Analogous Iron Research Articles

Solvent-Free Synthesis of ZIFs: A Route toward the Elusive Fe(II) Analogue of ZIF-8.

Herein we report the synthesis of an elusive metal-organic framework, the iron(II) analogue of ZIF-8 with the formula Fe(2-methylimidazolate)2, here denoted as MUV-3. The preparation of this highly interesting porous material, inaccessible by common synthetic procedures, occurs in a solvent-free reaction upon addition of an easily detachable template molecule, yielding single crystals of MUV-3. This methodology can be extended to other metals and imidazolate derivatives, allowing the preparation of ZIF-8, ZIF-67, and the unprecedented iron(II) ZIFs Fe(2-ethylimidazolate)2 and Fe(2-methylbenzimidazolate)2. The different performance of MUV-3 toward NO sorption, in comparison to ZIF-8, results from the chemisorption of NO molecules, which also causes a gate-opening behavior. Finally, the controlled pyrolysis of MUV-3 results in a N-doped graphitic nanocomposite that exhibits extraordinary performance for the oxygen evolution reaction (OER), with low overpotential at different current densities (316 mV at 10 mA cm-2), low Tafel slope (37 mV per decade), high maximum current density (710 mA cm-2 at 2.0 V vs RHE), and great durability (15 h).

The Effect of β‐Hydrogen Atoms on Iron Speciation in Cross‐Couplings with Simple Iron Salts and Alkyl Grignard Reagents

AbstractThe effects of β‐hydrogen‐containing alkyl Grignard reagents in simple ferric salt cross‐couplings have been elucidated. The reaction of FeCl3 with EtMgBr in THF leads to the formation of the cluster species [Fe8Et12]2−, a rare example of a structurally characterized metal complex with bridging ethyl ligands. Analogous reactions in the presence of NMP, a key additive for effective cross‐coupling with simple ferric salts and β‐hydrogen‐containing alkyl nucleophiles, result in the formation of [FeEt3]−. Reactivity studies demonstrate the effectiveness of [FeEt3]− in rapidly and selectively forming the cross‐coupled product upon reaction with electrophiles. The identification of iron‐ate species with EtMgBr analogous to those previously observed with MeMgBr is a critical insight, indicating that analogous iron species can be operative in catalysis for these two classes of alkyl nucleophiles.

The Effect of β-Hydrogen Atoms on Iron Speciation in Cross-Couplings with Simple Iron Salts and Alkyl Grignard Reagents.

The effects of β-hydrogen-containing alkyl Grignard reagents in simple ferric salt cross-couplings have been elucidated. The reaction of FeCl3 with EtMgBr in THF leads to the formation of the cluster species [Fe8 Et12 ]2- , a rare example of a structurally characterized metal complex with bridging ethyl ligands. Analogous reactions in the presence of NMP, a key additive for effective cross-coupling with simple ferric salts and β-hydrogen-containing alkyl nucleophiles, result in the formation of [FeEt3 ]- . Reactivity studies demonstrate the effectiveness of [FeEt3 ]- in rapidly and selectively forming the cross-coupled product upon reaction with electrophiles. The identification of iron-ate species with EtMgBr analogous to those previously observed with MeMgBr is a critical insight, indicating that analogous iron species can be operative in catalysis for these two classes of alkyl nucleophiles.

Electronic Effects of the Backbone on Bis(imino)pyridyliron(II)‐Catalyzed Ethylene Polymerization

A series of bis(imino)pyridyl iron(II) complexes 1–5 with electron‐donating and ‐withdrawing substituents on the backbone of the ligand have been synthesized and characterized. Activated with methylaluminoxane (MAO), ethylene polymerization by these iron(II) complexes was investigated in detail, involving the electronic effect and influence of reaction conditions on catalyst activity and polymer molecular weight. The electronic perturbations exert great influence on the catalytic activity of ethylene polymerization. Complexes bearing electron‐withdrawing substituents in the 4‐position of the pyridine ring showed the better activity than analogues bearing electron‐donating substituents. However, no clear trend was observed for the dependence of polymer molecular weight on these substituents. In particular, the binuclear iron(III) complex 2′ we obtained by accident displayed a similar catalytic behavior compared to its iron(II) analogue complex 2.

Synthesis of a gallium-functionalized polyoxovanadate-alkoxide cluster: Toward a general route for heterometal installation

Incorporating transition metal ions into Lindqvist polyoxovanadate-alkoxide (POV-alkoxide) clusters has been shown to be an effective method for tuning the electronic properties of these molecular assemblies. Herein, we report the synthesis of a gallium-functionalized POV-alkoxide cluster, extending the family of heterometal-functionalized clusters to a metalloid ion. Complex [V5O6(OCH3)12GaCl] was characterized by infrared and electronic absorption spectroscopies and cyclic voltammetry, revealing the retention of the extensive delocalization upon installation of the 3d10 ion. The new reaction conditions developed for [V5O6(OCH3)12GaCl] were shown to successfully generate the iron analogue, [V5O6(OCH3)12FeCl], from a ferric salt precursor. However, attempts at installing other group 13 ions, e.g. InIII, instead produced the cyclic hexavanadate cluster, [Bu4N][V6O6(OCH3)12Cl].

A monomeric manganese(II) catecholato complex: Synthesis, crystal structure, and reactivity toward molecular oxygen

A monomeric manganese(II) catecholato complex with hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate (TptBu,iPr) was synthesized and characterized. X-ray analysis of the compound shows a tetrahedral manganese coordinated to one oxygen atom of the catecholato ligand and three nitrogen atoms of the TptBu,iPr ligand. The MnO distance (1.952(5) Å) is longer than the corresponding FeO distance (1.874(7) Å) in the analogous iron(II) TptBu,iPr complex due to the weaker Lewis acidity of manganese(II). Upon reacting of the manganese(II) complex with molecular oxygen, a manganese(II) semiquinonato complex was obtained, thus serving as an important model for catechol dioxygenase. This differs from the reaction of the iron(II) analog, which generates an iron(III) catecholato complex.

Unexpected Solvent Effect in Electrocatalytic CO2 to CO Conversion Revealed Using Asymmetric Metalloporphyrins.

Rapid and efficient electrochemical CO2 reduction is an ongoing challenge for the production of sustainable fuels and chemicals. In this work, electrochemical CO2 reduction is investigated using metalloporphyrin catalysts (metal = Mn, Fe, Co, Ni, Cu) that feature one hydroxyphenyl group, and three other phenyl groups, in the porphyrin heterocycle (5-(2-hydroxyphenyl)-10,15,20-triphenylporphyrin, TPOH). These complexes, which are minimal versions of related complexes bearing up to eight proton relays, were designed to allow more straightforward determination of the role of the 2-hydroxylphenyl functional group. The iron-substituted version of TPOH supports robust reduction of CO2 in acetonitrile solvent, where carbon monoxide is the only detected product. Addition of weak Brønsted acids (1 M water or 8 mM phenol) gives rise to almost 100-fold enhancement in turnover frequency. Surprisingly, the iron analogue is a poor catalyst when the solvent is changed to dimethylformamide. These results lead to the proposal of a model where the hydroxyphenyl group behaves as a local proton source, a hydrogen bond donor to CO2-bound intermediates, and a hydrogen bonding partner to Brønsted acids. The observations from this model suggest improvements for existing electrocatalytic CO2 reduction systems.

High-temperature superconductivity as viewed from the maximum hardness principle

The Maximum Hardness Principle – and its reformulation by Chattaraj as the Minimum Polarizability Principle – is an immensely useful concept which works in support of a chemical intuition. As we show here, it may also be used to rationalize the scarcity of high-temperature superconductors, which stems – inter alia – from rarity of high-density of state metals in Nature. It is suggested that the high-temperature oxocuprate superconductors as well as their iron analogues – are energetically metastable at T ➔ 0 K and p ➔ 0 atm conditions, and their tendency for disproportionation is hindered only by the substantial rigidity of the crystal lattice, while the phase separation and/or superstructure formation is frequently observed in these systems. This hypothesis is corroborated by hybrid density functional theory theoretical calculations for Na- (thus: hole) or La- (thus: electron) doped CaCu(II)O2 precursor. Non-equilibrium synthetic methods are suggested to be necessary for fabrication of high-temperature superconductors of any sort.Graphical abstractDoped oxocuprate superconductors are shown to be unstable with respect to phase separation (disproportionation) in accordance with the Maximum Hardness Principle; their metastability is mostly due to rigidity of [CuO2] sheets and preparation using high-temperature conditions

Synthesis, characterization, and oxidation catalysis studies of a monofunctionalized copper pyridine-aza macrocycle

A copper-containing pyridine-aza macrocycle (PyMAC) with a propionic acid functionality was synthesized and characterized using X-ray crystallography and various spectroscopic techniques. The complex was then screened for peroxidase, olefin epoxidation, and phenol oxidation properties, using an ABTS assay, GC-MS, and UV–Vis spectrophotometry, respectively. The control copper (II) perchlorate salts were superior oxidation catalysts to the pentadentate carboxylic acid complex, CuLCOOH, and the unfunctionalized amine PyMAC, CuCRH. This might indicate that free copper ions facilitate the one-electron oxidations necessary for the reactions better than complexed copper. It also indicates that the copper PyMACs are far less effective than the analogous nickel and iron systems. We did, however, obtain valuable insights on the chemistry governing this family of PyMACs by probing the CuLCOOH’s solution and solid-phase protonation equilibria, which will be useful for isolating and studying the analogous iron complex, FeLCOOH.

Implications of structural differences between Cu-BTC and Fe-BTC on their hydrogen storage capacity

Hydrogen is an attractive energy vector because it is free of carbon and at the same time contains a high energy density. The main challenge for hydrogen as fuel for massive applications is its storage at high density under technologically viable conditions. In that sense, its physical adsorption in porous solid continues being an option under study. Copper benzene-1,3,5-tricarboxilate (Cu-BTC) is one of the most widely studied metal-organic framework (MOF)-based porous solids, including its potential application for hydrogen storage. Its iron analogue, Fe-BTC has received relatively minor attention probably because it is obtained as a material of low crystallinity and this is a handicap to understand the involved adsorption interactions. In this contribution, we are reporting the implications of their structural differences on the hydrogen storage capacity, with emphasis in the probable guest-host interactions that determine the adsorption process and considering the structural features of them. The samples to be study were prepared using a solvothermal route and then characterized from infrared (IR), Mössbauer, and X-ray photoelectron (XPS) spectroscopies, powder X-ray diffraction (XRD) patterns, thermogravimetric (TG) curves and adsorption data. Both, the H2 adsorption isotherms and the corresponding adsorption heats show significant differences for the two materials, which are explained according to their structural differences.

Iron(III) and aluminium(III) based mixed nanostructured hydroxyphosphates as potential vaccine adjuvants: Preparation and physicochemical characterization

Adjuvants are substances that, when combined with an antigen in a vaccine formulation, potentiate the immune response against that antigen. Aluminium hydroxyphosphate is among the currently used adjuvants in vaccines, while studies on its iron analogue, ferric hydroxyphosphate, are quite limited. In this article we report on the preparation and physicochemical characterization of novel nanostructured iron and aluminium based mixed hydroxyphosphates as potential vaccine adjuvants, prepared by controlled co-precipitation from mixed solutions of Fe(III) and Al(III) chlorides with sodium phosphate and sterilized by autoclaving. The obtained hydroxyphosphates consisted of a network of platy nanoparticles (20–40nm in size), which aggregated in aqueous medium to form secondary micron-sized particles. The obtained nanostructured hydroxyphosphates were characterized by energy dispersive X-ray spectroscopy for elemental composition, X-ray powder diffraction, infrared spectroscopy, zeta-potential measurements for determination of isoelectric point, protein adsorption, and dissolution rate in sodium citrate solution.

Reversible Complexation of Lewis Bases to Low-Coordinate Fe(II), Co(II), and Ni(II) Amides: Influence of the Metal, Donor Ligand, and Amide Substituent on Binding Constants.

The binding constants of several monodentate Lewis bases (L) having different donor atoms [L = tetrahydrofuran (THF), tetrahydrothiophene (THT), pyridine, 4-(dimethylamino)pyridine (DMAP), PMe3, PCy3, and 1,3-bis(1,3,5-trimethylphenyl)imidazol-2-ylidene (IMes)] to the linear two-coordinate bis(amido) transition-metal complexes, M{N(SiMe3)Dipp}2 [M = Fe (1), Co (2), and Ni (3)] were determined by electronic absorption spectroscopy. In addition, 1H NMR spectroscopy was used to study the complexation of M{N(SiMe3)Dipp}2 by THF. The binding constants of the less bulky amido complexes [M{N(SiMe3)2}2]2 (M = Fe or Co; the nickel species is unstable) were also measured and found to bind THF 102-103 times more strongly than M{N(SiMe3)Dipp}2. Furthermore, unlike the corresponding -N(SiMe3)Dipp complexes, where the iron species are more strongly complexed than those of cobalt, the --N(SiMe3)2 species [Co{N(SiMe3)2}2]2 displayed a higher binding constant to THF than [Fe{N(SiMe3)2}2]2, which is in agreement with the stronger association of the [Co{N(SiMe3)2}2]2 dimer in comparison to that of its iron analogue. The binding strengths of the ligands to 1-3 were found to be in the order of DMAP > pyridine > PMe3 > PCy3 ≈ IMes > THF > THT, and those of the respective metals were in the order of Fe > Co > Ni.

Synthesis and Mechanism of Formation of Hydride-Sulfide Complexes of Iron.

Iron-sulfide complexes with hydride ligands provide an experimental precedent for spectroscopically detected hydride species on the iron-sulfur MoFe7S9C cofactor of nitrogenase. In this contribution, we expand upon our recent synthesis of the first iron sulfide hydride complex from an iron hydride and a sodium thiolate ( Arnet, N. A.; Dugan, T. R.; Menges, F. S.; Mercado, B. Q.; Brennessel, W. W.; Bill, E.; Johnson, M. A.; Holland, P. L., J. Am. Chem. Soc. 2015 , 137 , 13220 - 13223 ). First, we describe the isolation of an analogous iron sulfide hydride with a smaller diketiminate supporting ligand, which benefits from easier preparation of the hydride precursor and easier isolation of the product. Second, we describe mechanistic studies on the C-S bond cleavage through which the iron sulfide hydride product is formed. In a key experiment, use of cyclopropylmethanethiolate as the sulfur precursor leads to products from cyclopropane ring opening, implicating an alkyl radical as an intermediate. Combined with the results of isotopic labeling studies, the data are consistent with a mechanism in which homolytic C-S bond cleavage is followed by rebound of the alkyl radical to abstract a hydrogen atom from iron to give the observed alkane and iron-sulfide products.

Halogen Substitution Effects on N2O Schiff Base Ligands in Unprecedented Abrupt FeII Spin Crossover Complexes

A family of halogen-substituted Schiff base iron(II) complexes, [FeII (qsal-X)2 ], (qsal-X=5-X-N-(8-quinolyl)salicylaldimines)) in which X=F (1), Cl (2), Br (3) or I (4) has been investigated in detail. Compound 1 shows a temperature invariant high spin state, whereas the others all show abrupt spin transitions, at or above room temperature, namely, 295 K (X=I) up to 342 K (X=Br), these being some of the highest T1/2 values obtained, to date, for FeII N/O species. We have recently reported subtle symmetry breaking in [FeII (qsal-Cl)2 ] 2 with two spin transition steps occurring at 308 and 316 K. A photomagnetic study reveals almost full HS conversion of [FeII (qsal-I)2 ] 4 at low temperature (T(LIESST)=54 °K). The halogen substitution effects on the magnetic properties, as well as the crystal packing of the [FeII (qsal-X)2 ] compounds and theoretical calculations, are discussed in depth, giving important knowledge for the design of new spin crossover materials. In comparison to the well known iron(III) analogues, [FeIII (qsal-X)2 ]+ , the two extra π-π and P4AE interactions found in [FeII (qsal-X)2 ] compounds, are believed to be accountable for the spin transitions occurring at ambient temperatures.

σ-CAM Mechanisms for the Hydrogenation of Alkenes by cis- and trans-Disilametallacyclic Carbonyl Complexes (M = Fe, Ru, Os): Experimental and Theoretical Studies

Abstract The hydrogenation of alkenes catalyzed by disilametallacyclic carbonyl complexes of iron, ruthenium or osmium was studied experimentally and theoretically. The disilaruthenacycle 2 with two CO ligands in the trans-configuration was prepared, characterized, and its ability to catalyze hydrogenation was studied. Similar to the corresponding iron analogue 1 in which the CO ligands are in the cis-configuration, 2 contains a H2MSi4 core with Si⋯H⋯Si SISHA (secondary interaction of silicon and hydrogen atoms) and catalyzed the hydrogenation of several alkenes under mild conditions. DFT calculations of 1 and 2 with cis- and trans-CO configurations (cis-1, trans-1, cis-2 and trans-2) revealed that the mechanism of ethylene hydrogenation comprises three catalytic cycles, and a key step involves the H-H bond of H2 being activated by an M-Si bond through oxidative hydrogen migration. These mechanisms are a variety of σ-CAM (σ-complex-assisted metathesis) mechanisms. Further calculations suggest that these catalytic cycles can apply to the catalytic hydrogenation of ethylene by osmium analogues of 1 and 2 (cis-3 and trans-3). Some of the elementary reactions in the cycles are dependent on the metal, and the osmium complexes show different performance from the iron and ruthenium analogues due to the characteristic natures of the third-row transition metals.

Activating Fe(I) Porphyrins for the Hydrogen Evolution Reaction Using Second-Sphere Proton Transfer Residues

Iron porphyrin complexes with second-sphere distal triazole residues show a hydrogen evolution reaction (HER) catalyzed by the Fe(I) state in both organic and aqueous media, whereas an analogous iron porphyrin complex without the distal residues catalyzes the HER in the formal Fe(0) state. This activation of the Fe(I) state by the second-sphere residues lowers the overpotential of the HER by these iron porphyrin complexes by 50%. Experimental data and theoretical calculations indicate that the distal triazole residues, once protonated, enhance the proton affinity of the iron center via formation of a dihydrogen bond with an Fe(III)-H- intermediate.

Synthesis of Optically Pure Helicene Metallocenes.

Two types of ruthenocenes and a ferrocene coordinated by rac-9H-cyclopenta[1,2-c:4,3-c']diphenanthrenyl anion(s), a [7]helicene with a cyclopentadienyl moiety at the center of its skeleton, were successfully synthesized: mono-helicene ruthenocene 1 and its iron analogue 1Fe with one [7]helicene ligand bound to the central metal, and bis-helicene ruthenocene 2 with two [7]helicenes. Starting from a racemic mixture of the ligand precursor, rac-2 and meso-2 were obtained in a 7:3 ratio. Since the [7]helicene has a high racemization barrier, enantiomers of the complexes were isolated in their pure forms; they showed large optical rotations and intense circular dichroism (CD) responses.

Synthesis of Optically Pure Helicene Metallocenes

AbstractTwo types of ruthenocenes and a ferrocene coordinated by rac‐9H‐cyclopenta[1,2‐c:4,3‐c′]diphenanthrenyl anion(s), a [7]helicene with a cyclopentadienyl moiety at the center of its skeleton, were successfully synthesized: mono‐helicene ruthenocene 1 and its iron analogue 1Fe with one [7]helicene ligand bound to the central metal, and bis‐helicene ruthenocene 2 with two [7]helicenes. Starting from a racemic mixture of the ligand precursor, rac‐2 and meso‐2 were obtained in a 7:3 ratio. Since the [7]helicene has a high racemization barrier, enantiomers of the complexes were isolated in their pure forms; they showed large optical rotations and intense circular dichroism (CD) responses.

Iron(III) Half‐Sandwich Complexes of the Two‐Legged Piano‐Stool [CpFe(aryl)Cl] Type from the Corresponding Aryliron(II) Precursors

Instead of reacting with the σ‐mesityliron(II) complex [Cp′′′Fe(σ‐C6H2Me3‐2,4,6)] (1; Cp′′′ = 1,2,4‐tri‐tert‐butylcyclopentadienyl) to form a cyclohexadienyl(cyclopentadienyl)iron(II) sandwich complex with palladium coordination to the ipso carbon atom of the six‐membered ring, palladium(II) chloride oxidized 1 to the mesityliron(III) complex [Cp′′′Fe(σ‐C6H2Me3‐2,4,6)Cl] (5) with a two‐legged piano‐stool geometry. The oxidation of [4CpFe(σ‐C6H2Me3‐2,4,6)] (4; 4Cp = tetraisopropylcyclopentadienyl) to [4CpFe(σ‐C6H2Me3‐2,4,6)Cl] (6) was accomplished with hexachloroethane. Two other derivatives, [4CpFe(σ‐C6H3iPr2‐2,6)Cl] (7) and [5CpFe(σ‐C6H3iPr2‐2,6)Cl] (8; 5Cp = pentaisopropylcyclopentadienyl), could also be obtained by the same method. The molecular structures of 5 and 6 are compared with the structure of the (tetraisopropylcyclopentadienyl)iron(III) dibromide [4CpFeBr2] (9), which was formed from the iron(II) analogue [4CpFe(µ‐Br)]2 during attempted nucleophilic substitution with an arylmagnesium bromide.

Metastable Layered Cobalt Chalcogenides from Topochemical Deintercalation.

We present a general strategy to synthesize metastable layered materials via topochemical deintercalation of thermodynamically stable phases. Through kinetic control of the deintercalation reaction, we have prepared two hypothesized metastable compounds, CoSe and CoS, with the anti-PbO type structure from the starting compounds KCo2Se2 and KCo2S2, respectively. Thermal stability, crystal structure from X-ray and neutron diffraction, magnetic susceptibility, magnetization, and electrical resistivity are studied for these new layered chalcogenides; both CoSe and CoS are found to be weak itinerant ferromagnets with Curie temperatures close to 10 K. Due to the weak van der Waals forces between the layers, CoSe is found to be a suitable host for further intercalation of guest species such as Li-ethylenediamine. From first-principles calculations, we explain why the Co chalcogenides are ferromagnets instead of superconductors as in their iron analogues. Bonding analysis of the calculated electronic density of states both explains their phase stability and predicts the limits of our deintercalation technique. Our results have broad implications for the rational design of new two-dimensional building blocks for functional materials.