Synthetic Models for Non-Heme Carboxylate-Bridged Diiron Metalloproteins: Strategies and Tactics

Alkane Oxidation: Methane Monooxygenases, Related Enzymes, and Their Biomimetics

Methane monooxygenases (MMOs) mediate the facile conversion of methane into methanol in methanotrophic bacteria with high efficiency under ambient conditions. Because the selective oxidation of methane is extremely challenging, there is considerable interest in understanding how these enzymes carry out this difficult chemistry. The impetus of these efforts is to learn from the microbes to develop a biomimetic catalyst to accomplish the same chemical transformation. Here, we review the progress made over the past two to three decades toward delineating the structures and functions of the catalytic sites in two MMOs: soluble methane monooxygenase (sMMO) and particulate methane monooxygenase (pMMO). sMMO is a water-soluble three-component protein complex consisting of a hydroxylase with a nonheme diiron catalytic site; pMMO is a membrane-bound metalloenzyme with a unique tricopper cluster as the site of hydroxylation. The metal cluster in each of these MMOs harnesses O₂ to functionalize the C-H bond using different chemistry. We highlight some of the common basic principles that they share. Finally, the development of functional models of the catalytic sites of MMOs is described. These efforts have culminated in the first successful biomimetic catalyst capable of efficient methane oxidation without overoxidation at room temperature.

Metal Ion Modeling Using Classical Mechanics

Metal ions play significant roles in numerous fields including chemistry, geochemistry, biochemistry, and materials science. With computational tools increasingly becoming important in chemical research, methods have emerged to effectively face the challenge of modeling metal ions in the gas, aqueous, and solid phases. Herein, we review both quantum and classical modeling strategies for metal ion-containing systems that have been developed over the past few decades. This Review focuses on classical metal ion modeling based on unpolarized models (including the nonbonded, bonded, cationic dummy atom, and combined models), polarizable models (e.g., the fluctuating charge, Drude oscillator, and the induced dipole models), the angular overlap model, and valence bond-based models. Quantum mechanical studies of metal ion-containing systems at the semiempirical, ab initio, and density functional levels of theory are reviewed as well with a particular focus on how these methods inform classical modeling efforts. Finally, conclusions and future prospects and directions are offered that will further enhance the classical modeling of metal ion-containing systems.

Novel nickel(ii) complexes of sterically modified linear N4 ligands: effect of ligand stereoelectronic factors and solvent of coordination on nickel(ii) spin-state and catalytic alkane hydroxylation

The donor atom type and diazacyclo backbone of the ligands and solvent of coordination dictate the Ni(ii) spin state (4, LS; 1–3, 5, HS) and catalytic activity of complexes.

Direct synthesis of phenol by novel [FeFe]-hydrogenase model complexes as catalysts of benzene hydroxylation with H2O2

Compared the catalytic performance of complexes 1–3, the complex 2 has the highest phenol yield (24.6%) and phenol selectivity (92%), which has the highest electron densities of the catalytically active sites.

Hydroxo-bridged diiron(iii) and dimanganese(iii) bisporphyrins: modulation of metal spins by counter anions

A brief account has been presented on how the inter-heme interactions in μ-hydroxo diiron(iii) bisporphyrins and counter anions can induce significant change in the structure and properties including the iron spin state without affecting the overall topology.

Remarkable Anion-Dependent Spin-State Switching in Diiron(III) μ-Hydroxo Bisporphyrins: What Role do Counterions Play?

Addition of 2,4,6-trinitrophenol (HTNP) to an ethene-bridged diiron(III) μ-oxo bisporphyrin (1) in CH₂ Cl₂ initially leads to the formation of diiron(III) μ-hydroxo bisporphyrin (2⋅TNP) with a phenolate counterion that, after further addition of HTNP or dissolution in a nonpolar solvent, converts to a diiron(III) complex with axial phenoxide coordination (3⋅(TNP)₂ ). The progress of the reaction from μ-oxo to μ-hydroxo to axially ligated complex has been monitored in solution by using ¹ H NMR spectroscopy because their signals appear in three different and distinct spectral regions. The X-ray structure of 2⋅TNP revealed that the nearly planar TNP counterion fits perfectly within the bisporphyrin cavity to form a strong hydrogen bond with the μ-hydroxo group, which thus stabilizes the two equivalent iron centers. In contrast, such counterions as I₅ , I₃ , BF₄ , SbF₆ , and PF₆ are found to be tightly associated with one of the porphyrin rings and, therefore, stabilize two different spin states of iron in one molecule. A spectroscopic investigation of 2⋅TNP has revealed the presence of two equivalent iron centers with a high-spin state (S=5/2) in the solid state that converts to intermediate spin (S=3/2) in solution. An extensive computational study by using a range of DFT methods was performed on 2⋅TNP and 2⁺ , and clearly supports the experimentally observed spin flip triggered by hydrogen-bonding interactions. The counterion is shown to perturb the spin-state ordering through, for example, hydrogen-bonding interactions, switched positions between counterion and axial ligand, ion-pair interactions, and charge polarization. The present investigation thus provides a clear rationalization of the unusual counterion-specific spin states observed in the μ-hydroxo bisporphyrins that have so far remained the most outstanding issue.

Classical Characterization Techniques to Reveal the Structural Model of Nanocomposites with Bimetallic Monolayers of Porphyrins

Nanocomposites with bimetallic monolayers of porphyrins were prepared. The well-ordered metalloporphyrin monolayers covalently linked to the gold surface produce an important increase of the B band (∼400 nm) shifted 20 nm relative to that of the related high-spin iron(III) complexes in solution. The position of the B band in the bimetallic architectures is highly dependent on the relative amount of the two porphyrins, showing the most significant shift for the SiO2/APTES/AuNp/Fe-TPyP&M-TPyP (1:1) (30 nm, M = Ni(II) or Cu(II)). Resonance Raman based on the oxidation state marker bands (1553, 1354, and 390 cm(-1)) indicates that Fe-TPyP attached on gold nanoparticles adopts a low-spin Fe(II) conformation, which changes to Fe(II) intermediate spin or a low-spin Fe(III) in the presence of Cu-TPyP or Ni-TPyP. Surface-enhanced Raman scattering studies confirmed the hypothesis. MALDI-TOF analysis of the composites on gold nanoparticles was very useful in the detection of oxygenated forms of the metal complexes.

Functional models of nonheme diiron enzymes: kinetic and computational evidence for the formation of oxoiron(iv) species from peroxo-diiron(iii) complexes, and their reactivity towards phenols and H2O2

The reactivity of the previously reported peroxo adducts [Fe2(μ-O2)(L(1))4(CH3CN)2](2+), and [Fe2(μ-O2)(L(2))4(CH3CN)2](2+), (L(1) = 2-(2'-pyridyl)benzimidazole and L(2) = 2-(2'-pyridyl)-N-methylbenzimidazole) towards H2O2 as catalase mimics, and towards various phenols as functional RNR-R2 mimics, is described. Kinetic, mechanistic and computational studies gave direct evidence for the involvement of the (μ-1,2-peroxo)diiron(iii) intermediate in the O-H activation process via formation of low-spin oxoiron(iv) species.

New mechanistic insight into intramolecular arene hydroxylation initiated by (μ-1,2-peroxo)diiron(III) complexes with dinucleating ligands

(μ-1,2-Peroxo)diiron(iii) complexes (-R) with dinucleating ligands (R-L) generated from the reaction of bis(μ-hydroxo)diiron(ii) complexes [Fe2(R-L)(OH)2](2+) (-R) with dioxygen in acetone at -20 °C provide a diiron-centred electrophilic oxidant, presumably diiron(iv)-oxo species, which is involved in aromatic ligand hydroxylation.

Deprotonation of Water Ligands in V, Cr, Mn, Fe, and Co Complexes Reduces Oxidation-Driven Carboxylate Ligand Frequency Shifts

In Mn complexes, it has been shown that oxidation-driven changes in carboxylate ligand vibrations are suppressed, if a water or hydroxo ligand is simultaneously deprotonated. Deprotonation with oxidation has also been shown to greatly reduce the dependence of Mn complex redox energies on the oxidation state of the metal. We have here investigated the effect of oxidation with deprotonation on the carboxylate ligand frequencies of V, Cr, Mn, Fe, and Co complexes. The effects of anionic ligand substitution (instead of deprotonation) and solvent dielectric were also investigated to determine the mechanism that drives carboxylate frequency shifts. It is shown that the effect of deprotonation was similar for all of the metals tested in this study. C-O bond lengths and O-C-O angle changes in the carboxylate ligand were also reduced by deprotonation. Furthermore, the effect of anionic ligand substitution was similar to deprotonation in the suppression of carboxylate frequency shifts. These shifts were also reduced by increases in the solvent dielectric, in the absence of charge conservation through deprotonation. Therefore, we conclude that carboxylate frequency shifts are largely driven by electrostatic effects.

Highly selective fluorescence sensors for the fluoride anion based on carboxylate-bridged diiron complexes

The diiron–sulfur clusters bearing urea and anthracene units showed rapid and selective recognition for the fluoride ion.

Non-heme μ-Oxo- and bis(μ-carboxylato)-bridged diiron(iii) complexes of a 3N ligand as catalysts for alkane hydroxylation: stereoelectronic factors of carboxylate bridges determine the catalytic efficiency

The stereoelectronic factors of carboxylate bridges in diiron(iii) complexes determine the efficiency of catalytic alkane hydroxylation with m-CPBA.

Design and synthesis of a dinucleating ligand system with varying terminal donor functions that provides no bridging donor and its application to the synthesis of a series of FeIII–μ-O–FeIII complexes

We have developed the dinucleating ligands H₄julia, susan, and H₄hilde^Me2 and present their μ-oxo diferric complexes.

Synthesis and photophysical studies of tetrazolate-based Eu(III) photoluminescent ternary complexes containing N-heterocyclic phosphine oxides auxiliary co-ligands

Two new ternary tetrazolate Eu(III) complexes with phosphine oxide co-ligands Eu(PTO)3 ·(P1/P2) [PTO = 5-(2-pyridyl-1-oxide)tetrazole, P1 = diphenylphosphorylamino-phenylphosphoryl-benzene, P2 = diphenylphosphorylpyridine)-bis-isobutyricphosphoryl] were synthesized and characterized using UV, fluorescence, IR and (1) H NMR spectroscopic techniques. The analytical data prove that the complexes are mononuclear in nature and the central Eu(III) ion is coordinated by three N and three O atoms of tetrazolate, and two O atoms of the corresponding bidentate phosphine oxide ligands. The ancillary ligand increased the photoluminescence efficiency of Eu(PTO)3 ·P1 (complex 3) by twofold compared with our previously reported Eu(PTO)3 complex (complex 1). Copyright © 2015 John Wiley & Sons, Ltd.

An Iron-based Film for Highly Efficient Electrocatalytic Oxygen Evolution from Neutral Aqueous Solution

An ultrathin Fe-based film was prepared by electrodeposition from an Fe(II) solution through a fast and simple cyclic voltammetry method. The extremely low Fe loading of 12.3 nmol cm(-2) on indium tin oxide electrodes is crucial for high atom efficiency and transparence of the resulted film. This Fe-based film was shown to be a very efficient electrocatalyst for oxygen evolution from neutral aqueous solution with remarkable activity and stability. In a 34 h controlled potential electrolysis at 1.45 V (vs NHE) and pH 7.0, impressive turnover number of 5.2 × 10(4) and turnover frequency of 1528 h(-1) were obtained. To the best of our knowledge, these values represent one of the highest among electrodeposited catalyst films for water oxidation under comparable conditions. The morphology and the composition of the catalyst film was determined by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray, and X-ray photoelectron spectroscopy, which all confirmed the deposition of Fe-based materials with Fe(III) oxidation state on the electrode. This study is significant because of the use of iron, the fast and simple cyclic voltammetry electrodeposition, the extremely low catalyst loading and thus the transparency of the catalyst film, the remarkable activity and stability, and the oxygen evolution in neutral aqueous media.

Synthesis of carboxylate-bridged iron-thiolate clusters from alcohols/aldehydes or carboxylate salts

A series of novel carboxylate-bridged cyclopentadienyl diiron complexes [Cp*Fe(μ-SEt)2(μ-η(2)-OOCR)FeCp*][PF6] (, R = H; , R = Me; , R = Et; , R = Pr-n; , R = Ph; , R = p-Me-C6H4; , R = PhCH[double bond, length as m-dash]CH; , CH[triple bond, length as m-dash]C) were obtained from alcohols/aldehydes or sodium carboxylates at room temperature. These eight complexes were fully characterized by spectroscopy, and some of them (, , and ) were further studied by X-ray crystallography. In addition, the electrochemical properties of clusters and are also discussed.

Synthesis, crystal structure and investigation of mononuclear copper(II) and zinc(II) complexes of a new carboxylate rich tripodal ligand and their interaction with carbohydrates in alkaline aqueous solution

A new carboxylate rich asymmetric tripodal ligand, N-[2-carboxybenzomethyl]-N-[carboxymethyl]-β-alanine (H3camb), and its di-copper(II), (NH4)2[1]2, and di-zinc(II), ((CH3)4N)2[2]2, complexes have been synthesized as carbohydrate binding models in aqueous solutions. The ligand and complexes have been fully characterized using several techniques, including single crystal X-ray diffraction. The interactions of (NH4)2[1]2 and ((CH3)4N)2[2]2 with D-glucose, D-mannose, D-xylose and xylitol in aqueous alkaline media were investigated using UV-Vis and (13)C-NMR spectroscopic techniques, respectively. The molar conductance, NMR and ESI-MS studies indicate that the complexes dissociate in solution to produce the respective complex anions, 1(-) and 2(-). Complexes 1(-) and 2(-) showed chelating ability towards the naturally abundant and biologically relevant sugars, D-glucose, D-mannose, D-xylose, and xylitol. The complex ions bind to one molar equivalent of the sugars, even in the presence of stoichiometric excess of the substrates, in solution. Experimentally obtained spectroscopic data and computational results suggest that the substrates bind to the metal center in a bidentate fashion. Apparent binding constant values, pK(app), between the complexes and the substrates were determined and a specific mode of substrate binding is proposed. The pK(app) and relativistic density functional theory (DFT) calculated Gibbs free energy values indicate that D-mannose displayed the strongest interaction with the complexes. Syntheses, characterizations, detailed substrate binding studies using spectroscopic techniques, single crystal X-ray diffraction and geometry optimizations of the complex-substrates with DFT calculations are also reported.

Site-selective formation of an iron(iv)-oxo species at the more electron-rich iron atom of heteroleptic μ-nitrido diiron phthalocyanines

A combination of MS and computation on μ-nitrido bridged diiron complexes reveals H₂O₂ binding to the complex and generates an oxidant capable of oxidizing methane.

Iron(iv)–oxo species have been identified as the active intermediates in key enzymatic processes, and their catalytic properties are strongly affected by the equatorial and axial ligands bound to the metal, but details of these effects are still unresolved. In our aim to create better and more efficient oxidants of H-atom abstraction reactions, we have investigated a unique heteroleptic diiron phthalocyanine complex. We propose a novel intramolecular approach to determine the structural features that govern the catalytic activity of iron(iv)–oxo sites. Heteroleptic μ-nitrido diiron phthalocyanine complexes having an unsubstituted phthalocyanine (Pc1) and a phthalocyanine ligand substituted with electron-withdrawing alkylsulfonyl groups (PcSO₂R) were prepared and characterized. A reaction with terminal oxidants gives two isomeric iron(iv)–oxo and iron(iii)–hydroperoxo species with abundances dependent on the equatorial ligand. Cryospray ionization mass spectrometry (CSI-MS) characterized both hydroperoxo and diiron oxo species in the presence of H₂O₂. When m-CPBA was used as the oxidant, the formation of diiron oxo species (PcSO₂R)FeNFe(Pc1) Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O was also evidenced. Sufficient amounts of these transient species were trapped in the quadrupole region of the mass-spectrometer and underwent a CID-MS/MS fragmentation. Analyses of fragmentation patterns indicated a preferential formation of hydroperoxo and oxo moieties at more electron-rich iron sites of both heteroleptic μ-nitrido complexes. DFT calculations show that both isomers are close in energy. However, the analysis of the iron(iii)–hydroperoxo bond strength reveals major differences for the (Pc1)FeN(PcSO₂R)Fe^IIIOOH system as compared to (PcSO₂R)FeN(Pc1)Fe^IIIOOH system, and, hence binding of a terminal oxidant will be preferentially on more electron-rich sides. Subsequent kinetics studies showed that these oxidants are able to even oxidize methane to formic acid efficiently.

Redox and acid-base properties of asymmetric non-heme (hydr)oxo-bridged diiron complexes

The diiron unit is commonly found as the active site in enzymes that catalyze important biological transformations. Two μ-(hydr)oxo-diiron(iii) complexes with the ligands 2,2'-(2-methyl-2-(pyridine-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)diphenol (H2L) and 2,2'-(2-methyl-2(pyridine-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)bis(4-nitrophenol) (H2L(NO2)), namely [(FeL)2(μ-O)] () and [(FeL(NO2))2(μ-OH)]ClO4 () were synthesized and characterized. In the solid state, both structures are asymmetric, with unsupported (hydr)oxo bridges. Intramolecular hydrogen bonding of the ligand NH groups to the phenolate O atoms hold the diiron cores in a bent configuration (Fe-O-Fe angle of 143.7° for and 140.1° for ). A new phenolate bridged diferrous complex, [(FeL)2] (), was synthesized and characterized. Upon exposure to air the diferrous complex is oxidized to the diferric . Cyclic voltammetry at different scan rates and chemical reduction of [(FeL)2(μ-OH)]BPh4 () with cobaltocene revealed disproportionation followed by proton transfer, and a mixed-valence species could not be trapped. Subsequent exposure to molecular oxygen results in the formation of . Electrochemical studies of indicate easier reduction of the diiron(iii/iii) to the mixed-valence state than for . The protonation of by benzoic acid to form [(FeL)2(μ-OH)](+) only changes the Fe-O-Fe angle by 5° (from 143.7° to 138.6°), and the pKa of the hydroxo bridge is estimated to be about 20.4. We attribute this high pKa partly to stabilization of the benzoate by hydrogen bonding to the ligand's amine proton. Magnetic susceptibility studies on solid samples of and yielded values of the antiferromagnetic exchange coupling constants, J, for these S = 5/2 dimers of -13.1 cm(-1) and -87.5 cm(-1), respectively, typical of such unsupported hydroxo- and oxo-bridges.