A Diiron Center Stabilized by a Bis‐TPA Ligand as a Model of Soluble Methane Monooxygenase: Predominant Alkene Epoxidation with H₂O₂

Abstract

Zweispänner: Der Dieisen(II)‐Komplex [Fe2(6‐HPA)(O)(OH2)2](ClO4)4 (1) katalysiert die Alkenepoxidierung mit H2O2 über einen Peroxodieisenkomplex. Ausführliche Isotopenmarkierungsexperimente mit μ‐18O‐1 ergaben, dass sowohl aus dem Oxo‐ als auch aus dem Peroxoliganden stammende Sauerstoffatome in das Epoxidprodukt eingebaut werden (siehe Schema). 6‐HPA=1,2‐Bis[2‐{bis(2‐pyridylmethyl)aminomethyl}‐6‐pyridyl]ethan.

Is the μ‐Oxo‐μ‐Peroxodiiron Intermediate of a Ribonucleotide Reductase Biomimetic a Possible Oxidant of Epoxidation Reactions?

“…In strongly distorted electronic systems in which the two iron atoms have different spin states, the peroxo group moves considerably out of the plane of the moxodiiron group due to orbital rearrangements. The calculated absorption spectra of 1,11 1 are in good agreement with experimental studies on biomimetics and RNR enzyme systems. Moreover, vibrational shifts in the spectrum due to 18 O 2 substitution of the oxygen atoms in the peroxo group follow similar trends as experimental observations.…”

“…reacts with alkenes to generate epoxides efficiently. [11] More recent studies by Suzuki et al on peroxodi-A C H T U N G T R E N N U N G iron complexes with a bridging carboxylate ligand gave different reactivity patterns for a bridging Ph 3 CCOO À versus Ph 3 COO À group. [12] In particular, the system with Ph 3 COO À ligand gave regioselective phenyl hydroxylation, whereas the one with Ph 3 CCOO À ligand exhibited reversible deoxygenation.…”

“…The undecaplet and open-shell singlet structures have the two metal 3d blocks completely singly occupied, and the five unpaired electrons on each iron center are ferromagnetically or antiferromagnetically coupled (Figure 2 b). Thus, 11 1 and 1 1 essentially can be seen as two neighboring sextet iron atoms: In the quintet spin state, there is also a doublet spin on Fe1, but it is ferromagnetically coupled to a quartet spin on Fe2 with p* xy 2 p* FeO 1 p* FeOO 1 s* z 2 1 occupation. Finally, the triplet spin state structure represents the ferromagnetically coupled system with a doublet spin on each metal atom, whereby one center has p* xy 2 p* FeO 2 p* FeOO 1 occupation, whereas the other p* xy 2 p* FeO 1 p* FeOO 2 occupation so that the two radicals are on opposite sides of the Fe 2 O 3 ring.…”

“…Geometrically, the ironligand distances show some fluctuations between the various spin states due to differences in charge and spin populations on the two iron centers. For instance, the p* FeO orbital on Fe1 is singly occupied in 11 1, but doubly occupied in other spin states; as a result a much longer Fe À O distance is obtained in 11 1 than in 3,5,7 1. Furthermore, single occupation of the two s* orbitals in 7 1 leads to enhanced distances between Fe2 and its ligands due to more antibonding character into these bonds, whereas the corresponding bonds on Fe1 are much shorter.…”

Biomimetic Iron‐Catalyzed Asymmetric Epoxidations: Fundamental Concepts, Challenges and Opportunities

“…Precedent for the equilibrium between peroxodiironA C H T U N G T R E N N U N G (III) 11 and dioxodiiron(IV) species 14 has been demonstrated with Fe complexes of HPA Figure 3) ligands. [147] This illustrates a clear discrepancy between enzymatic reactions which allow epoxidation by a peroxide (sMMOHperoxo) while high valent iron is needed by synthetic diiron catalysts for the same transformation. [147] This illustrates a clear discrepancy between enzymatic reactions which allow epoxidation by a peroxide (sMMOHperoxo) while high valent iron is needed by synthetic diiron catalysts for the same transformation.…”