Variation of the Molecular and Electronic Structures of μ‐Oxo Diferric Complexes with the Bridging Motive

Abstract

The complexes [(susan){FeIII(OAc)(μ‐O)FeIII(OAc)}](ClO4)2, [(susan){FeIII(μ‐O)(μ‐OAc)FeIII}](ClO4)3, and [(susan){FeIII(μ‐O)(μ‐CO3)FeIII}](ClO4)2 (susan = 4,7‐dimethyl‐1,1,10,10‐tetra(2‐pyridylmethyl)‐1,4,7,10‐tetraazadecane) were synthesized and characterized. Prominent IR vibrations do not shift from the solid state to CH3CN solutions demonstrating dissolution without structural rearrangements or substitutions. The acetates in [(susan){FeIII(OAc)(μ‐O)FeIII(OAc)}]2+ are in a trans conformation resulting in a C2 symmetry of the central core and the overall complex. On the other hand, a second bridging ligand enforces a CS symmetry of the central core, which is incommensurable with the wrapping of the ligand around the central core resulting in only overall C1 symmetry. The Fe–OAc bonds are shorter for the terminal acetates (1.93 Å) than for the bridging acetate (1.97 and 2.07 Å). The bridging dianionic carbonate also results in shorter Fe–Ocarb bonds (1.91 and 1.97 Å). Mössbauer spectroscopy shows a lower quadrupole splitting for [(susan){FeIII(μ‐O)(μ‐CO3)FeIII}](ClO4)2 in line with the shorter and thus more covalent Fe–Ocarb bonds. UV/Vis/NIR spectra differ in the d–d and in the LMCT regions for the mono‐ and doubly‐bridged complexes. The electrochemical characterization shows variations for the potentials for oxidations and reductions. This variation is discussed in light of the difference in π‐donor interactions and the overall charge of the complexes.

Reactivity Differences for the Oxidation of FeIIFeII to FeIII(µ‐O)FeIII Complexes Caused by Pyridyl versus 6‐Methyl‐Pyridyl Ligands

“…In accordance to the assignment of Solomon and coworkers, the transition at 17250 cm ‐1 should correspond to the 6 A 1 → 4 T 2 transition. However, in a recent study on doubly‐bridged diferric complexes of susan, we already found inconsistencies in the series for this assignment . Thus, it seems likely that the transitions between 17000 – 18000 cm ‐1 contain also some µ‐O→Fe III LMCT character.…”

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

“…We studied the μ-oxo-bridged diiron complexes intensively by magnetic, electrochemical, and spectroscopic means. [45,66,[69][70][71] It turned out, that the UV-Vis-NIR spectroscopic and magnetic properties are only slightly depending on the nature of the anionic ligand X À in the mono-μ-oxo-bridged complexes [(susan){Fe III X(μ-O)Fe III X}] 2 + . [66] However, the variation of the bridging mode from mono-bridged μ-oxo to doublybridged μ-oxo,μ-carboxylato results in strong differences in the UV-Vis-NIR spectra, which is known from tpa-based complexes.…”

“…[45,46] The electrochemical characterization of the μ-oxo-bridged diferric complexes of the ligand susan [(susan){Fe III X(μ-O)Fe III X}] 2 + reveal irreversible oxidation processes. [45,70,71] The peak potential in V vs Fc + /Fc of these oxidations span a large range depending on the exogenous ligand X: Cl À 1.48, OAc À 1.45, F À 1.40, OMe À 1.14, and OH À 0.79. It is interesting that in such an isostructural series the variation of only one ligand can influence this redox potential so strongly.…”

“…This variation demonstrates the different electron-density donation ability of the exogenous ligands. [45,46,70,71] Remarkable is the strong cathodic shift of 690 mV by substituting Cl À by OH À , which implies a strong stabilization of Fe IV by the strongly σ-and πdonating hydroxo ligand.…”

“…Going from [(susan){Fe(OAc)(μ-O)Fe(OAc)}] 2 + to [(susan){Fe(μ-O)(μ-OAc)Fe}] 3 + results in an anodic shift out of the potential window of our experiment. [70] This strong shift is mainly attributed to the change of the overall charge from 2 + to 3 + . According to the Born equation, an increase (decrease) of the total charge increases (decreases) redox potentials.…”