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

Abstract

Abstract 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 H 2 L R 2 in order to increase the electron density at the iron centers by the strong π donation of the phenolate donors to facilitate oxidation to FeIV. Indeed, the dinuclear complex [( L t -Bu 2 )FeIII(μ-O)FeIII( L t -Bu 2 )] can be oxidized at relatively low potentials of 0.27 and 0.44\u202fV vs Fc+/Fc but these oxidations are ligand-centered leading to the phenoxyl-radical complexes [( L t -Bu 2 )FeIII(μ-O)FeIII( L t -Bu 2 )]+ and [( L t -Bu 2 )FeIII(μ-O)FeIII( L t -Bu 2 )]2+. These oxidized complexes decay with half-lives of ∼27\u202fmin and ∼6\u202fh at −40\u202f°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 H4julia (terminal carboxylates), H 4 h i l d e M e 2 (terminal phenolates), and susan (terminal pyridines). The study of μ-oxo-bridged diferric complexes shows that the electronic structures are not only governed by the strong μ-oxo bridge but depend also on the terminal ligands. [(julia){Fe(OH2)(μ-O)Fe(OH2)}] and [(susan){Fe(OH)(μ-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)(μ-O)Fe(OAc)}]2+ and [(susan){Fe(μ-O)(μ-OAc)Fe}]3+ show a reversible carboxylate shift in solution depending on the addition of acid or base. [( h i l d e M e 2 ){Fe(μ-O)Fe}] and [(susan){FeCl(μ-O)FeCl}](ClO4)2 show a catalytic reactivity in the hydroxylation of cyclohexane. In the isostructural series [(susan){FeX(μ-O)FeX}]2+ with X\u202f=\u202fCl, F, OH, OAc, the potentials for the irreversible oxidation to FeIV strongly depend on the terminal ligand varying from 1.48\u202fV for X\u202f=\u202fCl to 0.79\u202fV vs Fc+/Fc for X\u202f=\u202fOH. Complex [(julia){Fe(OH2)(μ-O)Fe(OH2)}] can be deprotonated twice to a complex that is oxidized by O2 to a transient FeIVFeIII 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.