Ruthenium complexes electron-transfer reactions

Let us now examine sample sets of data. We shall consider two reactions, the formation of a biradical1 [Eq. (7-10)] and an electron transfer reaction between two ruthenium complexes [Eq. (7-11)], in which LN represent nitrogen-donor ligands specified in the original reference.2 The chemical equations are... 

The systems that we investigated in collaboration with others involved intermolecular and intramolecular electron-transfer reactions between ruthenium complexes and cytochrome c. We also studied a series of intermolecular reactions between chelated cobalt complexes and cytochrome c. A variety of high-pressure experimental techniques, including stopped-flow, flash-photolysis, pulse-radiolysis, and voltammetry, were employed in these investigations. As the following presentation shows, a remarkably good agreement was found between the volume data obtained with the aid of these different techniques, which clearly demonstrates the complementarity of these methods for the study of electron-transfer processes. 

In order to obtain further information on the magnitude of the overall reaction volume and the location of the transition state along the reaction coordinate, a series of intermolecular electron-transfer reactions of cytochrome c with pentaammineruthenium complexes were studied, where the sixth ligand on the ruthenium complex was selected in such a way that the overall driving force was low enough so that the reaction kinetics could be studied in both directions (153, 154). The selected substituents were isonicotinamide (isn), 4-ethylpyr-idine (etpy), pyridine (py), and 3,5-lutidine (lut). The overall reaction can be formulated as... 

Sabbatini N, Dellonte S, Bonazzi A et al (1986) Photoinduced electron-transfer reactions of poly(pyridine)ruthenium(II) complexes with europium(III/II) cryptates. Inorg Chem 25 1738-1742... 

The inverted region in electron transfer reactions is studied for the reaction of electronically-excited ruthenium(II) tris-bipyridyl ions with various metal(III) tris-bipyridyl complexes. Numerical calculations for the diffusion-reaction equation are summarized for the case where electron transfer occurs over a range of distances. Comparison is made with the experimental data and with a simple approximation. The analysis reveals some of the factors which can cause a flattening of the In k versus AG curve in the inverted... 

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). 

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. 

Intramolecular electron transfer from Ru(II) to Fe(III) in (NH3)3Ru(II) (His-33)cyt(Fe(III)) induced by pulse-radiolysis reduction of Ru(III) in the (NH3)5Ru(III) (His-33)cyt(Fe(III)) complex were investigated [84]. The results obtained differ from those of refs. 77-80 where flash photolysis was used to study the similar electron transfer reaction. It was found [84] that, over the temperature range 276-317 K the rate of electron transfer from Ru(II) to Fe(III) is weakly temperature dependent with EA 3.3 kcal mol 1. At 298 K the value of kt = 53 2 s"1. The small differences in the temperature dependence of the electron tunneling rate in ruthenium-modified cytochrome c reported in refs. 77-80 and 84 was explained [84] by the different experimental conditions used in these two studies. 

Luminescent ruthenium(II) polypyridine indole complexes such as [Ru (bpy)2(bpy-indole)]2+ (37) and their indole-free counterparts have been synthesised and characterised [77]. The ruthenium(II) indole complexes display typical MLCT (djt(Ru) tt (N N)) absorption bands, and intense and long-lived orange-red 3MLCT (djt(Ru) -> Ti (bpy-indolc)) luminescence upon visible-light irradiation in fluid solutions at 298 K and in alcohol glass at 77 K. In contrast to the rhenium(I) indole complexes, the indole moiety does not quench the emission of the ruthenium(II) polypyridine complexes because the excited complexes are not sufficiently oxidising to initiate electron-transfer reactions. Emission titrations show that the luminescence intensities of the ruthenium(II) indole complexes are only increased by ca. 1.38- to... 

PHOTO-INDUCED ELECTRON TRANSFER REACTIONS IN POLYMER-BOUND RUTHENIUM BIPYRIDYL COMPLEXES... 

Scott JR, Fairris JL, McLean M, et al. Intramolecular electron-transfer reactions of cytochrome 5 covalently bonded to ruthenium(II) polypyridine complexes reorgani-zational energy and pressure effects. Inorg Chim Acta 1996 243 193-200. 

Harris MR, Davis DJ, Durham B, Millett F. Temperature and viscosity dependence of the electron-transfer reaction between plastocyanin and cytochrome c labeled with a ruthenium(II) bipyridine complex. Biochim Biophys Acta 1997 1319 147-54. 

B. Stereoselective Photoinduced Electron Transfer Reaction of Ruthenium(ll) tris-Bipyridine Complexes... 

In summary, the stereoselectivity was certainly observed in the photoin-duced electron transfer reactions of chiral ruthenium(II) complexes with chiral viologen and Co(III) complexes. However, not only the photoinduced electron transfer reaction but also the charge separation in the encounter complex and the reverse reaction between the ruthenium(III) complex and Co(acac)2 + acac participate in the stereoselection. In the reactions between the ruthenium(II) and Co(III) complexes, the energy transfer also contributes to the quenching reaction, which makes difficult the observation of stereoselectivity in the quenching reaction. 

First, we will briefly discuss the electronic structure of the copper(I) complex in the excited state, because the electronic structure is less well known than that of the ruthenium(II) complex. In the usual transition-metal complexes, the d-d excited state exists in a lower energy than the MLCT excited state, as shown in Scheme 20A. This feature is not favorable for the electron transfer reaction, as was discussed in Sec. II.B. However, the d-d excited state does not exist in the copper(I) complexes, because the copper(I) atom takes ad10 electron configuration. 

Light-Induced Electron Transfer Reactions of Metalloporphyrins and Polypyridyl Ruthenium Complexes in Organized Assemblies... 

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