Nonheme Fe(IV) Oxo Complexes of Two New Pentadentate Ligands and Their Hydrogen-Atom and Oxygen-Atom Transfer Reactions

Abstract

Two new pentadentate {N5} donor ligands based on the N4Py (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) framework have been synthesized, viz. [N-(1-methyl-2-benzimidazolyl)methyl-N-(2-pyridyl)methyl-N-(bis-2-pyridyl methyl)amine] (L(1)) and [N-bis(1-methyl-2-benzimidazolyl)methyl-N-(bis-2-pyridylmethyl)amine] (L(2)), where one or two pyridyl arms of N4Py have been replaced by corresponding (N-methyl)benzimidazolyl-containing arms. The complexes [Fe(II)(CH3CN)(L)](2+) (L = L(1) (1); L(2) (2)) were synthesized, and reaction of these ferrous complexes with iodosylbenzene led to the formation of the ferryl complexes [Fe(IV)(O)(L)](2+) (L = L(1) (3); L(2) (4)), which were characterized by UV-vis spectroscopy, high resolution mass spectrometry, and Mössbauer spectroscopy. Complexes 3 and 4 are relatively stable with half-lives at room temperature of 40 h (L = L(1)) and 2.5 h (L = L(2)). The redox potentials of 1 and 2, as well as the visible spectra of 3 and 4, indicate that the ligand field weakens as ligand pyridyl substituents are progressively substituted by (N-methyl)benzimidazolyl moieties. The reactivities of 3 and 4 in hydrogen-atom transfer (HAT) and oxygen-atom transfer (OAT) reactions show that both complexes exhibit enhanced reactivities when compared to the analogous N4Py complex ([Fe(IV)(O)(N4Py)](2+)), and that the normalized HAT rates increase by approximately 1 order of magnitude for each replacement of a pyridyl moiety; i.e., [Fe(IV)(O)(L(2))](2+) exhibits the highest rates. The second-order HAT rate constants can be directly related to the substrate C-H bond dissociation energies. Computational modeling of the HAT reactions indicates that the reaction proceeds via a high spin transition state.

Mono- and binuclear non-heme iron chemistry from a theoretical perspective

“…These were dominant criteria in the aforementioned examples of improving the reactivity of existing iron-oxo complexes [152,153] (i.e. blocking the axial position for acetonitrile and employing weaker donor ligands).…”

Crystallographic Evidence for a Sterically Induced Ferryl Tilt in a Non‐Heme Oxoiron(IV) Complex that Makes it a Better Oxidant

“…[5],e xcept for its crystal structure, D value and Fe=Ostretch reported herein. [5],e xcept for its crystal structure, D value and Fe=Ostretch reported herein.…”

“…Complexes 2 and 3 exhibit Mçssbauer spectra with parameters quite similar to those for 1 ( Table 1; Figures S9,S10), and those of 2 match those reported by Mitra et al [5] Their behavior in applied magnetic fields ( Figure S10) indicates that they are typical of S = 1non-heme oxoiron(IV) complexes described in the past fifteen years. [1a, 11] Highfrequency and -field EPR (HFEPR) and frequency-domain spectroscopy with applied fixed magnetic fields (FIR magnetic spectroscopy or FIRMS) have also been found to be useful for characterizing the electronic structures of iron(IV) complexes (Figures S11-S14).…”

“…Indeed we find that the substrate oxidation rates of 1-3 increase as the average FeÀNb ond lengthens for the Figure 3. [5] b) Correlation of log k 2 ' values for thioanisole at À40 8 8C(squares) and cyclohexane at 25 8 8C(triangles)a nd at À40 8 8C(circles) versus the average FeÀNbond lengths for 1-3 based on crystallographic data. [5] b) Correlation of log k 2 ' values for thioanisole at À40 8 8C(squares) and cyclohexane at 25 8 8C(triangles)a nd at À40 8 8C(circles) versus the average FeÀNbond lengths for 1-3 based on crystallographic data.…”

NMR Reveals That a Highly Reactive Nonheme FeIV=O Complex Retains Its Six‐Coordinate Geometry and S=1 State in Solution

“…[18][19][20][21] COSY crossp eaks were also observedu nder favorable circumstances. [18] We first presentt he 1 HNMR spectrum of [Fe IV (O)(N2Py2B)] 2+ + [16,22] (N2Py2B = N,N-bis((1-methyl-1H-benzoimidazol-2-yl)-methyl)-1,1-di(pyridin-2-yl)-methanamine); 2, Scheme 2, Figure 4a and S9, Supporting Information) to guide us in interpreting the NMR data for 1.C omplex 2 has am irror plane of symmetry that gives rise to ar elatively simple 1 HNMR spectrumw ith seven sharp signals corresponding to protons on the heterocyclic ligands; all have unit intensity except for the peak at À0.9 ppm, which represents 3h ydrogen atoms and therefore is easily assigned to the NÀCH 3 group of the N-Me-benzimidazole donor.T he peaks at 48, 9, and À19 ppm can be assigned, respectively to pyridine b-, g-, and b'-protons by analogy to peaks in the spectrum of [Fe IV (O)(N4Py)] 2+ + (Figure S11; N4Py = N,N-bis(pyridyl-2-methyl)-bis(2-pyridyl)methylamine), [18] whereas peaks at 15, 12, and 5ppm can be associated with benzimidazole C4ÀH, C5ÀH, and C6ÀHp rotons. Their assignments follow from their relaxation times T 1 andl inewidths that reflect their varying distances from the metal center (Table S6).…”