Ligands redox processes

In our simple model, the expression in A2.4.135 corresponds to the activation energy for a redox process in which only the interaction between the central ion and the ligands in the primary solvation shell is considered, and this only in the fonn of the totally synnnetrical vibration. In reality, the rate of the electron transfer reaction is also infiuenced by the motion of molecules in the outer solvation shell, as well as by other... 

The advantages of titanium complexes over other metallic complexes is high selectivity, which can be readily adjusted by proper selection of ligands. Moreover, they are relative iaert to redox processes. The most common synthesis of chiral titanium complexes iavolves displacement of chloride or alkoxide groups on titanium with a chiral ligand, L ... 

Outer-sphere. Here, electron transfer from one reactant to the other is effected without changing the coordination sphere of either. This is likely to be the ea.se if both reactants are coordinatively. saturated and can safely be assumed to be so if the rate of the redox process is faster than the rates observed for substitution (ligand tran.sfer) reactions of the species in question. A good example is the reaction. 

Finally, we consider the alternative mechanism for electron transfer reactions -the inner-sphere process in which a bridge is formed between the two metal centers. The J-electron configurations of the metal ions involved have a number of profound consequences for this reaction, both for the mechanism itself and for our investigation of the reaction. The key step involves the formation of a complex in which a ligand bridges the two metal centers involved in the redox process. For this to be a low energy process, at least one of the metal centers must be labile. 

The hydrogen transfer reaction (HTR), a chemical redox process in which a substrate is reduced by an hydrogen donor, is generally catalysed by an organometallic complex [72]. Isopropanol is often used for this purpose since it can also act as the reaction solvent. Moreover the oxidation product, acetone, is easily removed from the reaction media (Scheme 14). The use of chiral ligands in the catalyst complex affords enantioselective ketone reductions [73, 74]. 

It is evident that the supported clusters have a strong affinity for hydride ligands provided by the support. The process by which the support delivers these ligands is referred to in the catalysis literature as reverse hydrogen spillover. The opposite process (spillover), well known for supported metals [36], is shown by the theoretical results to be a redox process in reverse spillover, the support hydroxyl groups oxidize the cluster. 

Since most Ni1 species with simple N-donor ligands are prone to disproportionation into Ni° and Ni11, relatively few Ni1 complexes with nonmacrocyclic N-donor ligands have been reported. Formation of Ni1 species is in most cases proposed on the basis of electrochemical data, although ligand-centered redox processes have to be considered. The ligands usually contain imine donor atoms or aromatic N-heterocycles, which because of their 7r-acceptor ability favor stabilization of lower oxidation states. 

Pendent arm 1,4,7-triazacyclononane macrocycles (91) and (92) have been used to stabilize the zinc-to-phenoxyl bond allowing characterization of these compounds.477 The interest in the zinc complexes comes from the wide potential range in which it is redox stable allowing observation of the ligand-based redox processes, this allows study of the radical by EPR and the electronic spectra is unperturbed by d-d transitions. Macrocycles of the type l,4,7-tris(2-hydroxybenzyl)-1,4,7-triazacylononane form a bound phenoxyl radical in a reversible one-electron oxidation of the ligand. The EPR, resonance Raman, electronic spectra, and crystal structure of the phenoxide complexes were reported. This compound can be compared to a zinc complex with a non-coordinated phenoxyl radical as a pendent from the ligand.735... 

V,/V-bis(2-hydroxy-di-3,5-/-butylphenyl)amine forms complexes of zinc which have ligand-based redox processes with four oxidation levels of the coordinated anion.864 2 1 and 1 1 complexes are formed in the presence of zinc with the 2 1 complex coordinated in an octahedral geometry and the 1 1 complex square planar with a triethylamine ligand completing the coordination sphere. The complexes, at the different redox levels, have been investigated by EPR, spectro-electrochemistry, l I NMR, and magnetochemistry, as appropriate. 

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. 

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. 

One of the earliest series of metal complexes which showed strong, redox-dependent near-IR absorptions is the well-known set of square-planar bis-dithiolene complexes of Ni, Pd, and Pt (Scheme 4). Extensive delocalization between metal and ligand orbitals in these non-innocent systems means that assignment of oxidation states is problematic, but does result in intense electronic transitions. These complexes have two reversible redox processes connecting the neutral, monoanionic, and dianionic species. 

The iron cycle shown in Fig. 10.14 illustrates some redox processes typically observed in soils, sediments and waters, especially at oxic-anoxic boundaries. The cycle includes the reductive dissolution of iron(lll) hydr)oxides by organic ligands, which may also be photocatalyzed in surface waters, and the oxidation of Fe(II) by oxygen, which is catalyzed by surfaces. The oxidation of Fe(II) to Fe(III)(hydr)-oxides is accompanied by the binding of reactive compounds (heavy metals, phosphate, or organic compounds) to the surface, and the reduction of the ferric (hydr) oxides is accompanied by the release of these substances into the water column. 

Give examples of photodissociation, ligand exchange and redox processes of d-block complexes. 

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