Osmium-bipyridyl complexes oxidation

The intense colors in 2,2 bipyridyl complexes of iron, ruthenium, and osmium are due to excitation of an electron from metal t2g orbitals to the empty jr -orbitals of the conjugated 2,2 bipyridyl. The photoexcitation of this MLCT excited state can lead to emission. However, not all complexes are luminescent because of the different competing deactivation pathways. This aspect is beyond the scope of this chapter the interested reader can refer to a number of publications on this subject [16-20]. The other potential deactivation pathways for the excited dye are donation of an electron (called oxidative quenching, Eq. 2) or the capture of an electron (reductive quenching, Eq. 3) or transfer of its energy to other molecules or... 

ECPs including coordination complexes are also able to show electrocatalytic properties e.g., toward the oxidation of nitric oxide in the case of porphyrin functionalized polypyrroles containing various metallic centres [275], for the oxygen or hydrogen peroxide reduction in the case of cobalt-salen PEDOT [276] or iron-containing polysalen [245], or for oxidation of ascorbic acid in the case of osmium bipyridyl functionalized PPy [277]. 

Evidence has been obtained for an inner-sphere mechanism in the iso-electronic peroxodiphosphate (P208 ) ion oxidation of metal complexes. For a series of reactants of the type Fe(LL)a + and Fe(LLL)a + (where LL = bipyridyl or phenanthroline and LLL = terpyridyl), there is no parallel to the outer sphere path observed in the SaOs " oxidation of the same complexes. The mechanism is considered to involve the rate-determining partial dissociation of one of the ligands, with PaOg " (or P04 ) approaching the vacant position, followed by a subsequent fast oxidation reaction. An exception may be in the case of the complex [Ru"(phen)2pyCl]+. In the case of osmium(ii) complexes, however, no reaction takes place in the absence of catalysts since no prior dissociation occurs. 

The electroreduction of some typically inorganic compoimds such as nitrogen oxides is catalysed by the presence of polymeric osmium complexes such as [Os(bipy)2(PVP)2oCl]Cl, where bipy denotes 2,2 -bipyridyl and PVP poly(4-vinylpyridine). This polymer modifies the reduction kinetics of nitrite relative to the reaction at a bare carbon electrode, and provides calibration graphs of slope 0.197 nA with detection limits of 0.1 pg/mL and excellent short-term reproducibility (RSD = 2.15% for n = 20). The sensor performance was found to scarcely change after 3 weeks of use in a flow system into which 240 standards and 30 meat extracts were injected [195]. 

Another interesting application is the study of the kinetics of thermodynamically unfavourable oxidations of a series of iron, ruthenium and osmium complexes with 2,2 -bipyridyl or 1,10-phenanthroline (ML32 ") (equation 28). If E ° for is more positive than E° for the... 

Resonance Raman spectroelectrochemistry was carried out on [Ru(bpy )2(box)] + and the osmium analogue in which the bipyridyl units were perdeuteriated [Os(dg-bpy)2(box)]+ by holding the cell potential beyond the first oxidation step for each sample. The resonance Raman spectra of [Rn(bpy)2(box)p+ and [Os(bpy)2(box)] are in essence analogous. This implies that the bipyridyl unit does not participate in the new optical transition in the oxidized complex and therefore confirmed that for both M(bpy) containing complexes the NIR band is a phenolate (tc) to M(III) (djt) Ugand to metal charge transfer LMCT transition. This example also serves to illustrate how a simple synthetic modification such as deuteriation can yield detailed information on electron transfer processes. If the bipyridyl unit were involved in the optical transition, shifts of between 30 and 60 cm would have been observed between the deuteriated and non-deuteriated spectra. 

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