Redox-active metal-polypyridine

Redox-Active Metal-Polypyridine Dendrimers as Light-Harvesting Antennae... 

Properties of Some Common Photo-redox-active Metal-Polypyridine Chromophores... 

Polynuclear complexes, molecular dyads, triads, and other supermolecules composed of redox- and photo-active metal polypyridine units have a great promise as components of future molecular electronic or photonic devices as optical switches, relays, memories, etc. [38, 46],... 

The hexanuclear Ru6 species has four outer and two inner metal centers oxidation active. Both in acetonitrile at room temperature ( 1/2 at + 1.52 V) and in liquid S02 at low temperature ( 1/2 at + 1.46 V), an oxidation process involving the practically simultaneous one-electron oxidation of the four outer Ru(II) centers is evidenced (Fig. 5.9 and Table 5.1). This confirms that the electronic interaction between metal centers that are not directly connected via a bridging ligand is negligible from an electrochemical viewpoint in the metal-polypyridine dendrimers. At more positive potentials, only recordable in liquid S02 at low temperature (Fig. 5.9), a bielectronic process, related to the simultaneous one-electron oxidation of the two inner metal centers at + 2.11 V, is found. This result was at a first sight surprising, since the redox... 

Insertion of redox-active functions in /3-diketonato-metal complexes increases their redox activity. Particularly useful has been the introduction of ferrocenyl or tetrathio-fulvenyl substituents other redox active fragments such as benzoquinone diimines or polypyridines have been also exploited. Frequently, the electrochemical investigation has been focussed on such redox-active substituents rather than on the whole metal-diketonato framework. This section will be limited to well known, representative, mononuclear complexes, but we shall omit complexes such as MCp2(acac) (M = Ti, the elec-... 

Most of the interesting redox properties of metal-polypyridine complexes originate from the electron-transfer activity of polypyridine ligands themselves. The free bpy ligand and its analogs are sequentially reduced in two one-electron electro-chemically reversible steps, producing the corresponding radical-anion and dianion, respectively ... 

Redox series of metal-polypyridines still await their practical exploration. The existence of multistep, reversible, sequential reduction processes, each step occurring at a defined potential and being localized at a specific molecular site, is very promising for possible applications in molecular electronics. This would require to organize the active complexes in films, polymers or supermolecules. Up to now, only the electrochromic behavior of some [Ru(N,N)a] + complexes has been explored with potential applications in electrochromic glasses, displays and redox sensors [206, 262, 264]. 

Since the ligand-field splitting of the d-orbital manifold of low-valent d metal atoms is relatively large, LF excited states of d -metals occur at higher energy than MLCT states, with the only exception of Fe F Hence, LF states are not involved in electron transfer reactivity but they can provide a non-radiative deactivation pathway for the reactive MLCT state, shortening its lifetime. LF states do not exist for d CuF The only polypyridine complexes with a redox-active LF state are [Cr(N,N)3] +, whose T/ E LF states are strong oxidants [278, 279]. 

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. 

In supramolecular systems, electronic interactions between metal-polypyridine and other redox-active or units are too small to perturb ground-state electrochemical and spectroscopic properties but are sufficient to enable very fast intramolecular electron-transfer reactions upon excitation. 

This review illustrates the above delineated characteristics of electron-transfer activated reactions by analyzing some representative thermal and photoinduced organometallic reactions. Kinetic studies of thermal reactions, time-resolved spectroscopic studies of photoinduced reactions, and free-energy correlations are presented to underscore the unifying role of ion-radical intermediates [29] in—at first glance—unrelated reactions such as additions, insertions, eliminations, redox reactions, etc. (Photoinduced electron-transfer reactions of metal porphyrin and polypyridine complexes are not included here since they are reviewed separately in Chapters 2.2.16 and 2.2.17, respectively.)... 

Osmium and ruthenium in high oxidation states (III, IV, or V) form polypyridine (bpy, phen or tpy) hydroxo and 0x0 complexes, which have a rich redox chemistry [166, 167]. Redox changes are metal-localized and accompanied by reactions of M=0 or M-OH bonds. These complexes are active as electrocatalysts of the oxidation of water to O2, or of CF to CI2, or they can transfer oxygen atoms and oxidize organic substrates like (CH3)2CHOH or C6H5-CH(CH3)2. 

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