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

Porphyrin Atropisomerism as a Molecular Engineering Tool in Medicinal Chemistry, Molecular Recognition, Supramolecular Assembly, and Catalysis

Porphyrin atropisomerism, which arises from restricted σ-bond rotation between the macrocycle and a sufficiently bulky substituent, was identified in 1969 by Gottwald and Ullman in 5,10,15,20-tetrakis(o-hydroxyphenyl)porphyrins. Henceforth, an entirely new field has emerged utilizing this transformative tool. This review strives to explain the consequences of atropisomerism in porphyrins, the methods which have been developed for their separation and analysis and present the diverse array of applications. Porphyrins alone possess intriguing properties and a structure which can be easily decorated and molded for a specific function. Therefore, atropisomerism serves as a transformative tool, making it possible to obtain even a specific molecular shape. Atropisomerism has been thoroughly exploited in catalysis and molecular recognition yet presents both challenges and opportunities in medicinal chemistry.

Current perspectives of artificial oxygen carriers as red blood cell substitutes: a review of old to cutting-edge technologies using in vitro and in vivo assessments

Background

Several circumstances such as accidents, surgery, traumatic hemorrhagic shock, and other causalities cause major blood loss. Allogenic blood transfusion can be resuscitative for such conditions; however, it has numerous ambivalent effects, including supply shortage, needs for more time, cost for blood grouping, the possibility of spreading an infection, and short shelf-life. Hypoxia or ischemia causes heart failure, neurological problems, and organ damage in many patients. To address this emergent medical need for resuscitation and to treat hypoxic conditions as well as to enhance oxygen transportation, researchers aspire to achieve a robust technology aimed to develop safe and feasible red blood cell substitutes for effective oxygen transport.

Area covered

This review article provides an overview of the formulation, storage, shelf-life, clinical application, side effects, and current perspectives of artificial oxygen carriers (AOCs) as red blood cell substitutes. Moreover, the pre-clinical (in vitro and in vivo) assessments for the evaluation of the efficacy and safety of oxygen transport through AOCs are key considerations in this study. With the most significant technologies, hemoglobin- and perfluorocarbon-based oxygen carriers as well as other modern technologies, such as synthetically produced porphyrin-based AOCs and oxygen-carrying micro/nanobubbles, have also been elucidated.

Expert opinion

Both hemoglobin- and perfluorocarbon-based oxygen carriers are significant, despite having the latter acting as safeguards; they are cost-effective, facile formulations which penetrate small blood vessels and remove arterial blockages due to their nano-size. They also show better biocompatibility and longer half-life circulation than other similar technologies.

The synthesis and crystal structures of two μ-oxo iron(III) porphyrin complexes

Two [Formula: see text]-oxo iron(III) porphyrin complexes, [Formula: see text]-oxo-bis[meso-tetra-[Formula: see text],[Formula: see text],[Formula: see text],[Formula: see text]-([Formula: see text]-nicotina-midophenyl)porphinatoiron(III)] ([Fe(TPyPP)]2O) and its copper adduct ([Fe(TPyPP)(CuCl)]2O[CF3SO3][Formula: see text], which are derived from Gunter’s porphyrin model for cytochrome [Formula: see text]oxidase are reported. The two complexes were isolated and characterized by single crystal X-ray diffraction, UV-vis and IR spectroscopies. Crystal structural analysis including comparisons to [Formula: see text]-oxo analogues and Hirshfeld surface calculations revealed several noteworthy structural features [Formula: see text] the various picket orientations which are partially attributed to intra- and/or inter-molecular nonbonded interactions.

Strategic Synthesis of 'Picket Fence' Porphyrins Based on Nonplanar Macrocycles

Traditional 'picket fence' porphyrin systems have been a topic of interest for their capacity to direct steric shielding effects selectively to one side of the macrocycle. Sterically overcrowded porphyrin systems that adopt macrocycle deformations have recently drawn attention for their applications in organocatalysis and sensing. Here we explore the combined benefits of nonplanar porphyrins and the old molecular design to bring new concepts to the playing field. The challenging ortho-positions of meso-phenyl residues in dodecasubstituted porphyrin systems led us to transition to less hindered para- and meta-sites and develop selective demethylation based on the steric interplay. Isolation of the symmetrical target compound [2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetrakis(3,5-dipivaloyloxyphenyl)porphyrin] was investigated under two synthetic pathways. The obtained insight was used to isolate unsymmetrical [2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetrakis(2-nitro-5-pivaloyloxyphenyl)porphyrin]. Upon separation of the atropisomers, a detailed single-crystal X-ray crystallographic analysis highlighted intrinsic intermolecular interactions. The nonplanarity of these systems in combination with 'picket fence' motifs provides an important feature in the design of supramolecular ensembles.

O2 Reduction by Iron Porphyrins with Electron Withdrawing Groups: To Scale or not to Scale

Iron porphyrins are synthesized by systematically introducing electron withdrawing groups (EWGs) on pyrroles to evaluate the relationship between rate (k) and overpotential (η). The results indicate that while EWGs lead...

Selective Convergence to Atropisomers of a Porphyrin Derivative Having Bulky Substituents at the Periphery

Four kinds of possible atropisomers of a porphyrin derivative (1), having mesityl groups at one of the o-positions of each meso-aryl group, can be selectively converged to targeted atropisomers among the four isomers (αααα, αααβ, αβαβ, and ααββ) under appropriate conditions for each atropisomer. For example, protonation and subsequent neutralization of a free base porphyrin (H₂-1) induces a convergence reaction to the αβαβ atropisomer, H₂-1-αβαβ, from an atropisomeric mixture. The αααα isomer, H₂-1-αααα, was also obtained by heating a solution of H₂-1 in CHCl₃ in 60% isolated yield, probably owing to a template effect of the solvent molecule. Remarkably, when an atropisomeric mixture of its zinc complex, Zn-1, was heated at 70 °C in a ClCH₂CH₂Cl/MeOH mixed solvent, crystals composed of only Zn-1-αααα were formed. The hydrophobic space formed by the four mesityl groups in the αααα isomer can be used for repeatable molecular encapsulation of benzene, and the encapsulation structure was elucidated by powder X-ray diffraction analysis. Heating the solid of an atropisomeric mixture of Zn-1 to 400 °C afforded the ααββ isomer almost quantitatively. On the other hand, the solid of H₂-1-αααα can be converted by heating, successively to H₂-1-αααβ at 286 °C and then to H₂-1-ααββ at 350 °C.

Beauty in Chemistry: Making Artistic Molecules with Schiff Bases

In 1864, Hugo Schiff, aged 30, discovered the reaction of aromatic aldehydes with primary amines to give imine derivatives. A C═N imine bond presents the unique properties of being strong, as expected for a covalent double bond, and of being reversible due to a fast hydrolytic process. In view of such features, Schiff base condensations are thermodynamically controlled, which, in the case of reactions involving multifunctional aldehydes and primary amines, allow the formation of complex and sophisticated structures through a trial-and-error mechanism. Back hydrolysis can be prevented by hydrogenating C═N bonds under mild conditions. In such a way, stable rings and cages of varying sizes can be synthesized. Moreover, transition and post-transition metal ions, establishing coordinative interactions with imine nitrogen atoms, can address Schiff base condensations of even more complex molecular systems, whose structure is controlled by the geometrical preferences of the metal. Metal template Schiff base condensations have produced multinuclear metal complexes exhibiting the shape of tetrahedral containers, of double helices, and, supreme wonder, of the Borromean rings. These molecular objects cannot be compared to the masterpieces of painting and sculpture of the macroscopic world, but they instill in the viewer aesthetical pleasure and admiration for their creators.

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

Cytochrome c oxidase (CcO) has a binuclear active site composed of a high-spin heme group and a tris-histidine-ligated copper ion (Cu_B). By using two different porphyrin models derived by Gunter (H₂TPyPP) and us (H₂TImPP), 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 (S = 5/2 Fe³⁺ and S = 1/2 Cu²⁺) but also revealed the intermolecular interactions and intramolecular couplings which are in accordance with the crystal structural features.

Proton mediated spin state transition of cobalt heme analogs

The spin state transition from low spin to high spin upon substrate addition is one of the key steps in cytochrome P450 catalysis. External perturbations such as pH and hydrogen bonding can also trigger the spin state transition of hemes through deprotonated histidine (e.g. Cytochrome c). In this work, we report the isolated 2-methylimidazole Cobalt(II) [Co(TPP)(2-MeHIm)] and [Co(TTP)(2-MeHIm)], and the corresponding 2-methylimidazolate derivatives where the N−H proton of axial 2-MeHIm is removed. Interestingly, various spectroscopies including EPR and XAFS determine a high-spin state (S = 3/2) for the imidazolate derivatives, in contrast to the low-spin state (S = 1/2) of all known imidazole analogs. DFT assisted stereoelectronic investigations are applied to understand the metal-ligand interactions, which suggest that the dramatically displaced metal center allowing a promotion e_g(d_π) → b_1g(\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$d_{x^2 - y^2}$$\end{document}dx2-y2) is crucial for the occurrence of the spin state transition.

Studying the electronic structures and spin transitions of synthetic heme analogs is crucial to advancing our understanding of heme enzyme mechanisms. Here the authors show that a Co(II) porphyrin complex undergoes an unexpected spin state transition upon deprotonation of its axial imidazole ligand.

Resonant inelastic X-ray scattering determination of the electronic structure of oxyhemoglobin and its model complex

Hemoglobin and myoglobin are oxygen-binding proteins with S = 0 heme {FeO₂}⁸ active sites. The electronic structure of these sites has been the subject of much debate. This study utilizes Fe K-edge X-ray absorption spectroscopy (XAS) and 1s2p resonant inelastic X-ray scattering (RIXS) to study oxyhemoglobin and a related heme {FeO₂}⁸ model compound, [(pfp)Fe(1-MeIm)(O₂)] (pfp = meso-tetra(α,α,α,α-o-pivalamido-phenyl)porphyrin, or TpivPP, 1-MeIm = 1-methylimidazole) (pfpO₂), which was previously analyzed using L-edge XAS. The K-edge XAS and RIXS data of pfpO₂ and oxyhemoglobin are compared with the data for low-spin Fe^II and Fe^III [Fe(tpp)(Im)₂]^0/+ (tpp = tetra-phenyl porphyrin) compounds, which serve as heme references. The X-ray data show that pfpO₂ is similar to Fe^II, while oxyhemoglobin is qualitatively similar to Fe^III, but with significant quantitative differences. Density-functional theory (DFT) calculations show that the difference between pfpO₂ and oxyhemoglobin is due to a distal histidine H bond to O₂ and the less hydrophobic environment in the protein, which lead to more backbonding into the O₂ A valence bond configuration interaction multiplet model is used to analyze the RIXS data and show that pfpO₂ is dominantly Fe^II with 6-8% Fe^III character, while oxyhemoglobin has a very mixed wave function that has 50-77% Fe^III character and a partially polarized Fe-O₂ π-bond.

Synthetic Fe/Cu Complexes: Toward Understanding Heme-Copper Oxidase Structure and Function

Heme-copper oxidases (HCOs) are terminal enzymes on the mitochondrial or bacterial respiratory electron transport chain, which utilize a unique heterobinuclear active site to catalyze the 4H+/4e- reduction of dioxygen to water. This process involves a proton-coupled electron transfer (PCET) from a tyrosine (phenolic) residue and additional redox events coupled to transmembrane proton pumping and ATP synthesis. Given that HCOs are large, complex, membrane-bound enzymes, bioinspired synthetic model chemistry is a promising approach to better understand heme-Cu-mediated dioxygen reduction, including the details of proton and electron movements. This review encompasses important aspects of heme-O2 and copper-O2 (bio)chemistries as they relate to the design and interpretation of small molecule model systems and provides perspectives from fundamental coordination chemistry, which can be applied to the understanding of HCO activity. We focus on recent advancements from studies of heme-Cu models, evaluating experimental and computational results, which highlight important fundamental structure-function relationships. Finally, we provide an outlook for future potential contributions from synthetic inorganic chemistry and discuss their implications with relevance to biological O2-reduction.

Picket fence porphyrin challenges. Unexpected atropisomerism

The synthesis and structural analysis of two new bis imidazole-ligated iron(II) porphyrinates are reported. The reacting porphyrin used in the studies was the four-coordinate [Formula: see text] atropisomer of [Fe(TpivPP)] (picket fence porphyrin); the axial ligands are 2-methylimidazole and 1,2-dimethylimidazole. Crystal structure analysis revealed that the [Fe(TpivPP)(2-MeHIm)[Formula: see text]] complex had a strongly ruffled porphyrin core that accommodated the hindered ligands on both the picket side and the open face of the porphyrin. Reaction with 1,2-dimethylimidazole with the four-coordinate [Fe(TpivPP)] starting material led to an isomerized form of the picket fence porphyrin. The structure analysis showed that the product obtained was the [Formula: see text] atropisomer. Strong ruffling caused by the bulky 1,2-dimethylimidazole ligand must allow the requisite rotation about the methine carbon to phenyl carbon single bond and yields what is probably the most stable form of the complex. The relative orientation of the two axial ligands in both complexes are approximately perpendicular to each other. Other structural parameters are in general accord with six-coordinate iron(II) porphyrinates.

Crystal structure of (methanol-κO)[5,10,15,20-tetra-kis-(2-amino-phen-yl)porphyrinato-κ4N]zinc(II)-chloro-form-methanol (1/1/1)

In the crystal structure of the title compound, [Zn(C44H32N8)(CH3OH)]·CHCl3·CH3OH, the ZnII cation is coordinated by four porphyrin N and one methanol O atom within a slightly distorted square-pyramidal environment and is shifted out of the porphyrin plane towards the direction of the methanol mol-ecule. The methyl group of the coordinating methanol mol-ecule is disordered over two sets of sites. The porphyrin backbone is nearly planar and the phenyl rings are almost perpendicular to the porphyrin plane. As is typical for picket-fence porphyrins, all four ortho substituents of the meso-phenyl groups (here the amino groups) are facing to the same side of the porphyrin mol-ecule. In the crystal structure, two neighbouring porphyrin complexes form centrosymmetric dimers that are connected via O-H⋯N hydrogen bonding. With the aid of additional N-H⋯N and C-H⋯N hydrogen bonding, these dimers are stacked into columns parallel to [010] that are finally arranged into layers parallel to (001). Between these layers channels are formed where chloro-form solvent mol-ecules are located that are connected to the porphyrin complexes by weak C-H⋯Cl hydrogen bonding. There are additional cavities in the structure where some small residual electron density is found, indicating the presence of disordered methanol mol-ecules, but a reasonable model could not be refined. Therefore the contribution of the electron density associated with the methanol solvent mol-ecule was removed with the SQUEEZE procedure [Spek (2015 ▸). Acta Cryst. C71, 9-18] in PLATON. Nevertheless, the given chemical formula and other crystal data take into account the methanol solvent mol-ecule.

How Does a Heme Carbene Differ from Diatomic Ligated (NO, CO, and CN-) Analogues in the Axial Bond?

Compared to well studied diatomic ligands (NO, CN^-, CO), the axial bonds of carbene hemes is much less known although its significance in biological chemistry. The unusually large quadrupole splitting (Δ E_Q = +2.2 mm·s^-1) and asymmetric parameter (η = 0.9) of the five-coordinate heme carbene [Fe(TTP)(CCl₂)], which is the largest among all known low spin ferrohemes, has driven investigations by means of Mössbauer effect Nuclear Resonance Vibrational Spectroscopy (NRVS). Three distinct measurements on one single crystal (two in-plane and one out-of-plane) have demonstrated comprehensive vibrational structures including stretch (429) and bending modes (472 cm^-1) of the axial Fe-CCl₂, and revealed iron vibrational anisotropy in three orthogonal directions for the first time. Frontier orbital analysis especially comparisons with diatomic analogues (NO, CN^-, CO) suggest that CCl₂, similar to NO, has led to strong but anisotropic π bonding in a ligand-based "4C"-coordinate which induced the vibrational anisotropies and very large Mössbauer parameters. This is contrasted to CN^- and CO complexes which possess a porphyrin-based "4N"-coordinate electronic and vibrational structures due to inherent on-axis linear ligation.

Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins

As a result of the adaptation of life to an aerobic environment, nature has evolved a panoply of metalloproteins for oxidative metabolism and protection against reactive oxygen species. Despite the diverse structures and functions of these proteins, they share common mechanistic grounds. An open-shell transition metal like iron or copper is employed to interact with O2 and its derived intermediates such as hydrogen peroxide to afford a variety of metal-oxygen intermediates. These reactive intermediates, including metal-superoxo, -(hydro)peroxo, and high-valent metal-oxo species, are the basis for the various biological functions of O2-utilizing metalloproteins. Collectively, these processes are called oxygen activation. Much of our understanding of the reactivity of these reactive intermediates has come from the study of heme-containing proteins and related metalloporphyrin compounds. These studies not only have deepened our understanding of various functions of heme proteins, such as O2 storage and transport, degradation of reactive oxygen species, redox signaling, and biological oxygenation, etc., but also have driven the development of bioinorganic chemistry and biomimetic catalysis. In this review, we survey the range of O2 activation processes mediated by heme proteins and model compounds with a focus on recent progress in the characterization and reactivity of important iron-oxygen intermediates. Representative reactions initiated by these reactive intermediates as well as some context from prior decades will also be presented. We will discuss the fundamental mechanistic features of these transformations and delineate the underlying structural and electronic factors that contribute to the spectrum of reactivities that has been observed in nature as well as those that have been invented using these paradigms. Given the recent developments in biocatalysis for non-natural chemistries and the renaissance of radical chemistry in organic synthesis, we envision that new enzymatic and synthetic transformations will emerge based on the radical processes mediated by metalloproteins and their synthetic analogs.

Spectroscopic studies of the cytochrome P450 reaction mechanisms

The cytochrome P450 monooxygenases (P450s) are thiolate heme proteins that can, often under physiological conditions, catalyze many distinct oxidative transformations on a wide variety of molecules, including relatively simple alkanes or fatty acids, as well as more complex compounds such as steroids and exogenous pollutants. They perform such impressive chemistry utilizing a sophisticated catalytic cycle that involves a series of consecutive chemical transformations of heme prosthetic group. Each of these steps provides a unique spectral signature that reflects changes in oxidation or spin states, deformation of the porphyrin ring or alteration of dioxygen moieties. For a long time, the focus of cytochrome P450 research was to understand the underlying reaction mechanism of each enzymatic step, with the biggest challenge being identification and characterization of the powerful oxidizing intermediates. Spectroscopic methods, such as electronic absorption (UV-Vis), electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), electron nuclear double resonance (ENDOR), Mössbauer, X-ray absorption (XAS), and resonance Raman (rR), have been useful tools in providing multifaceted and detailed mechanistic insights into the biophysics and biochemistry of these fascinating enzymes. The combination of spectroscopic techniques with novel approaches, such as cryoreduction and Nanodisc technology, allowed for generation, trapping and characterizing long sought transient intermediates, a task that has been difficult to achieve using other methods. Results obtained from the UV-Vis, rR and EPR spectroscopies are the main focus of this review, while the remaining spectroscopic techniques are briefly summarized. This article is part of a Special Issue entitled: Cytochrome P450 biodiversity and biotechnology, edited by Erika Plettner, Gianfranco Gilardi, Luet Wong, Vlada Urlacher, Jared Goldstone.

A structurally-characterized peroxomanganese(iv) porphyrin from reversible O2 binding within a metal-organic framework

Within a MOF, a side-on peroxomanganese(iv) porphyrin has been isolated and comprehensively examined.

The role of peroxometal species as reactive intermediates in myriad biological processes has motivated the synthesis and study of analogous molecular model complexes. Peroxomanganese(iv) porphyrin complexes are of particular interest, owing to their potential ability to form from reversible O₂ binding, yet have been exceedingly difficult to isolate and characterize in molecular form. Alternatively, immobilization of metalloporphyrin sites within a metal–organic framework (MOF) can enable the study of interactions between low-coordinate metal centers and gaseous substrates, without interference from bimolecular reactions and axial ligation by solvent molecules. Here, we employ this approach to isolate the first rigorously four-coordinate manganese(ii) porphyrin complex and examine its reactivity with O₂ using infrared spectroscopy, single-crystal X-ray diffraction, EPR spectroscopy, and O₂ adsorption analysis. X-ray diffraction experiments reveal for the first time a peroxomanganese(iv) porphyrin species, which exhibits a side-on, η² binding mode. Infrared and EPR spectroscopic data confirm the formulation of a peroxomanganese(iv) electronic structure, and show that O₂ binding is reversible at ambient temperature, in contrast to what has been observed in molecular form. Finally, O₂ gas adsorption measurements are employed to quantify the enthalpy of O₂ binding as h_ads = –49.6(8) kJ mol^–1. This enthalpy is considerably higher than in the corresponding Fe- and Co-based MOFs, and is found to increase with increasing reductive capacity of the M^II/III redox couple.

What Can Be Learned from Nuclear Resonance Vibrational Spectroscopy: Vibrational Dynamics and Hemes

Nuclear resonance vibrational spectroscopy (NRVS; also known as nuclear inelastic scattering, NIS) is a synchrotron-based method that reveals the full spectrum of vibrational dynamics for Mössbauer nuclei. Another major advantage, in addition to its completeness (no arbitrary optical selection rules), is the unique selectivity of NRVS. The basics of this recently developed technique are first introduced with descriptions of the experimental requirements and data analysis including the details of mode assignments. We discuss the use of NRVS to probe ⁵⁷Fe at the center of heme and heme protein derivatives yielding the vibrational density of states for the iron. The application to derivatives with diatomic ligands (O₂, NO, CO, CN^–) shows the strong capabilities of identifying mode character. The availability of the complete vibrational spectrum of iron allows the identification of modes not available by other techniques. This permits the correlation of frequency with other physical properties. A significant example is the correlation we find between the Fe–Im stretch in six-coordinate Fe(XO) hemes and the trans Fe–N(Im) bond distance, not possible previously. NRVS also provides uniquely quantitative insight into the dynamics of the iron. For example, it provides a model-independent means of characterizing the strength of iron coordination. Prediction of the temperature-dependent mean-squared displacement from NRVS measurements yields a vibrational “baseline” for Fe dynamics that can be compared with results from techniques that probe longer time scales to yield quantitative insights into additional dynamical processes.

Porphyrinoid rotaxanes: building a mechanical picket fence

Building on recent progress in the synthesis of functional porphyrins for a range of applications using the Cu-mediated azide-alkyne cycloaddition (CuAAC) reaction, we describe the active template CuAAC synthesis of interlocked triazole functionalised porphyrinoids in excellent yield. By synthesising interlocked analogues of previously studied porphyrin-corrole conjugates, we demonstrate that this approach gives access to rotaxanes in which the detailed electronic properties of the axle component are unchanged but whose steric properties are transformed by the mechanical "picket fence" provided by the threaded rings. Our results suggest that interlocked functionalised porphyrins, readily available using the AT-CuAAC approach, are sterically hindered scaffolds for the development of new catalysts and materials.

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

The five-coordinate iron porphyrin carbene complexes [Fe(TPP) (CCl₂)] (TPP = tetraphenylporphyrin), [Fe(TTP) (CCl₂)] (TTP = tetratolylporphyrin) and [Fe(TFPP) (CPh₂)] (TFPP = tetra(pentafluorophenyl)porphyrin), utilizing two types of carbene ligands (CCl₂ and CPh₂), 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_Q), determined the S = 0 electronic configuration. More importantly, combined structural and Mössbauer studies, especially the comparison with the low spin iron(II) porphyrin complexes with strong diatomic ligands (CS, CO and CN^-) revealed the covalent bond nature of the carbene ligands. A correlation between the iron isomer shifts (IS, δ) and the axial bond distances is established for the first time for these donor carbon ligands (:C-R).

3D Motions of Iron in Six-Coordinate {FeNO}(7) Hemes by Nuclear Resonance Vibration Spectroscopy

The vibrational spectrum of a six-coordinate nitrosyl iron porphyrinate, monoclinic [Fe(TpFPP)(1-MeIm)(NO)] (TpFPP=tetra-para-fluorophenylporphyrin; 1-MeIm=1-methylimidazole), has been studied by oriented single-crystal nuclear resonance vibrational spectroscopy (NRVS). The crystal was oriented to give spectra perpendicular to the porphyrin plane and two in-plane spectra perpendicular or parallel to the projection of the FeNO plane. These enable assignment of the FeNO bending and stretching modes. The measurements reveal that the two in-plane spectra have substantial differences that result from the strongly bonded axial NO ligand. The direction of the in-plane iron motion is found to be largely parallel and perpendicular to the projection of the bent FeNO on the porphyrin plane. The out-of-plane Fe-N-O stretching and bending modes are strongly mixed with each other, as well as with porphyrin ligand modes. The stretch is mixed with v50 as was also observed for dioxygen complexes. The frequency of the assigned stretching mode of eight Fe-X-O (X=N, C, and O) complexes is correlated with the Fe-XO bond lengths. The nature of highest frequency band at ≈560 cm(-1) has also been examined in two additional new derivatives. Previously assigned as the Fe-NO stretch (by resonance Raman), it is better described as the bend, as the motion of the central nitrogen atom of the FeNO group is very large. There is significant mixing of this mode. The results emphasize the importance of mode mixing; the extent of mixing must be related to the peripheral phenyl substituents.

Binding of molecular oxygen by an artificial heme analogue: investigation on the formation of an Fe–tetracarbene superoxo complex

The reactivity of a heme-like organometallic Fe–NHC complex with O₂ is studied. The formation of a superoxo Fe(iii) intermediate is observed. The reactivity of the intermediate in acetone and acetonitrile is described and the products are identified.

Theoretical view on a linear end-on manganese–dioxygen complex bearing a calix[4]arene ligand

The Mn–O–O angle of mononuclear manganese(iii)-superoxo complexes supported by zwitterionic calix[4]arene ligands can be modulated via solvent polarity perturbations and/or ligand size adjustment as indicated by DFT calculations.

A moderate distortion of the `picket-fence' porphyrin (cryptand-222)potassium chlorido[meso-α,α,α,α-tetrakis(o-pivalamidophenyl)porphyrinato]ferrate(II) n-hexane monosolvate

As representative porphyrin model compounds, the structures of `picket-fence' porphyrins have been studied intensively. The title solvated complex salt {systematic name: (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane)potassium(I) [5,10,15,20-tetrakis(2-tert-butanamidophenyl)porphyrinato]iron(II) n-hexane monosolvate}, [K(C18H36N2O6)][Fe(C64H64N8O4)Cl]·C6H14 or [K(222)][Fe(TpivPP)Cl]·C6H14 [222 is cryptand-222 or 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane, and TpivPP is meso-α,α,α,α-tetrakis(o-pivalamidophenyl)porphyrinate(2-)], [K(222)][Fe(TpivPP)Cl]·C6H14, is a five-coordinate high-spin iron(II) picket-fence porphyrin complex. It crystallizes with a potassium cation chelated inside a cryptand-222 molecule; the average K-O and K-N distances are 2.81 (2) and 3.05 (2) Å, respectively. One of the protecting tert-butyl pickets is disordered. The porphyrin plane presents a moderately ruffled distortion, as suggested by the atomic displacements. The axial chloride ligand is located inside the molecular cavity on the hindered porphyrin side and the Fe-Cl bond is tilted slightly off the normal to the porphyrin plane by 4.1°. The out-of-plane displacement of the metal centre relative to the 24-atom mean plane (Δ24) is 0.62 Å, indicating a noticeable doming of the porphyrin core.

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. Further structural examination reveals a strong correlation between the mutual ligand orientations (θ) and the axial Fe-N(Im) bond distances, which is detailed for the first time. Mössbauer spectroscopic characterization shows that the low spin derivative has a quadrupole splitting of 0.99 mm s(-1) at 100 K.

Structural study of a manganese(II) 'picket-fence' porphyrin complex

'Picket-fence' porphyrin compounds are used in the investigation of interactions of hemes with dioxygen, carbon monoxide, nitric monoxide and imidazole ligands. (Cryptand-222)potassium chlorido[meso-tetra(α,α,α,α-o-pivalamidophenyl)porphyrinato]manganese tetrahydrofuran monosolvate (cryptand-222 is 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane), [K(C18H36N2O6)][Mn(C64H64N8O4)Cl]·C4H8O or [K(222)][Mn(TpivPP)Cl]·THF [systematic name for TpivPP: 5,10,15,20-tetrakis(2-tert-butanamidophenyl)porphyrin], is a five-coordinate high-spin manganese(II) picket-fence porphyrin complex. It crystallizes with a potassium cation chelated inside a cryptand-222 molecule; the average K-O and K-N distances are 2.83 (4) and 2.995 (13) Å, respectively. All four protecting tert-butyl pickets of the porphyrin are ordered. The porphyrin plane is nearly planar, as indicated by the atomic displacements and the dihedral angles between the mean planes of the pyrrole rings and the 24-atom mean plane. The axial chloride ligand is located inside the molecular cavity on the hindered porphyrin side and the Mn-Cl bond is tilted slightly off the normal to the porphyrin plane by 3.68 (2)°. The out-of-plane displacement of the metal centre relative to the 24-atom mean plane (Δ24) is 0.7013 (4) Å, indicating a noticeable porphyrin core doming.

Geometric and electronic structures of five-coordinate manganese(ii) “picket fence” porphyrin complexes

The X-ray structural investigation rationalized the variable axial ligand distances. EPR revealed five resonance positions and the simulations gave reasonable zero field splitting parameters.

Transition metal complexes and the activation of dioxygen

In order to address how diverse metalloprotein active sites, in particular those containing iron and copper, guide O₂binding and activation processes to perform diverse functions, studies of synthetic models of the active sites have been performed. These studies have led to deep, fundamental chemical insights into how O₂coordinates to mono- and multinuclear Fe and Cu centers and is reduced to superoxo, peroxo, hydroperoxo, and, after O-O bond scission, oxo species relevant to proposed intermediates in catalysis. Recent advances in understanding the various factors that influence the course of O₂activation by Fe and Cu complexes are surveyed, with an emphasis on evaluating the structure, bonding, and reactivity of intermediates involved. The discussion is guided by an overarching mechanistic paradigm, with differences in detail due to the involvement of disparate metal ions, nuclearities, geometries, and supporting ligands providing a rich tapestry of reaction pathways by which O₂is activated at Fe and Cu sites.

Comprehensive Fe-ligand vibration identification in {FeNO}6 hemes

Oriented single-crystal nuclear resonance vibrational spectroscopy (NRVS) has been used to obtain all iron vibrations in two {FeNO}(6) porphyrinate complexes, five-coordinate [Fe(OEP)(NO)]ClO4 and six-coordinate [Fe(OEP)(2-MeHIm)(NO)]ClO4. A new crystal structure was required for measurements of [Fe(OEP)(2-MeHIm)(NO)]ClO4, and the new structure is reported herein. Single crystals of both complexes were oriented to be either parallel or perpendicular to the porphyrin plane and/or axial imidazole ligand plane. Thus, the FeNO bending and stretching modes can now be unambiguously assigned; the pattern of shifts in frequency as a function of coordination number can also be determined. The pattern is quite distinct from those found for CO or {FeNO}(7) heme species. This is the result of unchanging Fe-N(NO) bonding interactions in the {FeNO}(6) species, in distinct contrast to the other diatomic ligand species. DFT calculations were also used to obtain detailed predictions of vibrational modes. Predictions were consistent with the intensity and character found in the experimental spectra. The NRVS data allow the assignment and observation of the challenging to obtain Fe-Im stretch in six-coordinate heme derivatives. NRVS data for this and related six-coordinate hemes with the diatomic ligands CO, NO, and O2 reveal a strong correlation between the Fe-Im stretch and Fe-N(Im) bond distance that is detailed for the first time.