Nonheme FeIV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 1 ), [N-bis­(1-methyl-2-imidazolyl)­methyl-N-(bis-2-pyridylmethyl)­amine] (L 2 ), (N-(isoquinolin-3-ylmethyl)-1,1-di­(pyridin-2-yl)-N-(pyridin-2-ylmethyl)­methanamine (L 3 ), and N,N-bis­(isoquinolin-3-ylmethyl)-1,1-di­(pyridin-2-yl)­methanamine (L 4 ), 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 [FeII(CH3CN)­(L 1 )]2+ (1a), [FeII(CH3CN)­(L 2 )]2+ (2a), [FeII(CH3CN)­(L 3 )]2+ (3a), and [FeII(CH3CN)­(L 4 )]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–Namine distances observed by crystallography, which are 2a > 1a ≥ 4a > 3a ≥ [Fe­(N4Py)­(CH3CN)]2+. The corresponding ferryl complexes [FeIV(O)­(L 1 )]2+ (1b), [FeIV(O)­(L 2 )]2+ (2b), [FeIV(O)­(L 3 )]2+ (3b), and [FeIV(O)­(L 4 )]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) FeIVO complexes. UV/vis spectra of the four FeIVO complexes in acetonitrile show typical long-wavelength absorptions of around 700 nm, which are expected for FeIV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 ([FeIV(O) (N4Py)]2+), indicating that the replacement of the pyridyl arms with (N-methyl) imidazolyl moieties makes L 1 and L 2 exert weaker ligand fields than the parent N4Py ligand, while the ligand field strengths of L 3 and L 4 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 [FeIV(O)­(N4Py)]2+ complex: the half-life sequence at room temperature is 1.67 h (