Redox-active ligands polypyridines

A similar behaviour is exhibited by the isostructural [Zn(phen)3]2 +. 192 In MeCN solution it exhibits an irreversible reduction (Ep = — 1.40 V vs. SCE) followed by further ligand-centred irreversible processes (we must in fact remind that polypyridines are redox active ligands).193... 

It is interesting to note that in a further confirmation of the redox-active character of the polypyridine ligands, reductive electrocrystallization of [Mn(bipy)3]2+ and [Mn(terpy)2]2+ complexes (M = Fe, Ru, Os) afforded the corresponding neutral species [M(bipy)3]° and [M(terpy)2]°. Based on the relevant bond lengths, it seems likely that the two added electrons enter the polypyridine ligands according to the formulation [MII(bipy-)2(bipy)]° and [Mn(terpy )2]°.111,112... 

One of the more important aspects of complexes of the type cw-[Os(bipy)2(L2)] + (L = vinylpyridine) is then-use as derivatives to form electroactive polymer films. Thus polypyridine films containing redox-active Os centers can be generated by electropolymerization of the coordinated vinylpyridine ligands of the parent complex from homogeneous solution. Importantly, these polypyridyl films contain redox-active Os fragments at each unit. 

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 ... 

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]. 

Bis- and tris-polypyridine complexes of Ru are the best known and most useful photo-redox active coordination compounds. The possibility to vary the structure and number of polypyridine ligands and of the ancillary ligands in mono- and bis-polypyridine complexes gives rise to a virtually endless number of photoactive species and provides many opportunities for controlling their properties. Photophysical,... 

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... 

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. 

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