Oxidation Ru

Rate constants for azurin Ru(NH3)5-modified at His83 have been determined by two methods in which the fully oxidized Ru(III)Cu(II) protein is reduced to Ru(II)Cu(II) by (a) flash photolysis using [Ru(bipy)3] as re-ductant [128], and (b) pulse radiolysis with CO2 as reductant [50]. Intramolecular rate constants of 1.9 + 1.4s (22°C) and 2.5+0.8 s (four runs at 17 °C), respectively are in reasonably satisfactory agreement. Both studies were in 0.10 M phosphate, but solutions for pulse radiolysis also have in addition 0.10 M sodium formate present. Ionic strengths were therefore 0.22 and 0.32 M (the value of 0.31 M in Refs. [50] and [129] is closer to and should be adjusted to the latter value in Ref. [130] an ionic strength of 0.32 M applies to the and O2... 

Tetracyano ligands have been used to bridge between four Ru(NH3)5 moieties. The complexes [ Ru(NH3)5 4(/i-L)] + (L = tetracyanoethene, tetracyano-p-quinodimethane, 1,2,4,5-tetracyano-benzene, 2,3,5,6-tetracyanopyrazine) exhibit intense, long-wavelength electronic absorptions. Oxidation to [ Ru(NH3)5 4(yU-L)] °" " and reduction to [ Ru(NH3)5 4(//-L)] + and [ Ru(NH3)5 4-(/i-L)] + can readily be achieved. The species are fully delocalized with partially reduced ligands or partially oxidized Ru centers. Treatment of [5,10,15,20-tetrakis(4-cyanophenyl)porphyrinato] cobalt(II) or [5,10,15,20-tetrakis(4-cyano-2,6,-dimethylphenyl)porphyrinato]cobalt(II) with [Ru-(NH3)5(0S02CF3)] introduces cyano-bound pendant Ru (NH3)5 groups to the porphyrinato complexes. ... 

The tris(ethylenediamine)ruthenium(III) species is obtained by oxidation of [Ru(en)3]2+ with, for example, iodine4 or bromine.6 The oxidizing agent and conditions employed must be chosen carefully to avoid further oxidation of the ethylenediamine ligand to coordinated diimine.7 In the present procedure solid silver anthranilate is used to oxidize [Ru(en)3] [ZnCl4], and [Ru(en)3] Cl3 is isolated. In this heterogeneous procedure the desired [Ru(en)3]Cl3 is the only soluble product and can easily be separated from the insoluble silver, silver chloride, and zinc dianthranilate. Other less soluble [Ru(en)3]3+ compounds can be obtained easily from the soluble chloride. 

Some years later, at the beginning of the 1970s, first ECL system based on the luminescent transition metal complex tris(2,2 -bipyridine)ruthenium(II)-Ru (bipy)32 + -has been reported.11 It was shown that the excited state 3 Ru(bipy)32 + can be generated in aprotic media by annihilation of the reduced Ru(bipy)31 + and oxidized Ru(bipy)33 + ions. Due to many reasons (such as strong luminescence and ability to undergo reversible one-electron transfer reactions), Ru (bipy)32+ later has become the most thoroughly studied ECL active molecule. 

The generated S04" radical anion can also oxidize Ru(bipy)32+ ions to Ru... 

Figure 17.17 Transient absorbance decay kinetics of the oxidized Ru(dcbpy)2(NCS)2 at 620 nm in the presence and in the absence of 0.1 M Co(II) in acetonitrile/ethylene carbonate 40 60 v/v. Reprinted with permission from Ref. 55. Copyright 2001 American Chemical Society.

Ruthenium bipyridyl complexes are suitable photosensitizers because then-excited states have a long lifetime and the oxidized Ru(III) center has a longterm chemical stability. Therefore, Ru bipyridyl complexes have been studied intensively as photosensitizers for homogeneous photocatalytic reactions and dye-sensitization systems. The Ru bipyridyl complex, bis(2,2 -bipyridine)(2,2 -bipyri-dine-4, 4,-dicarboxylate)ruthenium(II), having carboxyl groups as anchors to the semiconductor surface was synthesized and single-crystal Ti02 photoelectrodes sensitized by this Ru complex were studied in 1979 and 1980 [5,6]. 

The triiodide ion, I3-, formed by the reaction of oxidized Ru ( ) dye with the I- ion, is reduced back to the I" ion at the surface of the counterelectrode. In order to reduce the triiodide ion effectively, the counterelectrode must have a high electrocatalytic activity. Pt sputtered (5-10 Tg/cm2, or approximately 200 nm thickness) FTO glass or carbon material is usually used as the counterelectrode. 

Electron transfer from I- into the oxidized Ru photosensitizer (cation), or regeneration of the Ru photosensitizer, is one of the primary processes needed to achieve effective charge separation. The kinetics of this reaction has also been investigated by time-resolved laser spectroscopy [48,51]. The electron-transfer rate from I into the Ru(III) cation of the N3 dye was estimated to be 100 nsec... 

This reaction rate is much faster than that of charge recombination between injected electrons and Ru dye cations. Thus, fast regeneration of the oxidized Ru dye photosensitizer also contributes to the accomplishment of effective charge separation. 

Photoinduced electron relay in a polymer solid phase was found to take place for a ethylenediaminetetraacetic acid (EDTA)-tris(2,2 -bipyridine) mthenium (II)(Ru(bpy)32+) - methylviologen (MV2+) system27) utilizing cellulose paper. Electron transfer from EDTA to the oxidized Ru(bpv)33+ formed after electron transfer from its excited state to MV + accumulated blue MV+ in the cellulose paper. 

Thus, C14MV2- quenches the excited state of [Ru(bipy)3]2+ with a rate constant fcq = 8 x 10s mol-1 dm3 s-1 and this is unaffected by cetyltrimethylammonium chloride (CTAC), up to concentrations of 5 x 10-2 mol dm-3, indicating that mixed CTAC/C14MV2- micelles are not formed.139 In the absence of CTAC, kb in this system is 4x 109 mol-1 dm3 s-1, but flash photolysis showed that this drops to kb s2x 107 mol-1 dm3 s-1 in micellar solution. Thus, the more hydro-phobic radical cation, C14MV+, is solubilized by CTAC micelles, which, having a positive surface, do not allow approach of the oxidized [Ru(bipy)3]3C This then gives an efficiency of 30% for the redox reaction. This study was extended by the removal of the CTAC from solution and the introduction of a Pt catalyst protected by a positively charged polysoap.138 This work is described in Section 61.5.4.7.2. 

Modern variations include the in situ, and thus catalytic, use of this high-valent selective reagent, not only for alcohols but also for ethers (see later). Ru(VII) (perruthenate) in the compounds tetra-n-butylammonium perruthenate (TBAP) and tetra-n-propylammonium perruthenate (TPAP) has found wide application in alcohol oxidation. Ru-oxo complexes with valence states of IV to VI are key intermediates in, for example, the selective oxygen transfer to alkenes, leading to epoxides. On the other hand 16-electron Ru(II) complexes can be used to catalyse hydrogen transfer thus these are excellent catalysts for oxidative dehydrogenation of alcohols. A separate section is included to describe the different mechanisms in more detail. 

Coteiro s group studied the influence of the solvent to the ternary oxide, Ru-Ti-Sn, electrode stability (Coteiro et al. 2006). The precursor mixtures were prepared by dissolving RuCh H20, TiCl4, and SnCl2 2H20 salts into the solvent. Two different solvents, HC1/H20 (1 1 v/v) or isopropanol, were applied. The results with isopropanol as solvent showed that tin loss can be eliminated and higher elec-trochemically active area and stability can be achieved, when compared with HC1 solution. 

The photoreduction of cobalt(III) complexes by these complexes takes place catalytically, in which one-electron oxidized [Ru(( — )-menbpy)3]3+ and [Ru(S( —) PhEtbpy)3]3 + are reduced by ethanol in the solvent [26]. This is because these complexes have more positive reduction potential than that of [Ru(bpy)3]2+. 

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