Electronic Configurations and the Effect of Peripheral Substituents of (Nitrosyl)iron Corroles.

There have been debates on the electronic configurations of (nitrosyl)iron corroles for decades. In this work, pentacoordinate [Fe(TPC)(NO)], [Fe(TTC)(NO)], and [Fe(TFC)(NO)] with different -substituted phenyl groups (TPC, TTC, and TFC = tris(phenyl, 4-tolyl, or 4-fluorophenyl)corrole, respectively) have been isolated and investigated by various techniques including single-crystal X-ray diffraction, UV-vis spectroscopy, cyclic voltammetry, Fourier transform infrared, NMR, and absorption fine structure spectroscopy. Multitemperature and high-magnetic-field (3, 6, and 9 T) Mössbauer spectroscopy was also applied on all three complexes, which determined the = 0 diamagnetic states, consistent with the magnetic susceptibility and electron paramagnetic resonance measurements.

Perspectives on Ligand Properties of N-Heterocyclic Carbenes in Iron Porphyrin Complexes.

There has been considerable research interest in the ligand nature of N-heterocyclic carbenes (NHCs). In this work, two six-coordinate NHC iron porphyrin complexes [Fe(TTP)(1,3-MeImd)] (TTP = tetratolylporphyrin, 1,3-MeImd = 1,3-dimethylimidazol-2-ylidene) and [Fe(TDCPP)(1,3-MeImd)]ClO (TDCPP = 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin) are reported. Single-crystal X-ray characterizations demonstrate that both complexes have strongly ruffled conformations and relatively perpendicular ligand orientations which are forced by the sterically bulky 1,3-MeImd NHC ligands.

Effects of Superparamagnetic Iron Nanoparticles on Electrocatalysts for the Reduction of Oxygen.

It is of great research interest to understand the nanostructures contributing to the activity observed in the reduction of oxygen by non-platinum group metal (PGM) electrocatalysts in acidic media. Iron- and nitrogen-containing carbon networks are often the most studied structures, among which single-atom iron-coordinated nitrogen (FeN) moieties have often been proposed to be the structures leading to the high activity in these non-PGM electrocatalysts. Iron nanoparticles embedded within a carbon support are also formed under certain conditions as a result of the synthetic processes in making non-PGM electrocatalysts.

Preparation of Nonprecious Metal Electrocatalysts for the Reduction of Oxygen Using a Low-Temperature Sacrificial Metal.

Non-platinum group metal (non-PGM) electrocatalysts for the oxygen reduction reaction (ORR) are generally composed of iron, nitrogen, and carbon synthesized through high-temperature pyrolysis. Among the various types of precursors, metal-organic frameworks (MOFs), zeolitic imidazolate framework (ZIF)-8 in particular, have often been used in the synthesis. The pyrolysis of ZIF-8 precursor relies on the use of Zn as a sacrificial metal (SM), and the optimal processing temperatures often exceed 1000 °C to generate active non-PGM catalysts.

Intermolecular Interactions and Intramolecular Couplings of Binuclear Porphyrin Models for Cytochrome Oxidase.

Cytochrome oxidase (CO) has a binuclear active site composed of a high-spin heme group and a tris-histidine-ligated copper ion (Cu). By using two different porphyrin models derived by Gunter (HTPyPP) and us (HTImPP), we have isolated several mono- and binuclear complexes including one carbonyl and three chloride derivatives which are determined by 100 K single-crystal X-ray. Low-temperature (4 K) EPR and multitemperature (295-25 K) Mössbauer investigations on the products not only confirmed the spin states of the two metal ions ( = 5/2 Fe and = 1/2 Cu) but also revealed the intermolecular interactions and intramolecular couplings which are in accordance with the crystal structural features.

New Insights into the Ligand Nature of Carbene: Synthesis and Characterizations of Six-Coordinate Iron(II) Carbene Porphyrin Complexes.

Several six-coordinate iron(II) carbene tetra(pentafluorophenyl)porphyrin (TFPP) complexes, [Fe(TFPP)(CPh)(1-EtIm)] (1-EtIm = 1-ethylimidazole) and [Fe(TFPP)(CPh)(1,2-MeIm)] (1,2-MeIm = 1,2-dimethylimidazole), are isolated and studied by UV-vis, single-crystal X-ray, and Mössbauer spectroscopies. The single-crystal structural studies revealed noteworthy features including strong and "hard" axial carbene bonds (Fe-C) but "flexible" trans ligand bonds (Fe-N). The Mössbauer spectra of [Fe(TFPP)(CPh)(1-EtIm)] and [Fe(TFPP)(CPh)(1,2-MeIm)] are obtained on solid-state samples between 25 and 295 K, which give very large Δ E values (1.

Hydrogen-Bonding Effects in Five-Coordinate High-Spin Imidazole-Ligated Iron(II) Porphyrinates.

The influence of hydrogen binding to the N-H group of coordinated imidazole in high-spin iron(II) porphyrinates has been studied. The preparation and characterization of new complexes based on [Fe(TPP)(2-MeHIm)] (TPP is the dianion of tetraphenylporphyrin) are reported. The hydrogen bond acceptors are ethanol, tetramethylene sulfoxide, and 2-methylimidazole.

Iron(II) Bis(imidazole) Derivatives of a Binuclear Porphyrin Model: Crystal Structures and Mössbauer Properties.

A new sterically hindered "picket fence-like" porphyrin with chelates for the second metal atom, HTImPP (TImPP = meso-tetrakis[α,α,α,α-o-(5-imidazolecarboxylaminophenyl)]porphyrinato), is developed and used in the synthesis of four iron(II) bis(imidazole) derivatives, which are characterized by single crystal X-ray and other spectroscopies. The comprehensive studies on the crystal structures revealed noteworthy features including new axial ligand arrangements, deformed porphyrin planes, and strongly tilted pickets which can be rationalized by analysis of the intra- and intermolecular interactions. Solid-state Mössbauer experiments on [Fe(TImPP)(1-MeIm)] were conducted at several temperatures from 295 to 25 K.

Electronic Configuration and Ligand Nature of Five-Coordinate Iron Porphyrin Carbene Complexes: An Experimental Study.

The five-coordinate iron porphyrin carbene complexes [Fe(TPP) (CCl)] (TPP = tetraphenylporphyrin), [Fe(TTP) (CCl)] (TTP = tetratolylporphyrin) and [Fe(TFPP) (CPh)] (TFPP = tetra(pentafluorophenyl)porphyrin), utilizing two types of carbene ligands (CCl and CPh), have been investigated by single crystal X-ray, XANES (X-ray absorption near edge spectroscopy), Mössbauer, NMR and UV-vis spectroscopies. The XANES suggested the iron(II) oxidation state of the complexes. The multitemperature and high magnetic field Mössbauer experiments, which show very large quadrupole splittings (QS, ΔE), determined the S = 0 electronic configuration.

Unique Axial Imidazole Geometries of Fully Halogenated Iron(II) Porphyrin Complexes: Crystal Structures and Mössbauer Spectroscopic Studies.

The synthesis and characterization of several electron-poor iron(II) porphyrin (FeTFPPBr) complexes with axial imidazole ligands are reported. The single-crystal X-ray structures have been studied by a combination of crystal packing and Hirshfeld surface calculations, which explained the unusual axial-ligand geometries, e.g.

Identification of carbon-encapsulated iron nanoparticles as active species in non-precious metal oxygen reduction catalysts.

The widespread use of fuel cells is currently limited by the lack of efficient and cost-effective catalysts for the oxygen reduction reaction. Iron-based non-precious metal catalysts exhibit promising activity and stability, as an alternative to state-of-the-art platinum catalysts. However, the identity of the active species in non-precious metal catalysts remains elusive, impeding the development of new catalysts.

All high-spin (S = 2) iron(ii) hemes are NOT alike.

A common structural motif in heme proteins is a five-coordinate species in which the iron is coordinated by a histidyl residue. The widely distributed heme proteins with this motif are essential for the well being of humans and other organisms. We detail the differences in molecular structures and physical properties of high-spin iron(ii) porphyrin derivatives ligated by neutral imidazole, hydrogen bonded imidazole, and imidazolate or other anions.

Synthesis and characterization of a modified "picket fence" porphyrin complex - stronger π bonding interactions between Fe(ii) and axial ligands.

A new, modified "picket fence" porphyrin is synthesized and its bis(imidazole)-ligated iron(ii) derivative [Fe(MbenTpivPP)(1-MeIm)(2)] is investigated. X-ray structure determinations demonstrate that [Fe(MbenTpivPP)(1-MeIm)(2)] has structural features of a near planar porphyrin plane, a relative perpendicular ligand orientation, and one unusually large absolute ligand orientation (φ). The combination of these features leads to a new type of species that is different from previously reported analogues.

Bis(cyano) Iron(III) Porphyrinates: What Is the Ground State?

The synthesis of six new bis(cyano) iron(III) porphyrinate derivatives is reported. The anionic porphyrin complexes utilized tetraphenylporphyrin, tetramesitylporphyrin, and tetratolylporphyrin as the porphyrin ligand. The potassium salts of Kryptofix-222 and 18-C-6 were used as the cations.

Measurement of extreme hyperfine fields in two-coordinate high-spin Fe2+ complexes by Mössbauer spectroscopy: essentially free-ion magnetism in the solid state.

Mössbauer studies of three two-coordinate linear high-spin Fe(2+) compounds, namely, Fe{N(SiMe3)(Dipp)}2 (1) (Dipp = C6H3-2,6-(i)Pr2), Fe(OAr')2 (2) [Ar' = C6H3-2,6-(C6H3-2,6-(i)Pr2)2], and Fe{C(SiMe3)3}2 (3), are presented. The complexes were characterized by zero- and applied-field Mössbauer spectroscopy (1-3), as well as zero- and applied-field heat-capacity measurements (3). As 1-3 are rigorously linear, the distortion(s) that might normally be expected in view of the Jahn-Teller theorem need not necessarily apply.

Redesigning the blue copper azurin into a redox-active mononuclear nonheme iron protein: preparation and study of Fe(II)-M121E azurin.

Much progress has been made in designing heme and dinuclear nonheme iron enzymes. In contrast, engineering mononuclear nonheme iron enzymes is lagging, even though these enzymes belong to a large class that catalyzes quite diverse reactions. Herein we report spectroscopic and X-ray crystallographic studies of Fe(II)-M121E azurin (Az), by replacing the axial Met121 and Cu(II) in wild-type azurin (wtAz) with Glu and Fe(II), respectively.

Iron nitrosyl "natural" porphyrinates: does the porphyrin matter?

The synthesis and spectroscopic characterization of three five-coordinate nitrosyliron(II) complexes, [Fe(Porph)(NO)], are reported. These three nitrosyl derivatives, where Porph represents protoporphyrin IX dimethyl ester, mesoporphyrin IX dimethyl ester, or deuteroporphyrin IX dimethyl ester, display notable differences in their properties relative to the symmetrical synthetic porphyrins such as OEP and TPP. The N-O stretching frequencies are in the range of 1651-1660 cm(-1), frequencies that are lower than those of synthetic porphyrin derivatives.

Spectroscopic and computational study of a nonheme iron nitrosyl center in a biosynthetic model of nitric oxide reductase.

A major barrier to understanding the mechanism of nitric oxide reductases (NORs) is the lack of a selective probe of NO binding to the nonheme FeB center. By replacing the heme in a biosynthetic model of NORs, which structurally and functionally mimics NORs, with isostructural ZnPP, the electronic structure and functional properties of the FeB nitrosyl complex was probed. This approach allowed observation of the first S=3/2 nonheme {FeNO}(7) complex in a protein-based model system of NOR.

Correlated ligand dynamics in oxyiron picket fence porphyrins: structural and Mössbauer investigations.

Disorder in the position of the dioxygen ligand is a well-known problem in dioxygen complexes and, in particular, those of picket fence porphyrin species. The dynamics of Fe-O2 rotation and tert-butyl motion in three different picket fence porphyrin derivatives has been studied by a combination of multitemperature X-ray structural studies and Mössbauer spectroscopy. Structural studies show that the motions of the dioxygen ligand also require motions of the protecting pickets of the ligand binding pocket.

Effect of the Ruffled Porphyrin Ring on Electronic Structures: Structure and Characterization of [Fe(TalkylP)(OClO)] and [Fe(TPrP)(THF)]ClO (alkyl = Ethyl, Et and -Propyl, Pr).

We report the synthesis of Fe(TalkylP)(OClO)] (alkyl = ethyl and propyl) and [Fe(TPrP)(THF)]ClO, which are characterized by UV-vis, EPR, X-ray crystallography, and solid-state magnetic susceptibilities. The macrocycles of all three complexes are ruffled, all of the structural features for [Fe(TEtP(OClO)] and [Fe(TPrP)(OClO)] are characteristic of the nearly pure S = 3/2 state, while the structural parameters for [Fe(TPrP)(THF)]ClO feature a pure intermediate-spin (S = 3/2) state, which are all consistent with EPR and magnetic data. It is clear from these studies that the ruffled conformation plays a significant role in affecting the extent of S = 3/2 character.

New perspectives on iron-ligand vibrations of oxyheme complexes.

We report our studies of the vibrational dynamics of iron for three imidazole-ligated oxyheme derivatives that mimic the active sites of histidine-ligated heme proteins complexed with dioxygen. The experimental vibrational data are obtained from nuclear resonance vibrational spectroscopy (NRVS) measurements conducted on both powder samples and oriented single crystals, and which includes several in-plane (ip) and out-of-plane (oop) measurements. Vibrational spectral assignments have been made through a combination of the oriented sample spectra and predictions based on density functional theory (DFT) calculations.

Metalloporphyrin mixed-valence π-cation radicals: [Fe(oxoOEC(•/2))(Cl)]2SbCl6, structure, magnetic properties, and near-IR spectra.

The preparation and characterization of a mixed-valence π-cation radical derivative of an iron(III) oxochlorinato complex is reported. The new complex has been synthesized by the one-electron oxidation of a pair of [Fe(oxoOEC)(Cl)] molecules to form the dimeric cation [Fe(oxoOEC)(Cl)]₂⁺. The cation has been characterized by X-ray analysis, Mössbauer spectroscopy, UV-vis and near-IR spectroscopy, and magnetic susceptibility measurements from 6-300 K.

Electronic configuration of five-coordinate high-spin pyrazole-ligated iron(II) porphyrinates.

Pyrazole, a neutral nitrogen ligand and an isomer of imidazole, has been used as a fifth ligand to prepare two new species, [Fe(TPP)(Hdmpz)] and [Fe(Tp-OCH(3)PP)(Hdmpz)] (Hdmpz = 3,5-dimethylpyrazole), the first structurally characterized examples of five-coordinate iron(II) porphyrinates with a nonimidazole neutral ligand. Both complexes are characterized by X-ray crystallography, and structures show common features for five-coordinate iron(II) species, such as an expanded porphyrinato core, large equatorial Fe-N(p) bond distances, and a significant out-of-plane displacement of the iron(II) atom. The Fe-N(pyrazole) and Fe-N(p) bond distances are similar to those in imidazole-ligated species.

Structural and magnetic effects of meso-substitution in alkyl-substituted metalloporphyrinate pi-cation radicals: characterization of [Fe(TalkylP*)(Cl)]SbCl6 (alkyl = ethyl and n-propyl).

We report the preparation and characterization of two meso-alkyl substituted porphyrin pi-cation radical derivatives, [Fe(TalkylP(*))(Cl)]SbCl(6) (alkyl = ethyl or propyl). Both complexes have been characterized by UV/vis/near-IR, IR, and Mossbauer spectroscopy, temperature-dependent solid-state magnetic susceptibility measurements, and X-ray structure determinations. All data for both oxidized species are consistent with the formulation of the complexes as ring-oxidized iron(III) porphyrin species.