A dinucleating ligand system with varying terminal donor functions but without bridging donor functions: Design, synthesis, and applications for diiron complexes

摘要

The active species in metalloenzymes frequently consists of a high-valent dinuclear diiron core. Based on the success of tris(pyridylmethyl)amine- and bis(pyridylmethyl)-ethylenediamine-derived ligands for the stabilization of high-valent diiron complexes, we have started a program to further stabilize such high-valent dinuclear complexes by bis(2-hydroxybenzyl)-ethylenediamine-derived ligands H2LR2 in order to increase the electron density at the iron centers by the strong it donation of the phenolate donors to facilitate oxidation to Fe-IV. Indeed, the dinuclear complex [(Lt-Bu2)Fe-III(mu-O)Fe-III(Lt-Bu2)] can be oxidized at relatively low potentials of 0.27 and 0.44 V vs Fc(+)/Fc but these oxidations are ligand-centered leading to the phenoxyl-radical complexes [(L-center dot(t-Bu2))Fe-III(mu-O)Fe-III(Lt-Bu2)](+) and [(L-center dot(t-Bu2))Fe-III(mu-O)Fe-III(L-center dot(t-Bu2))](2+). These oxidized complexes decay with half-lives of similar to 27 min and similar to 6 h at -40 degrees C, respectively. Therefore, we have optimized our ligand design based on the requirements that (i) the oxidations are metal centered and not ligand-centered and (ii) the oxidized species do not decay into mononuclear fragments. This results in the dinucleating ligands of the second generation H(4)julia (terminal carboxylates), H(4)hilde(Me2) (terminal phenolates), and susan (terminal pyridines). The study of mu-oxo-bridged diferric complexes shows that the electronic structures are not only governed by the strong mu-oxo bridge but depend also on the terminal ligands. [(julia){Fe(OH2)(mu-O)Fe(OH2)}] and [(susan) {Fe(OH)(mu-O)Fe(OH)}](ClO4)(2) are embedded in hydrogen-bond networks. Hydrogen-bonds to the first and second coordination sphere weakens the Fe-O bonds, while hydrogen-bonds acceptors strengthen the Fe-O bonds. The complexes [(susan){Fe(OAc)(mu-O)Fe(OAc)}](2+) and [(susan)(Fe(mu-O)(mu-OAc)Fe}](3+) show a reversible carboxylate shift in solution depending on the addition of acid or base. [(hilde(Me2)) {Fe(mu-O)Fe}] and [(susan){FeCl(mu-O)FeCl}](ClO4)(2) show a catalytic reactivity in the hydroxylation of cyclohexane. In the isostructural series [(susan){FeX(mu-O)FeX}](2+) with X = Cl, F, OH, OAc, the potentials for the irreversible oxidation to Fe-IV strongly depend on the terminal ligand varying from 1.48 V for X = Cl to 0.79 V vs Fc(+)/Fc for X = OH. Complex [(julia){Fe(OH2)(mu-O)Fe(OH2)}] can be deprotonated twice to a complex that is oxidized by O-2 to a transient (FeFeIII)-Fe-IV intermediate at room temperature in aqueous solution. These results are discussed in light of a rational improvement of the ligands for the stabilization of high-valent complexes. (C) 2018 Elsevier B.V. All rights reserved.