onto graphite electrodes are active in electrocatalytic oxygen reduction. While the monomeric species is unstable, the polymerized species is an effective and stable reduction catalyst over a wide pH range. Both the two-electron reduction of oxygen to hydrogen peroxide and the four-electron reduction of oxygen to water are characterized by cyclic voltammetry, rotating disc and rotating ring-disc studies with appropriate theoretical analysis. Some mechanistic information is obtained. This is the first cobalt phthalocyanine species to provide a four-electron reduction pathway which exists over a wide pH range and is stable. The stability is associated with the polymerization since the monomeric species is not stable.

Electrocatalytic activity of N,N′,N″,N‴-tetramethyl-tetra-3,4-pyridoporphyrazinocobalt(II) adsorbed on a graphite electrode towards the oxidation of hydrazine and hydroxylamine

The complex N,N′,N″,N‴-tetramethyl-tetra-3,4-pyridoporphyrazinocobalt(II) ([Co(II)Tmtppa]4+) irreversibly adsorbed on a graphite electrode displays electrocatalytic activity towards the oxidation of both hydrazine and hydroxylamine. The kinetics of the electrocatalytic four-electron oxidation of NH2NH2 and two-electron oxidation of NH2OH are examined using cyclic voltammetric and rotating disk electrode methods, and the corresponding kinetic parameters are obtained. The possible application of the [Co(II)Tmtppa]4+ modified electrode in sensing NH2NH2 and NH2OH is also discussed in this paper.

Correlation of electronic charge transfer transitions and electrochemical potentials.: The bipyrazine(tetpacarbonyl) molybdenum(0) system in various solvents

Abstract Using a free-energy diagram, a relationship is drawn between an optical charge transfer energy and the electrochemical potentials of the donor and acceptor orbitals concerned. The charge transfer spectroscopy and electrochemical potentials of the title complex were studied in various solvents. A linear correlation, with negative slope, was observed between the difference in oxidation and reduction potentials and an MLCT transition. Using some additional solvent data, a number of useful parameters were derived in a fashion which would not be possible through consideration of either technique alone.

Surface copper immobilization by chelation of alizarin complexone and electrodeposition on graphite electrodes, and related hydrogen sulfide electrochemistry; matrix isolation of atomic copper and molecular copper sulfides on a graphite electrode

Abstract The irreversibly adsorbed alizarin complexone (AC) was employed to immobilize and maintain Cu II ions on the graphite electrode. The coordination chemistry between the adsorbed alizarin complexone ligand and the Cu II ion on the surface was examined by surface cyclic voltammetry. Upon reduction of the Cu II center to a Cu 0 atom, a submonolayer of individual atoms of Cu 0 rather than a continuous layer is formed on the electrode surface. The immobilized surface displays electrocatalytic activity towards the oxidation of sulfide ion from [S 2− ] ion to S 0 . The electrocatalytic activity for the sulfide oxidation on a [Cu II (AC)] − adsorbed electrode is shown to be essentially identical with that of a electrode that contains an electrodeposited submonolayer of Cu 0 . The active catalyst in both cases is identified to be a submonolayer of cupric sulfide. The electrochemistry of the Cu 0 submonolayer-coated electrode in aqueous solution containing hydrogen sulfide was also examined. When the modified electrode was polarized from −1.1 V to 0.2 V, three electrode processes were observed. The first, near −0.7 V, is a surface reaction between surface Cu 0 and adsorbed [S 2− ] ion to form a submonolayer of cuprous sulfide. The second appeared near −0.23 V and is another surface process between Cu 2 S and adsorbed sulfide ion to form a submonolayer of cupric sulfide. The third reaction is the electrochemical oxidation of sulfide ion catalyzed by CuS to form sulphur which deposits on the electrode surface when the potential is positive of −0.2 V.

Poisoning effect of SCN−, H2S and HCN on the reduction of O2 and H2O2 catalyzed by a 1:1 surface complex of Cu: 1,10-phenanthroline adsorbed on graphite electrodes, and its possible application in chemical analysis

Abstract The copper complex with a single 1,10-phenanthroline ligand can be irreversibly adsorbed on graphite electrodes, catalyzing the reduction of both O 2 and H 2 O 2 . The electrocatalytic kinetics of both substrate reductions were studied by cyclic voltammetric and rotating disk electrode methods. The addition of a very small quantity of a species such as SCN − , H 2 S or HCN in the test solution poisons the electrocatalytic activity for O 2 and H 2 O 2 reduction. A theoretical model is proposed to describe this poisoning effect based on the coordination equilibrium between poisoning species and surface adsorbed catalyst, the inner-sphere mechanism of substrate reduction, and Koutecky-Levich theory. The model is supported by experimental results. The surface behaviour of the adsorbed [Cu(phen)] 2+ ads in the presence of the poison species clearly shows the formation of a new surface complex with [Cu(phen)] 2+ ads . These new “poisoned” surface complexes are electrocatalytically inactive towards O 2 and H 2 O 2 reduction. The possible application of this poisoning effect for the analysis of trace SCN − , H 2 S and HCN was explored.

Proton-Induced Disproportionation of a Ruthenium Noninnocent Ligand Complex Yielding a Strong Oxidant and a Strong Reductant

The complex Ru(II)(NH(3))(2)(o-benzoquinonediimine)Cl(2) undergoes a reversible apparent acid/base reaction, although it has no obvious basic lone pairs. The reaction is a proton-assisted disproportionation yielding an oxidant ([Ru(III)(NH(3))(2)(o-benzoquinonediimine)Cl(2)](+)) and a reductant ([Ru(III)(NH(3))(2)(o-phenylenediamine)Cl(2)](+)). These species were characterized by electrochemistry, ultraviolet-visible light (UV-vis), vibrational (infrared (IR) and Raman), mass and electron paramagnetic resonance (EPR) spectroscopy, and X-ray structural analysis. The reaction is shown to be downhill from an isodesmic calculation. Three different isosbestic interconversions of the parent and product species are demonstrated. The electronic structures of these species were analyzed, and their optical spectra assigned, using density functional theory (DFT) and time-dependent DFT. This disproportionation of a noninnocent ligand complex may be relevant to the application of noninnocent ligands in organometallic catalysis and in the biological milieu.

Surface electrochemistry of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide ([MTT]Br) adsorbed on a graphite electrode

Abstract 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide ([MTT]Br), which is of importance as a biological redox indicator, irreversibly adsorbs on a graphite electrode and exhibits two surface electrochemical redox processes. The mechanism of reduction and re-oxidation of this species on the surface is discussed as a function of pH. AM1 calculations and redox data are interpreted to show that protonation of the first reduced product leads to a ring-opening pathway, eventually generating the two-electron reduced formazan species. Cyclic voltammetry, rotating-disk voltammetry, pH dependence studies and electronic spectroscopy are used to characterize the system.

Electrochemical Reduction of Nitrous Oxide (N2O) Catalysed by Tetraaminophthalocyanatocobalt(II) Adsorbed on a Graphite Electrode in Aqueous Solution

The surface electrochemical response of the CoII/CoI redox process of tetraaminophthalocyaninatocobalt(II) (CoIITAPc) adsorbed on a graphite electrode, was studied in the pH range of 2–13. In aqueous solution, the CoIITAPc adsorbed graphite electrode displays very strong electrocatalytic activity toward N2O reduction to N2, a process which was examined by cyclic and rotating disk electrode voltammetries. The possible application of this CoIITAPc modified electrode in N2O analysis was explored.

Binuclear trans-μ-pyrazine-bis­[bromo­tetra­pyridine­ruthenium(II)] dihexa­fluoro­phosphate di­methyl­form­amide disolvate

The Ru atoms of the title compound, [Ru2Br2(C4H4N2)(C5H5N)8](PF6)2.2C3H7NO, are coordinated by the four N atoms of the pyridine (py) ligands and a bromide. Two such units are bridged by a pyrazine (pz) unit, which is disposed about a centre of inversion, so as to form a trans pseudo-octahedral geometry about Ru. The py units are in an eclipsed conformation when viewed down the internuclear axis. The Ru—N(pz) distance is 2.079 (3) A, while the Ru—Br distance is 2.5524 (4) A.