Redox potentials osmium complexes

Since the first report on the ferrocene mediated oxidation of glucose by GOx [69], extensive solution-phase studies have been undertaken in an attempt to elucidate the factors controlling the mediator-enzyme interaction. Although the use of solution-phase mediators is not compatible with a membraneless biocatalytic fuel cell, such studies can help elucidate the relationship between enzyme structure, mediator size, structure and mobility, and mediation thermodynamics and kinetics. For example, comprehensive studies on ferrocene and its derivatives [70] and polypy-ridyl complexes of ruthenium and osmium [71, 72] as mediators of GOx have been undertaken. Ferrocenes have come to the fore as mediators to GOx, surpassing many others, because of factors such as their mediation efficiency, stability in the reduced form, pH independent redox potentials, ease of synthesis, and substitutional versatility. Ferrocenes are also of sufficiently small size to diffuse easily to the active site of GOx. However, solution phase mediation can only be used if the future biocatalytic fuel cell... 

Figure 2.5 Schematic representation of the Au/MPS/PAH-Os/solution interface modeled in Refs. [118-120] using the molecular theory for modified polyelectrolyte electrodes described in Section 2.5. The red arrows indicate the chemical equilibria considered by the theory. The redox polymer, PAH-Os (see Figure 2.4), is divided into the poly(allyl-amine) backbone (depicted as blue and light blue solid lines) and the pyridine-bipyridine osmium complexes. Each osmium complex is in redox equilibrium with the gold substrate and, dependingon its potential, can be in an oxidized Os(lll) (red spheres) or in a reduced Os(ll) (blue sphere) state. The allyl-amine units can be in a positively charged protonated state (plus signs on the polymer...

A similar polymer, composed of osmium complexed with bis-dichlorobipyridine, chloride, and PVI in a PVI—poly(acrylamide) copolymer (Table 2, compound 3), demonstrated a lower redox potential, 0.57 V vs SHE, at 37.5 °C in a nitrogen-saturated buffer, pH 5 109,156 adduct of this polymer with bilirubin oxidase, an oxygen-reducing enzyme, was immobilized on a carbon paper RDE and generated a current density exceeding 9 mA/cm at 4000 rpm in an O2-saturated PBS buffer, pH 7, 37.5 °C. Current decayed at a rate of 10% per day for 6 days on an RDE at 300 rpm. The performance characteristics of electrodes made with this polymer are compared to other reported results in Table 2. 

Redox Potentials and Rate Constants for the Oxidation of Reduced GO from Aspergillus nicer by Osmium and Ruthenium Complexes at 25 °C and pH 7. Complexes are Shown as Introduced Neglecting Hydrolysis in Water. See Footnote for the Key... 

Comparison of Redox Potentials and Comproportionation Constants for Symmetrically Substituted Dinuclear Osmium and Ruthenium Complexes... 

In the first step, the precursor, typically a ruthenium or osmium bis(2,2,-bipyridyl) (bpy) complex, reacts with solvent (S) to produce a solvated complex. When solvents such as dry methanol and ethanol are used, only one chloride is exchanged and the species [Ru(bpy)2(PVP) Cl]+ is obtained as the sole product. The nature of the coordination sphere around the metal center can be determined by UV-visible (UV/Vis) spectroscopy (Xmax, 496 nm) and by its redox potential, (about 0.65 V (vs. SCE), depending on the electrolyte being used). By a systematic variation of the ratio of monomer units to redox-active centers, the loading of the polymer backbone ( n) can be varied systematically. (Here, n stands for the number of monomer units in the polymer per redox-active center, e.g. in a PVP-based, n = 10 polymer, there are 10 pyridine units for every redox center. 

The analogous osmium polymers have also been studied in great detail. The synthetic procedures required for these metallopolymers are the same as those described above for ruthenium however, the reaction times are longer. The similarity between the analogous mononuclear and polymeric species is further illustrated by the fact that the corresponding osmium polymers have considerably lower redox potentials and are also photostable, as expected on the basis of the behavior observed for osmium polypyridyl complexes. 

A systematic approach using Trametes versicolor laccase co-immobilized with osmium-based redox polymers has highlighted the importance of redox complex loading [144, 145], and of the redox potential of the redox polymer [22], on ORR current density. Increasing ORR current density is observed with increased difference in redox potential between the Tl site of laccase and redox polymer, but maximum power from an EFC is predicted to result using a redox polymer of redox potential 0.17 V more negative than the Tl site of laccase. It would be inter-... 

Non-chromophoric ligand variations have been carried out in the series of osmium complexes [Os(phen)L ](L-pyridine, MeCN, phosphine, arsine) and emission energies, excited-state redox potentials, and radiative and non-radiative rate const2uits found to vary systematically with the potential of the ground-state Os(III/II) couple.Phosphorescence from [Os(TTP)(CO)MeOH] au d [Os(TTP)(CO)pytIdlne] is quenched by electron donors auid acceptors by a reversible electron transfer mechauiism. 

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