Isabel S. Gonçalves

MCM-41 functionalized with bipyridyl groups and its use as a support for oxomolybdenum(VI) catalysts

The ordered mesoporous silica MCM-41 was covalently grafted with (3-chloropropyl)trimethoxysilane. Chloro substitution by the anion [4-CH2-4′-Me-2,2′-bipyridine]− gave a ligand-silica containing ca. 0.3 mmol bipyridyl groups per gram. Powder X-ray diffraction and nitrogen adsorption–desorption analysis demonstrated that the textural characteristics of the support were preserved during the grafting experiments and that the channels remained accessible, despite sequential reductions in surface area, pore volume and pore size. The coupling reactions were monitored by 29Si MAS NMR and 13C CP MAS NMR spectroscopy. Bipyridyl-functionalized MCM-41 exhibits a high encapsulating ability, as evidenced by its interaction with a dichloromethane solution of MoO2Cl2(THF)2. A material with a metal loading of 8.3 mass% was obtained. Molybdenum K-edge EXAFS analysis could not substantiate the formation of a tethered complex of the type MoO2Cl2(N–N), but instead indicated the formation of unidentate-bridged entities of the type [O2Mo–X–MoO2] with a metal–metal separation of 3.28 A. The molybdenum-containing MCM was active as a catalyst for the epoxidation of cyclooctene by tert-butyl hydroperoxide. However, this activity is due, at least in part, to leached molybdenum species in solution.

Organorhenium(VII) and organomolybdenum(VI) oxides: synthesis and application in oxidation catalysis†

Methyltrioxorhenium(VII) has found numerous applications in various catalytic processes. In olefin epoxidation its activity can be enhanced by the addition of aromatic Lewis base nitrogen donor ligands, e.g. pyridines and pyrazoles. Due to the comparatively weak coordination of these ligands, a significant excess has to be used. Therefore the MTO/chiral Lewis base/H 2 O 2 system is not very useful for chiral epoxidations. In contrast to this, dimethyldioxomolybdenum (VI) MoO 2 (CH 3 ) 2 undergoes a significantly stronger interaction with Lewis bases and seems, despite its generally somewhat lower activity, a reasonable candidate for application in chiral epoxidation reactions together with an appropriate chiral Lewis base ligand. Complexes of the type MoO 2 (CH 3 ) 2 L are accessible via MoO 2 X 2 L (X = Cl, Br). These latter compounds are even more active in olefin epoxidation than MoO 2 (CH 3 ) 2 L. Unfortunately, however, all the Mo(VI) complexes mentioned above need t-butyl hydroperoxide as oxidizing agent and do not show activity in the presence of H 2 O 2 .

(Dimethyl)dioxomolybdenum(VI) complexes: syntheses and catalytic applications

Abstract Reaction of MoO2Br2S2 complexes [S=THF, CH3CN] with bidentate nitrogen donor ligands (L2) leads to complexes of the type MoO2Br2L2 in good yields, L2=substituted bipyridylphenantroline, 1,4-R2-diazabutadiene and bipyrimidine. Treatment of the latter complexes with Grignard reagents at low temperatures yields complexes of the general formula MoO2(CH3)2L2 and MoO2(C2H5)2(diphenylphenantroline). 1H NMR and IR data are comparatively indifferent to the ligand changes. The 95Mo NMR data of selected complexes reflect the donor capability of the organic ligands. Mass spectroscopy and temperature-dependent 95Mo NMR spectroscopy show a significant stability of the MoN bond. The compound MoO2(CH3)2(bipyrimidine) was additionally examined by single crystal X-ray analysis. The catalytic activity of the MoO2R2L2 complexes in olefin epoxidation with t-butyl hydroperoxide as oxidizing agent is strongly influenced by the nature of the ligand L and its steric bulk in the equatorial plane. The title complexes with a Mo(CH3)2 moiety are slightly less active in catalysis than the MoBr2 precursor compounds. Increase of both reaction time and/or temperature lead to a significant increase in the product yield in all examined cases. At about 90°C catalyst decomposition hampers further product yield increase.

Chiral bis(oxazoline) and pyridyl alcoholate dioxo-molybdenum(VI) complexes: synthesis, characterization and catalytic examinations

Abstract A group of chiral molybdenum(VI) complexes comprising MoO2Cl2L**, MoO2Cl(THF)L* and MoO2L2* [L**=chiral bis(oxazoline) and L*=chiral 2′-pyridyl alcoholate] have been prepared in good yields by reaction of the solvent substituted complex MoO2Cl2(THF)2 with one or two equivalents of chiral ligand. Optically active aminoalcohols (L*) were obtained by reaction of the appropriate organolithium compound with (−)-menthone, (+)-8-phenylisomenthone, (−)-8-phenylmenthone, (+)-camphor and (−)-thujone. The molybdenum complexes were characterized by multinuclear NMR (1H, 13C, 17O, 95Mo) spectroscopy, IR spectroscopy and elemental analysis. 95Mo-NMR data reflected the donor capability of the organic ligands, whereas 1H-NMR and IR data were comparatively indifferent to the changes in the Lewis base ligand. The complexes were evaluated as catalysts for the asymmetric epoxidation of trans-β-methylstyrene by tert-butylhydroperoxide. The bis(oxazoline) complexes showed good catalytic activity but had low optical yields. Complexes of the type MoO2Cl(THF)L* (L*=chiral 2′-pyridyl alcoholate) also exhibited high catalytic activity and enantiomeric excesses of up to 23%. The corresponding MoO2L2* alcoholate complexes were considerably less active with comparable optical yields.

Ordered benzene–silica hybrids with molecular-scale periodicity in the walls and different mesopore sizes

Mesoporous benzene–silica hybrid materials have been obtained via surfactant-mediated synthesis using alkyltrimethylammonium surfactants with alkyl chain lengths from 14 to 18. By combining N2 adsorption–desorption experiments and powder X-ray diffraction (PXRD) analysis we showed that these samples exhibited mesopore sizes in the range 3.2 to 3.9 nm and that the molecular-scale periodicity in the walls of these materials is maintained. Transmission electron microscopy (TEM), solid state nuclear magnetic resonance (NMR) and thermogravimetric analyses (TGA) have also been used to characterise the structure and stability of the samples.

Chiral dioxomolybdenum(VI) complexes for enantioselective alkene epoxidation

Chiral dioxomolybdenum(VI) complexes of the type MoO2Cl2(L*) (L* = oxime), MoO2(THF)2L* (L*= cis-p-menthane-3,8diol) and MoO2Cl(THF)L* (L*=8-phenylthioneomenthol and 8-phenylthioisoneomenthol) have been prepared in good yields by reacting MoO2Cl2(THF)2 with the appropriate chiral organic bidentate O,O-, O,N- and O,S-ligands. The complexes were characterised by solution NMR ( 1 H, 13 C, 95 Mo) and IR spectroscopy as well as elementary analysis, and were evaluated as catalysts in solution for the asymmetric epoxidation of cis--methylstyrene by tert-butylhydroperoxide (TBHP). The cis-diol complex shows high catalytic activity and enantiomeric excesses of up to 25%. An attempt was made to immobilise the complex MoO2(THF)Cl[(−)-8-phenylthioneomenthol] within the channels of MCM-41 mesoporous silica by using a tethering ligand [L=NC(CH2)3Si(OEt)3]. The material was characterised by powder X-ray diffraction (XRD), IR spectroscopy and magic-anglespinning (MAS) NMR ( 13 C, 29 Si). Catalytic examinations demonstrated that it was active in the epoxidation of cyclooctene by TBHP.

Structural and Photoluminescence Studies of a Europium(III) Tetrakis(β-diketonate) Complex with Tetrabutylammonium, Imidazolium, Pyridinium and Silica-Supported Imidazolium Counterions

Tetrakis(naphthoyltrifluoroacetonato)lanthanate(III) complexes (Ln = Eu, Gd) containing the cations tetrabutylammonium, [NBu(4)](+); 1-butyl-3-methylimidazolium, [C(4)mim](+); and 1-butyl-3-methylpyridinium, [C(4)mpyr](+), have been prepared and structurally characterized by single-crystal X-ray diffraction. The {EuO(8)} coordination sphere in [NBu(4)][Eu(NTA)(4)] is best described as a distorted dodecahedron, where the metal ion is located at the 4-fold inversion axis with only one crystallographically independent NTA residue. In [C(4)mim][Eu(NTA)(4)] and [C(4)mpyr][Gd(NTA)(4)], the central Ln(3+) ions are coordinated by eight oxygen atoms from four distinct beta-diketonate ligands, in an overall distorted square-antiprismatic geometry. Besides electrostatic interactions, the crystal packing in all three structures is stabilized by offset pi-pi interactions involving the naphthyl rings of neighboring complexes (and, for [C(4)mim][Eu(NTA)(4)] and [C(4)mpyr][Gd(NTA)(4)], neighboring naphthyl/imidazolium and naphthyl/pyridinium rings) and C-H...pi contacts. The photoluminescence properties of the three Eu(III) complexes were studied at room temperature and -259 degrees C by measuring emission and excitation spectra, (5)D(0) emission decay curves, and absolute emission quantum yields. Under ligand excitation (lambda(ex) = 290-395 nm), the quantum yields (room temperature) were in the range 0.72-0.77 for the 1-butyl-3-methylimidazolium salt. An immobilized analogue of this complex was prepared by supporting [Eu(NTA)(4)](-) on an ordered mesoporous silica derivatized with 1-propyl-3-methylimidazolium groups. The disappearance of the intra-4f(6) lines in the excitation spectrum of the supported material indicated an increase in the ligands sensitization process of the Eu(3+) ions, relative to direct intra-4f(6) excitation. The emission quantum yield measured for the supported material (0.32-0.40, for excitations between 265 and 360 nm) is the highest so far reported for lanthanide-containing ordered mesoporous silicas.

BIS-ACETONITRILE(DIBROMO)DIOXOMOLYBDENUM(VI) AND DERIVATIVES : SYNTHESIS, REACTIVITY, STRUCTURES AND CATALYTIC APPLICATIONS

Abstract We report on the preparation and spectroscopic properties of complexes of the general formula MoBr2O2Ln with L=NCCH3 (n=2), NCC(CH3)3 (n=1), NCC6H5 (n=2). The complex MoBr2O2(CH3CN)2 is additionally examined by single crystal X-ray structure determination. The MoBr2O2Ln complexes are very electron deficient (17O-, 95Mo-NMR evidence) and readily react with electron donor N-ligands to give more stable complexes e.g. MoBr2O2(bipyrimidine). The structure of the latter compound has been determined by single crystal X-ray crystallography. Reactions with organometallic complexes of the type [Cp′Mo(CN)2(CO)2]K (Cp′=Cp or Cp*) and Cp(CO)2Fe(CN) or [Cp(CO)Fe(CN)2]K result in the immediate reduction of the Mo(VI) center, as shown by EPR spectroscopy. The MoBr2O2Ln complexes are catalysts for the epoxidation of cyclooctene but decompose slowly during the course of the reaction due to their pronounced moisture sensitivity.

Preparation and photophysical characterisation of Zn–Al layered double hydroxides intercalated by anionic pyrene derivatives

Zn–Al hydrotalcite-like compounds intercalated by 1,3,6,8-pyrenetetrasulfonate (PTS), 1-pyrenesulfonate (PS) and 1-pyrenecarboxylate (PC) anions were synthesised by an ion-exchange procedure. The materials were characterised by powder X-ray diffraction at different temperatures, thermogravimetric analysis, FTIR, 13C{1H} CP/MAS NMR and photoluminescence techniques. In the fully hydrated states, the interlayer distances are 13.7 A for Zn–Al–PTS, 18.9 A for Zn–Al–PS, and 24.8 A for Zn–Al–PC. These can be ascribed to a monolayer arrangement for intercalated PTS anions and bilayer arrangements for the 1-pyrenyl derivatives. The samples exhibit different thermal decomposition pathways, and in the case of Zn–Al–PTS the removal of physisorbed and interlayer water leads to a change in the orientation of the organic anion with respect to the hydroxide layers. The structural transformation is fully reversible upon hydration. The photophysical characterisation of the bulk materials was based on the determination of their emission and fluorescence excitation spectra, and the fluorescence lifetimes. From the steady-state (monomer and “excimer-like” bands) and time-resolved (triple exponential decays) data, evidence for the presence of a structure with similar characteristics to pyrene dimer together with monomer (by comparison with the emission of dilute solutions of PS and PC), pre-associated and (possibly) dynamic excimer species could be presented.

New insights into the reaction of t-butylhydroperoxide with dichloro- and dimethyl(dioxo)molybdenum(VI)

Abstract Lewis-base adducts of dichlorodioxomolybdenum(VI) and dimethyldioxomolybdenum(VI) react in an equilibrium reaction with excess t-butylhydroperoxide (TBHP) under the formation of a seven-coordinated molybdenum(VI) complexes displaying a η1-alkylperoxo-ligand. HCl/CH4 elimination or the protonation of the Lewis-base ligand is not observed, the TBHP hydrogen atom is instead transferred to one of the terminal oxo ligands under the formation of a molybdenum bound OH moiety. The peroxo species is assumed to be the active catalyst in olefin epoxidation.