André D. Lopes

Lewis base adducts of bis-(halogeno)dioxomolybdenum(VI): syntheses, structures, and catalytic applications

Abstract Reaction of solvent substituted MoO2X2(Solv)2 complexes ((Solv)=THF, CH3CN) with mono- and bidentate nitrogen and oxygen donor ligands leads to complexes of the type MoO2X2L2 in nearly quantitative yields at room temperature within a few minutes. The 95 Mo and 17 O NMR data of selected complexes as well as the MoO IR vibrations were used to probe the influence of the ligands on the electronic properties of the metal and the MoO bond. Two complexes have additionally been examined by single crystal X-ray analysis. The activity of the MoO2X2L2 complexes as catalysts in olefin epoxidation with t-butylhydroperoxide as oxidizing agent depends on both the nature of the organic ligand L and the halogeno ligand X. The difference in activity observed between Cl and Br substituted complexes is not very pronounced. In general, the Cl derivatives are more active than their Br analogues. The organic ligands L display a significant influence on the catalytic performance. Complexes with ligands bearing aromatic substituents at N are in all cases, much more active than those bearing aliphatic substituents. The less active complexes can be activated by raising the temperature and extending the reaction time. In all observed cases, these changes produce a significant increase of the product yield.

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.

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.

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.

Studies on olefin epoxidation with t-BuOOH catalysed by dioxomolybdenum(VI) complexes of a novel chiral pyridyl alcoholate ligand

The chiral dioxomolybdenum(VI) complexes [MoCl{(1R,2S,5S)-8-trimethylsilyloxy-1-(2-pyridyl)mentholato}(O)2(THF)] and [Mo{(1R,2S,5S)-8-trimethylsilyloxy-1-(2-pyridyl)mentholato}2(O)2] have been prepared in good yields by reaction of the solvent substituted complex [MoCl2O2(THF)2] with one or two equivalents of chiral 2′-pyridyl alcohol. The optically active aminoalcohol was obtained by reaction of 2-pyridyllithium with (−)-(2S,5S)-8-trimethylsilyloxymenthone. The complexes are active catalysts in the homogeneous epoxidation of cyclic and linear olefins, dienes and terpenes by t-BuOOH. They present remarkable activity and excellent product selectivity in cyclooctene epoxidation (cyclooctene oxide was obtained in quantitative yield). In the case of limonene, regioselectivity is high in favour of the epoxidation of the internal cyclic double bond. Ring opening activity was also observed for α-pinene oxide, producing campholenic aldehyde and epoxy campholenic aldehyde.

Mesoporous Silicas Modified with Dioxomolybdenum(VI) Complexes: Synthesis and Catalysis

The dioxomolybdenum(VI) fragment MoO2X2 has been confined within the ordered mesopores of pure siliceous hexagonal MCM-41 and cubic MCM-48 molecular sieves either by direct grafting (solvent impregnation) with MoO2X2(THF)2 or by using a spacer ligand [L = NC(CH2)2Si(OEt)3]. The materials have been characterised by elemental analysis, powder X-ray diffraction, N2 adsorption, IR spectroscopy, and magic-angle spinning NMR spectroscopy (13C, 29Si). All catalysts, homogeneous and heterogenised, are active in the epoxidation of cyclooctene with tert-butyl hydroperoxide.

Synthesis and catalytic properties of molybdenum(VI) complexes with tris(3,5-dimethyl-1-pyrazolyl)methane.

The complex [MoO(2)Cl{HC(3,5-Me(2)pz)(3)}]BF(4) (1) (HC(3,5-Me(2)pz)(3) = tris(3,5-dimethyl-1-pyrazolyl)methane) has been prepared and examined as a catalyst for epoxidation of olefins at 55 °C using tert-butyl hydroperoxide (TBHP) as the oxidant. For reaction of cis-cyclooctene, epoxycyclooctane is obtained quantitatively within 5 h when water is rigorously excluded from the reaction mixture. Increasing amounts of water in the reaction mixture lead to lower activities (without affecting product selectivity) and transformation of 1 into the trioxidomolybdenum(VI) complex [{HC(3,5-Me(2)pz)(3)}MoO(3)] (4). Complex 4 was isolated as a microcrystalline solid by refluxing a suspension of 1 in water. The powder X-ray diffraction pattern of 4 can be indexed in the orthorhombic Pnma system, with a = 16.7349(5) Å, b = 13.6380(4) Å, and c = 7.8513(3) Å. Treatment of 1 in dichloromethane with excess TBHP led to isolation of the symmetrical [Mo(2)O(4)(μ(2)-O){HC(3,5-Me(2)pz)(3)}(2)](BF(4))(2) (2) and unsymmetrical [Mo(2)O(3)(O(2))(2)(μ(2)-O)(H(2)O){HC(3,5-Me(2)pz)(3)}] (3) oxido-bridged dimers, which were characterized by single-crystal X-ray diffraction. Complex 2 displays the well-known (Mo(2)O(5))(2+) bridging structure where each dioxidomolybdenum(VI) center is coordinated to three N atoms of the organic ligand and one μ(2)-bridging O atom. The unusual complex 3 comprises dioxido and oxidodiperoxo molybdenum(VI) centers linked by a μ(2)-bridging O atom, with the former center being coordinated to the tridentate N-ligand. The dinuclear complexes exhibit a similar catalytic performance to that found for mononuclear 1. For complexes 1 and 2 use of the ionic liquids (ILs) 1-butyl-3-methylimidazolium tetrafluoroborate and N-butyl-3-methylpyridinium tetrafluoroborate as solvents allowed the complexes to be completely dissolved, and in each case the catalyst and IL could be recycled and reused without loss of activity.

Synthesis, structural elucidation, and application of a pyrazolylpyridine-molybdenum oxide composite as a heterogeneous catalyst for olefin epoxidation.

The reaction of [MoO(2)Cl(2)(pypzEA)] (1) (pypzEA = ethyl[3-(pyridin-2-yl)-1H-pyrazol-1-yl]acetate) with water in a Teflon-lined stainless steel autoclave (100 °C) or in an open reflux system leads to the isolation of the molybdenum oxide/pyrazolylpyridine composite material [Mo(2)O(6)(HpypzA)] (2; HpypzA = [3-(pyridinium-2-yl)-1H-pyrazol-1-yl]acetate). The solid state structure of 2 was solved through single crystal and powder X-ray diffraction analyses in conjunction with information derived from FT-IR and (13)C CP MAS NMR spectroscopies and elemental analyses. In the asymmetric unit of 2, two crystallographically distinct Mo(6+) centers are bridged by a syn,syn-carboxylate group of HpypzA. The periodic repetition of these units along the a axis of the unit cell leads to the formation of a one-dimensional composite polymer, (∞)(1)[Mo(2)O(6)(HpypzA)]. The outstretched pyrazolylpyridine groups of adjacent polymers interdigitate to form a zipper-like motif, generating strong onset π-π contacts between adjacent rings of coordinated HpypzA molecules. The composite oxide 2 is a stable heterogeneous catalyst for liquid-phase olefin epoxidation.