Mixed-valent derivatives

AE° = 120 mV) allows a Aicom of about 102 to be calculated. Therefore, the corresponding monocation should be classified as a Class I/II mixed-valent derivative, in which the two ferrocene groups are substantially non-communicating. 

It is interesting to note that these complexes are mixed-valent MnmMnIV complexes. Based on the relative structural data [the bond distances of the MnA atom are shorter than those of MnB], it has been concluded that in [Mn202(bipy)4]3+ one of the manganese ions is in the oxidation state IV [Mn(B)] and the other in the oxidation state III [Mn(A)]. Hence, the complex would have to be classified as a mixed-valent derivative with localized charge (Robin-Day Class I). Conversely, the two manganese sites are identical in [Mn202(phen)4]" +, from which one can infer that the charge is delocalized over the two centres (Robin-Day Class III). 

McCormick JM, Reem RC, Solomon EL 1991. Chemical and spectroscopic studies of the mixed-valent derivatives of the noriheme iron protein hemerythrin. J Am Chem Soc 113 9066-9079. 

Fig. 5. Plot of apparent electron self exchange rate constants kf P, derived from polymer De values for films containing the indicated metals, mixed valent states, and ligands, all in acetonitrile, using Equation 2, vs. literature heterogeneous electron transfer rate constants k° for the corresponding monomers in nitrile solvents. See Ref. 6 for details. (Reproduced from Ref. 6. Copyright 1987 American Chemical Society.)...

Our interest towards such derivatives is dictated by the fact that they are mixed-valent complexes, which means that electrons may easily flow along their metal frame. 

Reduction of the met derivatives by one electron produces the mixed-valent 1/2-met (Cu(II)Cu(l)] derivatives. A comparison of the chemical and spectroscopic properties of 1/2-met hemocyanln and 1/2-met T2D laccase reveals a number of important differences (Figure 12). 

Methylococcus capsulatus, diferric iron cluster, 43 362-363 mixed-valent state, 43 389 s derived from reaction mechanism, 43 391-393... 

A series of hemocyanin and tyrosinase active site derivatives (Fig. 23) can be prepared61"66), allowing systematic variation of the binuclear copper active site and chemical perturbation for spectral studies. In the simplest derivative, met-apo, one copper has been removed and the remaining copper oxidized to the spectroscopically accessible Cu(II). Next in complexity is a mixed-valent binuclear copper site. The Cu(II), in this half-met derivative, exhibits open-shell d9 spectroscopic features and the Cu(I), though spectroscopically inaccessible, can still be studied by comparison to the met-apo derivative. Two derivatives have formally binuclear cupric sites met, which is EPR-non-detect-able, and dimer, which exhibits an intense broad EPR signal. Spectroscopic study of these derivatives has led to the present picture of the coupled binuclear copper protein active site shown at the bottom of Fig. 23. 

Several Mn2(II,II) complexes exist with different bridging patterns including solely carboxylates, or those associated with aqua, hydroxo, alkoxo, or phenoxo bridges.Conversely, mixed-valent Mn2(II,III) derivatives have received much less attention. 

Potentially bidentate ligands, such as the formamidinates [RNCR NR], the monoanion derived from 2,6-bis(phenylamino)pyridine and 7-methyl-l,8-naphthyridin-2-one, and others, formed complexes with a paddle wheel structural motif. Figure 5(d), and some of these complexes could be oxidized by ferrocenium cations to give mixed-valent species. Generally, such oxidation led to a slight lengthening of the metal-metal bond. 

The same reasons for the interest in incorporating ferrocene units into the side group structure have also provided motivation for the synthesis of polymers in which ferrocenyl moieties are part of the main chain. An important additional reason for the synthesis of polymers with skeletal ferrocenyl units is provided by the observation that in molecular species in which two ferrocene units are linked together in close proximity the iron atoms can often interact to yield delocalized, mixed-valent species on one-electron oxidation. For example, the mixed-valence cations derived... 

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