Nonheme Fe^IV═O Complexes Supported by Four Pentadentate Ligands: Reactivity toward H- and O- Atom Transfer Processes

Four new pentadentate N5-donor ligands, [N-(1-methyl-2-imidazolyl)methyl-N-(2-pyridyl)-methyl-N-(bis-2-pyridylmethyl)-amine] (L), [N-bis(1-methyl-2-imidazolyl)methyl-N-(bis-2-pyridylmethyl)amine] (L), (N-(isoquinolin-3-ylmethyl)-1,1-di(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanamine (L), and N,N-bis(isoquinolin-3-ylmethyl)-1,1-di(pyridin-2-yl)methanamine (L), have been synthesized based on the N4Py ligand framework, where one or two pyridyl arms of the N4Py parent are replaced by (N-methyl)imidazolyl or N-(isoquinolin-3-ylmethyl) moieties. Using these four pentadentate ligands, the mononuclear complexes [Fe^II(CH₃CN)(L)]²⁺ (1a), [Fe^II(CH₃CN)(L)]²⁺ (2a), [Fe^II(CH₃CN)(L)]²⁺ (3a), and [Fe^II(CH₃CN)(L)]²⁺ (4a) have been synthesized and characterized. The half-wave potentials (E_1/2) of the complexes become more positive in the order: 2a < 1a < 4a ≤ 3a ≤ [Fe(N4Py)(CH₃CN)]²⁺. The order of redox potentials correlates well with the Fe-N_amine distances observed by crystallography, which are 2a > 1a ≥ 4a > 3a ≥ [Fe(N4Py)(CH₃CN)]²⁺. The corresponding ferryl complexes [Fe^IV(O)(L)]²⁺ (1b), [Fe^IV(O)(L)]²⁺ (2b), [Fe^IV(O)(L)]²⁺ (3b), and [Fe^IV(O)(L)]²⁺ (4b) were prepared by the reaction of the ferrous complexes with isopropyl 2-iodoxybenzoate (IBX ester) in acetonitrile. The greenish complexes 3b and 4b were also isolated in the solid state by the reaction of the ferrous complexes in CH₃CN with ceric ammonium nitrate in water. Mössbauer spectroscopy and magnetic measurements (using superconducting quantum interference device) show that the four complexes 1b, 2b, 3b, and 4b are low-spin (S = 1) Fe^IV═O complexes. UV/vis spectra of the four Fe^IV═O complexes in acetonitrile show typical long-wavelength absorptions of around 700 nm, which are expected for Fe^IV═O complexes with N4Py-type ligands. The wavelengths of these absorptions decrease in the following order: 721 nm (2b) > 706 nm (1b) > 696 nm (4b) > 695 nm (3b) = 695 nm ([Fe^IV(O) (N4Py)]²⁺), indicating that the replacement of the pyridyl arms with (N-methyl) imidazolyl moieties makes L and L exert weaker ligand fields than the parent N4Py ligand, while the ligand field strengths of L and L are similar to the N4Py parent despite the replacement of the pyridyl arms with N-(isoquinolin-3-ylmethyl) moieties. Consequently, complexes 1b and 2b tend to be less stable than the parent [Fe^IV(O)(N4Py)]²⁺ complex: the half-life sequence at room temperature is 1.67 h (2b) < 16 h (1b) < 45 h (4b) < 63 h (3b) ≈ 60 h ([Fe^IV(O)(N4Py)]²⁺). Compared to the parent complex, 1b and 2b exhibit enhanced reactivity in both the oxidation of thioanisole in the oxygen atom transfer (OAT) reaction and the oxygenation of C-H bonds of aromatic and aliphatic substrates, presumed to occur via an oxygen rebound process. Furthermore, the second-order rate constants for hydrogen atom transfer (HAT) reactions affected by the ferryl complexes can be directly related to the C-H bond dissociation energies of a range of substrates that have been studied. Using either IBX ester or H₂O₂ as an oxidant, all four new Fe^II complexes display good performance in catalytic reactions involving both HAT and OAT reactions.