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 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 [Fe II (CH 3 CN)( L 1 )] 2+ ( 1a ), [Fe II (CH 3 CN)( L 2 )] 2+ ( 2a ), [Fe II (CH 3 CN)( L 3 )] 2+ ( 3a ), and [Fe II (CH 3 CN)( 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)(CH 3 CN)] 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)(CH 3 CN)] 2+ . The corresponding ferryl complexes [Fe IV (O)( L 1 )] 2+ ( 1b ), [Fe IV (O)( L 2 )] 2+ ( 2b ), [Fe IV (O)( L 3 )] 2+ ( 3b ), and [Fe IV (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 CH 3 CN 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 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 [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 H 2 O 2 as an oxidant, all four new Fe II complexes display good performance in catalytic reactions involving both HAT and OAT reactions.