No abstract

Increasing attention has been dedicated to the development of nanomaterials rendering green and sustainable processes, which occur in benign aqueous reaction media. Herein, we demonstrate the synthesis of another family of green nanomaterials, layered double hydroxide (LDH) nanoclusters, which are concentrated (98.7 g/L in aqueous solvent), stably dispersed (transparent sol for >2 weeks), and catalytically active colloids of nano LDHs (isotropic shape with the size of 7.8 nm as determined by small-angle X-ray scattering). LDH nanoclusters are available as colloidal building blocks to give access to meso- and macroporous LDH materials. Proof-of-concept applications revealed that the LDH nanocluster works as a solid basic catalyst and is separable from solvents of catalytic reactions, confirming the nature of nanocatalysts. The present work closely investigates the unique physical and chemical features of this colloid, the formation mechanism, and the ability to act as basic nanocatalysts in benign aqueous reaction systems.

Mo K-edge XAFS spectra have been measured for ordered mesoporous silica MCM-41 grafted with the complexes [MoO2X2(thf)2] (X=Cl, Br). For grafting reactions in the absence of triethylamine, materials with 1 wt. % Mo are obtained; the Mo K-edge EXAFS results indicate the co-existence of isolated surface-fixed monomeric species [MoO2[(-O)3SiO]2(thf)(n)] and [MoO2[(-O)3SiO]X(thf)(n)]. When Et3N is used in the grafting reactions, materials with 4 wt. % Mo are obtained. The EXAFS data for the material prepared using [MoO2Cl2(thf)2] and Et3N indicate the presence of dinuclear species with two Mo(VI) centres, each with two Mo=O groups and each linked by one or two oxo bridges (Mo...Mo 3.27 A). The molybdenum centres in the material prepared using the dibromo complex comprise mainly isolated four-coordinate dioxomolybdenum(VI) and trioxomolybdenum(VI) monomeric species, with a small contribution from dimeric species. All materials were further characterised in the solid state by powder X-ray diffraction, N2 adsorption analysis, MAS NMR (13C, 29Si) and FTIR spectroscopy. The derivatised MCMs perform differently as catalysts in the liquid-phase oxidation of various olefins and alcohols with tert-butyl hydroperoxide. The highest alkene epoxidation activity was recorded for the catalysts with low metal loading, whereas the material containing oxo-bridged dimers had the highest activity for oxidation of alcohols. The recyclability of all the catalysts was tested: the catalytic activity of the derivatised materials tended to stabilize with ageing.

The dioxomolybdenum(VI) complex [MoO 2 Cl 2 (L-L)], containing the bidentate 1,4-diazabutadiene ligand RN= C(Ph)−C(Ph)=NR [R = (CH 2 ) 3 Si(OEt) 3 ], was immobilised in the ordered mesoporous silica MCM-41 by carrying out a grafting reaction in dichloromethane. The grafted material (3.3 wt.-% Mo) was tested as a catalyst for the epoxidation of cyclooctene with tert-butyl hydroperoxide (TBHP) at 55°C. Selectivity to the epoxide was very high and the observed kinetic profile was similar to that of the complex [MoO 2 Cl 2 (L-L)] in the homogeneous phase. On recycling several times, some activity was lost from the first to second runs, but thereafter stabilised. Tethered complexes of this type were also prepared by a stepwise approach. The mesoporous silicas MCM-41 and MCM-48 were first treated with a toluene solu-

The dioxomolybdenum(VI) complex [MoO 2 Cl 2 {p-tolyl(CH 3 DAB)}] has been prepared in good yield by reaction of the solvent adduct MoO 2 Cl 2 (THF) 2 with one equivalent of the bidentate ligand N,N-p-tolyl-2,3-dimethyl-1,4-diazabutadiene. Treatment of the dichloro complex with the Grignard reagent CH 3 MgCl gives the dimethyl derivative [MoO 2 (CH 3) 2 {p-tolyl(CH 3 DAB)}]. The complexes are highly active and selective catalysts for the homogeneous epoxidation of cyclooctene using tert-butyl hydroperoxide (TBHP) as the oxidant. In both cases, the initial activity is ca. 175 mol mol À1 Mo h À1 and cyclooctene oxide is obtained quantitatively within 4 h. It was possible to recover the dimethyl complex at the end of the reaction and reuse it in a second run with only a small decrease in activity. The complexes are also active and selective for the epoxidation of other olefins, such as 1-octene, 2-octene, cyclododecene and (R)-(þ)-limonene, with TBHP. The catalytic production of cyclooctene oxide was investigated in detail, varying either the reaction temperature or the initial concentrations of substrate, oxidant and catalyst precursor. Kinetic studies show that the catalyst precursor-oxygen donor complex formation is first-order in TBHP and in the metal complex [MoO 2 Cl 2 {p-tolyl(CH 3 DAB)}]. A specific rate of 3.2 mol À1 dm 3 s À1 was found for catalyst formation at 25 C. Activation parameters for this reaction have also been measured (DH 6 ¼ ¼ 48 AE 3 kJ mol À1 , DS 6 ¼ ¼ À112 AE 10 J mol À1 K À1).