A Dinucleating Ligand System with Varying Terminal Donors to Mimic Diiron Active Sites

Abstract

To mimic dinuclear active sites of metalloproteins, we have developed a dinucleating ligand system consisting of two tetradentate tripodal ligand compartments with varying terminal donors (carboxylates, phenolates, and pyridines). These ligands provide access to a series of μ-oxobridged diferric complexes. The spectroscopic study allows to investigate the molecular structures even in solution, e. g. depending on protonation/deprotonation of coordinated OH À and H 2 O ligands or to observe a reversible pH-dependent carboxylate-shift between terminal and bridging binding mode. The electrochemical behavior is strongly influenced by the exogenous ligands, e. g. OH À facilitates oxidation to Fe IV by 690 mV relative to Cl À. Using the terminal carboxylates and a {Fe III (μ-O) 2 Fe III } core even allows oxidation with O 2 to a high-valent species with Fe IV (S = 2). The implications of this study for further generation of highvalent or peroxo species and their utilization in catalysis is discussed.

Pathways of the Extremely Reactive Iron(IV)‐oxido complexes with Tetradentate Bispidine Ligands

“…However, depending on the ligand sphere, diiron(III) complexes can be oxidized to higher valent species with high oxidation power (e. g. model systems for the methane mono-oxygenase active site). [33,[39][40][41] The preferential formation of the dinuclear complex at low oxidant concentrations suggests that its putative precursor, the iron(IV)-oxido complex Scheme 1. Ligands and complexes discussed in this manuscript; X = Cl À or MeCN (the specification "trans-N3" will generally be omitted for simplicity since trans-N3-[(L n )Fe IV = O(X)] n + is the relevant species in the context of most discussions below).…”

“…However, depending on the ligand sphere, diiron(III) complexes can be oxidized to higher valent species with high oxidation power (e. g. model systems for the methane mono‐oxygenase active site). [ 33 , 39 , 40 , 41 ] The preferential formation of the dinuclear complex at low oxidant concentrations suggests that its putative precursor, the iron(IV)‐oxido complex [(L 1 )Fe IV =O(Cl)] + is very reactive and that, therefore, the formation of the diiron(III) decay product by a second order reaction from the ferryl complex [(L 1 )Fe IV =O(Cl)] + and unreacted iron(II) precursor is very fast. It emerges that the only chance to trap [(L 1 )Fe IV =O(Cl)] + is to use an excess of the s PhIO oxidant in order to significantly accelerate the formation reaction.…”

Molecular and Electronic Structures of a Series of Dinuclear CoII Complexes Varied by Exogeneous Ligands: Influence of π‐Bonding on Redox Potentials

“…[45] In order to mimic dinuclear metalloenzymes active sites and consequently to obtain new dinuclear catalysts we have recently developed a dinucleating ligand system comprised of two tetradentate tripodally coordinating ligand compartments with varying terminal donors that are bridged by an ethylene spacer. [46][47][48][49] One example is the ligand susan (susan = 4,7-dimethyl-1,1,10,10-tetra(2-pyridylmethyl)-1,4,7,10tetraazadecane), [46] that can be regarded as a dinucleating version of the ligand Me 2 -uns-penp [50][51][52] or of the family of tpabased ligands although one pyridine is formally substituted by one tert-amine (Scheme 1). Dinuclear iron [46][47][48][53][54][55][56][57] and copper [58] complexes of the ligand susan have already been investigated to mimic reactive intermediates in the catalytic cycles of dinuclear metalloenzymes and their reactivity.…”

“…[46][47][48][49] One example is the ligand susan (susan = 4,7-dimethyl-1,1,10,10-tetra(2-pyridylmethyl)-1,4,7,10tetraazadecane), [46] that can be regarded as a dinucleating version of the ligand Me 2 -uns-penp [50][51][52] or of the family of tpabased ligands although one pyridine is formally substituted by one tert-amine (Scheme 1). Dinuclear iron [46][47][48][53][54][55][56][57] and copper [58] complexes of the ligand susan have already been investigated to mimic reactive intermediates in the catalytic cycles of dinuclear metalloenzymes and their reactivity. The ligand susan provides a flexible coordination that allows various bridging modes, [47,48] which can be interconverted on a fast time-scale [55] as it is required for an efficient catalysis.…”

“…Dinuclear iron [46][47][48][53][54][55][56][57] and copper [58] complexes of the ligand susan have already been investigated to mimic reactive intermediates in the catalytic cycles of dinuclear metalloenzymes and their reactivity. The ligand susan provides a flexible coordination that allows various bridging modes, [47,48] which can be interconverted on a fast time-scale [55] as it is required for an efficient catalysis.…”