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The deprotonation-induced valence inversion is a concerted mechanism


The work undertaken in this study was aimed at deciphering the mechanism of an electron transfe​​r associated to a proton transfer within a dinuclear iron complex. The results obtained allow to propose that in the observed reaction the transfers of the electron and proton are concerted.​
Published on 5 March 2012

A huge number of processes essential to the living world (respiration, oxygen activation for instance) involve (multi)electronic transfer often over very large distances. Similarly, electron transfers are involved in very numerous chemical and catalytic reactions. Charge accumulation due to electron transfers constitutes a very large energetic penalty. It can be avoided by linking these electron transfers to proton transfers. This was recently demonstrated in many cases such as the oxidation of the tyrosine TyrH into the tyrosyl radical Tyr•. Therefore studying the mechanisms and the energetic consequences of these coupled electron and proton transfers constitute a research field very active by its implications in biology and for energy conversion processes [1].

A major project of the Physicochemistry of metals in biology team of the Chemistry and Biology of Metals Laboratory focuses on the electronic structure and reactivity of the dinuclear active sites of iron enzymes. In the course of these studies, a mixed-valent
FeIIFeIII complex has been isolated (Figure 1) where the FeII ion (drawn in green) is bound by an aniline ligand whose nitrogen (N20) is doubly protonated.

X-ray structure of the dicationic mixed-valent complex [FeIIIFeII(L-BnNH2)(mpdp)]2+. The FeIII ion is shown in pink, the FeII in green.

Physicochemical studies, based in particular on NMR and Mössbauer spectroscopies, have shown that in the presence of bases the aniline ligand is deprotonated to give an anilide bound to the Fe
III ion. Since the resulting complex is still a mixed-valent species the aniline deprotonation has induced an electron transfer within the mixed-valent pair, as depicted in scheme 1.

Scheme 1
Aniline deprotonation causes an electron transfer within the mixed-valent pair.

This process is reversible through addition of an acid. It is the second example reported in literature of a valence inversion induced by (de)protonation [2]. The only other one is found in hemerythrin, an oxygen carrier in invertebrates, whose mixed-valent forms invert as a function of pH [3].

This process can thus be viewed as an electron transfer associated to a proton transfer. The mechanism of this transfer has been studied in collaboration with the Laboratoire d'Electrochimie Moléculaire of the University of Paris 7.
Scheme 2 (M1 = M2 = Fe, n = 2) summarizes the three mechanistic possibilities that can occur: sequential transfers of electron and then proton (EPT) or the reverse (PET) or concerted transfer of electron and proton (CPET).

Scheme 2
The three mechanistic possibilities:
• EPT sequential transfers of electron and then proton,
• PET the reverse order in sequential transfers and
• concerted transfer of electron and proton.

In-depth electrochemical studies of the reaction, supported by mathematical simulations of the kinetics of the involved reactions, have been performed. They support that a concerted transfer of electron and proton is occurring in this reaction. This conclusion was confirmed by the observation that the kinetics of the transfer is different in the presence of CH
3OH and CD3OD. In presence of CD3OD, the aniline is deuterated and the overall process is slowed down by a factor of 2 owing to the fact that a deuteron transfer is slower than a proton transfer. The electrochemical methodology developed in this work has a general value and can be used in many other cases [4].

This work developed within the frame of the ANR Bi3Pro is aimed at deciphering the mechanism of a pH induced intramolecular electron transfer within a dinuclear iron complex. Accordingly, this project addresses important fundamental issues and may give rise to potential applications. It is being pursued by considering the different parameters that influence the overall process, with a particular emphasis put on the redox potentials of the two iron sites and the acidity of the protic ligand.

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