Osmium complexes electron-transfer reactions

Moderate enantioselectivity factors have also been found for electron transfer reactions between HRP or GO and resolved octahedral ruthenium or osmium complexes, respectively. In particular, the rate constants for the oxidation of GO(red) by electrochemically generated and enantiomers of [Os(4,4 - 2 ) ]3 + equal 1.68 x 106 and 2.34 x 106 M-1 s-1, respectively (25 °C, pH 7) (41). The spectral kinetic study of the HRP-catalyzed oxidation of and A isomers of the cyclo-ruthenated complex [Ru(phpy)(phen)2]PF6 (Pig. 21) by hydrogen peroxide has revealed similarities with the oxidation of planar chiral 2-methylferrocene carboxlic acid (211). In both cases the stereoseleci-vity factor is pH dependent and the highest factors are not observed at the highest rates. The kA/kA ratio for [Ru(phpy)(phen)2]PF6 is close to 1 at pH 5-6.5 but increases to 2.5 at pH around 8 (211). 

This article is intended to review the published work on the photochemistry and photophysics of osmium complexes that has appeared in the literature over the past several years. We have attempted to cover, albeit somewhat selectively, literature dating back to the year 2000. A variety of reviews pertaining to particular aspects of osmium photophysics and photochemistry were published prior to 2000. A few reviews discuss the photophysical behavior of primarily monometallic Os complexes in solution [1,2]. Several earlier reviews discuss light induced energy and electron transfer reactions involving osmium complexes in much of this work the Os complex is not the chro-mophore [3-6]. Finally, one review exists discussing the photochemistry of Os carbonyl complexes [7]. 

Moderate enantioselectivity factors have also been found for electron transfer reactions between HRP or GO and resolved octahedral ruthenium or osmium complexes, respectively. In particular, the rate constants for the oxidation of GO(red) by electrochemically generated A and A enantiomers of [Os(4,4 -Me2bpy)3] equal 1.68 x 10 and 2.34 X 10 respectively (25 °C, pH 7) 41). The spectral kinetic... 

The challenge in this field is to control both the physical architecture and chemical reactivity of the film so as to promote selected electron transfer reactions while inhibiting others. With polymer-modified electrodes (PMEs), the electrode is conferred with the molecular selectivity and specificity that is lacking at a conventional pristine electrode. For example, poly(4-vinyl)pyridine and poly(N-vinyl)imi-dazole can be functionalized with osmium and ruthenium polypyridyl complexes. These synthetic macromolecules act as useful model systems for... 

R. Leidner and R. W. Murray, Electron-transfer reactions of iron, ruthenium, and osmium bipyridine and phenanthroline complexes at polymer/solution interfaces, J.Am.Chem.Soc., 106 1606 (1984). 

Novel two-electron group transfer reactions also occur with the osmium(IV) sulfilimido complexes (Equations (71) and (72)) ... 

Figure 10 Plot of rate constants for back electron transfer from Sn02 to electrostatically bound ruthenium ( ) and osmium ( ) complexes as a function of the number of carbon atoms comprising alkyl spacers. Within experimental error, the driving force for each series of reactions is unaffected by changing the size of the alkyl spacer.

The kinetics of chromium(l 11 )-catalyscd oxidation of fonnic acid by Ce(TV) in aqueous H2SO4 can be rationalized in terms of initial formation of an outer-sphere complex involving oxidant, catalyst, and substrate (S), Ce(TV)(S)Cr(III), followed by an inner-sphere complex Ce(III)(S)Cr(IV). It is proposed that electron transfer occurs within this complex from substrate to Cr(TV) (with elimination of H+) followed by fast reaction to give CO2 (again with elimination of H+).54 In contrast, there was no kinetic evidence for the accumulation of a corresponding inner-sphere intermediate in the osmium(VIII)-catalysed Ce(TV) oxidation of DMSO to dimethyl sulfone here, the observed rate law was rationalized in terms of rate-determining bimolecular electron transfer from DMSO to Os(VHI) in an outer-sphere step.55 The kinetics of oxidation of 2-hydroxy-l-naphthalidene anil by cerium(IV) in aqueous sulfuric acid have been... 

Comproportionation between cA-RuIV(bpy)2(py)02 + and cis- Run(bpy)2(py)(H20)2+ takes place by proton-coupled electron transfer (PCET) and exhibits a KIE of 16.1. Other PCET reactions of these and related ruthenium and osmium complexes also feature large KIEs. For example, oxidations of H202 by RuIV(bpy)2 (py)O2 + and by Ruin(bpy)2(py)OH2 + have KIEs of22.1 and 16.7, respectively. Oxidation of benzyl... 

Reaction of osmium tetra-/>-tolylporphyrin dimer [Os(TTP)]2 with excess hexamethylsilacyclopropane at room temperature gave a metalloporphyrin dimethylsilylene complex (Equation (17)) <9lOM3977>. The mechanism for this reaction was thought to involve an initial electron transfer followed by a radical chain process. 

The oxidation potential of the osmium complex [fraw5,-Os(CO)3(PPh3)2] in the presence of silver trifluoroacetate in CH2CI2 solution is higher by 0.51 V than that of the complex [fraw5,-Os(CO)3(PPh3)2] alone. This points to the formation of the intermediate complex 146 (item 0-3 of Table 3), that can serve as a model for the intermediate electron transfer stage in oxidation reactions by Ag(I)224. 

Early reports on interactions between redox enzymes and ruthenium or osmium compounds prior to the biosensor burst are hidden in a bulk of chemical and biochemical literature. This does not apply to the ruthenium biochemistry of cytochromes where complexes [Ru(NH3)5L] " , [Ru(bpy)2L2], and structurally related ruthenium compounds, which have been widely used in studies of intramolecular (long-range) electron transfer in proteins (124,156-158) and biomimetic models for the photosynthetic reaction centers (159). Applications of these compounds in biosensors are rather limited. The complex [Ru(NHg)6] has the correct redox potential but its reactivity toward oxidoreductases is low reflecting a low self-exchange rate constant (see Tables I and VII). The redox potentials of complexes [Ru(bpy)3] " and [Ru(phen)3] are way too much anodic (1.25 V vs. NHE) ruling out applications in MET. The complex [Ru(bpy)3] is such a powerful oxidant that it oxidizes HRP into Compounds II and I (160). The electron-transfer from the resting state of HRP at pH <10 when the hemin iron(III) is five-coordinate generates a 7i-cation radical intermediate with the rate constant 2.5 x 10 s" (pH 10.3)... 

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