Design, synthesis and reactivity of dimolybdenum complex bearing quaterphenylene-bridged pyridine-based PNP-type pincer ligand.

Dimolybdenum complexes bearing 3,3'''-(1,1':3',1'':3'',1'''-quaterphenylene)-bridged pyridine-based PNP-type pincer ligand are designed and prepared according to DFT calculations on the cleavage step of dinitrogen-bridged dimolybdenum complexes bearing polyphenylene-bridged pyridine-based PNP-type pincer ligands. The dimolybdenum complexes are found to work as effective catalysts toward ammonia formation from dinitrogen with samarium diiodide as a reductant and water as a proton source under ambient reaction conditions.

Stacking of a Cofacially Stacked Iron Phthalocyanine Dimer on Graphite Achieved High Catalytic CH4 Oxidation Activity Comparable to That of pMMO.

Numerous biomimetic molecular catalysts inspired by methane monooxygenases (MMOs) that utilize iron or copper-oxo species as key intermediates have been developed. However, the catalytic methane oxidation activities of biomimetic molecule-based catalysts are still much lower than those of MMOs. Herein, we report that the close stacking of a µ-nitrido-bridged iron phthalocyanine dimer onto a graphite surface is effective in achieving high catalytic methane oxidation activity. The activity is almost 50 times higher than that of other potent molecule-based methane oxidation catalysts and comparable to those of certain MMOs, in an aqueous solution containing H2O2. It was demonstrated that the graphite-supported µ-nitrido-bridged iron phthalocyanine dimer oxidized methane, even at room temperature. Electrochemical investigation and density functional theory calculations suggested that the stacking of the catalyst onto graphite induced partial charge transfer from the reactive oxo species of the µ-nitrido-bridged iron phthalocyanine dimer and significantly lowered the singly occupied molecular orbital level, thereby facilitating electron transfer from methane to the catalyst in the proton-coupled electron-transfer process. The cofacially stacked structure is advantageous for stable adhesion of the catalyst molecule on the graphite surface in the oxidative reaction condition and for preventing decreases in the oxo-basicity and generation rate of the terminal iron-oxo species. We also demonstrated that the graphite-supported catalyst exhibited appreciably enhanced activity under photoirradiation owing to the photothermal effect.

Selective methane oxidation by molecular iron catalysts in aqueous medium.

Using natural gas as chemical feedstock requires efficient oxidation of the constituent alkanes-and primarily methane1,2. The current industrial process uses steam reforming at high temperatures and pressures3,4 to generate a gas mixture that is then further converted into products such as methanol. Molecular Pt catalysts5-7 have also been used to convert methane to methanol8, but their selectivity is generally low owing to overoxidation-the initial oxidation products tend to be easier to oxidize than methane itself. Here we show that N-heterocyclic carbene-ligated FeII complexes with a hydrophobic cavity capture hydrophobic methane substrate from an aqueous solution and, after oxidation by the Fe centre, release a hydrophilic methanol product back into the solution. We find that increasing the size of the hydrophobic cavities enhances this effect, giving a turnover number of 5.0 × 102 and a methanol selectivity of 83% during a 3-h methane oxidation reaction. If the transport limitations arising from the processing of methane in an aqueous medium can be overcome, this catch-and-release strategy provides an efficient and selective approach to using naturally abundant alkane resources.

Theoretical Study on the Contribution of Interfacial Functional Groups to the Adhesive Interaction between Epoxy Resins and Aluminum Surfaces.

To ensure the quality and reliability of products bonded by epoxy resin adhesives, elucidation of the microscopic adhesion mechanism is essential. The adhesive interaction and bonding strength between epoxy resins and hydroxylated γ-alumina (001) surfaces were investigated by using a combined molecular dynamics (MD) and density functional theory (DFT) study. The curing reaction of an epoxy resin consisting of diglycidyl ether of bisphenol A (DGEBA) and 4,4'-diaminodiphenyl sulfone (DDS) was simulated. The resin structure was divided into fragmentary structures to study the interaction of each functional group with the alumina surface using DFT calculations. From the characteristics of the adhesive structures and the calculated adhesion energies, it was found that the fragments forming hydrogen bonds with hydroxy groups on the alumina surface resulted in large adhesion energies. On the other hand, the fragments adsorbed on the alumina surface via dispersion interactions resulted in small adhesion energies. The adhesion forces evaluated from the Hellmann-Feynman force calculations indicated the significant contribution of the hydroxy groups and benzene ether moieties derived from DGEBA to the adhesive stress of the DGEBA/DDS epoxy resin. The direction of hydrogen bonding between the epoxy resin and the surface and the difference in geometry at the interface between the donor and acceptor of hydrogen bonding played a central role in maintaining the adhesive strength during the failure process of the adhesive interface.

Theoretical Investigation into Selective Benzene Hydroxylation by Ruthenium-Substituted Keggin-Type Polyoxometalates.

Benzene hydroxylation catalyzed by ruthenium-substituted Keggin-type polyoxometalates [RuV(O)XW11O39]n- (RuVOX; X = Al, Ga, Si, Ge, P, As, S; heteroatoms; 3 ≤ n ≤ 6) is investigated using the density functional theory approach. As a possible side reaction, the water oxidation reaction is also considered. We found that the rate-determining step for water oxidation by RuVOX requires a higher activation free energy than the benzene hydroxylation reaction, suggesting that all of the RuVOX catalysts show high chemoselectivity toward benzene hydroxylation. Additionally, the heteroatom effect in benzene hydroxylation by RuVOX is discussed. The replacement of Si by X induces changes in the bond length of µ4O-X, resulting in a change in the activation free energy for benzene hydroxylation by RuVOX. Consequentially, RuVOS is expected to be the most effective catalyst among the (RuVOX) catalysts for the benzene hydroxylation reaction.

Augmented Self-Association by Electrostatic Forces in Thienopyrrole-Fused Thiadiazoles that Contain an Ester instead of an Ether Linker.

During the self-assembly of π-conjugated molecules, linkers and substituents can potentially add supportive noncovalent intermolecular interactions to π-stacking interactions. Here, we report the self-assembly behavior of thienopyrrole-fused thiadiazole (TPT) fluorescent dyes that possess ester or ether linkers and dodecyloxy side chains in solution and the condensed phase. A comparison of the self-association behavior of the ester- and ether-bridged compounds in solution using detailed UV-vis, fluorescence, and NMR spectroscopic studies revealed that the subtle replacement of the ether linkers by ester linkers leads to a distinct increase in the association constant (ca. 3-4 fold) and the enthalpic contribution (ca. 3 kcal mol-1 ). Theoretical calculations suggest that the ester linkers, which are in close proximity to one another due to the π-stacking interactions, induce attractive electrostatic forces and augment self-association. The self-assembly of TPT dyes into well-defined 1D clusters with high aspect ratios was observed, and their morphologies and crystallinity were investigated using SEM and X-ray diffraction analyses. TPTs with ester linkers exhibit a columnar liquid crystalline mesophase in the condensed phase.

Mechanistic Insights into the Dicopper-Complex-Catalyzed Hydroxylation of Methane and Benzene Using Nitric Oxide: A DFT Study.

Although hydrocarbons are known to act as reductants for the catalytic reduction of nitric oxides (NOx) over copper-based catalysts, the reaction mechanism requires clarification. Herein, density functional theory (DFT) calculations were carried out to investigate the reduction mechanisms of NOx to dinitrogen coupled to the hydroxylation of methane or benzene using the dicopper complex reported by Zhang and co-workers [ J. Am. Chem. Soc. 2019, 141, 10159-10164]. The B3LYP functional was used to optimize the (µ-oxo)(µ-nitrosyl)dicopper complex in the quartet state and the (µ-η2:η2-NO2)dicopper complex in the doublet state, the latter of which was found to be the ground state. Then, we investigated the reactivities of the (µ-η2:η2-NO2)dicopper complex toward methane and benzene by considering the conversions of N2O to N2 in the presence and the absence of methane or benzene. In the presence of methane and benzene, the calculated activation energies were 27.0 and 21.0 kcal/mol, respectively, whereas that with N2O alone was prohibitively high (61.9 kcal/mol). Thus, the (µ-η2:η2-NO2)dicopper complex prefers the reactions with methane and benzene to that with N2O. The reaction of the (µ-η2:η2-NO2)dicopper complex with methane or benzene generated the (µ-nitrosyl)dicopper complex. The (µ-nitrosyl)dicopper complex then reacted with N2O to regenerate the (µ-η2:η2-NO2)dicopper complex and N2 with an activation barrier of 31.5 kcal/mol. The overall reactions for methane and benzene hydroxylation were calculated to be exothermic by 41.7 and 54.1 kcal/mol, respectively. These results suggest that the catalytic reduction of NOx using hydrocarbons is feasible at certain operating temperatures. Thus, our calculations provide new insights into the design of catalysts for NOx purification.

Three-Step Spin State Transition and Hysteretic Proton Transfer in the Crystal of an Iron(II) Hydrazone Complex.

A proton-electron coupling system, exhibiting unique bistability or multistability of the protonated state, is an attractive target for developing new switchable materials based on proton dynamics. Herein, we present an iron(II) hydrazone crystalline compound, which displays the stepwise transition and bistability of proton transfer at the crystal level. These phenomena are realized through the coupling with spin transition. Although the multi-step transition with hysteresis has been observed in various systems, the corresponding behavior of proton transfer has not been reported in crystalline systems; thus, the described iron(II) complex is the first example. Furthermore, because proton transfer occurs only in one of the two ligands and π electrons redistribute in it, the dipole moment of the iron(II) complexes changes with the proton transfer, wherein the total dipole moment in the crystal was canceled out owing to the antiferroelectric-like arrangement.

Intermediate-Spin Iron(III) Complexes Having a Redox-Noninnocent Macrocyclic Tetraamido Ligand.

An iron(III) complex having a dibenzotetraethyltetraamido macrocyclic ligand (DTTM4-), (NEt4)2[FeIII(DTTM)Cl] (1), was synthesized and characterized by crystallographic, spectroscopic, and electrochemical methods. Complex 1 has a square-pyramidal structure in the S = 3/2 spin state. The complex exhibited two reversible redox waves at +0.36 and +0.68 V (vs SCE) in the cyclic voltammogram measured in CH2Cl2 at room temperature. The stepwise oxidation of 1 using chemical oxidants allowed us to observe clear and distinct spectral changes with distinct isosbestic points for each step, in which oxidation occurred at the phenylenediamido moiety rather than the iron center. One-electron oxidation of 1 by 1 equiv of [RuIII(bpy)3](ClO4)3 (bpy = 2,2'-bipyridine) in CH2Cl2 afforded square-pyramidal (NEt4)[Fe(DTTM)Cl] (2), which was in the S = 1 spin state involving a ligand radical and showed a slightly distorted square-pyramidal structure. Complex 2 showed an intervalence charge-transfer band at 900 nm, which was assigned on the basis of time-dependent density functional theory calculations, to indicate that the complex is in a class IIA mixed-valence ligand-radical regime with Hab = 884 cm-1. Two-electron oxidation of 1 by 2 equiv of [(4-Br-Ph)3N•+](SbCl6) in CH2Cl2 afforded two-electron-oxidized species of 1, [Fe(DTTM)Cl] (3), which was in the S = 1/2 spin state; complex 3 exhibited a distorted square-pyramidal structure. X-ray absorption near-edge structure spectra of 1-3 were measured in both CH3CN solutions and BN pellets to observe comparable rising-edge energies for the three complexes, and Mössbauer spectra of 1-3 showed almost identical isomer shifts and quadruple splitting parameters, indicating that the iron centers of the three complexes are intact to be in the intermediate-spin iron(III) state. Thus, in complexes 2 and 3, it is evident that antiferromagnetic coupling is operating between the unpaired electron(s) of the ligand radical(s) and those of the iron(III) center.

Specific Enhancement of Catalytic Activity by a Dicopper Core: Selective Hydroxylation of Benzene to Phenol with Hydrogen Peroxide.

A dicopper(II) complex, stabilized by the bis(tpa) ligand 1,2-bis[2-[bis(2-pyridylmethyl)aminomethyl]-6-pyridyl]ethane (6-hpa), [Cu2 (µ-OH)(6-hpa)]3+ , was synthesized and structurally characterized. This complex catalyzed selective hydroxylation of benzene to phenol using H2 O2 , thus attaining large turnover numbers (TONs) and high H2 O2 efficiency. The TON after 40 hours for the phenol production exceeded 12000 in MeCN at 50 °C under N2 , the highest value reported for benzene hydroxylation with H2 O2 catalyzed by homogeneous complexes. At 22 % benzene conversion, phenol (95.2 %) and p-benzoquinone (4.8 %) were produced. The mechanism of H2 O2 activation and benzene hydroxylation is proposed.

Theoretical Study of the Catalytic Hydrogenation of Alkenes by a Disilaferracyclic Complex: Can the Fe-Si σ-Bond-Assisted Activation of H-H Bonds Allow Development of a Catalysis of Iron?

The mechanisms associated with the hydrogenation of alkenes catalyzed by the iron complex Fe(cis-CO)2{o-(SiMe2)2C6H4}2(H)2 (1) were investigated by DFT calculations. The complex 1 has a structure in which the iron center is bonded to four silicon atoms and two hydrides. Secondary Si···H···Si interactions were also observed. The exchange of a 1,2-bis(dimethylsilyl)benzene ligand with ethylene and hydrogen gives a disilaferracycle bearing η2-(CH2═CH2) and η2-H2 ligands. The catalytic cycle initiated from the disilaferracycle involves cleavage of a H-H linkage assisted by an Fe-Si bond to form Fe-H and η1-(H-Si) moieties (step 1), hydrogen migration from the Fe-H group to the η2-(CH2═CH2) ligand which accomplishes the insertion of ethylene into the Fe-H bond (step 2), and reaction of the resulting ß-agostic ethyl moiety with the η2-(H-Si) group to form ethane on the iron atom (step 3). The octahedral geometry of 1 as well as the presence of π-acidic CO ligands and Fe-Si σ-bonds contributes to all of the catalytic intermediates and the transition states being in the low-spin state. Steps 1 and 3 correspond to the σ-complex-assisted metathesis (σ-CAM) mechanisms proposed by Perutz and Sabo-Etienne, suggesting that these mechanisms can assist in the design of iron-based hydrogenation catalysts operating under mild conditions.

Catalytic transformation of dinitrogen into ammonia and hydrazine by iron-dinitrogen complexes bearing pincer ligand.

Synthesis and reactivity of iron-dinitrogen complexes have been extensively studied, because the iron atom plays an important role in the industrial and biological nitrogen fixation. As a result, iron-catalyzed reduction of molecular dinitrogen into ammonia has recently been achieved. Here we show that an iron-dinitrogen complex bearing an anionic PNP-pincer ligand works as an effective catalyst towards the catalytic nitrogen fixation, where a mixture of ammonia and hydrazine is produced. In the present reaction system, molecular dinitrogen is catalytically and directly converted into hydrazine by using transition metal-dinitrogen complexes as catalysts. Because hydrazine is considered as a key intermediate in the nitrogen fixation in nitrogenase, the findings described in this paper provide an opportunity to elucidate the reaction mechanism in nitrogenase.

Persistent four-coordinate iron-centered radical stabilized by π-donation.

Dinuclear iron carbonyl complex 2, which contains an elongated unsupported Fe-Fe bond, was synthesized by the reaction between Fe2(CO)9 and phosphinyl radical 1. Thermal Fe-Fe bond homolysis led to the generation of a four-coordinate carbonyl-based iron-centered radical, 3, which is stabilized by π-donation. Complex 3 exhibited high reactivity toward organic radicals to form diamagnetic five-coordinate Fe(ii) complexes.

Iron-catalysed transformation of molecular dinitrogen into silylamine under ambient conditions.

Although stoichiometric transformations using transition metal-N(2) complexes have been well investigated towards the goal of nitrogen fixation under mild reaction conditions, only a few examples of the catalytic transformations of N(2) using transition metal-N(2) complexes as catalysts have been reported. In almost all the catalytic systems, the use of Mo is essential to realize the catalytic transformation of N(2), where Mo-N(2) complexes are considered to work as effective catalysts. Here we show the first successful example of the Fe-catalysed transformation of N(2) into N(SiMe(3))(3) under ambient conditions, in which iron complexes such as iron pentacarbonyl [Fe(CO)(5)] and ferrocenes have been found to work as effective catalysts. A plausible reaction pathway is proposed, where Fe(II)-N(2) complex bearing two Me(3)Si groups as ancillary ligands has an important role as a key reactive intermediate, with the aid of density-functional-theory calculations.

Initial step of B12-dependent enzymatic catalysis: energetic implications regarding involvement of the one-electron-reduced form of adenosylcobalamin cofactor.

Density functional theory analysis was performed to elucidate the impact of one-electron reduction upon the initial step of adenosylcobalamin-dependent enzymatic catalysis. The transition state (TS) corresponding to the Co-C bond cleavage and subsequent hydrogen abstraction from the substrate was located. The intrinsic reaction coordinate calculations predicted that the reaction consisting of Co-C5' bond cleavage in [Co(III)(corrin(•))]-Rib (where Rib is ribosyl) and hydrogen-atom abstraction from the CH(3)-CH(2)-CHO substrate occurs in a concerted fashion. The computed activation energy barrier of the reaction (15.0 kcal/mol) was lowered by approximately 54.5% in comparison with the reaction involving the positively charged cofactor model (Im-[Co(III)(corrin)]-Rib(+), where Im is imidazole; energy barrier = 33.0 kcal/mol). The Im base was detached during the TS search in the reaction involving the one-electron-reduced analogue. Thus, to compare the energetics of the two reactions, the axial Im ligand detachment energy for the Im-[Co(III)(corrin(•))]-Rib model was computed [7.6 kcal/mol (gas phase); 4.6 kcal/mol (water)]. Consequently, the effective activation energy barrier for the reaction mediated by the Im-off [Co(III)(corrin(•))]-Rib was estimated to be 22.6 kcal/mol, which implied an overall 31.5% reduction in the energetic demands of the reaction. Considering that the lengthened Co-N(axial) bond has been observed in X-ray crystal structure studies of B(12)-dependent mutases, the catalytic impact induced by one-electron reduction of the cofactor is expected to be higher in the presence of the enzymatic environment.

Generation of adenosyl radical from S-adenosylmethionine (SAM) in biotin synthase.

A mechanism of the C-S bond activation of S-adenosylmethionine (SAM) in biotin synthase is discussed from quantum mechanical/molecular mechanical (QM/MM) computations. The active site of the enzyme involves a [4Fe-4S] cluster, which is coordinated to the COO(-) and NH(2) groups of the methionine moiety of SAM. The unpaired electrons on the iron atoms of the [4Fe-4S](2+) cluster are antiferromagnetically coupled, resulting in the S=0 ground spin state. An electron is transferred from an electron donor to the [4Fe-4S](2+)-SAM complex to produce the catalytically active [4Fe-4S](+) state. The SOMO of the [4Fe-4S](+)-SAM complex is localized on the [4Fe-4S] moiety and the spin density of the [4Fe-4S] core is calculated to be 0.83. The C-S bond cleavage is associated with the electron transfer from the [4Fe-4S](+) cluster to the antibonding σ* C-S orbital. The electron donor and acceptor states are effectively coupled with each other at the transition state for the C-S bond cleavage. The activation barrier is calculated to be 16.0 kcal/mol at the QM (B3LYP/SV(P))/MM (CHARMm) level of theory and the C-S bond activation process is 17.4 kcal/mol exothermic, which is in good agreement with the experimental observation that the C-S bond is irreversibly cleaved in biotin synthase. The sulfur atom of the produced methionine molecule is unlikely to bind to an iron atom of the [4Fe-4S](2+) cluster after the C-S bond cleavage from the energetical and structural points of view.

Theoretical analysis of the diradical nature of adenosylcobalamin cofactor-tyrosine complex in B12-dependent mutases: inspiring PCET-driven enzymatic catalysis.

Detailed theoretical and X-ray based structural analysis has been carried out in order to unmask the role of the tyrosine residue (Y) in the activation of the Co-C bond in AdoCbl-dependent mutases. In particular, methylmalonyl-CoA mutase (MCM) and glutamate mutase (GLM) enzymes have been studied; in the case of MCM, the significance of the Y89 residue has been analyzed extensively. Three different theoretical platforms encompassing the DFT, CASSCF/QDPT2, and QM/MM frameworks have been employed to elucidate the energetics of the AdoCbl-Y(-) complex while taking into account a varied degree of structural complexity. The diradical state, [AdoCbl](*-)-Y*, has been found to be the lowest electronic state of the AdoCbl-Y(-) complex, providing strong evidence that electron transfer from the Y89 residue to the cofactor is feasible. Crystallographic analysis of the active sites of MCM and GLM enzymes reveals that substrate binding can play a critical role in displacing the hydroxyl proton of the Y residue (Y89 in the case of MCM enzyme and Y181 in the case of GLM enzyme) that will facilitate the electron transfer (ET), hence making the activation process a case of proton-coupled electron transfer (PCET). PCET-inspired enzymatic catalysis implies that the cleavage of the Co-C bond takes place via one-electron reduced form of the AdoCbl cofactor (i.e., [AdoCbl](*-)), rather than its neutral analogue, thus providing an efficient mode of cleavage that can help in understanding the origin of the catalytic effect in such enzymes.

Nonradical mechanism for methane hydroxylation by iron-oxo complexes.

A nonradical mechanism for methane hydroxylation by the bare FeO+ complex, Fe-ZSM-5 zeolite, and soluble methane monooxygenase is proposed from quantum chemical calculations. This mechanism is applicable when a metal-oxo species is coordinatively unsaturated. Direct interaction between methane and a metal active center can form a weakly bound methane complex in the initial stages of this reaction. Subsequent C-H bond cleavage to form an intermediate with an HO-Fe-CH3 moiety in a nonradical manner and recombination of the resultant OH and CH3 ligands take place at a metal active center to form a final methanol complex. Thus, this is a nonradical, two-step reaction. The fact that methyl radical is 10-20 kcal/mol less stable than secondary and tertiary carbon radicals and benzyl radicals leads us to propose this mechanism.

How axial ligands control the reactivity of high-valent iron(IV)-oxo porphyrin pi-cation radicals in alkane hydroxylation: a computational study.

The push effect of anionic axial ligands of high-valent iron(IV)-oxo porphyrin pi-cation radicals, (Porp)(+.)Fe(IV)(O)(X) (X=OH(-), AcO(-), Cl(-), and CF(3)SO(3)(-)), in alkane hydroxylation is investigated by B3LYP DFT calculations. The electron-donating ability of anionic axial ligands influences the activation energy for the alkane hydroxylation by the iron(IV)-oxo intermediates and the Fe-O bond distance of the iron-oxo species in transition state.