Osmium polypyridine complexes

A dendrimer with metal complexes both in the core and in the branches was described by Balzani et al. The luminescent, heteroleptic (having different ligands), dendritic polypyridine-ruthenium or polypyridine-osmium complex can be prepared both divergently and convergently [130] (cf. Section 2.5.2). 

The electroactive units in the dendrimers that we are going to discuss are the metal-based moieties. An important requirement for any kind of application is the chemical redox reversibility of such moieties. The most common metal complexes able to exhibit a chemically reversible redox behavior are ferrocene and its derivatives and the iron, ruthenium and osmium complexes of polypyridine ligands. Therefore it is not surprising that most of the investigated dendrimers contain such metal-based moieties. In the electrochemical window accessible in the usual solvents (around +2/-2V) ferrocene-type complexes undergo only one redox process, whereas iron, ruthenium and osmium polypyridine complexes undergo a metal-based oxidation process and at least three ligand-based reduction processes. 

ECL of Ir(ppy)3/TPA [79] and Os(phen)2(dppene) /TPA [88] in the presence of Triton X-100 were also smdied. An osmium complex, Os(phen)3, was the first osmium complex that gave ECL response using S20g reduction to generate the excited state [89]. A series of osmium complexes containing bpy and phen ligands showed ECL enhancement via annihilation [10]. While using TPA as coreactant [90], Os(phen)2(dppene) exhibits ECL in aqueous and mixed aqueous/ nonaqueous solutions. Osmium polypyridine complexes have also been studied [88, 89]. 

Nitric oxide and iron nitrosyl complexes have been observed in the reduction of nitrite by bacterial nitrite reductases, which contain iron chlorin or iron isobac-terichlorin [151]. A specific nitric oxide reductase also exists to convert NO to nitrous oxide [9]. Iron complexes of chlorins, isobacteriochlorins, and porphyrins, as well as ruthenium and osmium polypyridines, and cobalt and nickel... 

Balzani et al. prepared dendrimers with metal complexes serving both as core [36] and as branching unit The metallodendrimer in Fig. 2.10 is constructed solely from polypyridine ligands and transition metal ions. Such dendritic transition metal complexes can be synthesised both convergently and divergently and different transition metal ions (ruthenium/osmium) can be incorporated. This provides a means of influencing the luminescence properties of the den-drimer. Thus the energy transfer process proceeds from the inside outwards in... 

Osmium and ruthenium in high oxidation states (III, IV, or V) form polypyridine (bpy, phen or tpy) hydroxo and 0x0 complexes, which have a rich redox chemistry [166, 167]. Redox changes are metal-localized and accompanied by reactions of M=0 or M-OH bonds. These complexes are active as electrocatalysts of the oxidation of water to O2, or of CF to CI2, or they can transfer oxygen atoms and oxidize organic substrates like (CH3)2CHOH or C6H5-CH(CH3)2. 

Bergkamp, M. A. Gutlich, P. Netzel, T. L. Sutin, N. Lifetimes of the ligand-to-metal charge-transfer excited states of iron(III) and osmium(III) polypyridine complexes. Effects of isotopic substitution and temperature. J. Phys. Chem. 1983, 87, 3877-3883. 

Figure 2.8 Three examples of small redox molecules for which in situ STM and single-molecule in situ STS have been recorded. Molecular structures and tunneling current-overpotential correlations are shown, (a) Two osmium polypyridine complexes on a Pt(lll) electrode surface and tunneling current-overpotential correlations at different bias voltages [55]. (b) Hexanethiol 4,4 -substi-tuted viologen (6V6) and 6-p-tetrathiafulvalene...

Osmium and ruthenium polypyridine complexes initially received much attention from Dwyer and coworkers because the M(II), M(III), and M(IV) oxidation states are substitution inert.Interest in them has been renewed because of their photochemical reactions and the role they play in the study of reactions of coordinated ligands " and of mixed valence ions and in the preparation of electroactive polymer films. The aqua complexes " also have important potential applications in the selective oxidation of organic molecules and water. We found that trifluoromethanesulfonato (triflato) complexes are convenient synthetic intermediates in the preparation of aqua and oxo species, " and we describe the syntheses of the cii-bis(2,2 -bipyridine) complexes here. 

Serroni S, Campagna S, Puntoriero F, Di Pietro C, Loiseau F, McClenaghan ND (2001) Dendrimers based on rutheninm(ll) and osmium(ll) polypyridine complexes and the approach of nsing complexes as ligands and complexes as metals. Chem Soc Rev 30 367-375... 

The rate constants and activation parameters (including AV ) for electron self-exchange in the [Mn(CNC(CH)3)6]-"/ -" and [Mn(CNC6Hu)6] couples have been determined by Mn NMR line broadening in several pure and binary organic solvent systems. The values of A V cover a range of about 12 cm moP (-9 to -21 cm mol ) with no simple correlation with solvent parameters observed. A self-exchange rate constant of 0.7 0.4 M" s" has been calculated for the [Mn(edta)(H20)] and [Mn(cdta)(H20)] couples from the application of the Marcus relationship to outer-sphere cross-reactions with a variety of metal complexes in aqueous solution. Deviations from the correlation were observed for the nonadiabatic reactions with osmium tris(polypyridine) complexes. 

We describe here the kinetics and mechanism of electron transport in an enzyme electrode where glucose oxidase is connected to the electrode surface with an electron transfer relay consisting of an osmium polypyridyl complex pendent on a polypyridine backbone. 

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