Redox polypyridine complexes

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. 

Table 6 Formal electrode potentials (V vs. SCE) for the redox processes exhibited by polypyridine complexes of chromium...

All the three polypyridyl complexes display the reversible reduction sequence 2 + / + /0. The relative potential values are reported in Table 7. As far as the nature of such redox changes is concerned, it is important to recall the ambiguity that exists in attributing metal-centred or ligand-centred redox processes for metal-polypyridine complexes. 

Table 13 summarizes the electrode potentials of the redox processes for the complete set of polypyridine complexes. 

The redox potentials of the nickel(II) polypyridine complexes are reported in Table 16.167 168... 

Luminescent and redox-reactive building blocks for the design of photochemical molecular devices mono-, di-,tri-, and tetranuclear ruthenium(n) polypyridine complexes. [G. Denti, S. Campagna, L. Sabatino, S. Serroni, M. Ciano, V. Balzani, Inorg. Chem. 1990, 29(23), 4750-4758] [ 830]. 

Schwarz HA, Creutz C, Sutin N. (1985) Homogeneous catalysis of the photoreduction of water by visible light. 4. Cobalt(I) polypyridine complexes. Redox and substitutional kinetics and thermodynamics in the aqueous 2,2 -bipyridine and 4,4 -dimethyl-2,2 -bipyridine series studied by the pulse-radiolysis technique. Inorg Chem 24 433H39. 

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.)... 

The great structural variability of polypyridine complexes enable us to tune and control their redox properties over a very broad range. For example, variations in polypyridine ligand structure can shift the reduction or oxidation potential within a range almost 2 V wide. Further control can be achieved by changing the nature of the metal and, in mixed-ligand complexes, of ancillary ligands. Polypyridine com-... 

The amount of research performed and literature published on electron-transfer reactions of metal-polypyridine complexes is enormous. Several excellent reviews [42, 74, 93-97] and books [38, 62, 98, 99] deal with polypyridine eomplexes, their redox chemistry, photochemistry, and applications. Hereinafter, the most prominent aspects of electron transfer reactivity of mononuclear metal-polypyridine eomplexes will be surveyed without attempting to cover exhaustively the vast original literature. Instead, the main purpose of this chapter is to single out the structural, thermodynamic, and kinetic factors which enable and eontrol the special and diverse electron-transfer behavior of metal-polypyridine complexes in their electronic ground and excited states. Although supramolecular eleetron-transfer chemistry of metal-polypyridines is not discussed here in detail, beeause it is covered in Volume 3 of this monograph, links connecting the redox behavior of mononuclear polypyridine eomplexes and their supramolecular counterparts will be briefly outlined. 

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 ... 

The redox behavior of complexes of the type [Re (X)(CO)3(R-dab)] X = halide is similar to that of their polypyridine counterparts—one oxidation and two successive reductions which ultimately produce pentacoordinated [Re (CO)3(R-dab)] species [134, 136, 142]. There are, however, some remarkable differences between the redox chemistry of Re dab and polypyridine complexes (i) Oxidation [134] to [Re (X)(CO)3(R-dab)]+ is reversible and less positive (-1-0.58 V for X = Cl, R = tBu), indicating the greater stability of Re Cdab complexes and smaller involvement of E ligand orbitals in the HOMO of the Re complex, (ii) The rate of dissociation of the axial ligand after the first reduction to [Re (Br)(CO)3(R-dab )] ... 

Labilization of an ancillary ligand X on reduction indicates the possibility of employing Ru and Os polypyridine complexes as redox catalysts (Section 5.3.5). Indeed, electrocatalysis of CO2 reduction to CO or formate by [Ru(bpy)2(CO)2] +, [Ru(bpy)2(CO)Cl]+, or [Os(bpy)2(CO)H]+ has been reported [158, 159]. The elec-trocatalytically most active Ru-bpy species is, however, a film of a -Ru-Ru-bonded polymer [- Ru(bpy)(CO)2 -]. It is formed on an electrode surface by reduction of various mono- or bis-bpy Ru carbonyl or carbonyl-chloro complexes [160 163], Dissociation of a CO ligand from the polymer upon further bpy-localized reduction is the crucial step which enables CO2 coordination and reduction. 

Similarly to polypyridine complexes of other d metals, Ir(III)-bipyridine [106, 186] and phenanthroline [186, 187] complexes form extensive ligand-based redox series. The [Ir(bpy)3] series comprises eight members related by one-electron steps [106], [Ir(bpy)3] + is reduced in six successive one-electron steps which occur in two groups of closely spaced three reductions spanning the range from —1.22 to -2.76 V. It also has an one-electron irreversible oxidation at ca -1-1.7 V, presumably to [Ir(bpy)3] +. The substituted complex [Ir(bpy)2(Cl)2] has an irreversible oxidation and four one-electron reductions, presumably ligand-based. The reduction mechanism is complicated by dissociation of Cl ligand(s) on electron uptake [186]. 

The systematic survey given above enables us to single out general features of the redox chemistry of transition metal polypyridine complexes ... 

Spectroelectrochemical (EPR, UV Vis, res. Raman, and to a lesser extent IR) characterization of redox products often reveals features characteristic of reduced polypyridine ligands for ligand-localized reductions or of oxidized metal atoms for metal-centered oxidations. Stretching CO frequencies, obtained by IR spectroelectrochemistry, are an especially useful marker of a metal oxidation state in carbonyl-polypyridine complexes. 

Electron-transfer Properties of Ground-state Polypyridine Complexes 823 5.3.2 Redox Patterns... 

Typical polypyridine complexes of second- or third-row d transition metals are characterized by metal-localized oxidation and a series of predominantly polypyridine-localized one-electron reductions. Their redox patterns are schematically shown in Figure 1. 

Figure 1. Redox patterns of a free polypyridine ligand and typical mono-, bis- and tris-polypyridine complexes [4, 8, 9, 212], is the orbital energy, Ua and are pair interaction energies between electrons placed on the same and different polypyridine ligands, respectively, t/g. Kg, and Gg are solvation energy terms. In the first approximation, interaction and solvation terms are assumed to be independent of the number of electrons, that is to be constant along the redox series. In reality, they show small changes. Interaction energies are positive and solvation terms are negative. Hence, solvation diminishes the redox potential differences. Dashed lines show the origin of reduction doublets and triplets in bis- and tris-polypyridine complexes, respectively.

Most polypyridine complexes of second- and third-row transition metals also display a predominantly metal-localized oxidation at positive potentials which are chemically either reversible or partly reversible. Further one-electron oxidations often occur at more positive potentials in liquid SO2 [57]. The first oxidation potential depends on the metal atom (for example, Ru > Os), the ancillary hgands in [M(W)(X)(Y)(Z)(N,N)j or [M(X)(Y)(N,N)2] and, also, on the structure of the polypyridine ligand. Empirically, oxidation potentials can be calculated using additive Lever electrochemical parameters which quantify the influence of the metal atom and individual hgands on metal-centered redox couples [9, 157, 220]. 

Redox patterns of first-row transition metal polypyridine complexes are very much... 

Variations in the structure of a polypyridine ligand provide a means of control of the number of reduction steps of polypyridine complexes, of their reduction and oxidation potentials, and of the extent of rr-delocalization between the metal atom and a polypyridine ligand. These aspects were discussed above in Sections 5.3.1 and 5.3.2. Here, the polypyridine influence on redox potentials will be discussed in more detail. 

Given the importance and great variety of electron-transfer reactions of polypyridine complexes, systematic kinetic studies are surprisingly scarce and only few kinetic data are available. Some representative rate constant values for homogeneous self-exchange redox reactions were reported ... 

Reductively induced dissociation of an ancillary ligand from polypyridine complexes has a great importance for redox catalysis ... 

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