Ligand-based reduction processes

The electroactive units in the dendrimers that we are going to discuss are the metal-based moieties. An important requirement for any kind of application is the chemical redox reversibility of such moieties. The most common metal complexes able to exhibit a chemically reversible redox behavior are ferrocene and its derivatives and the iron, ruthenium and osmium complexes of polypyridine ligands. Therefore it is not surprising that most of the investigated dendrimers contain such metal-based moieties. In the electrochemical window accessible in the usual solvents (around +2/-2V) ferrocene-type complexes undergo only one redox process, whereas iron, ruthenium and osmium polypyridine complexes undergo a metal-based oxidation process and at least three ligand-based reduction processes. 

Larger dendrimers based on a Ru(bpy)2+ core and containing up to 54 peripheral methylester units (12) have recently been obtained [29a]. Both the metal-centered oxidation and ligand-centered reduction processes become less reversible on increasing dendrimer size [29b]. 

The values obtained for the ligand-based reductions suggest strongly that these processes are bpy-based for most compounds. For the bpzt-based dinuclear compounds the first reduction appears to be based on the bridging ligand. This is indeed coi rmed by resonance Raman studies. [7] This difference in location is most likely caused by the presence of a pyrazine ring in the latter compounds. 

Similar Ru(II) polypyridine units were connected to dirhodium(ll) tetracarboxylate platforms to form the supramolecular assemblies 80-82 (Figures 9.37 and 9.38) [166], exhibiting severed metal-centered oxidation and ligand-centered reduction processes, and intense LC and MLCT bands centered on the Ru(II) units. Efficient energy transfer from MLCT (Ru-based) to the lowest-energy... 

Electrochemical studies on [WE(S2)(S2CNR2)2l (E = O, S R = Me, Et) reveal an irreversible one-electron ligand-based oxidation process involving the disulhde ligand, and a one-electron reduction generating radical anions that are inherently unstable and decay to unidentified products (879). 

The first attempt to construct a dendrimer with an electroactive Ru-polypyridi-ne core was based on the reaction of Ru(bpy)2Cl2 with a branched polyether-substituted phenanthroline ligand (11) [27]. In the potential window +2/-2V, this compound shows a one-electron oxidation process and three distinct one-electron reduction processes that, by comparison with the behavior of the... 

The complex Tb(TTFA) (o-phen) underwent a reduction at E --1.5 V vs. SCE which was partially reversible. An oxidation was not observed below +2 V. All redox reactions should be ligand-based processes. The potential difference of Ae > 3.5 V is energy sufficient to generate the IL triplet at 2.56 eV. The low eel intensity could be due to a competing irreversible decay of the primary redox pair. 

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. 

The bimetallic complex [Re(CO)3Cl]jtbpq synthesized in this work showed the typical spectroscopic and electrochemical behavior based on analogous polypyridyl complexes of rhenium(l). Re(l) dn tpbq n charge transfer transition and ligand-field n- n transitions are observed. Typical redox behavior of this system consists of Re /Re oxidation and tpbq/tpbq reduction. Such electrochemical activity, particularly in the reductive region, is found ideal for catalytic processes such as CO reduction. IR-SEC studies have shown that the reduction process occurring at -0.50... 

That the reduction takes place at the surface, receives support from an experiment in which the extent of reduction of various Fe oxides by Shewanella alga after 30 days was linearly correlated with the SAbet the exception was 2-line ferrihydrite for which a surface area of 600 m /g had to be assumed in order to fit the relationship (Roden Zachara, 1996). Although experimental (BET) surface areas of ferrihydrite are substantially lower than 600 m /g, calculated values based on particle size as well as those determined from ligand adsorption experiments (see Table 5.1) are in this range. Dissolved Fe was found to create a lag phase in the reduction process (in contrast to the behaviour in inorganic systems) because Fe is adsorbed at the cell surface (Liu et al. 2001). This effect can be overcome by complexing the Fe (e. g. 

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