Ana M. Santos

Trigonal-Bipyramidal Lewis Base Adducts of Methyltrioxorhenium(VII) and Their Bisperoxo Congeners: Characterization, Application in Catalytic Epoxidation, and Density Functional Mechanistic Study

Pyrazole and pyridine adducts of methyltrioxorhenium(VII) form bisperoxo complexes with excess H2O2 (see picture). Density functional calculations reveal that the increased efficiency of these complexes as catalysts for olefin epoxidation originates from their stability and from the moderate energies of the corresponding transition states. Nonaromatic nitrogen-base ligands reduce the catalytic performance of the adducts, in agreement with the computational results.

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 .

Molybdenum(VI)cis-dioxo complexes bearing sugar derived chiral Schiff-base ligands: synthesis, characterization, and catalytic applications

Molybdenum(VI)–cis-dioxo complexes bearing sugar derived chiral Schiff-base ligands of general formula MoO2(L)(Solv) have been synthesized (with L = N-salicylidene-D-glucosamine; N-salicylidene-1,3,4,6-tetraacetyl-α-D-glucosamine; N-5-chlorosalicylaldehyde-1,3,4,6-tetraacetyl-α-D-glucosamine; N-salicylaldehyde-1,3,4,6-tetraacetyl-β-D-glucosamine; N-5-chlorosalicylaldehyde-1,3,4,6-tetraacetyl-β-D-glucosamine; N-salicylidene-4,6-O-ethylidene-β-D-glucopyranosylamine, and Solv = methanol or ethanol). Analytical data including IR, 1D- and 2D-NMR, MS and EA are in accord with their descriptions as monometallic compounds with one ligand L and a coordinated solvent molecule. One of the complexes and two of the chiral ligands have been examined by X-ray crystallography. In the case of the sugar –OH groups being protected as acetyl groups, one of them is selectively deacetylated and coordinates to the metal centre during the reaction process. Furthermore, an inversion takes place at the C1 carbon atom. This uncommon behaviour has been examined in some detail. The high catalytic activity of the title compounds for epoxidation is also described as well as the moderate enantiomeric induction of up to 30% ee for cis-β-methyl styrene.

(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.

Organonitrile ligated silver complexes with perfluorinated weakly coordinating anions and their catalytic application for coupling reactions

Homogeneous catalytic processes mediated by silver(I) complexes are relatively rare. This work describes the synthesis and characterization of acetonitrile ligated silver salts with three weakly coordinating anions [B(C6F5)4]−, [B{C6H3(CF3)2}4]− and [(C6F5)3B–C3H4N2–B(C6F5)3]−. The silver cation is coordinated either by four or by two acetonitrile ligands. All examined Ag(I) complexes show catalytic activity in coupling reactions of terminal alkynes with aldehydes and amines.

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.

4-Ethynylpyridine as bridging moiety in mixed Ru/Re complexes

Two ruthenium(II) 4-ethynylpyridine-hydride complexes bearing one Lewis-basic nitrogen atom as coordination site, namely trans-Ru(dppe)2H(CCpy-4) (1) and trans-Ru(dppm)2H(CCpy-4) (2) (dppe=1,2-bis(diphenylphosphino) ethane, dppm=1,2-bis(diphenylphosphino) methane) and Ru(dmpe)2(CCpy-4)2 (3) (dmpe=1,2-bis(dimethylphosphino) ethane) with two nitrogen donor atoms were applied to synthesize the heterobimetallic mixed Ru/Re complexes 4–7. The X-ray crystal structure of the binuclear complex [Re(CO)3(t-bu2bipy)Ru(dppe)2(CCpy-4)H] [OS(O)2CF3] (t-bu2bipy=4, 4′-di (t-butyl)-2,2′-bipyridine, (7)) has been determined. Besides the usual characterization (IR, NMR, UV/Vis, EA) of the compounds 1–7, thermogravimmetry (TG) and cyclovoltammetry (CV) of selected complexes were measured in order to study the interaction between the organometallic building blocks. The existence of long-range Ru⋯Re interactions has been observed.