Chemical reactivity redox properties

In order to build up dendrimers crqrable of exhibiting redox activity and light-induced functions, appropriate building blocks have to be used. In the last 20 years, extensive investigations carried out on the photochemical and electrochemical properties of transition metal compounds have shown that Ru(II) and Os(ll) complexes of aromatic M-heterocycles (Figure 1), e.g., Ru(bpy)j and Os(bpy)j (bpy = 2,2 -bipyridine), exhibit a unique combination of chemical stability, redox properties, excited state reactivity, luminescence, and excited state lifetime. Furthermore all these properties can be tuned within rather broad ranges by... 

There is much interest in transition metal polypyridyl complexes, largely due to their numerous applications in a variety of fields (247-250). In particular, ruthenium(II) tris(2,2 -bipyridyl) has been one of the most extensively studied complexes of the last decade due to its chemical stability, redox properties, excited-state reactivity, and luminescent emission (251, 252). 

Electrochemical measurements are commonly carried out in a medium that consists of solvent containing a supporting electrolyte. The choice of the solvent is dictated primarily by the solubility of the analyte and its redox activity, and by solvent properties such as the electrical conductivity, electrochemical activity, and chemical reactivity. The solvent should not react with the analyte (or products) and should not undergo electrochemical reactions over a wide potential range. 

This considerable enhancement in redox properties may however remain chemically hidden. Several causes may converge to mask these properties. First of all electron transfer is an intermolecular act of reactivity even when thermodynamically feasible it may have to compete with very rapid intramolecular acts of deactivation (fluorescence, phosphorescence, internal conversion)99. The rate of electron transfer is given by the Rehm-Weller equation96,100... 

Very little is known about the nature of the weak interactions of CAs in solutions where a vast majority of their chemical reactions has been studied. Particularly, the study of donor-acceptor complexes of CAs by modern physical-chemical methods is still of great interest. Besides, complexation of CAs with donors or acceptors of electron density is a useful tool for modifying the stability, reactivity and spectral properties of CAs. Systematic investigations of the redox properties of CAs are needed in order to elucidate the role of electron transfer in the transformations of CAs. 

Most of the late transition metals (such as Fe, Co, Rh, Ir, Ni, Pd, Pt, Cu, Au, and Zn) have been found to form bis(dithiolene) complexes. A significant amount of work has been reported on the electronic structures and spectroscopy (32), redox properties (2), as well as the conductivity (33) of bis(dithiolene) complexes. Far less has been reported on their chemical reactivity. 

Compared to the large body of electrochemical data, there have been fewer studies on the chemical reactivity of bis(dithiolene) complexes. In light of the rich redox chemistry of bis(dithiolene) complexes and the redox-active nature of the dithiolene ligands, it is not surprising that much of the reactivity observed is related to the redox properties and is often centered on the dithiolene ligands. 

In summary, bis(dithiolene) complexes are clearly distinct from traditional inorganic or organometallic complexes in which the chemical reactivity is dominated by the metal center. The unique properties of dithiolene ligands such as redox activity, aromaticity, and unsaturation of the metal-ligand chelate rings, in combination with the metal-centered reactivity paths, have generated many unusual reactivity patterns for this class of complexes. 

Although it has already been studied extensively, the redox and redox-related chemistry remains a dominant theme in dithiolene chemistry. Much of the chemical reactivity reported so far is associated with the dithiolene ligand and is, in many cases, related to the redox properties. Clearly, the range of accessible charge levels of dithiolene complexes may be exploited for new reaction... 

One of the most important factors providing acceleration of enzymatic reactions as compared to chemical reactions is drastic changes of chemical reactivity catalytic groups inside and outside the enzyme protein globule. Drawing the charges of metal ions, carboxylate and protonated residues into the protein interior is accompanied by essential alternation of its acid-base and redox properties. This effect can be illustrated by the reaction of cleavage and formation of an a-C-H bond in enzymatic reactions of racemization, transamination, and isomerization (Ha et al., 2000 and references therein). 

Structure-reactivity relationship. This chapter is not a comprehensive review of the published work on radiation-induced chemical oxidation of benzene derivatives, nor does it cover redox properties and energetics of radical cations of substituted benzenes. The latter aspects have already been reviewed by Jonsson " earlier. In a series of papers,Jonsson and co-workers have clearly shown correlations between substituent pattern and redox properties of radical cations of substituted benzenes. Further, it has been shown by them that the product pattern is governed by the charge distribution on the radical cation and the electron density distribution on the corresponding substituted benzene. This chapter is an overview of the work carried out on radiation-induced oxidation of substituted benzenes with emphasis on the contribution to the area from our research group. 

Transition metal polypyridine complexes are highly redox-active, both in their electronic ground- and excited states. Their electron transfer reactivity and properties can be fine-tuned by variations in the molecular structure and composition. They are excellent candidates for applications in redox-catalysis and photocatalysis, conversion of light energy into chemical or electrical energy, as sensors, active components of functional supramolecular assemblies, and molecular electronic and photonic devices. 

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