The kinetics of a fast leuco-Methylene Blue (LMB) re-oxidation to Methylene Blue (MB) by copper()-halide (Cl Ϫ , Br Ϫ ) complexes in acidic aqueous media has been studied spectrophotometrically using a stopped-flow technique. The reaction follows a simple first order rate expression under an excess of the copper() species (and H ϩ (aq)), and the pseudo-first order rate constant (kЈ obs ) is largely independent of the atmosphere used (air, oxygen, argon). The rate law, at constant Cl Ϫ (Br Ϫ ) anion concentration, is given by the expression:where K is the protonation constant, and k a and k b are the pseudosecond order rate constants for protonated and deprotonated forms of LMB, respectively. The rate law was determined based on the observed kЈ obs vs. [Cu II ] and [H ϩ ] dependences. The rate dramatically increases with [Cl Ϫ ] over the range: 0.1-1.5 M, reflecting the following reactivity order: Cu 2ϩ (aq) Ӷ CuCl ϩ (aq) < . . . < CuCl 4 2Ϫ. The slow re-oxidation of LMB by oxygen has also been briefly examined at different [H ϩ ]. ESR results provide clear evidence for the formation of an intermediate radical. The mechanistic consequences of all these results are discussed.

Three water-soluble ruthenium(III) compounds, Y[cis-RuCl 2 (pic) 2 ]⋅nH 2 O (where pic = picolinate anion, Y = H(Hpic) 2 + (1), H 2 pic + (2) or K + (3), n = 2, 1.5 and 2.5, respectively), were synthesized and their X-ray structures determined. Compound 1 was fully characterized both as solid and in aqueous solution by elemental analysis, magnetic susceptibility measurements, EPR, IR and UV-Vis spectroscopy, cyclic voltammetry, microwave plasma atomic emission and mass spectrometry, capillary electrophoresis and chromatographic methods. It was shown that the coordination geometry around the low spin (S = ½) Ru III center is distorted octahedral with the chloride ligands in a cis configuration. Furthermore, all the measurements showed that the structures in the solid state and solution are absolutely identical with the cis-[RuCl 2 (pic) 2 ] − anion being inert in aqueous solution. Microbial studies showed that 1 exerts a strong inhibition on the growth and increased mortality level of the bacterial strains -E. coli, Pseudomonas sp., Sarcina sp., Micrococcus sp., Serratia sp., Bacillus cereus, B. subtilis and yeastSaccharomyces cerevisiae.

The crystal structures of [Ru(terpy)(bipy)Cl]Cl·2HO and [Ru(terpy)(en)Cl]Cl·3HO, where terpy = 2,2':6',2''-terpyridine, bipy = 2,2'-bipyridine and en = ethylenediamine, were determined and compared to the structure of the complexes in solution obtained by multi-nuclear NMR spectroscopy in DMSO as a solvent. In aqueous solution, both chlorido complexes aquate fully to the corresponding aqua complexes, viz. [Ru(terpy)(bipy)(HO)] and [Ru(terpy)(en)(HO)], within ca. 2 h and ca. 2 min at 37 °C, respectively. The spontaneous aquation reactions can only be suppressed by chloride concentrations as high as 2 to 4 M, i.e. concentrations much higher than that found in human blood. The corresponding aqua complexes are characterized by pK values of ca. 10 and 11, respectively, which suggest a more labile coordinated water molecule in the case of the [Ru(terpy)(en)(HO)] complex. Substitution reactions of the aqua complexes with chloride, cyanide and thiourea show that the [Ru(terpy)(en)(HO)] complex is 30-60 times more labile than the [Ru(terpy)(bipy)(HO)] complex at 25 °C. Water exchange reactions for both complexes were studied by O-NMR and DFT calculations (B3LYP(CPCM)/def2tzvp//B3LYP/def2svp and ωB97XD(CPCM)/def2tzvp//B3LYP/def2svp). Thermal and pressure activation parameters for the water exchange and ligand substitution reactions support the operation of an associative interchange (I) process. The difference in reactivity between these complexes can be accounted for in terms of π-back bonding effects of the terpy and bipy ligands and steric hindrance on the bipy complex. Consequences for eventual biological application of the chlorido complexes are discussed.

The kinetics of reduction of the mer-[Ru III (pic) 3 ] complex (pic -= picolinato) by ascorbic acid (AscH 2 ) leading to formation of a red ruthenium(II) species have been studied spectrophotometrically by using both conventional mixing and stopped-flow methods. The reaction was followed as a function of the reductant concentration over a wide pH range (1.0-7.4). Electron transfer proceeds by an outer-sphere mechanism involving three protolytic forms of ascorbic acid, [a]2529 AscH 2 , AscHand Asc 2-, for which specific rate constants have been determined. The Gibbs' energy of activation was found to correlate linearly with the HOMO energies of the protolytic forms of the reductant. The mer-[Ru III (pic) 3 ] complex is too sparingly soluble in water to inhibit the growth of the Escherichia coli (ATCC 8739) strain. Its cytotoxicity against non-tumorigenic cells precludes its potential use as an anticancer agent.Scheme 1. Acid/base properties of the different redox forms of lascorbic acid. [6]

In this contribution, we report the synthesis and full characterization of the first mixed-valence Ru(II)/Ru(III) ion-pair complex, [Ru II (bipy) 2 (pic)] + [cis-Ru III Cl 2 (pic) 2 ] − , in the solid state and in aqueous solution, where bipy = 2,2′bipyridine and pic − = picolinate. In addition, unexpected high-frequency electron paramagnetic resonance evidence for interactions between two neighboring Ru(III) ions, resulting in a triplet state, S = 1, was found.

Methylene blue degradation has been studied spectrophotometrically in alkaline and alkaline peroxide solutions. In both systems, the reaction proceeds via successive N-demethylation and deamination steps determined applying TLC and HPLC techniques. Disappearance of the dye obeys first-order kinetics under the excess of all other reactants. The rate expression for the hydrolytic process (in the absence of hydrogen peroxide) is as follows:)[MB]. Analogous forms of the reaction rate dependence on [OH -] (at constant hydrogen peroxide concentration) and on [HO 2 -] (at constant OH -ion concentration) are observed: -d[MB]/dt = ((a 0 ? b 0 [OH -])/(1 ? c 0 [OH -]))[MB] and -d[MB]/dt = ((a 00 ? b 00 [HO 2 -])/(1 ? c 00 [HO 2 -]))[MB], respectively. A higherorder than linear dependence of the pseudo first-order rate constant on the nucleophiles (OH -and/or HO 2 -) concentrations results from the competitive formation of the ion pair of the cationic dye with the chloride of the supporting electrolyte and adducts of the dye with OH -and/or HO 2 -anions.

The oxidation of methylene blue (MB + ) by cerium(IV) was studied in 0.1-5 M H 2 SO 4 . The reaction proceeds via MB radical (MB 2+• ) formed by one electron transfer to the oxidant. The radical is observed spectrophotometrically by a very intense absorbance at k max = 526 nm and by the e.p.r signal at g = 2.000. The kinetics of the fast radical formation are two orders of magnitude slower than its decomposition, which were examined using a stopped-flow method at 298 K under pseudo-first order conditions. The rate laws for the both steps were determined and a likely mechanism reported.

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