Ligand localized reductions

Distinct peaks at the positive (+) and negative (-) regions of the voltammogram (Fig. 17.4), are observed. The cyclic voltammogram of [Re(CO)3Cl]jtpbq is dominated by metal-localized oxidation and ligand-localized reductions, as observed in analogous complexes. Reduction in the complexes may be attributed to the BL/BL... 

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. 

Experimentally observed redox patterns can be compared with those calculated by quantum-chemical techniques for a particular localization [5, 212], This approach is especially useful for assignment of ligand-localized reductions in heteroleptic complexes. 

The role of the direct ligand localized reduction of the vinyl-containing ligand in the polymerization process. 

It is noted [166] that the similarity in energy between metal and quinone electronic levels is responsible for shifts in charge distribution between metal and ligands. These shifts result from the change in the donor character of the ligand with reduction from SQ to Cat, and the consequential inversion in the order of localized metal and quinone electronic levels. 

Substituted complexes of the type [M (N,N)2XY]" M = Ru, Os, X,Y = halides, CN , 204 , py, en, NH3, etc. show, in general, two doublets of N,N-localized reductions [154-156]. The reduction behavior is complicated by loss of an ancillary ligand X upon the second and, more rapidly, the 3rd bpy-localized reduction. The reduction potentials are only slightly X-dependent. On the other hand, the oxidation potential is highly dependent on X. Extensive tables of oxidation and the first reduction potentials of these complexes are available [15, 28, 74, 157]. Their values can be predicted by use of electrochemical ligand parameters [15, 28, 157]. 

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. 

Potentials of polypyridine-localized redox couples depend linearly on the reduction potential of free polypyridines with a slope close to unity, when measured for homologous series of structurally related complexes [26, 67, 74, 101, 151, 205, 206]. This argument should be used with caution because even the potentials of metal-centered oxidation depends on the free polypyridine reduction potential, albeit with a significantly lower slope [29, 67] (Sections 5.3.2 and 5.3.3). Nevertheless, correlations of redox potentials in a series of [M(N,N)3] complexes, N,N = bpy, 4,4 -Me2-bpy, and 5,5 -Me2-bpy, with those of free ligands were often sufficient to identify N,N-localized reductions [101, 111, 117]. 

A comparison of electrochemistry of some of the free polypyridine ligands and their Ru Re and Mo complexes shows [97, 123, 134, 136, 137, 142, 230, 231] that, all other factors being constant, the potential of the first N,N-localized reduction increases in the order ... 

Now, the excited state oxidation potential is related to the potential of the ground-state ligand-localized redox couple and the excited state reduction potential is related to the potential of the ground-state metal-localized redox couple, see Figure 5. These relations are very logical since oxidation of an MLCT-excited polypyridine complex actually amounts to oxidation of the reduced polypyridine ligand N,N . Similarly, reduction of an MLCT-excited polypyridine complex corresponds to re-... 

Figure 5. Relationship between ground- and excited state redox potentials. The upper diagram is valid generally, regardless of the excited state character. The lower diagram is applicable only to predominantly localized MLCT excited states of those complexes whose first oxidation and reduction are predominantly metal- and ligand-localized, respectively. See Figure 2 and Eqs 12, 13, 18, and 19.

In the case of the phen complex, reduction of the monovalent state leads to copper metal, the initial electron transfer occurring either in the Cu 4s orbital or via a phen n orbital. Fast dissociation follows this monoelectronic reduction step. The redox orbitals involved during the reduction process of the copper catenate are likely to be ligand-localized. This is also supported by the small difference between the redox potentials of the Cu+ / and Cu°/ couples (A i/2 200 mV). Electrolysis of Cu.5+ in the cavity of an EPR spectrometer confirms the radical anion nature of the formally copper(O) complex obtained by one-electron reduction of the catenate g = 2.000 + 0.002, Ai/ = 39 G. 

For Ru(trpy)(py)32+ this process can be assigned to the pyridine localized reduction (E = -1.96V) by analogy with that in Ru(py)62+ (En c = -1.93V) (22). Both Ru(HC(pz)3)(vpy)32+ and Ru-(trpy) (vpy) 3" show an irreversible reductive wave at potentials 400 and 200 mV more positive, respectively, which can reasonably be assigned to the direct reduction of coordinated 4-vinylpyri-dine. In the case of complexes such as Ru(bpy)2(vpy)22+ or Ru-(bpy)2(stilb)22+, however the vinyl ligand reduction is intensely masked by a second bpy-4>py reduction process. 

Figure 20 Low temperature ESR spectrum of the complex [Ru(bpz)3]° (bpz = 2,2 -bipyrazine), electrochemically reduced by two electrons in DMF showing the ligand localized nature of the orbitals involved in the reductions.

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