Spin States of the Ferryl Catalysts

It has been argued that for the reactivity the driving force involving the ferryl oxidant, that is, its redox potential, might be of more importance than its spin state [45]. However, information on the redox potentials of ferryl complexes only started to appear recently, and there is still some dispute on the experimental techniques to measure these potentials (see Section 6.5). Moreover, so far there is no experimental data that allows the unambiguous comparison of the oxidation 

3) Apart from the ferryl aqua ion which has been characterized by Mossbauer spectroscopy (7a), the L -based ferryl complex may be considered the first reported high-spin nonheme Fe =0 model system [7b], although a full spectroscopic characterization is still missing, and a crystal structure and full 

The first redox potentials of ferryl complexes were measured by cyclic voltammetry in dry acetonitrile [48] but the instability of the reduced form [(L)Fe -O] leads to observed data that are not unambiguous. Two other types of experiments have been described more recently to obtain valuable information on the oxidation power of ferryl complexes (i) the spectropotentiometric oxidation of the Fe - OH complex in acetonitrile with added water [49] and (ii) the titration of the ferryl complex with ferrocene derivatives (Fc) in dry acetonitrile to determine the Fc + Fe =0/Fc -1- Fe -0 electron transfer equilibrium constant and, together with the kno wn redox potential of the Fc derivative used, the Fe =0/Fe -0 potential [48b]. Note that the two potentials (i) and (ii) describe two entirely different processes, both of importance for ferryl-catalyzed oxidation reactions, that is, (i) a H -coupled electron transfer and (ii) a pure electron transfer. That is. 

4) An interestingveryrecent observation isthat oxidation of in water at 10°Cwith 

or cerflV) ammonium nitrate (see also Section 6.6.4) produces a transient (max. concentration after 0.2 s) with an absorption maximum at 804 nm at present there is no unambiguous characterization of this species (P. Comba, A. Waleska, unpublished results) [48]. 

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