Molybdenum-catalysed oxidation of cyclohexene with hydrogen peroxide in the presence of alcohols and X-ray structures of octamolybdate [C4mim]4[Mo8O26] and tetraperoxodimolybdate [Htmpy]2[{MoO(O2)2}2(μ-O)] complexes

A biradical oxo-molybdenum complex containing semiquinone and o-aminophenol benzoxazole-based ligands

We report a new mononuclear molybdenum(iv) complex, MoOLBISLSQ, in which LSQ (2,4-di-tert-butyl o-semibenzoquinone ligand) has been prepared from the reaction of the o-iminosemibenzoquinone form of a tridentate non-innocent benzoxazole ligand, LBIS, and MoO2(acac)2. The complex was characterized by X-ray crystallography, elemental analysis, IR and UV-vis spectroscopy and magnetic susceptibility measurements. The crystal structure of MoOLBISLSQ revealed a distorted octahedral geometry around the metal centre, surrounded by one O and two N atoms of LBIS and two O atoms of LSQ. The effective magnetic moment (μ eff) of MoOLBISLSQ decreased from 2.36 to 0.2 μB in the temperature range of 290 to 2 K, indicating a singlet ground state caused by antiferromagnetic coupling between the metal and ligand centred unpaired electrons. Also, the latter led to the EPR silence of the complex. Cyclic voltammetry (CV) studies indicate both ligand and metal-centered redox processes. MoOLBISLSQ was applied as a catalyst for the oxidative cleavage of cyclohexene to adipic acid and selective oxidation of sulfides to sulfones with aqueous hydrogen peroxide.

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.

Molybdenum(0) tricarbonyl and tetracarbonyl complexes with a cationic pyrazolylpyridine ligand: synthesis, crystal structures and catalytic performance in olefin epoxidation

The synthesis of molybdenum(0) tricarbonyl and tetracarbonyl complexes of the form [Mo(CO)3(ptapzpy)Br] (1) and cis-[Mo(CO)4(ptapzpy)]Br (2) is reported, where ptapzpy = 2-(1-propyltrimethylammonium-3-pyrazolyl)pyridine. Preparation of these derivatives was accomplished either through thermal replacement of CO in Mo(CO)6 (for 1) or substitution under milder conditions of piperidine ligands in the precursor cis-[Mo(CO)4(pip)2] (for 2). The crystal structures of the ligand [ptapzpy]Br and complexes 1 and 2 were determined. Thermal treatment of 2 at 125-150 °C leads to mono decarbonylation and formation of 1. On the other hand, oxidative decarbonylation of 1 and 2 by reaction with tert-butylhydroperoxide (TBHP, 10 equiv.) gives a molybdenum oxide hybrid material formulated as [Mo3O9([ptapzpy]Br)2]·nH2O (3), which was characterised by FT-IR and Raman spectroscopy, thermogravimetric analysis, and 13C{1H} CP MAS NMR spectroscopy. Compounds 1-3 were effective (pre)catalysts for the epoxidation of cis-cyclooctene at 55 °C with aqueous H2O2 or TBHP (slightly better results were obtained with the former). The characterisation of the Mo-containing solids isolated after the catalytic reaction showed that poorly soluble β-octamolybdate salts, (L) x [Mo8O26], were formed from 1-3 with TBHP and from 1 with H2O2, while soluble oxoperoxo species were formed from 3 with H2O2. These findings helped to explain the different catalytic performances obtained.

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.