Implication of oxidant activation on olefin epoxidation catalysed by Molybdenum catalysts with aroylhydrazonato ligands: Experimental and theoretical studies

The influence of hydrogen bonding on the structure of organic-inorganic hybrid catalysts and its application in the solvent-free epoxidation of α-olefins

In this study, two types of catalysts were prepared by the combination of gemini quaternary ammonium salt with two distinct species of phosphotungstic acid. Catalysts prepared by the Wells-Dawson type of phosphotungstic acid and Keggin-type phosphotungstic acid both exhibited dual-phase catalytic behavior, demonstrating both heterogeneous and homogeneous catalytic activities. In comparison to the catalyst prepared by the Keggin-type phosphotungstic acid, due to the higher size of Wells-Dawson type of phosphotungstic acid, hydrogen bonding could not effectively affect the catalyst prepared by H6P2W18O62. Subsequently, the influential factors on the catalytic reaction were investigated. Through the utilization of techniques such as XPS, FT-IR, Raman spectra and other characterization methods, two distinct structure and reaction mechanisms for these catalysts were elucidated under the influence of hydrogen bonding.

Preparative and Catalytic Properties of MoVI Mononuclear and Metallosupramolecular Coordination Assemblies Bearing Hydrazonato Ligands

A series of polynuclear, dinuclear, and mononuclear Mo(VI) complexes were synthesized with the hydrazonato ligands derived from 5-methoxysalicylaldehyde and the corresponding hydrazides (isonicotinic hydrazide (H2L1), nicotinic hydrazide (H2L2), 2-aminobenzhydrazide (H2L3), or 4-aminobenzhydrazide (H2L4)). The metallosupramolecular compounds obtained from non-coordinating solvents, [MoO2(L1,2)]n (1 and 2) and [MoO2(L3,4)]2 (3 and 4), formed infinite structures and metallacycles, respectively. By blocking two coordination sites with cis-dioxo ligands, the molybdenum centers have three coordination sites occupied by the ONO donor atoms from the rigid hydrazone ligands and one by the N atom of pyridyl or amine-functionalized ligand subcomponents from the neighboring Mo building units. The reaction in methanol afforded the mononuclear analogs [MoO2(L1-4)(MeOH)] (1a-4a) with additional monodentate MeOH ligands. All isolated complexes were tested as catalysts for cyclooctene epoxidation using tert-butyl hydroperoxide (TBHP) as an oxidant in water. The impact of the structure and ligand lability on the catalytic efficiency in homogeneous cyclooctene epoxidation was elucidated based on theoretical considerations. Thus, dinuclear assemblies exhibited better catalytic activity than mononuclear or polynuclear complexes.

Impedance Spectroscopy as a Powerful Tool for Researching Molybdenum-Based Materials with Schiff Base Hydrazones

Molybdenum coordination complexes are widely applied due to their biological and pharmacological potential, as well as their performance in different catalytic processes. Parent dioxidomolybdenum Schiff base complexes were prepared via the reaction of [MoO₂(acac)₂] with a hydrazone Schiff-base tetradentate ligand. A new hydrazone-Schiff base (H₂L^{1 and 2}) and its corresponding mononuclear and polynuclear dioxidomolybdenum(VI) complex were synthesized and characterized by spectroscopic methods and elemental analyses, and their thermal behavior was investigated by thermogravimetry. The crystal and molecular structures of H₂L² ligands and the complexes [MoO₂(L¹)(H₂O)], [MoO₂(L²)(H₂O)], [MoO₂(L¹)(MeOH)]∙MeOH, [MoO₂(L¹)(EtOH)]∙EtOH, [MoO₂(L¹)(2-PrOH)]∙2-PrOH, and [MoO₂(L¹)]_n were determined by single-crystal X-ray diffraction. Using the in situ impedance spectroscopy method (IS), the structural transformations of chosen complexes were followed, and their electrical properties were examined in a wide range of temperatures and frequencies.

Molybdenum, Vanadium, and Tungsten-Based Catalysts for Sustainable (ep)Oxidation

This article gives an overview of the research activity of the LAC2 team at LCC developed at Castres in the field of sustainable chemistry with an emphasis on the collaboration with a research team from the University of Zagreb, Faculty of Science, Croatia. The work is situated within the context of sustainable chemistry for the development of catalytic processes. Those processes imply molecular complexes containing oxido-molybdenum, -vanadium, -tungsten or simple polyoxometalates (POMs) as catalysts for organic solvent-free epoxidation. The studies considered first the influence of the nature of complexes (and related ligands) on the reactivity (assessing mechanisms through DFT calculations) with model substrates. From those model processes, the work has been enlarged to the valorization of biomass resources. A part concerns the activity on vanadium chemistry and the final part concerns the use of POMs as catalysts, from molecular to grafted catalysts, (ep)oxidizing substrates from fossil and biomass resources.

Counter Anion Effects on the Formation and Structural Transformations of Mo(vi)-Hydrazone Coordination Assemblies: Salts, Solvates, Co-Crystals, and Neutral Complexes

Complex salts [1H]X and [1H](XA)0.5·2MeOH, and co-crystals [1H]X·0.5VA (X = chloride or bromide, XA = chloranilate or bromanilate, VA = o-vanillin azine), comprising [MoO2(HL)(MeOH)]+ ([1H]+) cation (H2L = 3-methoxysalicylaldehyde isonicotinoyl hydrazone), were prepared either by solution-based synthesis or by mechanochemical synthesis. Whereas [1H]X salts were extremely sensitive to humidity, their stability could be reinforced by the azine incorporation into the complex network. Solvent-mediated transformations of [1H]X led to methanol co-ligand replacement and afforded complexes [MoO2(HL)X] (2Cl·MeOH, 2Cl, and 2Br·0.5MeCN). However, solvates [1H](XA)0.5·2MeOH, under the same conditions, gave stable complexes [1H](XA)0.5 in which methanol remained coordinated. The differences in the assembly’s behavior were attributed to the packing arrangements, the relative orientation of cations and anions, and interactions between them. Polymorph [MoO2(L)(MeOH)] (1), not attainable by other routes, was the only product when compounds [MoO2(HL)X] were treated with a weak base at low temperatures. Tetranuclear [MoO2(L)]4 and polynuclear [MoO2(L)]n (2) supramolecular isomers, concomitantly crystallized when the reaction was conducted solvothermally. All of the complexes were characterized using X-ray diffraction methods (SCXRD and PXRD), spectroscopic methods (ATR-IR and solution-state and solid-state MAS NMR), and elemental and thermal analyses. The cytotoxicity of the different types of compounds against THP-1 and HepG2 cells was also evaluated.