Octahedral iron(iv)-tosylimido complexes exhibiting single electron-oxidation reactivity.

High valent iron species are very reactive molecules involved in oxidation reactions of relevance to biology and chemical synthesis. Herein we describe iron(iv)-tosylimido complexes [FeIV(NTs)(MePy2tacn)](OTf)2 (1(IV)[double bond, length as m-dash]NTs) and [FeIV(NTs)(Me2(CHPy2)tacn)](OTf)2 (2(IV)[double bond, length as m-dash]NTs), (MePy2tacn = N-methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane, and Me2(CHPy2)tacn = 1-(di(2-pyridyl)methyl)-4,7-dimethyl-1,4,7-triazacyclononane, Ts = Tosyl). 1(IV)[double bond, length as m-dash]NTs and 2(IV)[double bond, length as m-dash]NTs are rare examples of octahedral iron(iv)-imido complexes and are isoelectronic analogues of the recently described iron(iv)-oxo complexes [FeIV(O)(L)]2+ (L = MePy2tacn and Me2(CHPy2)tacn, respectively). 1(IV)[double bond, length as m-dash]NTs and 2(IV)[double bond, length as m-dash]NTs are metastable and have been spectroscopically characterized by HR-MS, UV-vis, 1H-NMR, resonance Raman, Mössbauer, and X-ray absorption (XAS) spectroscopy as well as by DFT computational methods. Ferric complexes [FeIII(HNTs)(L)]2+, 1(III)-NHTs (L = MePy2tacn) and 2(III)-NHTs (L = Me2(CHPy2)tacn) have been isolated after the decay of 1(IV)[double bond, length as m-dash]NTs and 2(IV)[double bond, length as m-dash]NTs in solution, spectroscopically characterized, and the molecular structure of [FeIII(HNTs)(MePy2tacn)](SbF6)2 determined by single crystal X-ray diffraction. Reaction of 1(IV)[double bond, length as m-dash]NTs and 2(IV)[double bond, length as m-dash]NTs with different p-substituted thioanisoles results in the transfer of the tosylimido moiety to the sulphur atom producing sulfilimine products. In these reactions, 1(IV)[double bond, length as m-dash]NTs and 2(IV)[double bond, length as m-dash]NTs behave as single electron oxidants and Hammett analyses of reaction rates evidence that tosylimido transfer is more sensitive than oxo transfer to charge effects. In addition, reaction of 1(IV)[double bond, length as m-dash]NTs and 2(IV)[double bond, length as m-dash]NTs with hydrocarbons containing weak C-H bonds results in the formation of 1(III)-NHTs and 2(III)-NHTs respectively, along with the oxidized substrate. Kinetic analyses indicate that reactions proceed via a mechanistically unusual HAT reaction, where an association complex precedes hydrogen abstraction.

Oxoiron(IV) Tetramethylcyclam Complexes with Axial Carboxylate Ligands: Effect of Tethering the Carboxylate on Reactivity.

Oxoiron(IV) species are implicated as reactive intermediates in nonheme monoiron oxygenases, often acting as the agent for hydrogen-atom transfer from substrate. A histidine is the most likely ligand trans to the oxo unit in most enzymes characterized thus far but is replaced by a carboxylate in the case of isopenicillin N synthase. As the effect of a trans carboxylate ligand on the properties of the oxoiron(IV) unit has not been systematically studied, we have synthesized and characterized four oxoiron(IV) complexes supported by the tetramethylcyclam (TMC) macrocycle and having a carboxylate ligand trans to the oxo unit. Two complexes have acetate or propionate axial ligands, while the other two have the carboxylate functionality tethered to the macrocyclic ligand framework by one or two methylene units. Interestingly, these four complexes exhibit substrate oxidation rates that differ by more than 100-fold, despite having Ep,c values for the reduction of the Fe═O unit that span a range of only 130 mV. Eyring parameters for 1,4-cyclohexadiene oxidation show that reactivity differences originate from differences in activation enthalpy between complexes with tethered carboxylates and those with untethered carboxylates, in agreement with computational results. As noted previously for the initial subset of four complexes, the logarithms of the oxygen atom transfer rates of 11 complexes of the FeIV(O)TMC(X) series increase linearly with the observed Ep,c values, reflecting the electrophilicity of the Fe═O unit. In contrast, no correlation with Ep,c values is observed for the corresponding hydrogen atom transfer (HAT) reaction rates; instead, the HAT rates increase as the computed triplet-quintet spin state gap narrows, consistent with Shaik's two-state-reactivity model. In fact, the two complexes with untethered carboxylates are among the most reactive HAT agents in this series, demonstrating that the axial ligand can play a key role in tuning the HAT reactivity in a nonheme iron enzyme active site.

An Ultra-Stable Oxoiron(IV) Complex and Its Blue Conjugate Base.

Treatment of [FeII(L)](OTf)2 (4), (where L = 1,4,8-Me3cyclam-11-CH2C(O)NMe2) with iodosylbenzene yielded the corresponding S = 1 oxoiron(IV) complex [FeIV(O(L)](OTf)2 (5) in nearly quantitative yield. The remarkably high stability of 5 (t1/2 ≈ 5 days at 25 °C) facilitated its characterization by X-ray crystallography and a raft of spectroscopic techniques. Treatment of 5 with strong base was found to generate a distinct, significantly less stable S = 1 oxoiron(IV) complex, 6 (t1/2 ~ 1.5 hrs. at 0 °C), which could be converted back to 5 by addition of a strong acid; these observations indicate that 5 and 6 represent a conjugate acid-base pair. That 6 can be formulated as [FeIV(O)(L-H)](OTf) was further supported by ESI mass spectrometry, spectroscopic and electrochemical studies, and DFT calculations. The close structural similarity of 5 and 6 provided a unique opportunity to probe the influence of the donor trans to the FeIV=O unit upon its reactivity in H-atom transfer (HAT) and O-atom transfer (OAT), and 5 was found to display greater reactivity than 6 in both OAT and HAT. While the greater OAT reactivity of 5 is expected on the basis of its higher redox potential, its higher HAT reactivity does not follow the anti-electrophilic trend reported for a series of [FeIV(O)(TMC)(X)] complexes (TMC = tetramethylcyclam) and thus appears to be inconsistent with the Two-State Reactivity rationale that is the prevailing explanation for the relative facility of oxoiron(IV) complexes to undergo HAT.

Factors affecting the carboxylate shift upon formation of nonheme diiron-O2 adducts.

Several [Fe(II)2(N-EtHPTB)(µ-O2X)](2+) complexes (1·O2X) have been synthesized, where N-EtHPTB is the anion of N,N,N'N'-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane and O2X is an oxyanion bridge. Crystal structures reveal five-coordinate (µ-alkoxo)diiron(II) cores. These diiron(II) complexes react with O2 at low temperatures in CH2Cl2 (-90 °C) to form blue-green O2 adducts that are best described as triply bridged (µ-η(1):η(1)-peroxo)diiron(III) species (2·O2X). With one exception, all 2·O2X intermediates convert irreversibly to doubly bridged, blue (µ-η(1):η(1)-peroxo)diiron(III) species (3·O2X). Where possible, 2·O2X and 3·O2X intermediates were characterized using resonance Raman spectroscopy, showing respective νO-O values of ∼850 and ∼900 cm(-1). How the steric and electronic properties of O2X affect conversion of 2·O2X to 3·O2X was examined. Stopped-flow analysis reveals that oxygenation kinetics of 1·O2X is unaffected by the nature of O2X, and for the first time, the benzoate analog of 2·O2X (2·O2CPh) is observed.

Nonheme Oxoiron(IV) Complexes of Pentadentate N5 Ligands: Spectroscopy, Electrochemistry, and Oxidative Reactivity.

Oxoiron(IV) species have been found to act as the oxidants in the catalytic cycles of several mononuclear nonheme iron enzymes that activate dioxygen. To gain insight into the factors that govern the oxidative reactivity of such complexes, a series of five synthetic S = 1 [Fe(IV)(O)(L(N5))](2+) complexes has been characterized with respect to their spectroscopic and electrochemical properties as well as their relative abilities to carry out oxo transfer and hydrogen atom abstraction. The Fe=O units in these five complexes are supported by neutral pentadentate ligands having a combination of pyridine and tertiary amine donors but with different ligand frameworks. Characterization of the five complexes by X-ray absorption spectroscopy reveals Fe=O bonds of ca. 1.65 Å in length that give rise to the intense 1s→3d pre-edge features indicative of iron centers with substantial deviation from centrosymmetry. Resonance Raman studies show that the five complexes exhibit ν(Fe=O) modes at 825-841 cm(-1). Spectropotentiometric experiments in acetonitrile with 0.1 M water reveal that the supporting pentadentate ligands modulate the E(1/2)(IV/III) redox potentials with values ranging from 0.83 to 1.23 V vs. Fc, providing the first electrochemical determination of the E(1/2)(IV/III) redox potentials for a series of oxoiron(IV) complexes. The 0.4-V difference in potential may arise from differences in the relative number of pyridine and tertiary amine donors on the L(N5) ligand and in the orientations of the pyridine donors relative to the Fe=O bond that are enforced by the ligand architecture. The rates of oxo-atom transfer (OAT) to thioanisole correlate linearly with the increase in the redox potentials, reflecting the relative electrophilicities of the oxoiron(IV) units. However this linear relationship does not extend to the rates of hydrogen-atom transfer (HAT) from 1,3-cyclohexadiene (CHD), 9,10-dihydroanthracene (DHA), and benzyl alcohol, suggesting that the HAT reactions are not governed by thermodynamics alone. This study represents the first investigation to compare the electrochemical and oxidative properties of a series of S = 1 Fe(IV)=O complexes with different ligand frameworks and sheds some light on the complexities of the reactivity of the oxoiron(IV) unit.

Modeling the cis-oxo-labile binding site motif of non-heme iron oxygenases: water exchange and oxidation reactivity of a non-heme iron(IV)-oxo compound bearing a tripodal tetradentate ligand.

The spectroscopic and chemical characterization of a new synthetic non-heme iron(IV)-oxo species [Fe(IV)(O)((Me,H) Pytacn)(S)](2+) (2, (Me,H)Pytacn=1-(2'-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane, S=CH(3)CN or H(2)O) is described. Complex 2 was prepared by reaction of [Fe(II)(CF(3)SO(3))(2)((Me,H) Pytacn)] (1) with peracetic acid. Complex 2 bears a tetradentate N(4) ligand that leaves two cis sites available for binding an oxo group and a second external ligand but, unlike the related iron(IV)-oxo species with tetradentate ligands, it is remarkably stable at room temperature (t(1/2)>2 h at 288 K). Its ability to exchange the oxygen atom of the oxo ligand with water has been analyzed in detail by means of kinetic studies, and a mechanism is proposed on the basis of DFT calculations. Hydrogen-atom abstraction from C-H bonds and oxygen-atom transfer to sulfides by 2 have also been studied. Despite its thermal stability, 2 proves to be a very powerful oxidant that is capable of breaking the strong C-H bond of cyclohexane (bond dissociation energy=99.3 kcal mol(-1)).

Characterization of two distinct adducts in the reaction of a nonheme diiron(II) complex with O2.

Two [Fe(II)(2)(N-EtHPTB)(mu-O(2)X)](2+) complexes, where N-EtHPTB is the anion of N,N,N'N'-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane and O(2)X is O(2)PPh(2) (1 x O(2)PPh(2)) or O(2)AsMe(2) (1 x O(2)AsMe(2)), have been synthesized. Their crystal structures both show interiron distances of 3.54 A that arise from a (mu-alkoxo)diiron(II) core supported by an O(2)X bridge. These diiron(II) complexes react with O(2) at low temperatures in MeCN (-40 degrees C) and CH(2)Cl(2) (-60 degrees C) to form long-lived O(2) adducts that are best described as (mu-eta(1):eta(1)-peroxo)diiron(III) species (2 x O(2)X) with nu(O-O) approximately 850 cm(-1). Upon warming to -30 degrees C, 2 x O(2)PPh(2) converts irreversibly to a second (mu-eta(1):eta(1)-peroxo)diiron(III) intermediate (3 x O(2)PPh(2)) with nu(O-O) approximately 900 cm(-1), a value which matches that reported for [Fe(2)(N-EtHPTB)(O(2))(O(2)CPh)](2+) (3 x O(2)CPh) (Dong et al. J. Am. Chem. Soc. 1993, 115, 1851-1859). Mossbauer spectra of 2 x O(2)PPh(2) and 3 x O(2)PPh(2) indicate that the iron centers within each species are antiferromagnetically coupled with J approximately 60 cm(-1), while extended X-ray absorption fine structure analysis reveals interiron distances of 3.25 and 3.47 A for 2 x O(2)PPh(2) and 3 x O(2)PPh(2), respectively. A similarly short interiron distance (3.27 A) is found for 2 x O(2)AsMe(2). The shorter interiron distance associated with 2 x O(2)PPh(2) and 2 x O(2)AsMe(2) is proposed to derive from a triply bridged diiron(III) species with alkoxo (from N-EtHPTB), 1,2-peroxo, and 1,3-O(2)X bridges, while the longer distance associated with 3 x O(2)PPh(2) results from the shift of the O(2)PPh(2) bridge to a terminal position on one iron. The differences in nu(O-O) are also consistent with the different interiron distances. It is suggested that the O...O bite distance of the O(2)X moiety affects the thermal stability of 2 x O(2)X, with the O(2)X having the largest bite distance (O(2)AsMe(2)) favoring the 2 x O(2)X adduct and the O(2)X having the smallest bite distance (O(2)CPh) favoring the 3 x O(2)X adduct. Interestingly, neither 3 x O(2)AsMe(2) nor the benzoate analog of 2 x O(2)X (2 x O(2)Bz) are observed.

A synthetic high-spin oxoiron(IV) complex: generation, spectroscopic characterization, and reactivity.

High versus low: The high-yield generation of a synthetic high-spin oxoiron(IV) complex, [Fe(IV)(O)(TMG(3)tren)](2+) (see picture, TMG(3)tren = 1,1,1-tris{2-[N2-(1,1,3,3-tetramethylguanidino)]ethyl}amine), has been achieved by using the very bulky tetradentate TMG(3)tren ligand, in order to both sterically protect the oxoiron(IV) moiety and enforce a trigonal bipyramidal geometry at the iron center, for which an S = 2 ground state is favored.

Spectroscopic and computational studies of (mu-oxo)(mu-1,2-peroxo)diiron(III) complexes of relevance to nonheme diiron oxygenase intermediates.

With the goal of gaining insight into the structures of peroxo intermediates observed for oxygen-activating nonheme diiron enzymes, a series of metastable synthetic diiron(III)-peroxo complexes with [Fe(III)(2)(mu-O)(mu-1,2-O(2))] cores has been characterized by X-ray absorption and resonance Raman spectroscopies, EXAFS analysis shows that this basic core structure gives rise to an Fe-Fe distance of approximately 3.15 A; the distance is decreased by 0.1 A upon introduction of an additional carboxylate bridge. In corresponding resonance Raman studies, vibrations arising from both the Fe-O-Fe and the Fe-O-O-Fe units can be observed. Importantly a linear correlation can be discerned between the nu(O-O) frequency of a complex and its Fe-Fe distance among the subset of complexes with [Fe(III)(2)(mu-OR)(mu-1,2-O(2))] cores (R = H, alkyl, aryl, or no substituent). These experimental studies are complemented by a normal coordinate analysis and DFT calculations.