A vapoluminescent thin film including a coordinatively unsaturated Cu( i ) complex acts as an efficient light-on sensor for N-heteroaromatic vapours and exhibits intense emission of different colours.

The development of transition metal clusters is an active area of research in inorganic chemistry, as they can be used as catalysts to perform chemically or biologically relevant reactions. Computational chemistry, employing density functional theory (DFT), plays a key role in rationalizing the electronic structure and properties of transition metal clusters. This article reviews recent quantum chemical studies of Mo3S4M clusters (M = Fe, Co, Ni), their CO- or N2-bound variants, and metal–hydride clusters. The ground state of the cluster systems was computed, and properties such as metal–metal bonding, orbital interactions, fluxional behavior of ligands, spectroscopy, and reaction mechanisms were rationalized and compared with available experimental results. Our research findings evidence that computational studies employing quantum chemical methods can guide experimental researchers to develop novel transition metal clusters for potential applications in catalysis.

The mechanism of the reaction between CO2 and OH- (anion) in ice cluster models was determined using density functional theory (DFT), employing the omega B97X-D functional and def2-TZVP basis sets for all atoms. A range of reaction barriers, 0.08-0.43 eV, were found, and the lowest energy path has a barrier of 0.08 eV, giving rise to the bicarbonate ion (HCO3-). Computed rate constants, accounting for quantum tunneling by employing the Eckart potential, suggest that the CO2 + OH- -> HCO3- reaction can operate in ice at low temperatures (e.g., 10 K). In contrast, relatively high reaction barriers (0.52-0.74 eV) were found for the CO2 + OH center dot (radical) -> HCO3 center dot (radical) reaction, and the computed rate constants at low temperatures (e.g., 10 K) are extremely small. Based on the computed data, we argue that OH- can react with CO2 trapped in interstellar ice at 10 K, and the product of the reaction, HCO3-, is stable in ice. On the other hand, the OH radical does not react with CO2 in ice. Therefore, we propose that OH anions in interstellar ice play a role in the formation of precursors of complex organic molecules (COMs) in the interstellar medium. The present findings will open a new dimension to explore the chemical evolution in the interstellar medium through the chemistry of anions in interstellar ices.

Sulfur dioxide (SO2) is a sulfur-containing molecule expected to exist as a solid in the interstellar medium. In this study, we have performed laboratory experiments and computational studies on the surface reactions of solid SO2 with hydrogen atoms on amorphous solid water (ASW) at low temperatures. After 40 minutes of exposure of SO2 deposited on ASW to H atoms, approximately 80% of the solid SO2 was lost from the substrate at 10-40 K, and approximately 50% even at 60 K, without any definite detection of reaction products. Quantum chemical calculations suggest that H atoms preferentially add to the S atom of solid SO2, forming the HSO2 radical. Further reactions of the HSO2 radical with H atoms result in the formation of several S-bearing species, including HS(O)OH, the S(O)OH radical, HO-S-OH, HS-OH, and H2S. In codeposition experiments involving H and SO2, we have confirmed the formation of H2S, HS(O)OH, and/or HO-S-OH. However, the yields of these S-bearing species are insufficient to account for the complete loss of the initial SO2 reactant. These findings suggest that some products are desorbed into the gas phase upon formation. This study indicates that a portion of the SO2 in ice mantles may remain unreacted, avoiding hydrogenation, while the remainder is converted into other species, some of which may be subject to chemical desorption.

Copper-catalyzed azide–alkyne cycloaddition click (CuAAC) reaction is widely used to synthesize drug candidates and other biomolecule classes. Homogeneous catalysts, which consist of copper coordinated to a ligand framework, have been optimized for high yield and specificity of the CuAAC reaction, but CuAAC reaction with these catalysts requires the addition of a reducing agent and basic conditions, which can complicate some of the desired syntheses. Additionally, removing copper from the synthesized CuAAC-containing biomolecule is necessary for biological applications but inconvenient and requires additional purification steps. We describe here the design and synthesis of a PNN-type pincer ligand complex with copper (I) that stabilizes the copper (I) and, therefore, can act as a CuAAC catalyst without a reducing agent and base under physiologically relevant conditions. This complex was immobilized on two types of resin, and one of the immobilized catalyst forms worked well under aqueous physiological conditions. Minimal copper leaching was observed from the immobilized catalyst, which allowed its use in multiple reaction cycles without the addition of any reducing agent or base and without recharging with copper ion. The mechanism of the catalytic cycle was rationalized by density functional theory (DFT). This catalyst’s utility was demonstrated by synthesizing coumarin derivatives of small molecules such as ferrocene and sugar.

The self-assembly of d8 transition metal complexes is essential for the development of optoelectronic and sensing materials with superior photofunctional properties. However, detailed insight into the electronic delocalization of excited states across multiple molecules, particularly in comparing 5d8 (Pt(ii)) and 4d8 (Pd(ii)) systems, remains ambiguous but important. In this study, we have successfully evaluated the differences in the excited-state delocalization and thermal responses of self-assembled Pt(ii) and Pd(ii) complexes. Although the complexes presented herein, K[M(CN)2(dFppy)]·H2O (M = Pt or Pd, dFppy = 2-(4,6-difluorophenyl)pyridinate), are crystallographically isomorphous with similarly short metal⋯metal contacts, only the Pt(ii) complex exhibited thermal equilibria between delocalized excited states, resulting in a drastic thermochromic luminescence with a red-shift of greater than 100 nm. In contrast, the dimeric localized emission from the Pd(ii) complex showed a significant increase in the quantum yield upon cooling, approaching almost unity.

Ni has been known as a relatively inert transition metal for N2 capturing and reduction. Based upon our recent report on N2 activation at the Fe site of cubic [CpR3Mo3S4Fe] clusters (CpR = C5Me5 (Cp*), C5Me4SiMe3 (CpL), C5Me4SiEt3 (CpXL)), we report herein a rare example of catalytic N2 silylation by the Ni congeners, [CpR3Mo3S4Ni] clusters. Even though the reduction of [CpR3Mo3S4Ni] precatalysts under a N2 atmosphere did not give stable and isolable N2-bound species, the clusters displayed up to 100 equiv. per cluster of N(SiMe3)3 formation from N2 in the presence of excess Na and ClSiMe3. This number is as high as four times the only previous report in lieu of molecular Ni catalyst. Computational studies on the potential N2-bound state of the [CpR3Mo3S4Ni] catalyst and its Fe analog gave quantitative insights into the role of the M(d) to N2(π*) π back-donation in the activation of the N-N bond.

The CH _3 O and CH _2 OH radicals can be important precursors of complex organic molecules (COMs) in interstellar dust. The COMs presumably originating from these radicals were abundantly found in various astronomical objects. Because each radical leads to different types of COMs, determining the abundance ratio of CH _3 O to CH _2 OH is crucial for a better understanding of the chemical evolution into various COMs. Recent work suggested that the reaction between CH _3 OH and OH on ice dust plays an important role in forming CH _3 O and CH _2 OH radicals. However, quantitative details on the abundance of these radicals have not been presented to date. Herein, we experimentally determined the branching ratio (CH _3 O/CH _2 OH) resulting from the CH _3 OH + OH reaction on the water ice surface at 10 K to be 4.3 ± 0.6. Furthermore, the CH _3 O product in the reaction would participate in subsequent diffusive reactions even at a temperature as low as 10 K. This fact should provide critical information for COMs formation models in cold molecular clouds.

A systematic mechanistic survey was performed for the CH3OH + OH reaction on ice. ONIOM(ωB97X-D/Def2-TZVP:AMOEBA09) calculations suggested a range of binding energies for the CH2OH radical (0.29-0.69 eV) and CH3OH (0.15-0.72 eV) molecule on hexagonal water ice (Ih) and amorphous solid water (ASW). Computed average binding energies of CH2OH radical (0.49 eV) and CH3OH (0.41 eV) are relatively stronger compared to the CH3O radical binding energies (0.32 eV, Sameera et al., J. Phy. Chem. A, 2021, 125, 387-393). Thus, the CH3OH molecule, CH2OH and CH3O radicals can adsorb on ice, where the binding energies follow the order CH2OH > CH3OH > CH3O. The multi-component artificial force-induced reaction (MC-AFIR) method systematically determined the reaction mechanisms for the CH3OH + OH reaction on ice, where two reaction paths, giving rise to CH2OH and CH3O radicals, were confirmed. A range of reaction barriers, employing the ωB97X-D/Def2-TZVP level of theory, was found for each reaction (0.03-0.11 eV for CH2OH radical formation, and 0.03-0.44 eV for CH3O radical formation). Based on the lowest energy reaction paths, we suspect that both reactions operate on ice. The computed data in this study evidence that the nature of the binding site or the reaction site has a significant effect on the computed binding energies or reaction barriers. Thus, the outcomes of the present study will be very useful for the computational astrochemistry community to determine reliable binding energies and reaction barriers on ice.

A cubic metal-sulfur cluster containing three Mo ions and a Pd ion, [(Cp3Mo3S4Pd)-Mo-SiEt3]Cl (Mo3Pd, Cp-SiEt3=C5Me4SiEt3), was synthesized by the incorporation of the Pd ion into a Mo3S4 cluster [(Cp3Mo3S4)-Mo-SiEt3] (Mo-3). Mo3Pd was characterized by H-1 NMR, UV-vis, X-ray crystallography, and cyclic voltammetry measurements. The electrochemical measurements demonstrated reversible one- and two-electron reduction processes for Mo3Pd, which suggested potential catalytic activity for two-electron substrate reductions such as hydrogen evolution reaction. Controlled potential electrolysis in the presence of Mo3Pd and trifluoroethanol in THF solvent displayed H-2 formation with a constant current over 60 min. The amount of generated H-2 by Mo3Pd was two times higher than Mo-3, indicating the catalytic activity facilitated by the Pd center. The mechanism of the catalytic cycle was determined by density functional theory.

We have performed experimental investigations of methanol formation via the reactions of low energy CH_3^+ ions with an amorphous solid water (ASW) surface around 10 K. A newly developed experimental apparatus enabled irradiation of the ASW surface by several eV ions and detection of trace amounts of reaction products on the surface. It was found that methanol molecules were produced by low-energy CH_3^+ irradiation of the ASW surface and that hydroxy groups in produced methanol originated from water molecules in ASW, as predicted in a previous theoretical study. Little temperature dependence of observed methanol intensity is apparent in the temperature range 12 - 60 K. Ab-initio molecular dynamics simulations under constant temperature conditions of 10 K suggested that this reaction spontaneously produced a methanol molecule and an H_3O^+ ion, regardless of the contact point of CH_3^+ on the ASW surface. We have performed simulation with an astrochemical model under molecular-cloud conditions, where the reaction between CH_3^+ and H_2O ice, leading to methanol formation, was included. We found that the impact of the reaction on methanol abundance was limited only at the edge of the molecular cloud (< 1 mag) because of the low abundance of CH_3^+ in the gas phase, whereas the reaction between the abundant molecular ion HCO^+ and H_2O ice, which has not yet been confirmed experimentally, can considerably affect the abundance of a complex organic molecule. This work sheds light on a new type of reaction between molecular ions and ice surfaces that should be included in astrochemical models.

Methyl mercaptan (CH 3 SH) is one of the S-bearing organic compounds found in the interstellar medium (ISM). In this study, we investigated the surface reactions of solid CH 3 SH with H atoms on amorphous solid water using experimental and computational methods to examine their physicochemical behavior in the ISM. Consequently, the primary product was discovered to be CH 4 . As the computational studies show that the dominant reaction pathway is H + CH 3 SH → CH 3 + H 2 S, the observed CH 4 would result from H addition to CH 3 . As relatively minor routes, the H abstraction processes from the –CH 3 and –SH functional groups of CH 3 SH, giving rise to CH 2 SH and CH 3 S radicals, are confirmed. Although these radicals may form CH 3 SH again by reactions with H atoms, the loss of CH 3 SH from the ice surface by chemical desorption would be minor.

Abstract We have previously shown that cyclopentadienyl (CpR)-supported [Mo3S4] platforms capture and stabilize halides of hetero-metals (M) under reducing conditions to give [Mo3S4M] cubes. Here we report Co and Ni variants with CpXL ligands (CpXL = C5Me4SiEt3) and CO binding to the [Mo3S4M] clusters (M = Fe, Co, Ni). Properties of the isolated CO-bound [Mo3S4M] cubes were investigated by X-ray diffraction, IR, and electrochemical analyses. Density functional theory (DFT) calculations were performed for the isolated CO-bound clusters to evaluate M-CO interactions. These analyses constitute foundations to develop bio-mimetic molecular catalysts for the direct conversion of CO and/or CO2 into hydrocarbons, which can contribute to the reduction of carbon emissions.

Heterogeneous radical processes on ice surfaces play a vital role in the formation of building blocks of the biologically relevant molecules in space. Therefore, quantitative mechanistic details of the radical binding and radical reactions on ices are crucial in rationalizing the chemical evolution in the Universe. The radical chemistry on ice surfaces was explored at low temperatures by combining quantum chemical calculations and laboratory experiments. A range of binding energies was observed for OH, HCO, CH3, and CH3O radicals binding on ices. Computed reaction paths of the radical reactions on ices, OCS + H and PH3 + D, explained the experimentally observed products. In both radical reactions, quantum tunnelling plays a key role in achieving the reactions at low temperatures. Our findings give quantitative insights into radical chemistry on ice surfaces in interstellar space and the planetary atmospheres.

We have developed a Python interface, PyQM/MM, to perform ONIOM(QM:MM) calculations with the AMOEBA09 polarizable force field. The ONIOM(QM:MM) implementation in PyQM/MM uses the Gaussian16 program for quantum mechanical (QM) computations and the Tinker program for molecular mechanics (MM) computations with the AMOEBA09 polarizable force field. We have used PyQM/MM, employing ONIOM(QM:AMOEBA09) method, to calculate binding energy of SH and OH radicals on amorphous solid water (ASW). Computed binding energies of SH radical are in the range of 0.10-0.36 eV, where the average binding energy is 0.22 eV. Compared to SH radical, OH radical binding energies are stronger (0.21 – 0.52 eV, and the average value is 0.36 eV). We propose that both SH and OH radicals adsorb on ASW, and SH radical binding preference is smaller than OH radical. Also, we have rationalized the mechanism for the reaction between OH anion and CO in ice using ONIOM(wB97X-D:AMOEBA09) method. The computed reaction mechanism showed a relatively low energy path to form HC(O)OH, where the OH anion is recovered during the reaction. In contrast, the reaction between OH radical CO gives rise to HOCO radical. We propose that OH anions and OH radicals in interstellar ices can react with the molecules trapped in ices to synthesize complex organic molecules. These examples evidence that PyQM/MM is a user-friendly strategy to perform ONIOM(wB97XD:AMOEBA09) calculations to study chemical processes in the interstellar medium.

This chapter presents recent progress in developing computational methods and applications of automated mechanism discovery in chemistry. Systematic determination of the reaction mechanisms has been a challenging topic in modern computational chemistry. For this purpose, various computational methods have been developed to find multiple reaction paths starting from a known local minimum (LM). Applying such techniques to various LMs one-after-another explores reaction path networks in a broad sense and rationalizes the mechanisms of the known, unknown, or unexpected reactions. Thus, automated mechanism discovery can guide experimental researchers to develop novel chemical reactions and materials.

Nitrogen (N2) fixation by nature, which is a crucial process for the supply of bio-available forms of nitrogen, is performed by nitrogenase. This enzyme uses a unique transition-metal-sulfur-carbon cluster as its active-site co-factor ([(R-homocitrate)MoFe7S9C], FeMoco)1,2, and the sulfur-surrounded iron (Fe) atoms have been postulated to capture and reduce N2 (refs. 3-6). Although there are a few examples of synthetic counterparts of the FeMoco, metal-sulfur cluster, which have shown binding of N2 (refs. 7-9), the reduction of N2 by any synthetic metal-sulfur cluster or by the extracted form of FeMoco10 has remained elusive, despite nearly 50 years of research. Here we show that the Fe atoms in our synthetic [Mo3S4Fe] cubes11,12 can capture a N2 molecule and catalyse N2 silylation to form N(SiMe3)3 under treatment with excess sodium and trimethylsilyl chloride. These results exemplify the catalytic silylation of N2 by a synthetic metal-sulfur cluster and demonstrate the N2-reduction capability of Fe atoms in a sulfur-rich environment, which is reminiscent of the ability of FeMoco to bind and activate N2.

Quantitative details of the reaction mechanisms of transition metal homogeneous catalysis can be determined from modern computational methods. A computed reaction mechanism gives an atomic-scale picture of the chemical events of the catalytic processes. Thus, important lessons on the reaction mechanisms and selectivity can be learned. These properties are crucial for experimental researches to design novel transition metal homogeneous catalysis. This chapter presents recent computational studies, employing density functional theory or density functional theory/molecular mechanics methods in cross-coupling and nitrogen reduction reactions. Detailed computational studies rationalized the electronic structure of the transition metal catalysts and the reaction mechanism of the full catalytic cycles. Also, systematic surveys of the selectivity determining transition states of the asymmetric cross-coupling reactions rationalized the origin of the selectivity. Thus, lessons from the computational studies can guide the experimental researchers to develop novel homogeneous catalysis for potential applications in industry or academia.

We have synthesized two ligand systems, N(SO2)(R1)dpa (L1) and N(SO2)(R2)dpa (L2), where R1 = biphenyl and R2 = azobenzene, which are sulfonamide derivatives of the NNN-donor chelating dipicolylamine. Both L1 and L2 can be used as sensors for detecting Fe3+ and are highly sensitive and selective over a wide range of common cations. Time-dependent density functional theory (TDDFT) calculations confirmed that the key excitations of L2 and the [Fe(L2)(H2O)3]3+ model complex involve -R2-unit-based π and π* charge transfer. L2 demonstrates a relatively high photostability, a fluorescence turn-on mechanism, and a detection limit of 0.018 μM with 1.00 μM L2 concentration, whereas L1 has a detection limit of 0.67 μM. Thus, both ligands have the potential to be used as fluorosensors for the detection of Fe3+ in aqueous solutions.