Abstract
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]/MoVI supramolecular adduct to function as an acid-oxidation multifunctional catalyst, and its recovery and reuse via relatively simple procedures.
Highlights
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)
In the resultant crystalline supramolecular compound (2), the diaqua complexes are embedded between barrelshaped hydrogen-bonded entities, {CB[6]·10(H2O)}, which are arranged in layers
The activity of compound 2 for the catalytic oxidation of cis-cyclooctene is moderate, it has the advantage of using H2O2 as oxidant, and the selectivity towards the epoxide is very high (90-100%) for reaction temperatures up to 70 °C and different solvents
Summary
Two main avenues have been pursued to isolate airstable crystalline solids containing the diaqua complex: 1) adduct formation with polyethers such as diglyme or crown ethers such as 18-crown-67; 2) co-crystallisation with ammonium ions such as pyridinium, triethylammonium,9 1,3bis(4-pyridinium)propane (H2dipy-pra) and 2,2′dipyridylammonium (Hdpa) (all with chloride as the counterion) These compounds display intricate hydrogenbonding networks that give rise to supramolecular frameworks. Transition-metal aqua complexes generally interact with CBs indirectly via hydrogen bonds between the coordinated water molecules and the portal oxygen atoms of the CB. This gives rise to supramolecular adducts which crystallise with complex network structures, such as that found with CB[6] and the trinuclear molybdenum oxo cluster [Mo3O4(H2O)6Cl3]+.14c. Reported in the Cambridge Structural Database (version 5.39 – 2018): CSD codes YEYWOO, FEKCED,7a FEKCED01,7b KEQBUE,7b XMOPYC10,8 LAPPOH, KICNIU, MUPGIO, and VONXUS. The MoVI coordination centre displays a distorted octahedral geometry, with the four O-atoms (two oxido groups, Mo=O, and two water molecules, Mo–Ow) in the basal plane, and two chlorine atoms, Mo–Cl, occupying the axial positions (Fig. 1; Table 1 gives details about the coordination bond lengths and internal angles)
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