Ligand centered second reduction

The first reduction in the cobalt-based polymer is metal-centered, resulting in the appearance of a new MLCT transition, with the second reduction being ligand-centered. For the nickel-based polymer, in contrast, both redox processes are ligand-based. 

These dendrimers show one oxidation, due to Ru(III)/Ru(II) couple, and two reduction processes, attributed to the first and second reduction of the ligands. Scanning the potential to more negative values, adsorption onto Pt electrode occurs and charge trapping peaks are observed. These peaks likely arise from redox centers that are electronically isolated from the electrode surface, so that their redox reactions are mediated by adjacent redox sites. Morphological changes of the deposited film have been observed upon application of a potential difference and they have been attributed to the deposition or dissolution of the dendrimer and/or to ejection or adsorption of counterions and/or solvent into the film. 

The tris-bidentate complexes, [Os(phen)2(das)]2+, [Os(bpy)2(das)]2+, [Os(phen)2(dpae)]2+, [Os(phen)(das)2]2+, and [Os(bpy)(das)2]2+, are prepared by refluxing cis-[Os(N-N)2Cl2] or [Os(N-N)C14] with the appropriate diarsine in ethylene glycol. These complexes exhibit reversible Os(III/II) oxidations and a reversible reduction that is centred at the phen or bpy ligand. In the case of [Os(bpy)2(das)]2+, a second ligand-centered reduction is observed in acetonitrile. The electronic absorption and emission spectra of these complexes have also been reported (97). [Os(bpy)(das)2]2+ has been used as an effective sensitizer in a photoelec-trochemical half-cell (312). 

Despite lack of sequence homology, the function of the quinone reduction site (Qi site) is similar to that of the secondary quinone-binding site (Qb site) of bacterial reaction centers. Both sites have a conserved histidine residue as quinone ligand and both quinone molecules are reduced to hydroquinone in two consecutive one-electron transfer steps. The midpoint potential for the first step is pH-independent at near neutrality, whereas that for the second reduction varies by 120mV per pH unit (Robertson et al., 1984). This suggests a reaction pathway Q —> Q" QH2, with both protons added concomitantly with the second electron. A stable semi-quinone anion intermediate can be detected by EPR spectroscopy of samples frozen during turnover (Yu et al., 1980 de Vries et al., 1980) or with the redox potential adjusted near the midpoint of ubiquinone (Robertson et al., 1984 Ohnishi and Trumpower, 1980). The semiquinone signal is not observed in the presence of antimycin, which is consistent with the proposal that antimycin inhibits the reaction at the site (Mitchell, 1976 Mitchell, 1975). 

Typical cyclic voltammograms (CVs) of the TRPyP complexes can be seen in Fig. 18. The Ru(III/II) redox processes associated with the peripheral ruthenium complexes are responsible for a reversible wave 0.9 V, exhibiting four times the intensity of those waves centered in the metalloporphyrin moiety (120-122, 171, 184). This process is followed by an oxidation wave of the metalloporphyrin at E > 1.2 V (Table III). At negative potentials, two successive monoelec-tronic waves corresponding to the reduction of the porphyrin to the radical anion (—0.7 V) and the dianion species (—1.1 V) can be observed, followed by the first and second reduction of the bpy ligands at —1.4 and — 1.6 V, respectively. 

The cyclic voltammograms (CV) of the bipyridine nitrosyl ruthenium complexes may reveal multiple couples resulting from redox processes centered at the metal, the nitrosyl ligand, and the bpy ligands. Two couples are evident in the CV electrochemical potential range of +1.0 to —1.0 V vs. Ag/AgCl of a 5/ira s-[Ru(bpy)2L(NO)] complexes in organic solvent or aqueous solution. The reduction peak at the more positive value is chemically reversible, whereas the second reduction peak is practically irreversible in the CV time scale such peaks correspond to the NO "° and pro-... 

The Tc complex [TcI(N—Ar)s] (28) (see Section 5.2.2.1.2) can be reduced with Na° in THE to yield the green, nonbridged, dinuclear compound [Tc2(NAr)6] (46), in which three imido-ligands are bound to the Tc center and connected by a single bond to the second technetium. The molecule has a staggered, ethane-like structure and is diamagnetic. Reduction of (28) yields another homoleptic imido-complex of Tc, the imido-bridged, tetrahedral, dinuclear compound [Tc2(/u-NAr )2(NAr )4] (47) (Ar = 2,6-diisopropylphenyl). The conformation could be confirmed by X-ray structure analysis the assumption of a Tc Tc bond is confirmed by the... 

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