Iron Chemistry of a Pentadentate Ligand That Generates a Metastable Fe^III−OOH Intermediate

Abstract

In an effort to gain more insight into the factors controlling the formation of low-spin non-heme Fe(III)-peroxo intermediates in oxidation catalysis, such as activated bleomycin, we have synthesized a series of iron complexes based on the pentadentate ligand N4Py (N4Py = N,N-bis(2-pyridylmethyl)-N-(bis-2-pyridylmethyl)amine). The following complexes have been prepared: [(N4Py)Fe(II)(CH(3)CN)](ClO(4))(2) (1), [(N4Py)Fe(II)Cl](ClO(4)) (2), [(N4Py)Fe(III)OMe](ClO(4))(2) (3), and [(N4Py)(2)Fe(2)O](ClO(4))(4) (4). Complexes 1 and 2 have low- and high-spin Fe(II) centers, respectively, whereas 3 is an Fe(III) complex that undergoes a temperature-dependent spin transition. The iron centers in the oxo-bridged dimer 4 are antiferromagnetically coupled (J = -104 cm(-)(1)). Comparison of the crystal structures of 1, 3, and 4 shows that the ligand is well suited to accommodate both Fe(II) and Fe(III) in either spin state. For the high-spin Fe(III) complexes 3 and 4 the iron atoms are positioned somewhat outside of the cavity formed by the ligand, while in the case of the low-spin Fe(II) complex 1 the iron atom is retained in the middle of the cavity with approximately equal bond lengths to all nitrogen atoms from the ligand. On the basis of UV/vis and EPR observations, it is shown that 1, 3, and 4 all react with H(2)O(2) to generate the purple low-spin [(N4Py)Fe(III)OOH](2+) intermediate (6). In the case of 1, titration experiments with H(2)O(2) monitored by UV/vis and (1)H NMR reveal the formation of [(N4Py)Fe(III)OH](2+) (5) and the oxo-bridged diiron(III) dimer (4) prior to the generation of the Fe(III)-OOH species (6). Raman spectra of 6 show distinctive Raman features, particularly a nu(O-O) at 790 cm(-)(1) that is the lowest observed for any iron-peroxo species. This observation may rationalize the reactivity of low-spin Fe(III)-OOH species such as "activated bleomycin".

A Non‐Heme Iron Photocatalyst for Light‐Driven Aerobic Oxidation of Methanol

“…The 1 H NMR spectrum of 2 a in CD 3 CN (see Figure S21) is similar to that reported for its N4Py analogue29 and shows moderate paramagnetic line broadening and shift, which is consistent with strong antiferromagnetic coupling of the Fe III centers, and also further confirmed by the absence of signals in its EPR spectrum at 77 K (see Figure S22). The UV/Vis absorption spectrum of 2 a in anhydrous acetonitrile shows the strong absorption at 312 nm (see Figure S23), which has been assigned as an oxo → Fe charge‐transfer band,30 with symmetric and asymmetric bands of a near‐linear Fe−O−Fe core31 at 407 and 810 cm −1 , respectively, observed in its resonance Raman ( λ exc =355 nm) spectrum (see Figure S24).…”

“…The electronic nature of the photoreaction and the thermodynamic energies of possible dissociation products were explored by DFT methods (see the Supporting Information). In brief, the electronic structure of the μ‐oxido‐bridged dinuclear complex 2 a and all accessible spin states revealed an antiferromagnetically coupled ground state (see Table S4 in the Supporting Information), in accordance with the experimental data 29. The excited states of 2 a are predicted to result in Fe−O bond elongation owing to the charge‐transfer character of the spin‐allowed transitions to low‐lying excited states.…”

“…In non‐heme systems, the equilibrium between mononuclear and μ‐oxido‐bridged dinuclear Fe III complexes with pentadentate ligands (N4Py, P2DA, 6 ‐OC 6 H 4 ‐TPA, etc. ),29, 32, 33 has been shown earlier to be rapid. Addition of base (NaOAc) and proton sources (H 2 O or TfOH) shifts the equilibrium towards complexes, such as mononuclear Fe III −OH and Fe III −OH 2 and dinuclear Fe III −O−Fe III complexes.…”

The autoxidation activity of new mixed-ligand manganese and iron complexes with tripodal ligands

“…Often in oxidation catalysis with iron complexes, polypyridyl ligands are used [31][32][33][34]. These polypyridyl ligands are generally good σ -donors, and moderate-to-weak π -donors [35].…”

Hydrocarbon Oxidation with H2O2, Catalyzed by Iron Complexes with a Polydentate Pyridine-Based Ligand

“…Others include iron(II) complexes containing pyridine-based pentadentate ligands, with two to five pyridine rings [9][10][11][12][13][14][15][16]. These complexes can react with H 2 O 2 , forming metastable Fe-OOH intermediates, and in some cases to catalyze alkane hydroxylation [11,15].…”