electrochemical generation of a nonheme oxoiron(IV) complex

The complex [Fe(IV)O(N4Py)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) has been prepared by bulk electrolysis in aqueous CH3CN and CH2Cl2, and its redox properties characterized. Bulk chronocoulometry and spectropotentiometry experiments in CH3CN show that [Fe(II)(N4Py)(NCCH3)]2+ can be oxidized quantitatively to its iron(III) derivative at an applied potential of +0.71 V vs ferrocene and then to the oxoiron(IV) complex (in the presence of added water) at potentials above +1.3 V. The E1/2 value for the Fe(IV/III) couple has been estimated to be +0.90 V from spectropotentiometric titrations in CH3CN and cyclic voltammetric measurements in CH2Cl2.     

A trigonal-bipyramidal oxorhenium(V) complex with a bidentate nitrogen-donor ligand

The oxorhenium(V) complex [ReO(Hapb)(apb)] (1) (H₂apb =?2-(2-aminophenyl)-1-benzimidazole) was prepared by reaction of trans-[ReO₂(py)₄]Cl with a twofold molar amount of H₂apb. The complex was characterized by spectroscopy and X-ray crystallography. The results show that the rhenium atom lies in a distorted trigonal-bipyramidal environment, with the two imidazole nitrogen atoms in apical positions, and the oxo and two amido nitrogens in the trigonal plane.     

Photocatalytic generation of a non-heme oxoiron(IV) complex with water as an oxygen source

The photocatalytic formation of a non-heme oxoiron(IV) complex, [(N4Py)Fe(IV)(O)](2+) [N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine], efficiently proceeds via electron transfer from the excited state of a ruthenium complex, [Ru(II)(bpy)(3)](2+)* (bpy = 2,2'-bipyridine) to [Co(III)(NH(3))(5)Cl](2+) and stepwise electron-transfer oxidation of [(N4Py)Fe(II)](2+) with 2 equiv of [Ru(III)(bpy)(3)](3+) and H(2)O as an oxygen source. The oxoiron(IV) complex was independently generated by both chemical oxidation of [(N4Py)Fe(II)](2+) with [Ru(III)(bpy)(3)](3+) and electrochemical oxidation of [(N4Py)Fe(II)](2+).     

Unexpected kinetic complexity in the formation of a nonheme oxoiron(IV) complex

The stoichiometric formation of [FeIV(O)(TPA)(NCMe)]2+ (TPA = tris(2-pyridylmethyl)amine) from the reaction of [FeII(TPA)(NCMe)2]2+ with 1 equiv. peracetic acid exhibits more kinetic complexity than might be expected from the simple stoichiometry. A multiple-pathway mechanism with an FeIV-peracetic acid species, [(TPA)FeIV(O)((H)O3CR)]2+/+, as the primary oxidant is proposed.     

A high-spin iron(IV)-oxo complex supported by a trigonal nonheme pyrrolide platform

We report the generation and characterization of a new high-spin iron(IV)-oxo complex supported by a trigonal nonheme pyrrolide platform. Oxygen-atom transfer to [(tpa(Mes))Fe(II)](-) (tpa(Ar) = tris(5-arylpyrrol-2-ylmethyl)amine) in acetonitrile solution affords the Fe(III)-alkoxide product [(tpa(Mes2MesO))Fe(III)](-) resulting from intramolecular C-H oxidation with no observable ferryl intermediates. In contrast, treatment of the phenyl derivative [(tpa(Ph))Fe(II)](-) with trimethylamine N-oxide in acetonitrile solution produces the iron(IV)-oxo complex [(tpa(Ph))Fe(IV)(O)](-) that has been characterized by a suite of techniques, including mass spectrometry as well as UV-vis, FTIR, M?ssbauer, XAS, and parallel-mode EPR spectroscopies. Mass spectral, FTIR, and optical absorption studies provide signatures for the iron-oxo chromophore, and M?ssbauer and XAS measurements establish the presence of an Fe(IV) center. Moreover, the Fe(IV)-oxo species gives parallel-mode EPR features indicative of a high-spin, S = 2 system. Preliminary reactivity studies show that the high-spin ferryl tpa(Ph) complex is capable of mediating intermolecular C-H oxidation as well as oxygen-atom transfer chemistry.     

Formation, stability, and reactivity of a mononuclear nonheme oxoiron(IV) complex in aqueous solution

A mononuclear nonheme oxoiron(IV) complex bearing a pentadentate N5 ligand was prepared in aqueous solution; the pH dependence of its stability and reactivities was reported along with the mechanistic details of sulfide oxidation by the oxoiron(IV) species.     

Nonheme oxoiron(IV) complexes of tris(2-pyridylmethyl)amine with cis-monoanionic ligands

Treatment of [Fe(IV)(O)(TPA)(NCMe)](CF3SO3)2 [TPA, N,N,N-tris(2-pyridylmethyl)amine] with 3 equiv of NR4X (X = CF3CO2, Cl, or Br) in MeCN at -40 degrees C affords a series of metastable [Fe(IV)(O)(TPA)(X)]+ complexes. Some characteristic features of the S = 1 oxoiron(IV) unit are quite insensitive to the ligand substitution in the equatorial plane, namely, the Fe-O distances (1.65-1.66 A), the energy ( approximately 7114.5 eV) and intensity [25(2) units] of the 1s-to-3d transition in the X-ray absorption spectra, and the M?ssbauer isomer shifts (0.01-0.06 mm.s(-1)) and quadrupole splittings (0.92-0.95 mm.s(-1)). The coordination of the anionic X ligand, however, is evidenced by red shifts of the characteristic near-IR ligand-field bands (720-800 nm) and spectroscopic observation of the bound anion by (19)F NMR for X = CF3CO2 and by EXAFS analysis for X = Cl (r(Fe-Cl) = 2.29 A) and Br (r(Fe-Br) = 2.43 A). Density functional theory calculations yield M?ssbauer parameters and bond lengths in good agreement with the experimental data and produce excited-state energies that follow the trend observed in the ligand-field bands. Despite mitigating the high effective charge of the iron(IV) center, the substitution of the MeCN ligand with monoanionic ligands X- decreases the thermal stability of [Fe(IV)(O)(TPA)]2+ complexes. These anion-substituted complexes model the cis-X-Fe(IV)=O units proposed in the mechanisms of oxygen-activating nonheme iron enzymes.     

First direct evidence for stereospecific olefin epoxidation and alkane hydroxylation by an oxoiron(IV) porphyrin complex

We report in this study that an oxoiron(IV) porphyrin complex bearing electron-deficient porphyrin ligand, (TPFPP)FeIV=O (TPFPP = meso-tetrakis(pentafluorophenyl)porphinato dianion), shows reactivities similar to those found in oxoiron(IV) porphyrin pi-cation radicals. In the epoxidation of olefins by the (TPFPP)FeIV=O complex, epoxides were yielded as major products; cyclohexene oxide was the sole product formed in the epoxidation of cyclohexene, and stilbenes were stereospecifically oxidized to the corresponding epoxide products. More striking results were obtained in alkane hydroxylation reactions; the hydroxylation of adamantane afforded a high degree of selectivity for tertiary C-H bonds over secondary C-H bonds, and the hydroxylation of cis-1,2-dimethylcyclohexane yielded a tertiary alcohol product with >99% retention of stereochemistry. The latter result demonstrates that an oxoiron(IV) porphyrin complex hydroxylates alkanes with a high stereospecificity. Isotope labeling studies performed with H218O and 18O2 in the olefin epoxidation and alkane hydroxylation reactions demonstrated that oxygen atoms in oxygenated products derived from the oxoiron(IV) porphyrin complex.     

Magnetic anisotropy of a high-spin octanuclear nickel(II) complex with a meso-helicate core

The octanickel(II) cluster 2b has been synthesized from the novel ferromagnetically coupled dinickel(II) metallacryptand 1b assembled from the m-phenylene-bis(oxamate) ligand. Complex 2b exhibits a dimer-of-tetramers structure, with two oxamate-bridged propeller-shaped tetranuclear units connected through three meta-substituted phenylenediamidate bridges, giving a metallacryptand core of the meso-helicate type. Complex 2b behaves as a ferromagnetically coupled dimer of two S=2 Ni(II)4 units with appreciable magnetic anisotropy.     

CrCpTp: a high-spin Cr(II) sandwich complex with a large structural distortion

CrCpTp was synthesized from the reaction of [CrCpCl](2) and KTp. Magnetic measurements indicate it to have a high-spin (S = 2) electronic configuration from 5 to 300 K. A single-crystal X-ray study reveals bond lengths typical for a high-spin configuration and a pronounced Jahn-Teller distortion. The nature of this distortion was probed by DFT calculations and the variation in bond lengths successfully reproduced. The metal-based HOMO is significantly antibonding with respect to a single pyrazolyl ring only.     

Kinetic study of the reactions of oxoiron(IV) with aromatic substrates in aqueous solutions

The kinetics of the reactions of oxoiron(IV) (FeO²⁺) with phenol, nitrobenzene, m‐, o‐, and p‐nitrophenol in 1 M HClO₄ was investigated by the stopped‐flow technique. The rate constants of these reactions decrease with increasing the one‐electron reduction potentials of the corresponding radical cations of the substrates and with the Hammett parameter of the NO₂ group in the phenol ring. A reaction mechanism is proposed, which accounts for the observed trends and for the nature of the reaction products. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 488–494, 2002     

Catalytic asymmetric conjugate reduction with ethanol: A more reactive system Pd(II)-Pr-DUPHOS complex with molecular sieves 4A

We describe herein the catalytic asymmetric conjugate reduction of α,β-unsaturated carbonyl compounds using a novel cationic Pd-ⁱPr-DUPHOS complex. In this reaction, EtOH worked well as a solvent and a reducing agent, and the reaction was completed within several hours in most cases to afford the reduced compounds almost quantitatively with modest to good enantioselectivity (up to 72% ee). It was found that the Pd-ⁱPr-DUPHOS complex was more reactive than the previously reported Pd-BINAP complex when molecular sieves 4A was added as an additive. Based on an X-ray structural analysis of [Pd{(S,S)-ⁱPr-duphos}](OTf)₂ complex, a working hypothesis of the reaction mechanism is also described.