Redox congeners

Comparisons among Metal-Alkynyl Redox Congeners... 

Unfortunately, in spite of the fact that electrochemistry allows access to redox congeners of many heterometallic complexes that could be isolated, we were unable to find examples of solid-state structures of redox congeners. So the main goal of judging quantitatively the effects of electron-transfer processes on these types of complexes could not be fulfilled. 

We must note that in many cases the electrochemical recognition has been limited to the simple evaluation of the redox potentials of the electron transfer steps, together with a superficial examination of the chemical stability of the complexes in various oxidation states. This is of little help to synthetic chemists, who would have to prepare redox congeners for complete chemical, physico-chemical, and structural characterization. Controlled potential coulometry and macroelectrolysis tests must be routinarily performed, both to define the number of electrons involved in each redox change and to obtain redox congeners, even if in low quantity, at least for a preliminary determination of their stability and of their spectroscopic properties. 

It is interesting to note how the capping unit in the present triangular PtCo2 assembly does not improve the stability of redox congeners with respect to that of the linear PtCo2 assembly in Pt(RNC)2[Co(CO)4]2 discussed in Sect. 2.1. 

Table 25. Selected bond distances (in A) for the redox congeners [Sb2Co4(CO)u]"- [156, 157]...

The occurrence of such redox congeners encouraged the search for the relevant thermodynamic redox potential. As a matter of fact, electrochemistry showed that the Fe6M2 assembly is able to support, without molecular destruction, two subsequent one-electron transfers, namely [Fe6(S)6(X)6 M(CO)3 2]3-/4-/5-. The relevant redox potentials are summarized in Table 27. 

The instability of the redox congeners of the C03M and RU4 complexes is due to decarbonylation reactions. Exhaustive one-electron reduction of [Co3Ru(CO)io-(PhC CPh)] in the presence of PPh3 affords the stable dianion [Co3Ru(CO)9-(PhC=CPh)] , and exhaustive two-electron reduction of [Ru4(CO)i2-(PhC=CPh)] affords the stable dianion [Ru4(CO)n(PhC=CPh)] The redox potentials of these electron transfers are reported in Table 8. 

Also in this case, the peak-to-peak separation of the different one-electron transfers, approaching the theoretical value of 59 mV, preludes to a substantial maintaining of the starting molecular frameworks. As a matter of fact, the redox congeners we succeeded in crystallizing showed to be isostructural with the parent precursors. Table 1 summarizes the relevant minimal structural rearrangements. 

As illustrated in Figure 15, [NiiiBi2(CO)is] undergoes both a one-electron oxidation and a one-electron reduction. Even if we were unable to obtain crystals of the relevant redox congeners, the... 

The few exaaqples of the redox aptitude of metal clusters here reported are sufficient to put in evidence that electrochemistry must becspectroscopic techniques do. As a matter of fact, the electrochemical investigation not only provides with an unequivocal fingerprint of the cluster, but also gives a prompt information about the existence and the relative stability of redox congeners, which otherwise requires laborious chemical manipulations. 

The redox properties elicited for Rh(bpy)3 + and its congeners are thus entirely consistent with the description of these species as bound-ligand radicals. On the other hand, the disproportionation reactions eq 2-6 are not known to be characteristic of ligand-centered radicals, but are consistent with behavior expected for rhodium(II). Furthermore the substitution lability deduced for Rh(bpy)3 + and Rh(bpy)2 +> while consistent with that expected for Rh(II), is orders of magnitude too great for Rh(lII). Finally the spectrum observed for the intermediate Rh(bpy)3 + is not that expected for [RhIII(bpy)2(bpy")]2+. The spectrum measured has an absorption maximum at 350 nm with e 4 x 10 M 1 cm l and a broad maximum at 500 nm with e = 1 x 1()3 M 1 cm l. The spectra of free and bound bpy radical anions are quite distinctive (23.35-38) very intense absorption maxima (e 1 x 10 to 4 x 10 M - cm l) are found at 350-390 nm and are accompanied by less intense maxima (e 5 x 10 cm ) at 400 to 600 nm. While the Rh(bpy)3 +... 

The corresponding neutral congener possesses a substantially similar geometry. Table 10 compiles the most significant bond lengths of the redox couple. 

Comparison with the redox aptitude of [Zn(TPP)] (Figure 52) points out a minor redox flexibility of phthalocyaninates with respect to porphyrinates, in that in the former either the second reduction or the second oxidation are complicated by degradation of the corresponding congeners. We also note that the HOMO/LUMO separation for [Zn(Pc)] is 1.55 eV, i.e. lower than that of [Zn(TPP)] (2.15 eV). 

The heavier congeners, molybdenum and tungsten, have a less interesting redox chemistry. The emfs are small and the differences relatively unimportant. The chemistry of these elements in iso- and heteropoly adds, multiple bonds, etc. is generally of more interest (see Chapter 6)... 

As illustrated in Fig. 7.32, in CH2CI2 the two complexes display quite different redox propensities. In fact, the phenyl-substituted complex undergoes reduction to the transient cobalt(ii) congener (E° = —1.13 V), as well as a few closely-spaced irreversible oxidation steps at potential values higher than - - 0.9 V, probably ligand centered. In contrast, the ferrocenyl substituted species simply undergoes a single-stepped three-electron oxidation ( = -1-0.68 V), centered on the ferrocenyl... 

Due to their structural and electrochemical properties, it would appear almost obvious to combine ferrocene-containing fragments and derivatives of tetra-thiafulvalene to construct new donors for conducting CT complexes. This approach would lead to new multistage redox systems that are likely to display different solid state properties to those of their congeners. However, only very few derivatives of this type have been so far reported. What appears to be the first compound belonging to this class was prepared by Ueno et al. in 1980 [64]. Bis(ferrocenyl)tetrathiafulvalene was obtained as a transjcis isomeric mixture (6 and 7, respectively) and was shown to form 1 1 CT complexes with TCNQ and DDQ. These materials possess... 

Some mixed-valence systems are sufficiently stable to be isolated, such as Prussian Blue (Fe )4[Fe"(CN)6]3 or the much studied Creutz-Taube ion (p-pz)[Ru(NH3)5]2 (1), pz=pyrazine Flowever, in many (but not al ) cases they are less robust than their homovalent congeners, often being formed at inconveniently high or low redox potentials, or existing only in a very limited electrochemical potential range as quantified by a small comproportionation constant K. ... 

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