X-ray Absorption Spectroscopy and Reactivity of Thiolate-Ligated Fe^III−OOR Complexes

Abstract

The reaction of a series of thiolate-ligated iron(II) complexes [FeII([15]aneN4)(SC6H5)]BF4 (1), [FeII([15]aneN4)(SC6H4-p-Cl)]BF4 (2), and [FeII([15]aneN4)(SC6H4-p-NO2)]BF4 (3) with alkylhydroperoxides at low temperature (−78 °C or −40 °C) leads to the metastable alkylperoxo-iron(III) species [FeIII([15]aneN4)(SC6H5)(OOtBu)]BF4 (1a), [FeIII([15]aneN4)(SC6H4-p-Cl)(OOtBu)]BF4 (2a), and [FeIII([15]aneN4)(SC6H4-p-NO2)(OOtBu)]BF4 (3a), respectively. X-ray absorption spectroscopic studies (XAS) were conducted on the FeIII-OOR complexes and their iron(II) precursors. The edge energy for the iron(II) complexes (~7118 eV) shifts to higher energy upon oxidation by ROOH, and the resulting edge energies for the FeIII-OOR species range from 7121 – 7125 eV and correlate with the nature of the thiolate donor. EXAFS analysis of the iron(II) complexes 1 – 3 in CH2Cl2 show that their solid state structures remain intact in solution. The EXAFS data on 1a – 3a confirm their proposed structures as mononuclear, 6-coordinate FeIII-OOR complexes with 4N and 1S donors completing the coordination sphere. The Fe-O bond distances obtained from EXAFS for 1a – 3a are 1.82 – 1.85 Å, significantly longer than other low-spin FeIII-OOR complexes. The Fe-O distances correlate with the nature of the thiolate donor, in agreement with the previous trends observed for ν(Fe-O) from resonance Raman (RR) spectroscopy, and supported by optimized geometries obtained from density functional theory (DFT) calculations. Reactivity and kinetic studies on 1a – 3a show an important influence of the thiolate donor.

Acid‐Triggered O−O Bond Heterolysis of a Nonheme FeIII(OOH) Species for the Stereospecific Hydroxylation of Strong C−H Bonds

“…Compound 3 exhibits a rising edge energy at 7123.4 eV, % 2eVh igher than that found for the startingi ron(II) compound 1,c onsistent with an iron(III) center. [46][47][48][49] Table S1). In particular, the 1.82-scatterer found for 3 can be assigned to the O-atom of an HOO ligand, as observed at 1.76 and1 .81 in the EXAFS analysis of two relatedl owspin Fe III (OOH) complexes.…”

“…This assignment is further supported by XAS (X‐ray absorption spectroscopy) data (Figure ). Compound 3 exhibits a rising edge energy at 7123.4 eV, ≈2 eV higher than that found for the starting iron(II) compound 1 , consistent with an iron(III) center . Furthermore, EXAFS analysis of 3 shows a six‐coordinate metal center with 0.6 N/O atom at 1.82 Å (to match the percentage of 3 present in the sample), 2 N/O atoms at 1.97 Å and 3 N/O atoms at 2.17 Å (see Supporting Information, Table S1), compared to 1 , which has a first coordination sphere having 2 N/O atoms at 2.05 Å and 4 N/O atoms at 2.19 Å (Figure S1, Table S1).…”

Structural and Spectroscopic Characterization of Metastable Thiolate-Ligated Manganese(III)–Alkylperoxo Species

“…50–53 The Mn-O-O angle (109.2(4)°) in 2a is remarkably acute relative to most metal-alkylperoxo species, 51–53 and similar to that (105.1(2)°) of the only other structurally characterized Mn–alkylperoxo, Tp tBu,iPr Mn II (O 2 CMe 2 Ph). 53 This suggests that there is little to no π-bonding interaction between the S=2 Mn(III) center of 2a and the proximal peroxide oxygen.…”

Thioether-ligated iron(ii) and iron(iii)-hydroperoxo/alkylperoxo complexes with an H-bond donor in the second coordination sphere

“…1 also include a sulfur donor in the first coordination sphere, but the incorporation of sulfur or other heteroatoms in model complexes is rare. 23,25–41 In addition, little work has been done to determine the effect of secondary sphere interactions, such as hydrogen-bonding networks, on the structures and spectroscopic properties of iron-peroxo complexes, 42,43 despite their roles in many of the corresponding enzyme intermediates. Recently our lab has focused on determining the first and second coordination sphere effects on non-heme iron model complexes, including the incorporation of S donors in the first coordination sphere, and oxidizable substrates or H-bond donors in the second coordination sphere.…”