Intramolecular Oxidative O‐Demethylation of an Oxoferryl Porphyrin Complexed with a Per‐O‐methylated β‐Cyclodextrin Dimer

The intramolecular oxidation of ROCH₃ to ROCH₂OH, where the latter compound spontaneously decomposed to ROH and HCHO, was observed during the reaction of the supramolecular complex (met‐hemoCD3) with cumene hydroperoxide in aqueous solution. Met‐hemoCD3 is composed of meso‐tetrakis(4‐sulfonatophenyl)porphinatoiron(III) (Fe^IIITPPS) and a per‐O‐methylated β‐cyclodextrin dimer having an ‐OCH₂PyCH₂O‐ linker (Py=pyridine‐3,5‐diyl). The O=Fe^IVTPPS complex was formed by the reaction of met‐hemoCD3 with cumene hydroperoxide, and isolated by gel‐filtration chromatography. Although the isolated O=Fe^IVTPPS complex in the cyclodextrin cage was stable in aqueous solution at 25 °C, it was gradually converted to Fe^IITPPS (t_1/2=7.6 h). This conversion was accompanied by oxidative O‐demethylation of an OCH₃ group in the cyclodextrin dimer. The results indicated that hydrogen abstraction by O=Fe^IVTPPS from ROCH₃ yields HO‐Fe^IIITPPS and ROCH₂^.. This was followed by radical coupling to afford Fe^IITPPS and ROCH₂OH. The hemiacetal (ROCH₂OH) immediately decomposed to ROH and HCHO. This study revealed the ability of oxoferryl porphyrin to induce two‐electron oxidation.     

Three Redox States of Nitrosyl: NO+, NO., and NO−/HNO Interconvert Reversibly on the Same Pentacyanoferrate(II) Platform

Not so elusive : [Fe^II(CN)₅(HNO)]^3? has been characterized spectroscopically after the two‐electron reduction of nitroprusside (see scheme). The complex is stable at pH 6, slowly decomposing to [Fe(CN)₆]^4? and N₂O. It is deprotonated at increasing pH value with oxidation of bound NO^? to [Fe^II(CN)₅(NO)]^3?. [Fe^II(CN)₅(HNO)]^3? is the first non‐heme iron–nitroxyl complex prepared in aqueous solution that is reversibly redox‐active under biologically relevant conditions.

     

Gold Nanoparticles Carrying Diatomic Molecules (O2 and CO) in Aqueous Solution

Gold nanoparticles (AuNPs) prepared by citrate reduction of aurochloric acid (HAuCl₄) were functionalized by tris(4‐sulfonatophenyl)porphinatoiron(III) (Fe^IIIP2) and poly(ethylene glycol) with thiolated arms (PEG‐SH). Fe^IIIP2 on the AuNP surface existed as its μ‐oxo dimer, which was reduced by Na₂S₂O₄ to yield monomeric Fe^IIP2. Fe^IIP2‐bearing AuNPs were further functionalized through inclusion of two sulfonatophenyl groups of Fe^IIP2 by a per‐O‐methylated β‐cyclodextrin dimer with a pyridine linker (Py3CD) to obtain AuNPs capable of carrying diatomic molecules in the body. The resulting AuNPs (hemoCD‐AuNPs) bound O₂ as well as CO in an aqueous solution. Although a noncolloidal 1:1 complex of 5,10,15,20‐tetrakis(4‐sulfonatophenyl)porphinatoiron(II) and Py3CD injected into the femoral vein of a rat was rapidly excreted in the urine, no excretion was observed with ferric hemoCD‐AuNPs, which were gradually accumulated in the spleen and liver of a rat. These results suggest that hemoCD‐AuNPs can be used as a carrier of diatomic molecules such as O₂ and CO in vivo.     

Six,Seven or Eight Coordinate FeII,CoII or NiII Complexes of Amide‐Appended Tetraazamacrocycles for ParaCEST Thermometry

Fe^II, Co^II and Ni^II complexes of two tetraazamacrocycles (1,4,8,11‐tetrakis(carbamoylmethyl)‐1,4,8,11‐tetraazacyclotetradecane ( L1 ) and 1,4,7,10‐tetrakis(carbamoylmethyl)‐1,4,7,10‐tetraazacyclododecane ( L2 ) show promise as paraCEST agents for registration of temperature (paraCEST=paramagnetic chemical exchange saturation transfer). The Fe^II, Co^II and Ni^II complexes of L1 show up to four CEST peaks shifted ≤112 ppm, whereas analogous complexes of L2 show only a single CEST peak at ≤69 ppm. Comparison of the temperature coefficients (C_T) of the CEST peaks of [Co( L2 )]²⁺, [Fe( L2 )]²⁺, [Ni( L1 )]²⁺ and [Co( L1 )]²⁺ showed that a CEST peak of [Co( L1 )]²⁺ gave the largest C_T (?0.66 ppm ^oC^?1 at 4.7 T). NMR spectral and CEST properties of these complexes correspond to coordination complex symmetry as shown by structural data. The [Ni( L1 )]²⁺ and [Co( L1 )]²⁺ complexes have a six‐coordinate metal ion bound to the 1‐, 4‐amide oxygen atoms and four nitrogen atoms of the tetraazamacrocycle. The [Fe( L2 )]²⁺ complex has an unusual eight‐coordinate Fe^II bound to four amide oxygen atoms and four macrocyclic nitrogen atoms. For [Co( L2 )]²⁺, one structure has seven‐coordinate Co^II with three bound amide pendents and a second structure has a six‐coordinate Co^II with two bound amide pendents.     

A Myoglobin Functional Model Composed of a Ferrous Porphyrin and a Cyclodextrin Dimer with an Imidazole Linker

A 1:1 inclusion complex (Fe^IIPImCD) of 5,10,15,20‐tetrakis‐ (4‐sulfonatophenyl)porphinatoiron(II) (Fe^IIP) and an O‐methylated β‐cyclodextrin dimer with an imidazole linker (ImCD) was found to bind dioxygen in aqueous solution. The half‐saturation pressure of dioxygen (P_1/2^O2) is 1.7 torr at 25 °C, which is 10 times lower than that for a previous myoglobin functional model (hemoCD) with a pyridine linker. Meanwhile, the half‐life of oxygenated Fe^IIPImCD is 3 h, which is 10 times shorter than that of oxygenated hemoCD. The covering of the iron(II) center by a microscopic environment is essential for preventing autoxidation of oxygenated ferrous porphyrin, which is promoted by nucleophilic attack of H₂O and/or nucleophiles such as inorganic anions. Due to structural requirements, covering of the Fe^II center of Fe^IIPImCD is insufficient compared with the case of hemoCD. As a result, water molecules can penetrate more easily the cleft of the O₂–Fe^IIPImCD complex and act as an autoxidation inducer. This structure also causes poorer selectivity against carbon monoxide (M=1040). In contrast, the dioxygen affinity of Fe^IIPImCD is much higher than that of hemoCD because the imidazole moiety is a stronger electron donor than pyridine.     

Air‐Free Synthesis of a Ferrous Metal‐Organic Framework Featuring HKUST‐1 Structure and its Mössbauer Spectrum

Single crystals of the Fe^II metal‐organic framework (MOF) with 1,3,5‐benzenetricarboxylate (BTC) as a linker were solvothermally obtained under air‐free conditions. X‐ray diffraction analysis of the crystals demonstrated a structure for Fe^II‐MOF analogous to that of [Cu₃(BTC)₂] (HKUST‐1). Unlike HKUST‐1, however, the Fe^II‐MOF did not retain permanent porosity after exchange of guest molecules. The Mössbauer spectrum of the Fe^II‐MOF was recorded at 80 K in zero field yielding an apparent quadrupole splitting of ΔE_Q = 2.43 mm · s^–1, and an isomer shift of δ = 1.20 mm · s^–1, consistent with high‐spin central iron(II) atoms. Air exposure of the Fe^II‐MOF was found to result in oxidation of the metal atoms to afford Fe^III. These results demonstrate that Fe^II‐based MOFs can be prepared in similar fashion to the [Cu₃(BTC)₂], but that they lack permanent porosity when degassed.     

A Heterometallic FeII–DyIII Single‐Molecule Magnet with a Record Anisotropy Barrier

A record anisotropy barrier (319 cm^?1) for all d‐f complexes was observed for a unique Fe^II‐Dy^III‐Fe^II single‐molecule magnet (SMM), which possesses two asymmetric and distorted Fe^II ions and one quasi‐D_5h Dy^III ion. The frozen magnetization of the Dy^III ion leads to the decreased Fe^II relaxation rates evident in the Mössbauer spectrum. Ab initio calculations suggest that tunneling is interrupted effectively thanks to the exchange doublets.     

Synthesis,crystal structure and magnetic properties of a two-dimensional mixed-valence assembly [Fe(salen)]2[Fe(CN)5NO]

A cyanide-bridged Fe^III–Fe^II mixed-valence assembly, [Fe^III(salen)]₂[Fe^II(CN)₅NO] [salen = N,N-ethylenebis(salicylideneiminato)dianion], prepared by slow diffusion of an aqueous solution of Na₂[Fe(CN)₅NO] · 2H₂O and a MeOH solution of [Fe(salen)NO₃] in an H tube, has been characterized by X-ray structure analysis, i.r. spectra and magnetic measurements. The product assumes a two-dimensional network structure consisting of pillow-like octanuclear [—Fe^II—CN—Fe^III—NC—]₄ units with dimensions: Fe^II—C = 1.942(7) Å, C—N = 1.139(9) Å, Fe^III—N = 2.173(6) Å, Fe^II—C—N = 178.0(6)°, Fe^III—N—C = 163.4(6)°. The Fe^II—N—O bond angle is linear (180.0°). The variable temperature magnetic susceptibility, measured in the 4.8–300 K range, indicates the presence of a weak intralayer antiferromagnetic interaction and gives an Fe^III–Fe^III exchange integral of –0.033 cm^–1.     

catena‐Poly­[[di­aqua­di­chloro­iron(II)]‐μ‐1,4‐dioxane‐κ2O:O′]

The crystal structure of the title compound, [FeCl₂(C₄H₈O₂)(H₂O)₂]_n, contains six‐coordinate Fe^II atoms in approximately octahedral environments. The Fe^II atoms have symmetry, i.e. all pairs of identical ligands are trans. The structure consists of polymeric chains made up of dioxane mol­ecules, in the chair conformation with symmetry, linking the Fe^II centers. The chains are crosslinked by O—H?Cl hydrogen bonds.     

Molecular magnet based on the FeII,III complex with squaric acid

The reaction of Fe^II and Fe^III salts with tetrabutylammonium squarate in an aqueous solution gives rise to a new molecular magnet (T _c = 8.6 K).     

Trapping of a Highly Reactive Oxoiron(IV) Complex in the Catalytic Epoxidation of Olefins by Hydrogen Peroxide

The generation of a nonheme oxoiron(IV) intermediate, [(cyclam)Fe^IV(O)(CH₃CN)]²⁺ ( 2 ; cyclam=1,4,8,11‐tetraazacyclotetradecane), is reported in the reactions of [(cyclam)Fe^II]²⁺ with aqueous hydrogen peroxide (H₂O₂) or a soluble iodosylbenzene (sPhIO) as a rare example of an oxoiron(IV) species that shows a preference for epoxidation over allylic oxidation in the oxidation of cyclohexene. Complex 2 is kinetically and catalytically competent to perform the epoxidation of olefins with high stereo‐ and regioselectivity. More importantly, 2 is likely to be the reactive intermediate involved in the catalytic epoxidation of olefins by [(cyclam)Fe^II]²⁺ and H₂O₂. In spite of the predominance of the oxoiron(IV) cores in biology, the present study is a rare example of high‐yield isolation and spectroscopic characterization of a catalytically relevant oxoiron(IV) intermediate in chemical oxidation reactions.     

Bis(thio­carbohydrazide)dithio­cyanato­iron(II)

In the title compound, [Fe^II(NCS)₂(CH₆N₄S)₂], the Fe^II cation is surrounded by two S atoms and two N atoms from thio­carbohydrazide groups and by two N atoms from thio­cyanate groups. The geometry around the Fe^II cation, which is located on a center of inversion, is distorted octa­hedral. The thio­carbohydrazide mol­ecule assumes a cis–trans conformation, which is reinforced by an N—H⋯N hydrogen bond. Mol­ecules of the title compound are connected via inter­molecular N—H⋯S and N—H⋯N hydrogen bonds to form a three‐dimensional network structure.     

Exploring the Strength of the H‐Bond in Synthetic Models for Heme Proteins: The Importance of the N−H Acidity of the Distal Base

The distal hydrogen bond (H‐bond) in dioxygen‐binding proteins is crucial for the discrimination of O₂ with respect to CO or NO. We report the preparation and characterization of a series of Zn^II porphyrins, with one of three meso‐phenyl rings bearing both an alkyl‐tethered proximal imidazole ligand and a heterocyclic distal H‐bond donor connected by a rigid acetylene spacer. Previously, we had validated the corresponding Co^II complexes as synthetic model systems for dioxygen‐binding heme proteins and demonstrated the structural requirements for proper distal H‐bonding to Co^II‐bound dioxygen. Here, we systematically vary the H‐bond donor ability of the distal heterocycles, as predicted based on pK_a values. The H‐bond in the dioxygen adducts of the Co^II porphyrins was directly measured by Q‐band Davies‐ENDOR spectroscopy. It was shown that the strength of the hyperfine coupling between the dioxygen radical and the distal H‐atom increases with enhanced acidity of the H‐bond donor.     

FeII Spin‐Crossover Phenomenon in the Pentadecanuclear {Fe9[Re(CN)8]6} Spherical Cluster

The self‐assembly of iron(II) ions with rare octacyanidorhenate(V) metalloligands in a methanolic solution results in the formation of a nanometric pentadecanuclear {Fe^II₉[Re^V(CN)₈]₆(MeOH)₂₄}?10 MeOH ( 1 ) molecule with a six‐capped body‐centered cubic topology. The cluster demonstrates a thermally‐induced spin‐crossover phase transition at T_1/2=195 K which occurs selectively for a single Fe^II ion embedded in the center of a cluster core.     

Construction of Insulin 18‐mer Nanoassemblies Driven by Coordination to Iron(II) and Zinc(II) Ions at Distinct Sites

Controlled self‐assembly (SA) of proteins offers the possibility to tune their properties or to create new materials. Herein, we present the synthesis of a modified human insulin (HI) with two distinct metal‐ion binding sites, one native, the other abiotic, enabling hierarchical SA through coordination with two different metal ions. Selective attachment of an abiotic 2,2′‐bipyridine (bipy) ligand to HI, yielding HI–bipy, enabled Zn^II‐binding hexamers to SA into trimers of hexamers, [[HI–bipy]₆]₃, driven by octahedral coordination to a Fe^II ion. The structures were studied in solution by small‐angle X‐ray scattering and on surfaces with AFM. The abiotic metal ligand had a higher affinity for Fe^II than Zn^II ions, enabling control of the hexamer formation with Zn^II and the formation of trimers of hexamers with Fe^II ions. This precise control of protein SA to give oligomers of oligomers provides nanoscale structures with potential applications in nanomedicine.     

Cyano‐bridged Molecular Square: [Ni(iprtacn)Fe(phen)2(CN)2]2(PF6)4·6CH3CN – Preparation,Structure, Magnetic Properties and Binding with DNA

The cyano‐bridged molecular square Ni(iprtacn)]₂[Fe(phen)₂(CN)₂]₂(PF₆)₄ · 6CH₃CN ( 1 ) (iprtacn = 1,4,7‐triisopropyl‐1,4,7‐triazacyclononane, phen = 1, 10‐phenanthroline) was prepared and its crystal structure, magnetic properties, and binding with DNA were characterized. The four metal ions Ni^IIFe^IINi^IIFe^II of the complex 1 are almost coplanar. Magnetic susceptibilities measured over the range of 2–300 K show weak antiferromagnetic interactions between the two nickel(II) ions; best fitting for the experimental data leads to J = –1.27 cm^–1. UV/Vis and fluorescence spectra show that the complex is able to displace DNA‐bound EB and bind to DNA with strong interactions.     

Guest‐Induced Transformation of a Porphyrin‐Edged FeII4L6 Capsule into a CuIFeII2L4 Fullerene Receptor

The combination of a bent diamino(nickel(II) porphyrin) with 2‐formylpyridine and Fe^II yielded an Fe^II₄L₆ cage. Upon treatment with the fullerenes C₆₀ or C₇₀, this cage was found to transform into a new host–guest complex incorporating three Fe^II centers and four porphyrin ligands, in an arrangement that is hypothesized to maximize π interactions between the porphyrin units of the host and the fullerene guest bound within its central cavity. The new complex shows coordinative unsaturation at one of the Fe^II centers as the result of the incommensurate metal‐to‐ligand ratio, which enabled the preparation of a heterometallic cone‐shaped Cu^IFe^II₂L₄ adduct of C₆₀ or C₇₀.     

Redox Non‐Innocence of Nitrosobenzene at Nickel

Nitrosobenzene (PhNO) serves as a stable analogue of nitroxyl (HNO), a biologically relevant, redox‐active nitric oxide derivative. Capture of nitrosobenzene at the electron‐deficient β‐diketiminato nickel(I) complex [ⁱPr₂NN_F6]Ni results in reduction of the PhNO ligand to a (PhNO)^./? species coordinated to a square planar Ni^II center in [ⁱPr₂NN_F6]Ni(η²‐ONPh). Ligand centered reduction leads to the (PhNO)^2? moiety bound to Ni^II supported by XAS studies. Systematic investigation of structure–reactivity patterns of (PhNO)^./? and (PhNO)^2? ligands reveals parallels with superoxo (O₂)^./? and peroxo (O₂)^2? ligands, respectively, and forecasts reactivity patterns of the more transient HNO ligand.     

An Artificial Enzyme Made by Covalent Grafting of an FeII Complex into β‐Lactoglobulin: Molecular Chemistry,Oxidation Catalysis,and Reaction‐Intermediate Monitoring in a Protein

An artificial metalloenzyme based on the covalent grafting of a nonheme Fe^II polyazadentate complex into bovine β‐lactoglobulin has been prepared and characterized by using various spectroscopic techniques. Attachment of the Fe^II catalyst to the protein scaffold is shown to occur specifically at Cys121. In addition, spectrophotometric titration with cyanide ions based on the spin‐state conversion of the initial high spin (S=2) Fe^II complex into a low spin (S=0) one allows qualitative and quantitative characterization of the metal center’s first coordination sphere. This biohybrid catalyst activates hydrogen peroxide to oxidize thioanisole into phenylmethylsulfoxide as the sole product with an enantiomeric excess of up to 20 %. Investigation of the reaction between the biohybrid system and H₂O₂ reveals the generation of a high spin (S=5/2) Fe^III(η²‐O₂) intermediate, which is proposed to be responsible for the catalytic sulfoxidation of the substrate.     

A Brønsted‐Ligand‐Based Iron Complex as a Molecular Switch with Five Accessible States

A mononuclear Fe^II complex, prepared with a Brønsted diacid ligand, H₂L (H₂L=2‐[5‐phenyl‐1H‐pyrazole‐3‐yl] 6‐benzimidazole pyridine), shows switchable physical properties and was isolated in five different electronic states. The spin crossover (SCO) complex, [Fe^II(H₂L)₂](BF₄)₂ ( 1_A ), exhibits abrupt spin transition at T_1/2=258 K, and treatment with base yields a deprotonated analogue [Fe^II(HL)₂] ( 1_B ), which shows gradual SCO above 350 K. A range of Fe^III analogues were also characterized. [Fe^III(HL)(H₂L)](BF₄)Cl ( 1_C ) has an S=5/2 spin state, while the deprotonated complexes [Fe^III(L)(HL)], ( 1_D ), and (TEA)[Fe^III(L)₂], ( 1_E ) exist in the low‐spin S=1/2 state. The electronic properties of the five complexes were fully characterized and we demonstrate in situ switching between multiple states in both solution and the solid‐state. The versatility of this simple mononuclear system illustrates how proton donor/acceptor ligands can vastly increase the range of accessible states in switchable molecular devices.