Understanding the Regioselectivity of Aromatic Hydroxylation over Divanadium-Substituted γ-Keggin Polyoxotungstate

A density functional study of the photocatalytic degradation of polycaprolactone by the decatungstate anion in acetonitrile solution

A recent experimental study has reported that decatungstate [W10O32]4- can degrade various polyesters in the presence of light and molecular oxygen [Li et al., Nanoscale, 2023, 15, 15038]. We apply density functional theory to the photocatalyst-polycaprolactone model complex in acetonitrile solution and elucidate the degradation mechanisms and catalytic cycle. We consider hydrogen atom transfer (HAT) and single electron transfer (SET) mechanisms. The potential energy profiles show that the former proceeds exergonically in a single step but that the latter involves a subsequent proton transfer and finally yields HAT products as well. Oxygenated polymer species can regain the transferred hydrogen and regenerate the reduced photocatalyst. We propose a photocatalytic cycle that realizes both the photocatalyst regeneration and the polymer degradation.

Resolving the Mechanism for H2O2 Decomposition over Zr(IV)-Substituted Lindqvist Tungstate: Evidence of Singlet Oxygen Intermediacy

The decomposition of hydrogen peroxide (H2O2) is the main undesired side reaction in catalytic oxidation processes of industrial interest that make use of H2O2 as a terminal oxidant, such as the epoxidation of alkenes. However, the mechanism responsible for this reaction is still poorly understood, thus hindering the development of design rules to maximize the efficiency of catalytic oxidations in terms of product selectivity and oxidant utilization efficiency. Here, we thoroughly investigated the H2O2 decomposition mechanism using a Zr-monosubstituted dimeric Lindqvist tungstate, (Bu4N)6[{W5O18Zr(μ-OH)}2] ({ZrW5}2), which revealed high activity for this reaction in acetonitrile. The mechanism of the {ZrW5}2-catalyzed H2O2 degradation in the absence of an organic substrate was investigated using kinetic, spectroscopic, and computational tools. The reaction is first order in the Zr catalyst and shows saturation behavior with increasing H2O2 concentration. The apparent activation energy is 11.5 kcal·mol-1, which is significantly lower than the values previously found for Ti- and Nb-substituted Lindqvist tungstates (14.6 and 16.7 kcal·mol-1, respectively). EPR spectroscopic studies indicated the formation of superoxide radicals, while EPR with a specific singlet oxygen trap, 2,2,6,6-tetramethylpiperidone (4-oxo-TEMP), revealed the generation of 1O2. The interaction of test substrates, α-terpinene and tetramethylethylene, with H2O2 in the presence of {ZrW5}2 corroborated the formation of products typical of the oxidation processes that engage 1O2 (endoperoxide ascaridole and 2,3-dimethyl-3-butene-2-hydroperoxide, respectively). While radical scavengers tBuOH and p-benzoquinone produced no effect on the peroxide product yield, the addition of 4-oxo-TEMP significantly reduced it. After optimization of the reaction conditions, a 90% yield of ascaridole was attained. DFT calculations provided an atomistic description of the H2O2 decomposition mechanism by Zr-substituted Lindqvist tungstate catalysts. Calculations showed that the reaction proceeds through a Zr-trioxidane [Zr-η2-OO(OH)] key intermediate, whose formation is the rate-determining step. The Zr-substituted POM activates heterolytically a first H2O2 molecule to generate a Zr-peroxo species, which attacks nucleophilically to a second H2O2, causing its heterolytic O-O cleavage to yield the Zr-trioxidane complex. In agreement with spectroscopic and kinetic studies, the lowest-energy pathway involves dimeric Zr species and an inner-sphere mechanism. Still, we also found monomeric inner- and outer-sphere pathways that are close in energy and could coexist with the dimeric one. The highly reactive Zr-trioxidane intermediate can evolve heterolytically to release singlet oxygen and also decompose homolytically, producing superoxide as the predominant radical species. For H2O2 decomposition by Ti- and Nb-substituted POMs, we also propose the formation of the TM-trioxidane key intermediate, finding good agreement with the observed trends in apparent activation energies.

Organoantimony Dihydroperoxides: Synthesis, Crystal Structures, and Hydrogen Bonding Networks

Despite growing interest in the potential applications of p-block hydroperoxo complexes, the chemistry of inorganic hydroperoxides remains largely unexplored. For instance, single-crystal structures of antimony hydroperoxo complexes have not been reported to date. Herein, we present the synthesis of six triaryl and trialkylantimony dihydroperoxides [Me₃Sb(OOH)₂, Me₃Sb(OOH)₂·H₂O, Ph₃Sb(OOH)₂·0.75(C₄H₈O), Ph₃Sb(OOH)₂·2CH₃OH, pTol₃Sb(OOH)₂, pTol₃Sb(OOH)₂·2(C₄H₈O)], obtained by the reaction of the corresponding dibromide antimony(V) complexes with an excess of highly concentrated hydrogen peroxide in the presence of ammonia. The obtained compounds were characterized by single-crystal and powder X-ray diffraction, Fourier transform infrared and Raman spectroscopies, and thermal analysis. The crystal structures of all six compounds reveal hydrogen-bonded networks formed by hydroperoxo ligands. In addition to the previously reported double hydrogen bonding, new types of hydrogen-bonded motifs formed by hydroperoxo ligands were found, including infinite hydroperoxo chains. Solid-state density functional theory calculation of Me₃Sb(OOH)₂ revealed reasonably strong hydrogen bonding between OOH ligands with an energy of 35 kJ/mol. Additionally, the potential application of Ph₃Sb(OOH)₂·0.75(C₄H₈O) as a two-electron oxidant for the enantioselective epoxidation of olefins was investigated in comparison with Ph₃SiOOH, Ph₃PbOOH, t-BuOOH, and H₂O₂.

Immobilization of Polyoxometalates on Carbon Nanotubes: Tuning Catalyst Activity, Selectivity and Stability in H2O2-Based Oxidations

In recent years, carbon nanotubes (CNTs), including N-doped ones (N-CNTs), have received significant attention as supports for the construction of heterogeneous catalysts. In this work, we summarize our progress in the application of (N)-CNTs for immobilization of anionic metal-oxygen clusters or polyoxometalates (POMs) and use of (N)-CNTs-supported POM as catalysts for liquid-phase selective oxidation of organic compounds with the green oxidant–aqueous hydrogen peroxide. We discuss here the main factors, which favor adsorption of POMs on (N)-CNTs and ensure a quasi-molecular dispersion of POM on the surface and their strong attachment to the support. The effects of the POM nature, N-doping of CNTs, acid additives, and other factors on the POM immobilization process and catalytic activity/selectivity of the (N)-CNTs-immobilized POMs are analyzed. Particular attention is drawn to the critical issue of the catalyst stability and reusability. The scope and limitations of the POM/(N)-CNTs catalysts in H2O2-based selective oxidations are discussed.

Hydroperoxo double hydrogen bonding: stabilization of hydroperoxo complexes exemplified by triphenylsilicon and triphenylgermanium hydroperoxides

Triphenylsilicon and germanium hydroperoxo complexes were characterized by single crystal X-ray analysis revealing hydroperoxo double hydrogen bonding.

Polyoxometalates in solution: speciation under spotlight

The review covers stability and transformations of classical polyoxometalates in aqueous solutions and provides their ion-distribution diagrams over a wide pH range.

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.

Recent advances of polyoxometalate-catalyzed selective oxidation based on structural classification

The structural diversity and tenability observed in POMs has encouraged extensive investigations into their catalytic activity. Based on the structural classification of POMs, this review summarizes recent advances relating to POM-catalyzed selective oxidation and places most emphasis on dynamic developments from 2015 onwards. Work which contributes to comparing the catalytic performance of POMs with delicate structural differences (e.g. the same type of POM structure with differences of the heteroatom, addenda, protonated state or counter-ion) and in elucidating the origin/distinction of catalytic activity, as well as reasonable mechanisms, are especially highlighted.

Catalytic Applications of Vanadium: A Mechanistic Perspective

The chemistry of vanadium has seen remarkable activity in the past 50 years. In the present review, reactions catalyzed by homogeneous and supported vanadium complexes from 2008 to 2018 are summarized and discussed. Particular attention is given to mechanistic and kinetics studies of vanadium-catalyzed reactions including oxidations of alkanes, alkenes, arenes, alcohols, aldehydes, ketones, and sulfur species, as well as oxidative C-C and C-O bond cleavage, carbon-carbon bond formation, deoxydehydration, haloperoxidase, cyanation, hydrogenation, dehydrogenation, ring-opening metathesis polymerization, and oxo/imido heterometathesis. Additionally, insights into heterogeneous vanadium catalysis are provided when parallels can be drawn from the homogeneous literature.

Activation mechanism of hydrogen peroxide by a divanadium-substituted polyoxometalate [γ-PV2W10O38(μ-OH)2]3-: A computational study

In the present paper, the reaction mechanism corresponding to activation of hydrogen peroxide (H₂O₂) by a divanadium-substituted polyoxometalate (POM) [γ-PV₂W₁₀O₃₈(μ-OH)₂]^3- (I) to form catalytic active species, peroxo complex [γ-PV₂W₁₀O₃₈(μ-η²,η²-O₂)]^3- (III), was studied by using the density functional theory (DFT) calculations method with B3LYP functional. The results indicate that coordination of H₂O₂ to I proceeds via a vanadium-center-assisted proton transfer pathway to remove the first water molecule and form a hydroperoxy intermediate [γ-PV₂W₁₀O₃₈(μ-OH) (μ-OOH)]^3- (II). And intermediate II occurs through three successive water-assisted proton transfer steps to remove the second water molecule and finally forms catalytic active species. The calculated overall energy profiles show that coordination of H₂O₂ to vanadium center requires a proton transfer barrier of about 24 kcal mol^-1. A detailed comparison of molecular geometries and electronic structure shows that the catalytic active species has a very interesting structural feature, where a superoxide radical (O₂^-) was embedded into two vanadium centers, and may be a potential nucleophile. The unique withdrawing electron properties and flexible bonding ability of the γ-Keggin-type POM ligand contribute to the formation of O₂^- radical. The tunable alternate arrangement of W-O bond series in γ-Keggin-type POM ligand contributes to the flexibility of the γ-Keggin-type POM ligand. Meanwhile, our DFT calculations show a good performance of B3LYP-gauge-independent atomic orbital (IGAIM) method for the calculation of ¹H NMR parameters of divanadium-substituted phosphotungstate.