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] (L1), [N-bis(1-methyl-2-imidazolyl)methyl-N-(bis-2-pyridylmethyl)amine] (L2), (N-(isoquinolin-3-ylmethyl)-1,1-di(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanamine (L3), and N,N-bis(isoquinolin-3-ylmethyl)-1,1-di(pyridin-2-yl)methanamine (L4), 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)(L1)]2+ (1a), [FeII(CH3CN)(L2)]2+ (2a), [FeII(CH3CN)(L3)]2+ (3a), and [FeII(CH3CN)(L4)]2+ (4a) have been synthesized and characterized. The half-wave potentials (E1/2) 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)(L1)]2+ (1b), [FeIV(O)(L2)]2+ (2b), [FeIV(O)(L3)]2+ (3b), and [FeIV(O)(L4)]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 L1 and L2 exert weaker ligand fields than the parent N4Py ligand, while the ligand field strengths of L3 and L4 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 (2b) < 16 h (1b) < 45 h (4b) < 63 h (3b) ≈ 60 h ([FeIV(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 FeII complexes display good performance in catalytic reactions involving both HAT and OAT reactions.


Synthesis of complexes
If not otherwise specified，the syntheses described below were performed at room temperature in a glovebox.

Synthesis of complexes 1a-4a•(ClO 4 ) 2
The Fe II complexes 1a•(ClO 4 ) 2 , 2a•(ClO 4 ) 2 , 3a•(ClO 4 ) 2 , and 4a•(ClO 4 ) 2 were prepared using the same method: 0.2 mmol of each ligand (L 1 /L 2 /L 3 /L 4 ) was combined with one equivalent of Fe(ClO 4 ) 2 •xH 2 O (0.2 mmol) in 3ml anhydrous and degassed acetonitrile and the resultant mixture was stirred overnight at room temperature in a glovebox.After that, the concentrated solvent mixture was transferred into a test tube and the relevant Fe II complex was crystallized by liquid layer diffusion of degassed diethyl ether into the concentrated solution inside a glove box, after which the complex could be isolated as an air-stable solid (yields:～60%).

Synthesis of complexes 1a-4a•(OTf) 2
To a solution of 0.2 mmol of each ligand (L 1 /L 2 /L 3 /L 4 ) in anhydrous and degassed acetonitrile (3 mL), one equivalent of Fe(OTf) 2 •2MeCN was added.After stirring for overnight, the resultant mixture was transferred into a test tube and the relevant Fe II complex was crystallized by liquid layer diffusion of degassed diethyl ether into the concentrated solution inside a glove box, after which the complex could be isolated as an air-stable solid (yields:～55%).

Figure S4 .
Figure S4.The HSQC NMR spectrum of ligand L 2 in d 6 -DMSO measured at 298 K.

Figure S22 .
Figure S22.Cyclic voltammograms of 2a•(ClO 4 ) 2 in acetonitrile at different scan rates.The very small impurity peak near -0.4V was observed on both glassy carbon and platinum working electrodes.

Figure S27 .
Figure S27.Cyclic voltammograms of 4a•(ClO 4 ) 2 in acetonitrile at 50 mV/s with varying scan widths using a glassy carbon working electrode.Scanning negative, even if a reduction peak is not observed, results in irreversible oxidation peaks near -0.2 V vs Fc/Fc+.Similar results are observed for all the complexes, and can be observed in FigureS26depending on the starting potential of the cyclic scan.

Figure S29 .
Figure S29.Determination of the kinetic isotope effect (KIE) for separate reactions with toluene and d 8 -toluene with complex 4 at room temperature.
(ClO 4) 2 in d 6 -DMSO measured at 298 K Isomer shifts are given relative to iron metal at ambient temperature.
Simulation of the experimental data was performed with the MX program by diagonalization of the spin Hamiltonian for the electronic and nuclear spins: E. Bill, Max-Planck Institute for Chemical Energy Conversion, Mülheim/Ruhr, Germany.

Table S9 .
Selected bond lengths and angles for the calculated 4a within PBE0/Def2-TZVP(-f) level of theory and some crystallographic data values for comparison.The numbering of atoms is the same as used for the crystallographic data.