Abstract
The non-heme ferryl active sites are of significant interest for their application in biomedical and green catalysis. These sites have been shown to have an S = 1 or S = 2 ground spin state; the latter is functional in biology. Low-temperature magnetic circular dichroism (LT MCD) spectroscopy probes the nature of the excited states in these species including ligand-field (LF) states that are otherwise difficult to study by other spectroscopies. In particular, the temperature dependences of MCD features enable their unambiguous assignment and thus determination of the low-lying excited states in two prototypical S = 1 and S = 2 NHFe(IV)[double bond, length as m-dash]O complexes. Furthermore, some MCD bands exhibit vibronic structures that allow mapping of excited-state interactions and their effects on the potential energy surfaces (PESs). For the S = 2 species, there is also an unusual spectral feature in both near-infrared absorption and MCD spectra - Fano antiresonance (dip in Abs) and Fano resonance (sharp peak in MCD) that indicates the weak spin-orbit coupling of an S = 1 state with the S = 2 LF state. These experimental data are correlated with quantum-chemical calculations that are further extended to analyze the low-lying electronic states and the evolution of their multiconfigurational characters along the Fe-O PESs. These investigations show that the lowest-energy states develop oxyl Fe(III) character at distances that are relevant to the transition state (TS) for H-atom abstraction and define the frontier molecular orbitals that participate in the reactivity of S = 1 vs. S = 2 non-heme Fe(IV)[double bond, length as m-dash]O active sites. The S = 1 species has only one available channel that requires the C-H bond of a substrate to approach perpendicular to the Fe-oxo bond (the π channel). In contrast, there are three channels (one σ and two π) available for the S = 2 non-heme Fe(IV)[double bond, length as m-dash]O system allowing C-H substrate approach both along and perpendicular to the Fe-oxo bond that have important implications for enzymatic selectivity.
Highlights
Mononuclear non-heme iron (NHFe) enzymes, ubiquitous in living organisms, are involved in many vital biological processes including the regulation of hypoxia, demethylation of DNA, antibiotic, and natural product biosynthesis and bioremediation, and are related to disease states.[1,2] Most of these enzymes use an S = 2 FeII center to activate 3O2 to perform various ‘difficult’ and formally spin-forbidden reactions including hydroxylation, halogenation, desaturation, and electrophilic aromatic substitution on unreactive singlet organic substrates that require the cleavage of strong aliphatic or aromatic C—H bonds
For the S = 2 species, there is an unusual spectral feature in both near-infrared absorption and MCD spectra – Fano antiresonance and Fano resonance that indicates the weak spin– orbit coupling of an S = 1 state with the S = 2 LF state. These experimental data are correlated with quantum-chemical calculations that are further extended to analyze the low-lying electronic states and the evolution of their multiconfigurational characters along the Fe–O potential energy surfaces (PESs). These investigations show that the lowest-energy states develop oxyl FeIII character at distances that are relevant to the transition state (TS) for H-atom abstraction and define the frontier molecular orbitals that participate in the reactivity of S = 1 vs. S = 2 non-heme FeIVvO active sites
14 100 1000 0.02 28 714i 506 0.25 a States are labeled as in Fig. 8. b Electronic vertical transition from 5A1 ground state to highest-intensity component of Frank-Condon progression. c Frequency of excited-state Fe–oxo stretching mode. d Distortion of excited-state geometry along Fe–oxo stretching mode calculated from eqn (1) in the text. e Electronic vertical transition from 5A1 ground state at CASPT2 equilibriumpffiffigffiffieffiffioffi metry. f Estimated from 1=2π k=μ where k obtained from fit of CASPT2 potential energy surfaces with third-order Taylor expansion surfaces. g The ΔrFe–O taken as difference between equilibrium geometries of 5A1 state and excited state. h Taken from ref. 34. i Taken from Table S2 in ref. 22
Summary
For the S = 2 species, there is an unusual spectral feature in both near-infrared absorption and MCD spectra – Fano antiresonance (dip in Abs) and Fano resonance (sharp peak in MCD) that indicates the weak spin– orbit coupling of an S = 1 state with the S = 2 LF state These experimental data are correlated with quantum-chemical calculations that are further extended to analyze the low-lying electronic states and the evolution of their multiconfigurational characters along the Fe–O PESs. These experimental data are correlated with quantum-chemical calculations that are further extended to analyze the low-lying electronic states and the evolution of their multiconfigurational characters along the Fe–O PESs These investigations show that the lowest-energy states develop oxyl FeIII character at distances that are relevant to the transition state (TS) for H-atom abstraction and define the frontier molecular orbitals that participate in the reactivity of S = 1 vs S = 2 non-heme FeIVvO active sites. His work centered on elucidating the electronic structures and reaction mechanisms of lowand high-spin Fe(IV)–oxo species in enzymes and model complexes
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