¹⁹F NMR study of ligand dynamics in carboxylate-bridged diiron(ii) complexes supported by a macrocyclic ligand

Abstract

A series of asymmetrically carboxylate-bridged diiron(II) complexes featuring fluorine atoms as NMR spectroscopic probes, [Fe2(PIM)(Ar4F-PhCO2)2] (10), [Fe2(F2PIM)(ArTolCO2)2] (11), and [Fe2(F2PIM)(Ar4F-PhCO2)2] (12), were prepared and characterized by X-ray crystallography, Mössbauer spectroscopy, and VT 19F NMR spectroscopy. These complexes are part of a rare family of syn-N diiron(II) complexes, [Fe2(X2PIM)(RCO2)2], that are structurally very similar to the active site of the hydroxylase enzyme component of reduced methane monooxygenase (MMOHred). Solution characterization of these complexes demonstrates that they undergo intramolecular carboxylate rearrangements, or carboxylate shifts, a dynamic feature relevant to the reactivity of the diiron centers in bacterial multicomponent monooxygenases.

Reversible Carboxylate Shift in a μ-Oxo Diferric Complex in Solution by Acid-/Base-Addition

“…This family includes bacterial multicomponent monooxygenases (BMMs), ribonucleotide reductase (RNR), Δ 9 desaturase, and human deoxyhypusine hydroxylase (h DoHH). − Besides at least one bridging carboxylate, additional carboxylates are coordinated in various coordination modes. It has been proposed that these carboxylate ligands can change their coordination modes − in a dynamic process termed carboxylate shift. , This carboxylate shift can open coordination sites for substrate or O 2 binding, store and release protons in the enzyme’s active site, or modulate the molecular and thus electronic structure for function. − An example for the carboxylate shift in a peroxo-bridged diferric biomimetic complex was reported, although other assignments have been made in the literature. ,, Furthermore, a dynamic carboxylate shift in the solid state has been established by temperature-dependent X-ray crystallography in a pyrazolate-based diferrous complex …”

Tuning the Diiron Core Geometry in Carboxylate-Bridged Macrocyclic Model Complexes Affects Their Redox Properties and Supports Oxidation Chemistry

“…As depicted in Figure , both internal and external carboxylates bind to the diiron center in the μ-1,3 mode, differing from the μ-1,3 and the μ–η 1 :η 2 carboxylate bridges observed in PIM-based diiron­(II) analogues , and sMMOH red . The Fe–O and Fe–N bond distances of the diiron core structures in 7 and 8 are very similar to those of [Fe 2 (PIM)­(RCO 2 ) 2 ] and [Fe 2 (F 2 PIM)­(RCO 2 ) 2 ].…”

“…As depicted in Figure 3, both internal and external carboxylates bind to the diiron center in the μ-1,3 mode, differing from the μ-1,3 and the μ−η 1 :η 2 carboxylate bridges observed in PIM-based diiron(II) analogues 12,13 and sMMOH red . 3 7 and 8, however, closely mimic the active site of the reduced Class I RNR R2 protein.…”

“…Because of their kinetic lability, iron carboxylate complexes tend to form polymeric species in the absence of sufficient kinetic stabilization by the ligand framework. , In addition to using multidentate ligands with simple carboxylates, the desired dinuclearity could also be achieved with the use of bulky carboxylate ligands and/or a macrocyclic ligand framework. We previously employed the latter tactic using phenol-imine macrocycles (H 2 PIM and H 2 F 2 PIM, Figure ) together with two bulky external carboxylates (RCO 2 – ). , The diiron­(II) complexes of these ligands not only provide the desired dinuclearity but also orient the two N-donors in a syn fashion with respect to the Fe···Fe vector.…”

“…We previously employed the latter tactic using phenolimine macrocycles (H 2 PIM and H 2 F 2 PIM, Figure 1) together with two bulky external carboxylates (RCO 2 − ). 12,13 The diiron(II) complexes of these ligands not only provide the desired dinuclearity but also orient the two N-donors in a syn fashion with respect to the Fe•••Fe vector.…”