Computational Study of Metal–Dinitrogen Keggin-Type Polyoxometalate Complexes [PW11O39MIIN2)]5– (M = Ru, Os, Re, Ir): Bonding Nature and Dinitrogen Splitting

Keggin-type SiW12 encapsulated in MIL-101(Cr) as efficient heterogeneous photocatalysts for nitrogen fixation reaction

The incorporation of polyoxometalates (POMs) in metal-organic frameworks (MOFs) with host-guest structure have proven to be effective strategy to rational design of heterogeneous catalysis. In this study, the Keggin-type POM@MIL-101(Cr) composite catalysts (PMo₁₂, PW₁₂ and SiW₁₂) are synthesized for nitrogen fixation reaction without sacrificial agents at room temperature in the first time. The SiW₁₂ molecules are encapsulated in smaller cavities of MIL-101(Cr) by solvothermal method and in larger cavities by impregnation method, respectively. Solvothermal synthesized catalyst has a performance of 75.56 μmol·h^-1·g^-1_cat and TOF value of 1.95 h^-1, which are about 10 and 88 times than that of Na₄SiW₁₂O₄₀. The excellent performance is ascribed to the synergistic effect of SiW₁₂ and MIL-101(Cr). The MIL-101(Cr) adsorbs a large amount of N₂ and generates sufficiently photogenerated electrons under sunlight irradiation, and electrons quickly transfer to the SiW₁₂ through hydrogen bonds. Moreover, the agglomeration effect of the homogeneous catalyst SiW₁₂ is weakened due to encapsulation with more exposed active sites. This work provides a feasible route to design and synthesize nanocomposite materials with exceptional performance for photocatalytic nitrogen fixation.

Formation of the N≡N Triple Bond from Reductive Coupling of a Paramagnetic Diruthenium Nitrido Compound

Construction of nitrogen-nitrogen triple bonds via homocoupling of metal nitrides is an important fundamental reaction relevant to a potential Nitrogen Economy. Here, we report that room temperature photolysis of Ru₂(chp)₄N₃ (chp^- = 2-chloro-6-hydroxypyridinate) in CH₂Cl₂ produces N₂ via reductive coupling of Ru₂(chp)₄N nitrido species. Computational analysis reveals that the nitride coupling transition state (TS) features an out-of-plane "zigzag" geometry instead of the anticipated planar zigzag TS. However, with intentional exclusion of dispersion correction, the planar zigzag TS geometry can also be found. Both the out-of-plane and planar zigzag TS geometries feature two important types of orbital interactions: (1) donor-acceptor interactions involving intermolecular donation of a nitride lone pair into an empty Ru-N π* orbital and (2) Ru-N π to Ru-N π* interactions derived from coupling of nitridyl radicals. The relative importance of these two interactions is quantified both at and after the TS. Our analysis shows that both interactions are important for the formation of the N-N σ bond, while radical coupling interactions dominate the formation of N-N π bonds. Comparison is made to isoelectronic Ru₂-oxo compounds. Formation of an O-O bond via bimolecular oxo coupling is not observed experimentally and is calculated to have a much higher TS energy. The major difference between the nitrido and oxo systems stems from an extremely large driving force, ∼-500 kJ/mol, for N-N coupling vs a more modest driving force for O-O coupling, -40 to -140 kJ/mol.

Reduction of N2 to NH3 catalyzed by a Keggin-type polyoxometalate-supported dual-atom catalyst

In the present paper, a polyoxometalate-supported dual-atom catalyst has been designed for the nitrogen reduction reaction based on our density functional theory calculations.

Roles of Organic Fragments in Redirecting Crystal/Molecular Structures of Inorganic-Organic Hybrids Based on Lacunary Keggin-Type Polyoxometalates

Lacunary polyoxometalates (LPOMs) are key precursors for the synthesis of functional POMs. To date, reviews dedicated to behavioral studies of LPOMs often comprise the role of metal ions, including transition metal (TM) and rare earth (RE) ions, in extending and stability of high-nuclearity clusters. In contrast, the role of organic ligands in the structures and properties of lacunary-based hybrids has remained less explored. In this review, we focus on the role of organic fragments in the self-assembling process of POM-based architectures and discuss relationships between the nature and structure of organic ligand and properties such as the topology of hybrid inorganic–organic material in RE and TM-RE heterometallic derivatives of lacunary Keggin-type POMs. The effects of organic fragment in mixed ligand hybrids are also briefly reviewed.

DFT study of α-Keggin, lacunary Keggin, and ironII-VI substituted Keggin polyoxometalates: the effect of oxidation state and axial ligand on geometry, electronic structures and oxygen transfer

Herein, the geometry, electronic structure, Fe-ligand bonding nature and simulated IR spectrum of α-Keggin, lacunary Keggin, iron(ii/iii)-substituted and the important oxidized high-valent iron derivatives of Keggin type polyoxometalates have been studied using the density functional theory (DFT/OPTX-PBE) method and natural bond orbital (NBO) analysis. The effects of different Fe oxidation states (ii-vi) and H2O/OH-/O2- ligand interactions have been addressed concerning their geometry and electronic structures. It has been revealed that the d-atomic orbitals of Fe and 2p orbitals of polyoxometalate's oxygen-atoms contribute in ligand binding. Compared with other high valent species, the considered polyoxometalate system of [PW11O39(FeVO)]4-, possesses a high reactivity for oxygen transfer.

Theoretical study on the microscopic mechanism of lignin solubilization in Keggin-type polyoxometalate ionic liquids

Keggin-type polyoxometalate derived ionic liquids (POM-ILs) have recently been presented as effective solvent systems for biomass delignification. To investigate the mechanism of lignin dissolution in POM-ILs, the system involving POM-IL ([C4C1Im]₃[PW₁₂O₄₀]) and guaiacyl glycerol-β-guaiacyl ether (GGE), which contains a β-O-4 bond (the most dominant bond moiety in lignin), was studied using quantum mechanical calculations and molecular dynamics simulations. These studies show that more stable POM-IL structures are formed when [C4C1Im]⁺ is anchored in the connecting four terminal oxygen region of the [PW₁₂O₄₀]^3- surface. The cations in POM-ILs appear to stabilize the geometry by offering strong and positively charged sites, and the POM anion is a good H-bond acceptor. Calculations of POM-IL interacting with GGE show the POM anion interacts strongly with GGE through many H-bonds and π-π interactions which are the main interactions between the POM-IL anion and GGE and are strong enough to force GGE into highly bent conformations. These simulations provide fundamental models of the dissolution mechanism of lignin by POM-IL, which is promoted by strong interactions of the POM-IL anion with lignin.

Jahn-Teller Distorted Effects To Promote Nitrogen Reduction over Keggin-Type Phosphotungstic Acid Catalysts: Insight from Density Functional Theory Calculations

Molecular geometry, electronic structure, and possible reaction mechanism of a series of mono-transition-metal-substituted Keggin-type polyoxometalate (POM)-dinitrogen complexes [PW₁₁O₃₉M(N₂)] ^n- (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Tc, Ru, Rh, Pd, Ag, Cd, W, Re, Os, Ir, Pt, Au, and Hg) have been investigated by using density functional theory (DFT) calculations with M06L functional. The calculated adsorption energy of N₂ molecule, N-N bond length, N-N stretching frequency, and the NBO charge on the coordinated N₂ moiety indicate that Mo^II-, Tc^II-, W^II-, Re^II-, and Os^II-POM complexes are significant for binding and activation of the inert N₂ molecule. The degree of the N₂ activation can be classified into the "moderately activated" category according to Tuczek's sense [ J. Comput. Chem. 2006 , 27 , 1278 ]. Electronic structure and NBO analysis indicate that the terminal N atom of the coordinated N₂ molecule in these POM-dinitrogen complexes possesses more negative charge relative to the bridge N atom because Jahn-Teller distorted effects lead to an effective orbital mixture between σ_2s* orbital of N₂ and d _z² orbital of transition metal center. And the mono-lacunary Keggin-type POM ligand with five oxygen donor atoms serves as a strong electron donor to the bivalent metal center. Meanwhile, a catalytic cycle for direct conversion of N₂ into NH₃ has been systematically investigated based on a Re-POM complex along distal, alternating, and enzymatic pathways. The calculated free energy profile of the three catalytic cycles indicates that the distal mechanism is the favorable pathway in the presence of proton and electron donors.

Three novel polyoxometalate-based inorganic-organic hybrid materials based on 2,6-bis(1,2,4-triazol-1-yl)pyridine

Three novel inorganic-organic hybrid materials [Co(btp)2(W5O16)(H2O)] n (1), [Cd3(btp)6(PW12O40)2(H2O)6·6H2O] n (2), and [Ag3(btp)2(PMo12O40)·1.5H2O] n (3) (btp = 2,6-bis(1,2,4-triazol-1-yl)pyridine) have been hydrothermally synthesized and characterized by IR spectroscopy, single-crystal X-ray diffraction, powder X-ray diffraction (PXRD), elemental analysis and thermal gravimetric analysis (TGA). The most striking structure feature of compound 1 is a 3D polycatenation framework, interpenetrated by a 2D 4-connected sql topology layer and a 3D 6-connected rob topology framework. Compound 2 exhibits a rare meso-helices 3D network with different chiralities crossing coexistence. Compound 3 also holds a 3D framework formed by linking terminal oxygen atoms of [α-PMo12O40]3- anions and silver ions in a 2D metal-organic layer. Compound 1 displays antiferromagnetic behavior. The luminescence, electrochemical and photocatalytic properties of compounds 1-3 have also been investigated. Compound 3 exhibits significant electrochemical activity for the reduction of H2O2 while compounds 1 and 2 show efficient photocatalytic activities for the degradation of Rhodamine B (RhB). Furthermore, the three compounds display luminescence behaviors in the solid state.

Computational study on epoxidation of propylene by dioxygen using the silanol-functionalized polyoxometalate-supported osmium oxide catalyst

The silanol-functionalized POM-supported single-site Os oxide catalyst has been theoretically considered for epoxidation of propylene in the presence of dioxygen based on density functional theory calculations.

Calculations of NO reduction with CO over a Cu1/PMA single-atom catalyst: a study of surface oxygen species, active sites, and the reaction mechanism

Density functional theory calculations have been employed to probe the reaction mechanism of NO reduction with CO over a Cu₁/PMA (PMA is the phosphomolybdate, Cs₃PMo₁₂O₄₀) single-atom catalyst.

New insight into the catalytic cycle about epoxidation of alkenes by N2O over a Mn-substituted Keggin-type polyoxometalate

Although epoxidation of alkenes by N₂O catalyzed by Mn-substituted polyoxometalates (POMs) has been studied both experimental and theoretical methods, a complete catalytic cycle has not been established currently. In the present paper, density functional theory (DFT) calculations were employed to explore possible reaction mechanism about this catalytic cycle. Our DFT studies reveal that the reaction pathway starts from a low-valent Keggin-type POM aquametal derivative [PW₁₁O₃₉Mn^IIIH₂O]^4-. In the presence of N₂O pressure, the formation of the active catalytic species [PW₁₁O₃₉Mn^VO]^4- involves a ligand-substituted reaction about replacement of the aqua ligand with N₂O to generation of POM/N₂O adduct [PW₁₁O₃₉Mn^IIION₂]^4- and dissociation of N₂ from this adduct. The calculated free energy indicates that the ligand-substituted reaction is endergonic both in gas phase or various solvents. The partial optimization method reveals that the dissociation of N₂ from [PW₁₁O₃₉Mn^IIION₂]^4- involves crossing of the quintet state with a low-lying triplet state. Due to the high reactivity, the high-valent Mn^V-oxo species, [PW₁₁O₃₉Mn^VO]^4-, may react with the excess N₂O and alkenes. Thus, two alternative reaction pathways corresponding to activation of N₂O and epoxidation of alkenes have been considered in this work. The calculated free energy profile indicates that epoxidation of alkenes pathway is the favorable routes. Finally, a complete catalytic cycle for this reaction has been proposed. The rate-determining step in this catalytic cycle is the dissociation of N₂ from the low-valent POM/N₂O adduct according to our DFT-M06L calculations.