Post-Synthesis Strategies to Prepare Mesostructured and Hierarchical Silicates for Liquid Phase Catalytic Epoxidation

Olefin epoxidation is an important transformation for the chemical valorization of olefins, which may derive from renewable sources or domestic/industrial waste. Different post-synthesis strategies were employed to introduce molybdenum species into mesostructured and hierarchical micro-mesoporous catalysts of the type TUD-1 and BEA, respectively, to confer epoxidation activity for the conversion of relatively bulky olefins (e.g., biobased methyl oleate, DL-limonene) to epoxide products, using tert-butyl hydroperoxide as an oxidant. The influences of (i) the type of metal precursor, (ii) type of post-synthesis impregnation method, (iii) type of support and (iv) top-down versus bottom-up synthesis methodologies were studied to achieve superior catalytic performances. Higher epoxidation activity was achieved for a material prepared via (post-synthesis) incipient wetness impregnation of MoO2(acac)2 (acac = acetylacetonate) on (pre-treated) siliceous TUD-1 and calcination; for example, methyl oleate was converted to the corresponding epoxide with 100% selectivity at 89% conversion (70 °C). Catalytic and solid-state characterization studies were conducted to shed light on material stability phenomena.

Ionic Liquids Based on Oxidoperoxido-Molybdenum(VI) Complexes with a Chelating Picolinate Ligand for Catalytic Epoxidation

Ionic oxidoperoxido-molybdenum(VI) complexes of the type [Cat][MoO(O2)2(pic)], with pic = N,O-chelated picolinate ligand and Cat = monocation, were prepared in high yields (82–95%) from the precursor complex [H3O][MoO(O2)2(pic)] via [H]+ cation exchange for 1-ethyl-3-methylimidazolium [EMIM]+, 1-butyl-3-methylimidazolium [BMIM]+, 1-octyl-3-methylimidazolium [OMIM]+, N-cetylpyridinium [C16Py]+, and N-methyl-N,N,N-trioctylammonium [Aliquat]+. The structure and purity of the ionic compounds were assessed by 1H and 13C NMR, FT-IR, and elemental analysis (C, H, N), and the electrochemical properties were studied by differential pulse voltammetry (DPV) and cyclic voltammetry (CV). The [Cat][MoO(O2)2(pic)] compounds showed promising catalytic epoxidation activity based on the model reaction of cis-cyclooctene with tert-butyl hydroperoxide as oxidant. The type of cation influenced the physical state of the compound and the catalytic performance.

A hydrogen-bonded assembly of cucurbit[6]uril and [MoO2Cl2(H2O)2] with catalytic efficacy for the one-pot conversion of olefins to alkoxy products

The reaction of the macrocyclic cavitand cucurbit[6]uril (CB[6]) and the diaqua complex [MoO₂Cl₂(H₂O)₂] in hydrochloric acid solution gave a water insoluble supramolecular compound with the general composition 2[MoO₂Cl₂(H₂O)₂]·CB[6]·xH₂O·yHCl·z(CH₃COCH₃) (2). Single crystal X-ray diffraction (XRD) analysis revealed the presence of barrel-shape supramolecular entities, {CB[6]·10(H₂O)}, aligned in layers which are shifted relative to adjacent layers to form a brick-like pattern. The CB[6]/water hydrogen-bonded entities further engage in intermolecular interactions with water, HCl and [MoO₂Cl₂(H₂O)₂] molecules to form a three-dimensional (3D) framework. Compound 2 was characterised by thermogravimetric analysis (TGA), IR and Raman vibrational spectroscopy, and ¹³C{¹H} CP MAS NMR. The reference complex [MoO₂Cl₂(H₂O)₂]·(diglyme)₂ (1) and compound 2 were studied for the oxidative catalytic conversion of olefins (cis-cyclooctene, cyclohexene and styrene) with aqueous H₂O₂ as oxidant. Using alcohols as solvents, 2 was employed in a one-pot two-stage strategy for converting olefins to alkoxy products, which involves oxidation (with H₂O₂) and acid chemistry. Mechanistic studies were carried out using different intermediates as substrates, and the type of solvent and substrate scope were investigated. The results demonstrated the ability of the CB[6]/Mo^VI supramolecular adduct to function as an acid-oxidation multifunctional catalyst, and its recovery and reuse via relatively simple procedures.

High-yield synthesis and catalytic response of chainlike hybrid materials of the [(MoO3)m(2,2′-bipyridine)n] family

MoO₃/2,2′-bipyridine hybrids have been prepared (hydro)thermally from [MoO₃(2,2′-bipy)] and compared as catalysts in styrene oxide ethanolysis and olefin epoxidation.

Oxidoperoxidomolybdenum(vi) complexes with acylpyrazolonate ligands: synthesis, structure and catalytic properties

Oxidoperoxido–molybdenum(vi) complexes Ph₄P[Mo(O)(O₂)₂(Q^R)] and [Mo(O)(O₂)(Q^R)₂] (Q^R = acylpyrazolonate ligands) were synthesised and tested as catalysts in epoxidation, sulphoxidation and epoxide deoxygenation.

Study of temperature dependent three component dynamic covalent assembly via Hantzsch reaction catalyzed by dioxido- and oxidoperoxidomolybdenum(vi) complexes under solvent free conditions

Dioxido- and oxidoperoxidomolybdenum(vi) complexes, and their catalytic potential for the Hantzsch type reaction under solvent free condition, giving corresponding dihydropyridine and pyridine, are reported.

Templated C-C and C-N Bond Formation Facilitated by a Molybdenum(VI) Metal Center

Preparation of molybdenum dioxido complexes with novel iminophenolate ligands bearing pendant secondary amide functionalities led to unprecedented C-C and C-N coupling reactions of two α-iminoamides upon coordination. The diastereoselective cyclization to asymmetric imidazolidines occurs at the metal center in two consecutive steps via a monocoupled intermediate. A meaningful mechanism is proposed on the basis of full characterization of intermediate and final molybdenum-containing products by spectroscopic means and by single-crystal X-ray diffraction analyses. This process constitutes the first example of a diastereoselective self-cyclization of two α-iminoamides.

Activation of molecular oxygen by a molybdenum complex for catalytic oxidation

A sterically demanding molybdenum(VI) dioxo complex was found to catalytically activate molecular oxygen and to transfer its oxygen atoms to phosphines. Intermediate peroxo as well as reduced mono-oxo complexes were isolated and fully characterized. Monomeric Mo(IV) monooxo species proved to be of an unusual nature with the coordinated phosphine trans to the oxo group. The reduced molybdenum centers can activate O2 to form a stable Mo(VI) oxo-peroxo complex unambiguously characterized by single crystal X-ray diffraction analysis. NMR experiments demonstrate that both oxygen atoms of the peroxo unit are transferred to an accepting substrate, generating the Mo(IV) intermediate and restarting the catalytic cycle.

Hafnium catalysts for direct alkene epoxidation using molecular oxygen as oxidant

Direct alkene epoxidation catalytic cycles were computed for hafnium-based catalysts containing bis(dinitrone) and bis(imine nitrone) ligation using molecular oxygen as oxidant without co-reductant.

Oxygen atom transfer reactions from Mimoun complexes to sulfides and sulfoxides. A bonding evolution theory analysis

In this research, a comprehensive theoretical investigation has been conducted on oxygen atom transfer (OAT) reactions from Mimoun complexes to sulfides and sulfoxides. The joint use of the electron localization function (ELF) and Thom's catastrophe theory (CT) provides a powerful tool to analyze the evolution of chemical events along a reaction pathway. The progress of the reaction has been monitored by structural stability domains from ELF topology while the changes between them are controlled by turning points derived from CT which reveal that the reaction mechanism can be separated in several steps: first, a rupture of the peroxo O1-O2 bond, then a rearrangement of lone pairs of the sulfur atom occurs and subsequently the formation of S-O1 bond. The OAT process involving the oxidation of sulfides and sulfoxides is found to be an asynchronous process where O1-O2 bond breaking and S-O1 bond formation processes do not occur simultaneously. Nucleophilic/electrophilic characters of both dimethyl sulfide and dimethyl sulfoxide, respectively, are sufficiently described by our results, which hold the key to unprecedented insight into the mapping of electrons that compose the bonds while the bonds change.

Selective oxidation passing through η3-ozone intermediates: applications to direct propene epoxidation using molecular oxygen oxidant

An unprecedented catalytic route for selective oxidation is designed that passes through η³-ozone intermediates and uses molecular oxygen as the oxidant without requiring a coreductant. DFT-computed reaction cycles identify a new catalyst type with the lowest computed overall activation energy for direct propene epoxidation for any catalyst to date.