Effect of Condensed Water at an Alumina/Epoxy Resin Interface on Curing Reaction.

The adhesion of epoxy adhesives to aluminum materials is an important issue in assembling parts for lightweight mobility. Aluminum surfaces typically possess an oxide layer, which readily adsorbs water. In this study, the aggregation states of water and its effect on the curing reaction were examined by placing a water layer between an amorphous alumina surface and a mixture of epoxy and amine components. This study used molecular dynamics simulations and density functional theory calculations. Before the reaction, water molecules strongly adsorbed onto the alumina surface, aggregating excess water. Some water diffused into the epoxy/amine mixture, accelerating the diffusion of unreacted substances. This led to faster reaction kinetics, particularly in proximity to the alumina surface. The adsorption of water molecules onto the alumina surface and the aggregation of excess water were similarly observed even after the curing process. Subsequently, the interaction between the alumina surface and various functional groups of the epoxy/amine mixture was evaluated before and after the reaction. Epoxy monomers had little interaction with the alumina surface before the reaction, whereas hydroxy groups formed by the ring-opening reaction of epoxy groups exhibited notable interaction. Conversely, sulfonyl and amino groups in amine compounds formed hydrogen bonds with OH groups on the alumina surface before the reaction. However, after the reaction, amino groups weakened their interaction with the alumina OH groups as they transformed from primary to tertiary during the curing reaction. Both epoxy and amine monomers/fragments similarly interacted with water molecules, both before and after the reaction. The insights gained from this study are expected to contribute to a better understanding of the impact of moisture absorption on the application of epoxy resins.

Polar Crystals Using Molecular Chirality: Pseudosymmetric Crystallization toward Polarization Switching Materials.

Polar compounds with switchable polarization properties are applicable in various devices such as ferroelectric memory and pyroelectric sensors. However, a strategy to prepare polar compounds has not been established. We report a rational synthesis of a polar CoGa crystal using chiral cth ligands (SS-cth and RR-cth, cth = 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). Both the original homo metal Co crystal and Ga crystal exhibit a centrosymmetric isostructure, where the dipole moment of metal complexes with the SS-cth ligand and those with the RR-cth ligand are canceled out. To obtain a polar compound, the Co valence tautomeric complex with SS-cth in the homo metal Co crystal is replaced with the Ga complex with SS-cth by mixing Co valence tautomeric complexes with RR-cth and Ga complexes with SS-cth. The CoGa crystal exhibits polarization switching between the pseudononpolar state at a low temperature and the polar state at a high temperature because only Co complexes exhibit changes in electric dipole moment due to metal-to-ligand charge transfer. Following the same strategy, the polarization-switchable CoZn complex was synthesized. The CoZn crystal exhibits polarization switching between the polar state at a low temperature and the pseudononpolar state at a high temperature, which is the opposite temperature dependence to that of the CoGa crystal. These results revealed that the polar crystal can be synthesized by design, using a chiral ligand. Moreover, our method allows for the control of temperature-dependent polarization changes, which contrasts with typical ferroelectric compounds, in which the polar ferroelectric phase typically occurs at low temperatures.

Electronic Interaction of Epoxy Resin with Copper at the Adhered Interface.

A better understanding of the aggregation states of adhesive molecules in the interfacial region with an adherend is crucial for controlling the adhesion strength and is of great inherent academic interest. The adhesion mechanism has been described through four theories: adsorption, mechanical, diffusion, and electronic. While interfacial characterization techniques have been developed to validate the aforementioned theories, that related to the electronic theory has not yet been thoroughly studied. We here directly detected the electronic interaction between a commonly used thermosetting adhesive, cured epoxy of diglycidyl ether of bisphenol A (DGEBA) and 4,4'-diaminodiphenylmethane (DDM), and copper (Cu). This study used a combination of density functional theory (DFT) calculations and femtosecond transient absorption spectroscopic (TAS) measurements as this epoxy adhesive-Cu pairing is extensively used in electronic device packaging. The DFT calculations predicted that π electrons in a DDM molecule adsorbed onto the Cu surface flowed out onto the Cu surface, resulting in a positive charge on the DDM. TAS measurements for the Cu/epoxy multilayer film, a model sample containing many metal/adhesive interfaces, revealed that the electronic states of excited DDM moieties at the Cu interface were different from those in the bulk region. These results were in good accordance with the prediction by DFT calculations. Thus, it can be concluded that TAS is applicable to characterize the electronic interaction of adhesives with metal adherends in a nondestructive manner.

Efficient Synthesis and Structural Analysis of Chiral 4,4'-Biazulene.

4,4'-Biazulene is a potentially attractive key component of an axially chiral biaryl compound, however, its structure and properties have not been clarified owing to the lack of its efficient synthesis. We report a breakthrough in the reliable synthesis of 4,4'-biazulene, which is achieved by the access to azulen-4-ylboronic acid pinacol ester and 4-iodoazulene as novel key synthetic intermediates for the Suzuki-Miyaura cross-coupling reaction. The X-ray crystallographic analysis of 4,4'-biazulene confirmed its axial chirality. The enantiomers of 4,4'-biazulene were successfully resolved by HPLC on the chiral stationary phase column. The kinetic experiments and DFT calculations indicate that the racemization energy barrier of 4,4'-biazulene is comparable to that of 1,1'-binaphthyl.

A rhodium(II)/rhodium(III) redox couple for C-H bond amination with alkylazides: a rhodium(III)-nitrenoid intermediate with a tetradentate [14]-macrocyclic ligand.

The catalytic activity of a rhodium(II) dimer complex, [RhII(TMAA)]2 (TMAA = tetramethyltetraaza[14]annulene), in C-H amination reactions with organic azides is explored. Organic azides (N3-R) with an electron-withdrawing group such as a sulfonyl group (trisylazide; R = S(O)2iPr3C6H2 (Trs)) and a simple alkyl group (R = (CH2)4Ph, (CH2)2OCH2Ph, CH2Ph, or C6H4NO2) are employed in intra- and intermolecular C-H bond amination reactions. The spectroscopic analysis using ESI-mass and EPR spectroscopy techniques on the reaction intermediate generated from [RhII(TMAA)]2 and N3-R reveals that a rhodium(III)-nitrenoid species is an active oxidant in the C-H bond amination reaction. DFT calculations suggest that the species can feature a radical localised nitrogen atom. The DFT calculation studies also indicate that the amination reaction involves hydrogen atom abstraction from the organic substrate R'-H by the NR moiety of 2N˙R and successive rebound of the generated organic radical intermediate R'˙ to [RhIII(NH-R)(TMAA)], giving [RhII(TMAA)] and R'-NH-R (amination product).

Elucidating the Effects of Chemisorbed Water Molecules on the Adhesive Interactions of Epoxy Resin to γ-Alumina Surfaces.

The adhesion mechanism of epoxy resin to the γ-alumina (110) surface was investigated using first-principles density functional theory (DFT). Aluminum materials are lightweight and are used in a wide range of industrial fields. Its surface is oxidized to alumina, and the stable surface is known as the γ-alumina (110) surface. The coverage of hydroxy groups by chemisorbed water molecules on this surface varied depending on the pretreatment temperature. In this study, we investigated the adhesive interactions of epoxy resin on four alumina surfaces with different densities of surface hydroxy groups (0, 3, 6, and 9 OH/nm2) and have discussed their effects. At each interface, the energy curves of the vertically displaced epoxy resin were calculated and the adhesive forces were estimated by differentiating these curves. As the coverage of the surface hydroxy groups increased from 0 to 6 OH/nm2, the adhesive strength gradually decreased. However, the adhesive strength at 9 OH/nm2 was relatively large and almost equal to that at 3 OH/nm2. This inverse volcano-type behavior was analyzed via the decomposition of adhesive forces and the crystal orbital Hamilton population (COHP). The decomposition of adhesive forces into DFT and dispersion components revealed that the inverse volcano-type behavior is derived from the DFT component, and the interfacial interactions owing to the DFT component are accompanied by charge transfer. These were investigated using a COHP analysis, which revealed that this behavior was caused by changes in the activity of the aluminum atoms on the surface and surface reconstruction by chemisorbed water molecules. It is noteworthy that the adhesive strength for 9 OH/nm2 was only 6.9% lower than that for 0 OH/nm2 wherein the chemisorbed water molecules were completely removed from the surface. These results are expected to provide a guideline for the adhesion of epoxy resin to aluminum materials.

Hückel Molecular Orbital Analysis for Stability and Instability of Stacked Aromatic and Stacked Antiaromatic Systems.

Face-to-face stacking of aromatic compounds leads to stacked antiaromaticity, while that of antiaromatic compounds leads to stacked aromaticity. This is a prediction with a long history; in the late 2000s, the prediction was confirmed by high-precision quantum chemical calculations, and finally, in 2016, a π-conjugated system with stacked aromaticity was synthesized. Several variations have since been reported, but essentially, they are all the same molecule. To realize stacked aromaticity in a completely new and different molecular system and to trigger an extension of the concept of stacked aromaticity, it is important to understand the origin of stacked aromaticity. The Hückel method, which has been successful in giving qualitatively correct results for π-conjugated systems despite its bold assumptions, is well suited for the analysis of stacked aromaticity. We use this method to model the face-to-face stacking systems of benzene and cyclobutadiene molecules and discuss their stacked antiaromaticity and stacked aromaticity on the basis of their π-electron energies. By further developing the discussion, we search for clues to realize stacked aromaticity in synthesizable molecular systems.

Enhancement of Reactivity of a RuIV-Oxo Complex in Oxygen-Atom-Transfer Catalysis by Hydrogen-Bonding with Amide Moieties in the Second Coordination Sphere.

We have synthesized and characterized a RuII-OH2 complex (2), which has a pentadentate ligand with two pivalamide groups as bulky hydrogen-bonding (HB) moieties in the second coordination sphere (SCS). Complex 2 exhibits a coordination equilibrium through the coordination of one of the pivalamide oxygens to the Ru center in water, affording a η6-coordinated complex, 3. A detailed thermodynamic analysis of the coordination equilibrium revealed that the formation of 3 from 2 is entropy-driven owing to the dissociation of the axial aqua ligand in 2. Complex 2 was oxidized by a CeIV salt to produce the corresponding RuIII(OH) complex (5), which was characterized crystallographically. In the crystal structure of 5, hydrogen bonds are formed among the NH groups of the pivalamide moieties and the oxygen atom of the hydroxo ligand. Further 1e--oxidation of 5 yields the corresponding RuIV(O) complex, 6, which has intramolecular HB of the oxo ligand with two amide N-H protons. Additionally, the RuIII(OH) complex, 5, exhibits disproportionation to the corresponding RuIV(O) complex, 6, and a mixture of the RuII complexes, 2 and 3, in an acidic aqueous solution. We investigated the oxidation of a phenol derivative using complex 6 as the active species and clarified the switch of the reaction mechanism from hydrogen-atom transfer at pH 2.5 to electron transfer, followed by proton transfer at pH 1.0. Additionally, the intramolecular HB in 6 exerts enhancing effects on oxygen-atom transfer reactions from 6 to alkenes such as cyclohexene and its water-soluble derivative to afford the corresponding epoxides, relative to the corresponding RuIV(O) complex (6') lacking the HB moieties in the SCS.

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.

Exploring Metal Nanocluster Catalysts for Ammonia Synthesis Using Informatics Methods: A Concerted Effort of Bayesian Optimization, Swarm Intelligence, and First-Principles Computation.

This paper details the use of computational and informatics methods to design metal nanocluster catalysts for efficient ammonia synthesis. Three main problems are tackled: defining a measure of catalytic activity, choosing the best candidate from a large number of possibilities, and identifying the thermodynamically stable cluster catalyst structure. First-principles calculations, Bayesian optimization, and particle swarm optimization are used to obtain a Ti8 nanocluster as a catalyst candidate. The N2 adsorption structure on Ti8 indicates substantial activation of the N2 molecule, while the NH3 adsorption structure suggests that NH3 is likely to undergo easy desorption. The study also reveals several cluster catalyst candidates that break the general trade-off that surfaces that strongly adsorb reactants also strongly adsorb products.

Stepwise Sulfite Reduction on a Dinuclear Ruthenium Complex Leading to Hydrogen Sulfide.

Sulfite reduction by dissimilatory sulfite reductases is a key process in the global sulfur cycle. Sulfite reductases catalyze the 6e- reduction of SO32- to H2S using eight protons (SO32- + 8H+ + 6e- → H2S + 3H2O). However, detailed research into the reductive conversion of sulfite on transition-metal-based complexes remains unexplored. As part of our ongoing research into reproducing the function of reductases using dinuclear ruthenium complex {(TpRu)2(µ-Cl)(µ-pz)} (Tp = HB(pyrazolyl)3), we have targeted the function of sulfite reductase. The isolation of a key SO-bridged complex, followed by a sulfite-bridged complex, eventually resulted in a stepwise sulfite reduction. The reduction of a sulfite to a sulfur monoxide using 4H+ and 4e-, which was followed by conversion of the sulfur monoxide to a disulfide with concomitant consumption of 2H+ and 2e-, proceeded on the same platform. Finally, the production of H2S from the disulfide-bridged complex was achieved.

Mechanistic Study of Reduction of Nitrite to NO by the Copper(II) Complex: Different Concerted Proton-Electron Transfer Reactivity between Nitrite and Nitro Complexes.

The literature contains numerous reports of copper complexes for nitrite (NO2-) reduction. However, details of how protons and electrons arrive and how nitric oxide (NO) is released remain unknown. The influence of the coordination mode of nitrite on reactivity is also under debate. Kundu and co-workers have reported nitrite reduction by a copper(II) complex [J. Am. Chem. Soc. 2020, 142, 1726-1730]. In their report, the copper(II) complex reduced nitrite using a phenol derivative as a reductant, resulting in NO, a hydroxyl copper(II) complex, and the corresponding biphenol. Also, the involvement of proton-coupled electron transfer was proposed by mechanistic studies. Herein, density functional theory calculations were performed to determine a mechanism for reduction of nitrite by a copper(II) complex. As a result of geometry optimization of an initial complex, two possible structures were obtained: Cu-ONO and Cu-NO2. Two possible reaction pathways initiated from Cu-ONO or Cu-NO2 were then considered. The calculation results indicated that the Cu-ONO pathway is energetically favorable. When changes in the electronic structure were considered, both pathways were found to involve concerted proton-electron transfer (CPET). In addition, an intrinsic reaction coordinate analysis revealed that the two pathways were achieved by different types of CPET. Furthermore, an intrinsic bond orbital analysis clearly indicated that, in the Cu-ONO pathway, the chemical events involved proceeded concertedly yet asynchronously.

Complexation-Triggered Fluctuation of π-Conjugation on an Antiaromatic Dicyanoanthracene Dianion.

The formation of Lewis pairs is an important chemical concept. Recently, the complexation of Lewis acidic tris(pentafluorophenyl)borane with Lewis basic moieties and subsequent reduction has emerged as a fascinating strategy for designing novel reactions and structures. The impact of the complexation and subsequent reduction of antiaromatic systems bearing Lewis base moieties has been investigated. We found how Lewis adduct formation stabilizes an antiaromatic system consisting of 9,10-dicyanoanthracene and tris(pentafluorophenyl)borane by using synthesis, X-ray crystallography, spectroscopic analysis, and quantum chemical calculations.

Catalytic Activity of Molybdenum Complexes Bearing PNP-Type Pincer Ligand toward Ammonia Formation.

We newly designed and prepared a novel molybdenum complex bearing a 4-[3,5-bis(trifluoromethyl)phenyl]pyridine-based PNP-type pincer ligand, based on the bond dissociation free energies (BDFEs) of the N-H bonds in molybdenum-imide complexes bearing various substituted pyridine-based PNP-type pincer ligands. The complex worked as an excellent catalyst toward ammonia formation from the reaction of an atmospheric pressure of dinitrogen with samarium diiodide as a reductant and water as a proton source under ambient reaction conditions, where up to 3580 equivalents of ammonia were formed based on the molybdenum atom of the catalyst. The catalytic activity was significantly improved by one order of magnitude larger than that observed when using the complex before modification.

Effect of Macrocycles on the Photochemical and Electrochemical Properties of Cobalt-Dehydrocorrin Complex: Formation and Investigation of Co(I) Species.

Co(II)-pyrocobester (P-Co(II)), a dehydrocorrin complex, was semisynthesized from vitamin B12 (cyanocobalamin), and its photochemical and electrochemical properties were investigated and compared to those of the cobester (C-Co(II)), the cobalt-corrin complex. The UV-vis absorptions of P-Co(II) in CH2Cl2, ascribed to the π-π* transition, were red-shifted compared to those of C-Co(II) due to the π-expansion of the macrocycle in the pyrocobester. The reversible redox couple of P-Co(II) was observed at E1/2 = -0.30 V vs Ag/AgCl in CH3CN, which was assigned to the Co(II)/Co(I) redox couple by UV-vis, ESR, and molecular orbital analysis. This redox couple was positively shifted by 0.28 V compared to that of C-Co(II). This is caused by the high electronegativity of the dehydrocorrin macrocycle, which was estimated by DFT calculations for the free-base ligands. The reactivity of the Co(I)-pyrocobester (P-Co(I)) was evaluated by the reaction with methyl iodide in CV and UV-vis to form a photosensitive Co(III)-CH3 complex (P-Co(III)-CH3). The properties of the excited state of P-Co(I), *Co(I), were also investigated by femtosecond transient absorption (TA) spectroscopy. The lifetime of *Co(I) was estimated to be 29 ps from the kinetic trace at 587 nm. The lifetime of *Co(I) became shorter in the presence of Ar-X, such as iodobenzonitrile (1a), bromobenzonitrile (1b), and chlorobenzonitrile (1c), and the rate constants of electron transfer (ET) between the *Co(I) and Ar-X were determined to be 2.9 × 1011 M-1 s-1, 4.9 × 1010 M-1 s-1, and 1.0 × 1010 M-1 s-1 for 1a, 1b, and 1c, respectively.

Site-selective radical reactions of kinetically stable open-shell singlet diradicaloid difluorenoheteroles with tributyltin hydride and azo-based radical initiators.

We have demonstrated site-selective radical reactions of the kinetically stable open-shell singlet diradicaloids difluoreno[3,4-b:4',3'-d]thiophene (DFTh) and difluoreno[3,4-b:4',3'-d]furan (DFFu) with tributyltin hydride (HSn(n-Bu)3) and azo-based radical initiators. Treatment of these diradicaloids with HSn(n-Bu)3 induces hydrogenation at the ipso-carbon in the five-membered rings, while treatment with 2,2'-azobis(isobutyronitrile) (AIBN) induces substitution at the carbon atoms in the peripheral six-membered rings. We have also developed one-pot substitution/hydrogenation reactions of DFTh/DFFu with various azo-based radical initiators and HSn(n-Bu)3. The resulting products can be converted into substituted DFTh/DFFu derivatives via dehydrogenation. Theoretical calculations unveiled a detailed mechanism of the radical reactions of DFTh/DFFu with HSn(n-Bu)3 and with AIBN, and that the site-selectivity of these radical reactions is controlled by the balance of the spin density and the steric hindrance in DFTh/DFFu.

Frontier Orbital Views of Stacked Aromaticity.

Recent studies have theoretically and experimentally demonstrated that antiaromatic molecules with 4n π electrons exhibit stacked aromaticity according to π-π stacking when arranged in a face-to-face manner. However, the mechanism of its occurrence has not been clearly studied. In this study, we investigated the mechanism of stacked aromaticity using cyclobutadiene. When the antiaromatic molecules are stacked in a face-to-face manner, the orbital interactions between the degenerate singly occupied molecular orbitals (SOMOs) of the monomer unit cause a larger energy gap between the degenerate highest-occupied molecular orbitals (HOMOs) and the lowest-unoccupied molecular orbitals (LUMOs) of the dimer. However, the antiaromatic molecules are more stable in less symmetric conformations, mainly because of pseudo-Jahn-Teller distortions. In the case of cyclobutadiene, the two SOMOs of the monomer unit split into HOMO and LUMO because of the bond alternation. When the molecules are stacked in a face-to-face manner, the HOMO-LUMO gap of the dimer is smaller than that of the monomer due to the interactions between the HOMOs and LUMOs of the two monomer units. When the monomer units are within a specific distance of each other, the HOMO and LUMO of the dimer, which correspond to antibonding and bonding between the units, respectively, are interchanged. This alternation of molecular orbitals may result in an increase in the bond strength between the monomer units, exhibiting stacked aromaticity. We demonstrated that it is possible to control the distance exhibited by stacked aromaticity by engineering the HOMO-LUMO gap of the monomer units.

Intramolecular Hiyama Coupling: Synthesis of 1,8,13-Trisubstituted Chiral Triptycenes with Three Different Substituents by Intramolecular Substituent Transfer.

Herein, we describe Hiyama coupling via intramolecular substituent transfer from silicon on one blade of triptycenes to another to yield 1,8,13-trisubstituted chiral triptycenes. This reaction is attributed to the proximity effect of substituents on triptycene, which plays an important role in not only the formation of the oxy-palladacycle but also the activation of the silyl group to facilitate σ-bond metathesis. After bromination and nucleophilic ring opening, the second intramolecular Hiyama coupling provided various 1,8,13-trisubstituted chiral triptycenes. The optical resolution of 1,8,13-triptycene afforded an optically active form for the first time.

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.