Proton-triggered bistable bimetallic bioinspired systems – Bi3Pro

Several blocks can be identified as components of the complexes investigated here. A great advantage is the independent modification of these blocks. For instance, the chemist can modify (i) the acidic ligand that exhibits the exchangeable proton and accordingly modulate the pH value controlling the electron transfer, (ii) the environment of the Fe(II) site and that of the Fe(III) site and consequently influence the driving force of the electron transfer between the Fe(II) and the Fe(III) centres, (iii) the separation and the chemical link between the two metal centres and therefore alter the kinetics of the electron transfer, and (iv) the separation between the location of the exchangeable proton and the nearest metal site and hence impact the interaction between the proton transfer and the electron transfer. Moreover, the two forms Fe(III)Fe(II)–LH and Fe(II)Fe(III)–L present very distinctive spectroscopic signatures (colour for example) allowing a simple and straightforward identification.
This high degree of modularity allows the generation of a great variety of compounds and as a consequence the full study the key parameters that control the proton-coupled electron transfer reactions within this family of complexes.

The electrochemical studies performed up to now allowed to set up a new methodology to discriminate between a concerted transfer of the proton and the electron and a sequential mechanism. The detailed studies run on the complex containing an aniline as the acidic ligand have demonstrated a concerted process.
Reactions involving a change in the positions of the ligands on one metal center have been evidenced. These reactions lead to a difference in reactivity between the different forms of the complex. These reactions appeared to be an intrinsic feature of the mixed valent FeIIFeIII complexes synthesized within this project. This example can be helpful for the community of inorganic chemists.
On the theoretical aspects, the precise description of the charge transfer systems took benefit from the existence of these kind of systems that are well characterized. In particular, the energetic cost associated to the FeIIIFeII ? FeIIFeIII conversion has been determined by original theoretical calculations. The electronic relaxation and of the choice of the molecular orbitals appeared to be crucial.

The physical properties of the Fe(III)Fe(II)–LH and Fe(II)Fe(III)–L compounds are very distinctive and discriminant. As a result, these systems present all the required capacity for molecular switches, the switch being controlled by a change in pH.
Unexpectedly, the theoretical work may have implications in a research field that is in great upheaval, where in particular sub-femtosecond spectroscopies are concerned.

Two publications have already been published and two have been recently submitted. The detailed study on the first characterized complex with an aminobenzyl group was published in 2012. The performed electrochemical studies clearly demonstrated a concerted process.

This project is aimed at deciphering the mechanism of a pH-triggered intramolecular electron transfer within a binuclear iron complex. This proton-coupled electron transfer (PCET) bears relevance to a number of biological redox events intervening in the function of non heme diiron enzymes involved in dioxygen metabolism. Alternatively, this system can be viewed also as a pH-triggered molecular switch. Thus, this project crosses important fundamental and potentially applied issues and the detailed understanding that one will gain from this system will benefit to a large scientific community.
This peculiar system was discovered in the course of our biomimetic studies of diiron enzymes and associates a mixed valent (MV) iron pair to a protic ligand (aniline, NH2-L) that is bound to a single iron and can be deprotonated. The remarkable fact in this system is that the deprotonation of the aniline by an external base induces a switch of the valences within the iron pair: FeIIIFeII-(NH2-L) + B -> FeIIFeIII-(NH-L) + BH+ and that this reaction is reversible. Only two examples of such pH-triggered electron transfer have been reported in the literature but the processes were not analyzed in depth.
This system associates two different aspects: the intervalence transfer characteristic of MV species and its induction by proton exchange. It must be noted that the dissymetrical nature of the system makes it more difficult to study than the usual MV systems described so far. General consideration allows one to identify as critical parameters that will govern this PCET the acid strength of the aniline ligand, the redox potentials of each Fe ion and their ability to communicate with each other. While the first two parameters will affect the thermodynamics of the electron transfer, the last one will principally control its kinetics.
An utmost advantage of this system resides in the modular nature of the binucleating ligand used to complex the iron pair. Indeed, the binucleating ligand binds one Fe through a three-nitrogen arm and the other Fe through a different three-nitrogen arm that incorporates the aniline. The two iron ions communicate through a bridging phenolate to which the nitrogen arms are connected and an additional dicarboxylate. Both the arms and the bridges can be varied independently so as to tune the redox potential of each iron, the pKa of the aniline, and the Fe-Fe distance. A second advantage resides in its spectroscopic reachness that provides distinct signatures for the two forms in a number of spectroscopies: UV-visible, EPR, NMR, Mössbauer. A third advantage is the fact that these two forms can be reduced or oxidized producing a number of closely related species that can be used to study the electron transfer.
The project associates the expertises in synthesis and spectroscopic techniques, in electrochemistry and in theoretical chemistry of the three groups to address the fundamental question of the intramolecular PCET in binuclear iron species along three complementary aspects. First, the intrinsic ET within the MV system will be considered on non protic symmetric and dissymetric systems where the redox potentials of each Fe ion and their distance will be varied. The results derived from spectroscopic techniques will be validated by extensive calculations at the CAS level. Second, the thermodynamics of the PCET will be studied on systems where the redox potentials of each Fe ion and the pK of the aniline will be varied. These effects will be analyzed by combining spectroscopic and electrochemical techniques and the conclusions validated by quantum calculations. Third, the kinetics of the PCET will be analyzed by combining optical stopped flow and electrochemical techniques supported by H/D exchange.
It is anticipated that the detailed analyses of all relevant paremeters will eventually allow us to describe the global PCET in terms of the specific properties (redox potential, pKa, …) of every constituant of the system.