Ruthenium couple, electronic

A number of mechanistic pathways have been identified for the oxidation, such as O-atom transfer to sulfides, electrophilic attack on phenols, hydride transfer from alcohols, and proton-coupled electron transfer from hydroquinone. Some kinetic studies indicate that the rate-determining step involves preassociation of the substrate with the catalyst.507,508 The electrocatalytic properties of polypyridyl oxo-ruthenium complexes have been also applied with success to DNA cleavage509,5 and sugar oxidation.511... 

The electrochemical behavior of ruthenium(IV) 0x0 complexes has been extensively studied. They exhibit interesting electrochemical behavior, particularly in aqueous solutions, where proton-coupled electron transfer processes occur. In general, two couples are observed in the cyclic voltammetry of a Ru =0 complex in aqueous solutions (Equations (84) and (85)) ... 

Comproportionation between cA-RuIV(bpy)2(py)02 + and cis- Run(bpy)2(py)(H20)2+ takes place by proton-coupled electron transfer (PCET) and exhibits a KIE of 16.1. Other PCET reactions of these and related ruthenium and osmium complexes also feature large KIEs. For example, oxidations of H202 by RuIV(bpy)2 (py)O2 + and by Ruin(bpy)2(py)OH2 + have KIEs of22.1 and 16.7, respectively. Oxidation of benzyl... 

A doubly metallated 15 base-pair double helix containing ruthenium and rhodium at each end of the strands [106] showed the efficiency of DNA for coupling electron donors and acceptors over a very long range, greater than 40 A. The DNA double helix was found to behave like a piece of molecular wire with fast electron-transfer rates (>1010 s l) for the photoinduced electron transfer between the metallointercalators [107-109] and semiempirical Hartree-Fock calculations of HAB for DNA mediated electron transfer [110] were described. 

Within the Hush formalism of electron transfer [129], the electronic coupling through hydrogen bond interfaces may be deduced from the intensity of a mixed-valence transition between juxtaposed donor-acceptor pairs. We highlight the only such detailed study in this section. Curtis and coworkers have studied a collection of hydrogen-bonded mixed-valence adducts formed between ruthenium(II) electron-donor and ruthenium(III) electron-acceptor complexes in solution [130]. Using acetonitrile or nitromethane as a solvent, hydrogen-bonded assemblies of the type [(tpy)(bpy)Ru (CN)]2, (en)2Ru (bpy) 5+ 45 (bpy = 2,2 -bipyridine, tpy = 2, 6", 2 -terpyridine, en = ethylenediamine) and [(bpy)2Ru (CN)2]2, (en)2Ru (bpy) +... 

N. lORDANOVA, S. HamMES-SchIFFER, Theoretical investigation of large kinetic isotope effects for proton-coupled electron transfer in ruthenium polypyridyl complexes, J. Am. Chem. Soc. 124, 4848-4856 (2002). 

External reflectance spectroscopy has been used in the study of proton coupled electron transfers at a dinuclear ruthenium complex [(phen)2Ru(tatpp)Ru-(phen)2] (Figure 14.10) (37). 

These transition-metal catalysts contain electronically coupled hydridic and acidic hydrogen atoms that are transferred to a polar unsaturated species under mild conditions. The first such catalyst was Shvo s diruthenium hydride complex reported in the mid 1980s [41 14], Noyori and Ikatiya developed chiral ruthenium catalysts showing excellent enantioselectivity in the hydrogenation of ketones [45,46]. 

Foyt et al. [137] interpreted the quadrupole-splitting parameters of low-spin ruthenium(II) complexes in terms of a crystal field model in the strong-field approximation with the configuration treated as an equivalent one-electron problem. They have shown that, starting from pure octahedral symmetry with zero quadrupole splitting, A q increases as the ratio of the axial distortion to the spin-orbit coupling increases. 

The nitrogen on ruthenium work is consistent with the observation made on the H/Cl/Au Eley-Rideal chemistry and, taken together, the implications of these two pieces of work are quite profound, suggesting that an accurate theory of surface reactions cannot be constructed without accounting for strong coupling between the reaction coordinate and the metals electron... 

Very recently a new kind of electrocatalyst has been propounded using the dinuclear quinone-containing complex of ruthenium (25).492,493 Controlled-potential electrolysis of the complex at 1.70 V vs. Ag AgCl in H20 + CF3CH2OH evolves dioxygen with a current efficiency of 91% (21 turnovers). The turnover number of 02 evolution increases up to 33,500 when the electrolysis is carried out in water (pH 4.0) with an indium-tin oxide(ITO) electrode to which the complex is bound. It has been suggested that the four-electron oxidation of water is achieved by redox reactions of not only the two Run/Ruin couples, but also the two semiquinone/quinone couples of the molecule. 

Techniques for attaching such ruthenium electrocatalysts to the electrode surface, and thereby realizing some of the advantages of the modified electrode devices, have been developed.512-521 The electrocatalytic activity of these films have been evaluated and some preparative scale experiments performed. The modified electrodes are active and selective catalysts for oxidation of alcohols.5 6-521 However, the kinetics of the catalysis is markedly slower with films compared to bulk solution. This is a consequence of the slowness of the access to highest oxidation states of the complex and of the chemical reactions coupled with the electron transfer in films. In compensation, the stability of catalysts is dramatically improved in films, especially with complexes sensitive to bpy ligand loss like [Ru(bpy)2(0)2]2 + 51, 519 521... 

---