Insights into the Mechanism of Nitrobenzene Reduction to Aniline by Phosphomolybdic Acid Supported TM1 Single-Atom Catalysts.

Catalytic hydrogenation of nitrobenzene (Ph-NO2) to aniline (Ph-NH2) is a model reaction in the field of catalysis, in which the development of efficient catalysts remains a great challenge due to the lack of strategies to solve activity and selectivity problems. In this work, the mechanism of Ph-NO2 hydrogenation over Pt1 supported on phosphomolybdic acid (α-PMA) was proposed by density functional theory (DFT) calculations. The results show that the dissociation of the first and second N-O bonds is triggered by single H-induced and double H-induced mechanisms, respectively. The limiting potential of the reaction process is -0.19 V, which is the smallest potential in the field of Ph-NO2 reduction reaction to date. In the whole reaction process, the catalytic active site is the Pt atom, and polyoxometalate plays the role of an electronic sponge in the reaction. Additionally, based on experimentally confirmed Pt1/Na3PMA, the reduction capacity of Pd1/Na3PMA toward Ph-NO2 was predicted by DFT calculation. The distinctive adsorption patterns of Ph-NO2 on Pt1/Na3PMA and Pd1/Na3PMA were elucidated using the DOS diagram and fragment molecular orbital analysis. We anticipate that our theoretical calculations can provide novel perspectives for experimental researchers.

Regulating Efficient and Selective Single-atom Catalysts for Electrocatalytic CO2 Reduction.

Anchoring transition metal (TM) atoms on suitable substrates to form single-atom catalysts (SACs) is a novel approach to constructing electrocatalysts. Graphdiyne with sp-sp2 hybridized carbon atoms and uniformly distributed pores have been considered as a potential carbon material for supporting metal atoms in a variety of catalytic processes. Herein, density functional theory (DFT) calculations were performed to study the single TM atom anchoring on graphdiyne (TM1 -GDY, TM=Sc, Ti, V, Cr, Mn, Co and Cu) as the catalysts for CO2 reduction. After anchoring metal atoms on GDY, the catalytic activity of TM1 -GDY (TM=Mn, Co and Cu) for CO2 reduction reaction (CO2 RR) are significantly improved comparing with the pristine GDY. Among the studied TM1 -GDY, Cu1 -GDY shows excellent electrocatalytic activity for CO2 reduction for which the product is HCOOH and the limiting potential (UL ) is -0.16 V. Mn1 -GDY and Co1 -GDY exhibit superior catalytic selectivity for CO2 reduction to CH4 with UL of -0.62 and -0.34 V, respectively. The hydrogen evolution reaction (HER) by TM1 -GDY (TM=Mn, Co and Cu) occurs on carbon atoms, while the active sites of CO2 RR are the transition metal atoms . The present work is expected to provide a solid theoretical basis for CO2 conversion into valuable hydrocarbons.

Constructing Oxygen Vacancies via Engineering Heterostructured Fe3 C/Fe3 O4 Catalysts for Electrochemical Ammonia Synthesis.

Electrocatalytic nitrogen reduction reaction (NRR) under ambient conditions provides an intriguing pathway to convert N2 into NH3 . However, significant kinetic barriers of the NRR at low temperatures in desirable aqueous electrolytes remain a grand challenge due to the inert N≡N bond of the N2 molecule. Herein, we propose a unique strategy for in situ oxygen vacancy construction to address the significant trade-off between N2 adsorption and NH3 desorption by building a hollow shell structured Fe3 C/Fe3 O4 heterojunction coated with carbon frameworks (Fe3 C/Fe3 O4 @C). In the heterostructure, the Fe3 C triggers the oxygen vacancies of the Fe3 O4 component, which are likely active sites for the NRR. The design could optimize the adsorption strength of the N2 and Nx Hy intermediates, thus boosting the catalytic activity for the NRR. This work highlights the significance of the interaction between defect and interface engineering for regulating electrocatalytic properties of heterostructured catalysts for the challenging NRR. It could motivate an in-depth exploration to advance N2 reduction to ammonia.

Transition-Metal-Modified Vanadoborate Clusters as Stable and Efficient Photocatalysts for CO2 Reduction.

Photocatalytic carbon dioxide reduction (CO2RR) is considered to be a promising sustainable and clean approach to solve environmental issues. Polyoxometalates (POMs), with advantages in fast, reversible, and stepwise multiple-electron transfer without changing their structures, have been promising catalysts in various redox reactions. However, their performance is often restricted by poor thermal or chemical stability. In this work, two transition-metal-modified vanadoborate clusters, [Co(en)2]6[V12B18O54(OH)6]·17H2O (V12B18-Co) and [Ni(en)2]6[V12B18O54(OH)6]·17H2O (V12B18-Ni), are reported for photocatalytic CO2 reduction. V12B18-Co and V12B18-Ni can preserve their structures to 200 and 250 °C, respectively, and remain stable in polar organic solvents and a wide range of pH solutions. Under visible-light irradiation, CO2 can be converted into syngas and HCOO- with V12B18-Co or V12B18-Ni as catalysts. The total amount of gaseous products and liquid products for V12B18-Co is up to 9.5 and 0.168 mmol g-1 h-1. Comparing with V12B18-Co, the yield of CO for V12B18-Ni declines by 1.8-fold, while that of HCOO- increases by 35%. The AQY of V12B18-Co and V12B18-Ni is 1.1% and 0.93%, respectively. These values are higher than most of the reported POM materials under similar conditions. The density functional theory (DFT) calculations illuminate the active site of CO2RR and the reduction mechanism. This work provides new insights into the design of stable, high-performance, and low-cost photocatalysts for CO2 reduction.

Hollow Porous MnFe2 O4 Sphere Grown on Elm-Money-Derived Biochar towards Energy-Saving Full Water Electrolysis.

The development of inexpensive and efficient bifunctional electrocatalysts is significant for widespread practical applications of overall water splitting technology. Herein, a one-pot solvothermal method is used to prepare hollow porous MnFe2 O4 spheres, which are grown on natural-abundant elm-money-derived biochar material to construct MnFe2 O4 /BC composite. When the overpotential is 156 mV for both the oxygen evolution reaction and the hydrogen evolution reaction, the current density reaches up to 10 mA cm-2 , and its duration is 10 h. At 1.51 V, the overall water decomposition current density of 10 mA cm-2 can be obtained in 1 m KOH. This work proves that elm-money-derived biochar is a valid substrate for growing hollow porous spheres. MnFe2 O4 /BC give a promising general strategy for preparing the effective and stable bifunctional catalysis that can be expand to multiple transition metal oxide.

Configuration effect in polyoxometalate-based dyes on the performance of DSSCs: an insight from a theoretical perspective.

The electronic properties of dyes can be readily tuned by modifying the structure. Herein, the polyoxometalate (POM)-based dyes derived from dye XW11 with new patterns, donor-acceptor-π linker-acceptor (D-A-π-A) structure (dye 1), and D-π-A-π-A structure (dye 2) were designed by inserting a POM moiety besides the extensively exploited D-π-A structure (dye 3). Based on density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, the configuration effect on the designed dyes was investigated. The results indicate that dye 3 possesses the largest short-circuit photocurrent density JSC due to the red-shifted absorption spectra, superior intramolecular charge transfer (ICT) parameters and the largest electron injection efficiency. At the same time, dye 1 with a D-A-π-A structure not only benefits the conduction band energy shift, but also retards the charge recombination and dye aggregation effect, which is beneficial for open-circuit photovoltage VOC. Moreover, the dynamics analysis of interfacial electron transfer shows that the electrons in dye 1 are almost completely injected after 14 fs, while it takes a long time for dyes 2 and 3. The present work is expected to establish a structure-property relationship for future dye design.

Theoretical Insight into the Performance of MnII/III-Monosubstituted Heteropolytungstates as Water Oxidation Catalysts.

The performance of MnII/III-monosubstituted heteropolytungstates [MnIII(H2O)GeW11O39]5- ([GT-MnIII-OH2]5-, where GT = GeW11O39) and [MnII(H2O)GeW11O39]6- ([GT-MnII-OH2]6-) as water oxidation catalysts at pH 9 was explored using density functional theory calculations. The counterion effect was fully considered, in which five and six Na+ ions were included in the calculations for water oxidation catalyzed by [GT-MnIII-OH2]5- and [GT-MnII-OH2]6-, respectively. The process of water oxidation catalysis was divided into three elemental stages: (i) oxidative activation, (ii) O-O bond formation, and (iii) O2 evolution. In the oxidative activation stage, two electrons and two protons are removed from [Na5-GT-MnIII-OH2] and three electrons and two protons are removed from [Na6-GT-MnII-OH2]. Therefore, the MnIV-O• species [Na5-GT-MnIV-O•] is obtained. Two mechanisms, (i) water nucleophilic attack and (ii) oxo-oxo coupling, were demonstrated to be competitive in O-O bond formation triggered from [Na5-GT-MnIV-O•]. In the last stage, the O2 molecule could be readily evolved from the peroxo or dinuclear species and the catalyst returns to the ground state after the coordination of a water molecule(s).

A Calix[4]resorcinarene-Based [Co12] Coordination Cage for Highly Efficient Cycloaddition of CO2 to Epoxides.

The design and synthesis of polynuclear metal cluster-based coordination cages is of considerable interest due to their appealing structural characteristics and potential applications. Herein, we report a calix[4]resorcinarene-based [Co12] coordination cage, [Co12(TPC4R-I)2(1,3-BDC)10(µ3-OH)4(H2O)10(DMF)2]·7DMF·23H2O (1), assembled with 2 bowl-shaped calix[4]resorcinarenes (TPC4R-I), 10 angular 1,3-benzenedicarboxylates (1,3-BDC), and 12 Co(II) cations. Remarkably, it is shown to be a highly efficient recyclable heterogeneous catalyst for CO2 conversion due to its exposed Co(II) Lewis acid sites.

Two-State Reactivity Mechanism of Benzene C-C Activation by Trinuclear Titanium Hydride.

The cleavage of inert C-C bonds is a central challenge in modern chemistry. Multinuclear transition metal complexes would be a desirable alternative because of the synergetic effect of multiple metal centers. In this work, carbon-carbon bond cleavage and rearrangement of benzene by a trinuclear titanium hydride were investigated using density functional theory. The reaction occurs via a novel "two-state reactivity" mechanism. The important elementary steps consist of hydride transfer, benzene coordination, dehydrogenation, oxidative addition, hydride-proton exchange, and reductive elimination. Most importantly, the ground-state potential energy surface switches from nearly degenerate triplet and antiferromagnetic singlet states to a closed-shell singlet state in the dearomatization of benzene, which effectively decreases the activation barrier. Furthermore, the roles of the transition metal centers and hydrides were clarified.

Bonding interactions between sulfur dioxide (SO2) and mono-ruthenium(II)-substituted Keggin-type polyoxometalates: electronic structures of ruthenium-SO2 adducts.

Density functional theory (DFT) calculations and natural bond orbital (NBO) analysis were carried out to investigate the electronic structures and bonding features between the ruthenium(ii) atom and the SO2 molecule in two ruthenium-sulfur dioxide (SO2) adducts, trans-Ru(NH3)4(SO2)Cl(+) and [{SiW11O39}Ru(II)(SO2)](6-). In addition, the bonding interactions between SO2 and the metal-ruthenium fragment were determined by binding energy (ΔEabs) calculation and electronic structures. The results indicate that the η(1)-S-planar model in both trans-Ru(NH3)4(SO2)Cl(+) and [{SiW11O39}Ru(II)(SO2)](6-) are more favorable. NBO analysis of the bonding interaction between ruthenium and sulfur centers in the [{SiW11O39}Ru(II)(SO2)](6-) complex shows that it possesses a σ and a π bond. It predicts that the polyoxometalate [SiW11O39Ru](6-) can serve as a potential adsorbent for the SO2 molecule because of the strong Ru-S bond relative to Ru(NH3)4Cl(+).

TDDFT studies on the determination of the absolute configurations and chiroptical properties of Strandberg-type polyoxometalates.

The electronic circular dichroism (ECD) and UV-visible absorption (UV-vis) spectra of Strandberg-type polyoxometalates (POMs) (R, R)-[(R*PO3)2M5O15](2-) (R* = CH3CH(NH3), (M = Mo, W)) have been explored using the time-dependent density functional theory (TDDFT) method. It demonstrates that the absolute configurations of chiral systems can be determined by chiroptical spectroscopic methods combined with DFT calculations. The calculated ECD spectra of the Strandberg-type molybdate were produced over the range of 3.3-6.5 eV, which are generally in agreement with the experimental spectra. In addition, the ECD spectra of (R, R)-[(R*PO3)2W5O15](2-) (R* = CH3CH(NH3)) were produced over the range of 4.5-8.5 eV. The Becke's half-and-half hybrid exchange-correlation functional (BHandHLYP) with the HF exchange fraction to 55% hybrid functional was found to well predict the excitation energies of studied systems. The origins of the ECD bands of two systems are mainly ascribed to charge-transfer (CT) transitions from oxygen atoms to metal atoms in polyanion. The results suggest that the polyanion are chiroptical chromophores. The polyanion plays a role as an optically active chromophore and contribute to the absorptions of ECD spectra. The difference of the UV-vis/ECD spectra between two systems shows that the transition metal atom significantly influences on the chiroptical properties of the studied Strandberg-type POMs.

Theoretical exploration of photoisomerization-switchable second-order nonlinear optical responses of two-dimendional λ- and w-shaped polyoxometalate derivatives of dithienylperfluorocyclopentene.

The switchable second-order nonlinear optical (NLO) properties on two-dimensional (2D) molecules based on Lindqvist-type [Mo6O19](2-) and dithienylperfluorocyclopentene (DTE) have been investigated at density functional theory (DFT) level. The CAM-B3LYP and M06-2X functionals were employed to study the switching behavior on NLO properties by photoisomerization reaction. The ßtot value of system 2c (closed-ring form) is 15920.5 au, which is 150.1 times larger than that of the corresponding open-ring form (system 2o). The time-dependent DFT calculations predict that the charge transfer from DTE to polyoxometalate, and DTE intramolecular charge transfer in closed-ring systems effectively improve the static first hyperpolarizability. Furthermore, the Λ-shaped systems possess a larger u value than those of W-shaped systems owing to different orientation for substituent groups.

TDDFT studies on the electronic structures and chiroptical properties of mono-tin-substituted Wells-Dawson polyoxotungstates.

The UV/CD spectra of tin-bearing acetonyl-substituted Wells-Dawson polyoxotungstates α(1)- and α(2)-[P(2)W(17)O(61){SnCH(2)CH(2)C(═O)}](6-) were systematically investigated using the time-dependent density functional theory (TDDFT) method. The electronic circular dichroism (ECD) spectra were produced over the range of 3.3-5.8 eV. The calculated ECD spectra of the α(1)-R isomer were generally in agreement with the experimental spectra. The CAM-B3LYP hybrid functional was found to predict the excitation energies of tin-containing polyoxotungstates well. The fact that the UV/ECD spectra of α(1)-isomers are different from those of α(2)-isomers demonstrates the effect of the tin substitution site on the chiroptical properties of the studied isomers. The origins of the ECD bands are mainly ascribed to charge-transfer (CT) transitions from oxygen atoms to W atoms, from organic fragments to W atoms, or from the combination of two CT transitions. The results suggest that the organic fragment and polyoxometalate (POM) cage are chiroptical chromophores.

Isomeric effects on the acidity of Al13 Keggin clusters in porous ionic crystals.

We demonstrate a facile synthesis method of a porous ionic crystal (PIC) composed of the little-known δ-Keggin-type cationic polyoxoaluminum cluster ([δ-Al13O4(OH)24(H2O)12]7+, δ-Al13) with an oppositely-charged polyoxometalate, which enabled us to investigate the activity as a solid acid. The δ-Al13 based PIC exhibited much higher activity in pinacol rearrangement, a typical acid-catalyzed reaction, than the PIC based on the well-known and thermodynamically stable rotational isomer (ε-Al13). This work is a rare example of rotational isomers of polyoxoaluminum clusters exhibiting remarkably different catalytic activities.

The interesting luminescence behavior and rare nonlinear optical properties of the {Ag55Mo6} nanocluster.

The remarkably reversible thermochromic luminescence behavior and the rare nonlinear optical (NLO) properties of the [Ag55(MoO4)6(C[triple bond, length as m-dash]C t Bu)24(CH3COO)18(CH3COO)]·2H2O ({Ag55Mo6} for short) nanocluster reported were investigated experimentally. The important contributions of Ag+, C[triple bond, length as m-dash]C- ions and MoO4 2- groups to the NLO properties were proved by further density functional theory (DFT) calculations.

Multielectron transportation of polyoxometalate-grafted metalloporphyrin coordination frameworks for selective CO2-to-CH4 photoconversion.

Photocatalytic CO2 reduction into energy carriers is of utmost importance due to the rising concentrations of CO2 and the depleting energy resource. However, the highly selective generation of desirable hydrocarbon fuel, such as methane (CH4), from CO2 remains extremely challenging. Herein, we present two stable polyoxometalate-grafted metalloporphyrin coordination frameworks (POMCFs), which are constructed with reductive Zn-ϵ-Keggin clusters and photosensitive tetrakis(4-carboxylphenyl)porphyrin (H2TCPP) linkers, exhibiting high selectivity (>96%) for CH4 formation in a photocatalytic CO2-reduction system. To our knowledge, the high CH4 selectivity of POMCFs has surpassed all of the reported coordination-framework-based heterogeneous photocatalysts for CO2-to-CH4 conversion. Significantly, the introduction of a Zn-ϵ-keggin cluster with strong reducing ability is the important origin for POMCFs to obtain high photocatalytic selectivity for CH4 formation, considering that eight MoV atoms can theoretically donate eight electrons to fulfill the multielectron reduction process of CO2-to-CH4 transformation.

Quantum chemical studies on high-valent metal nitrido derivatives of Keggin-type polyoxometalates ([PW11O39{M(VI)N}]4- (M = Ru, Os, Re)): M(VI)-N bonding and electronic structures.

High-valent M(VI)N (M = Ru, Os) species are important reagents in nitrogen transfer reactions; the unique withdrawing properties of polyoxometalate (POMs) ligands would possibly modify the reactivity of the M(VI)N functional group. In the present paper, density functional theory (DFT) and natural bond orbital (NBO) analysis have been employed to calculate electronic structures, M(VI)-N bonding, and redox properties of high-valent metal nitrido derivatives of Keggin-type POMs, [PW(11)O(39) {M(VI)N}](4-) (M = Ru, Os, Re). Our calculations show that [PW(11)O(39){RuN}](4-) possesses stronger antibonding interaction between metal and nitrogen atoms compared with anions [PW(11)O(39){OsN}](4-) and [PW(11)O(39){ReN}](4-). A large increase in the Ru-N bond length of anion [PW(11)O(39){RuN}](4-) in the excited states has been found; the effective order and composition of the molecular orbital in anion [PW(11)O(39){RuN}](4-) is a key factor in determination of the increase of the Ru-N bond length in the excited states. The substitution effects of central tetrahedron heteroatoms (XO(4), X = Al, Si, P, As) in anions [XW(11)O(39){RuN}](4-) affect the relative energy of the LUMO; the relevant orbital energy increases in the order Al(III) < Si(IV) < P(V) approximately As(V). The RuN unit is the reduced center. NBO analysis of the extent of the bonding interaction between the ruthenium and the nitrogen centers in [PW(11)O(39){Ru(VI)N}](4-) shows that the Ru-N bond possesses a covalent feature and displays triple-, double-, and single-bond character when moving along the change of spin state ((1)1 --> (3)1 --> (5)1).

Redox-switchable second-order nonlinear optical responses of push-pull monotetrathiafulvalene-metalloporphyrins.

The redox-active tetrathiafulvalene (TTF) is a good electron donor, and porphyrin is highly delocalized in cyclic pi-conjugated systems. The direct combination of the two interesting building units into the same molecule provides an intriguing molecular system for designing nonlinear optical (NLO) molecular materials. In the present paper, the second-order NLO properties of a series of monoTTF-porphyrins and metalloporphyrins have been calculated by density functional theory (DFT) combined with the finite field (FF) method. Our calculations show that these compounds possess considerably large static first hyperpolarizabilities, approximately 400 x 10(-30) esu. Since the TTF unit is able to exist in three different stable redox states (TTF, TTF(*+), and TTF(2+)), the redox switching of the NLO response of the zinc(II) derivative of monoTTF-metalloporphyrin has been studied, and a substantial enhancement in static first hyperpolarizability has been obtained in its oxidized species according to our DFT-FF calculations. The beta values of one- and two-electron-oxidized species are 3.6 and 8.7 times as large as that of the neutral compound, especially for two-electron-oxidized species, with a value of 3384 x 10(-30) esu. This value is about 3 times that for a push-pull metalloporphyrin, which has an exceptionally large hyperpolarizability among reported organic NLO chromophores. Meanwhile, to give a more intuitive description of band assignments of the electron spectrum and trends in NLO behavior of these compounds, the time-dependent (TD)DFT method has been adopted to calculate the electron spectrum. The TDDFT calculations well-reproduce the soret band and Q-type bands of the monoTTF-porphyrin, and these absorption bands can be assigned to the pi --> pi* transition of the porphyrin core. On the other hand, the oxidized process significantly affects the geometrical structures of the TTF unit and porphyrin ring, and the two-electron-oxidized species has a planar TTF unit and a high conjugative porphyrin ring. This effect reduces the excited energy, changes the CT feature, and thus enhances its static first hyperpolarizability.

Prediction of remarkably large second-order nonlinear optical properties of organoimido-substituted hexamolybdates.

The dipole polarizabilities, dipole moments, density of states, and second-order nonlinear optical (NLO) properties of organoimido derivatives of hexamolybdates have been investigated by using time-dependent density functional response theory. This class of organic-inorganic hybrid compounds possesses remarkably large and eye-catching molecular second-order NLO response, especially [Mo(6)O(17)(NC(16)H(12)NO(2))(FeNC(10)H(9))](2-) (7) and [Mo(6)O(17)(NC(16)H(12)NO(2))(NC(6)H(2)(NH(2))(3))](2-) (6) with static second-order polarizability (beta(vec)) computed to be 15766.27 x 10(-30) esu and 6299.59 x 10(-30) esu, respectively. Thus, these systems have the possibility to be excellent second-order nonlinear optical materials. Analysis of the major contributions to the beta(vec) value suggests that the charge transfer (CT) from polyanion to organic segment (D-A) along the z-axis plays the key role in NLO response; the polyanion acts as a donor (D) whereas organoimido acts as an acceptor (A) in all the studied systems. The computed beta(vec) values increase by incorporation of an electron acceptor (-NO(2)) at the end of the phenyl ring of the organoimido segment. Furthermore, substitution of amino (-NH(2)) or ferrocenyl (-FeC(10)H(9)) at the outer side of polyanion and an electron acceptor (-NO(2)) at the end of the phenyl ring in organoimido segment simultaneously is more important to enhance the optical nonlinearity. Orbital analysis shows that the degree of CT between the polyanion and organoimido segments was increased when ferrocenyl donor was introduced. The present investigation provides important insight into the remarkably large NLO properties of organoimido-substituted hexamolybdates.

Theoretical study on the considerable second-order nonlinear optical properties of naphthylimido-substituted hexamolybdates.

The static first hyperpolarizabilities and origin of nonlinear optical (NLO) properties of [(2-methylnaphthyl)imido]hexamolybdates derivatives have been investigated by density functional theory (DFT). The [(2-methylnaphthyl)imido]hexamolybdate has considerable large first hyperpolarizability, 6.780 x 10(-30) esu, and it is larger than that of [(2,6-dimethylphenyl)arylimido]hexamolybdate due to the double aromatic rings in the naphthylimido ligand. The naphthylimido ligand acts as an electron-donor and the polyanion acts as an electron-acceptor. The substituent position on the naphthylimido is a key factor to determine the first hyperpolarizability of (naphthylimido)hexamolybdate derivatives. The derivative, which the iodine atom locates on the para nitrogen on the naphthylimido ligand, has the largest betao(o) value among the iodine-substituted derivatives. It suggests that the iodine atom is quasi linear with nitrogen and Mo, which is bonded to thenitrogen atom, could generate a large static electronic field and give the large contribution to NLO response.The introducing of electron-donors significantly enhances the first hyperpolarizabilities of (naphthylimido)hexamolybdates comparing with the electron-acceptors as the electron-donating ability is significantly enhanced when the electron-donor is attached to the naphthylimido segment. The present investigation provides important insight into NLO properties of (arylimido)molybdate derivatives.