Bidentate Lewis base adducts of methyltrioxorhenium(VII) and their application in catalytic epoxidation

Dendrimer crown-ether tethered multi-wall carbon nanotubes support methyltrioxorhenium in the selective oxidation of olefins to epoxides

Benzo-15-crown-5 ether supported on multi-wall carbon nanotubes (MWCNTs) by tethered poly(amidoamine) (PAMAM) dendrimers efficiently coordinated methyltrioxorhenium in the selective oxidation of olefins to epoxides.

Confinement Effects on Chemical Equilibria: Pentacyano(Pyrazine)Ferrate(II) Stability Changes within Nanosized Droplets of Water

Nanoscale confinement is known to impact properties of molecules and we observed changes in the reactivity of an iron coordination complex, pentacyano(pyrazine)ferrate(II). The confinement of two coordination complexes in a sodium AOT/isooctane reverse micellar (RM) water droplet was found to dramatically increase the hydrolysis rate of [Fe(CN)₅pyz]3- and change the monomer-dimer equilibria between [Fe(CN)₅pyz]3- and [Fe₂(CN)10pyz]6-. Combined UV-Vis and ¹H-NMR spectra of these complexes in RMs were analyzed and the position of the monomer-dimer equilibrium and the relative reaction times were determined at three different RM sizes. The data show that the hydrolysis rates (loss of pyrazine) are dramatically enhanced in RMs over bulk water and increase as the size of the RM decreases. Likewise, the monomer-dimer equilibrium changes to favor the formation of dimer as the RM size decreases. We conclude that the effects of the [Fe(CN)₅pyz]3- stability is related to its solvation within the RM.

Oxidomolybdenum complexes for acid catalysis using alcohols as solvents and reactants

With ethanol as solvent and reactant, a molybdenum (pre)catalyst promotes selective acid-catalysed reactions. Depending on the conditions, the Mo^VIcomplex is converted to Mo^VIoxidoalkoxido, Mo^Voxido-bridged dinuclear, and Mo^VIoctanuclear complexes.

Rhenium oxohalides: Synthesis and crystal structures of ReO3Cl(THF)2, ReOCl4(THF), Re2O3Cl6(THF)2, and Re2O3Cl6(H2O)2

Convenient methods to prepare solvated rhenium oxochlorides are described; these compounds should serve as useful starting materials for rhenium chemistry. Treatment of perrhenic acid, HReO(4), with chlorotrimethylsilane or with thionyl chloride, followed by addition of tetrahydrofuran, forms the new oxochloride complexes ReO(3)Cl(THF)(2) and ReOCl(4)(THF), respectively. Small amounts of two dinuclear oxochlorides, which evidently resulted from adventitious hydrolysis, were also isolated: Re(2)O(3)Cl(6)L(2), where L = THF or H(2)O. All four compounds were characterized by X-ray crystallography. The rhenium(vii) complex ReO(3)Cl(THF)(2) adopts a distorted octahedral geometry in which the three oxo ligands are in a facial arrangement; the rhenium(vi) complex ReOCl(4)(THF) adopts a trans octahedral structure. The two dinuclear rhenium(vi) compounds both have a single, nearly linear, bridging oxo group; on each Re center, the three terminal chlorides adopt a mer arrangement, and the terminal oxo and the coordinated Lewis base are mutually trans. The water ligand in the aqua complex is hydrogen bonded to nearby THF molecules. IR data are given.

An improved methyltrioxorhenium-catalyzed epoxidation of alkenes with hydrogen peroxide

Methyltrioxorhenium (MTO)-catalyzed epoxidation of alkenes with H(2)O(2) has been significantly improved by using 3-methylpyrazole as an additive. A system consisting of 35% H(2)O(2) and MTO-3-methylpyrazole in CH(2)Cl(2) catalyzes the epoxidation of various alkenes in excellent yields. The catalytic activity of MTO-3-methylpyrazole surpasses MTO-pyrazole and MTO-pyridine catalysts. Quantitative yields of epoxides from cyclic and internal alkenes were obtained with only 0.05-0.1 mol% of MTO in the presence of 10 mol% of 3-methylpyrazole.

MTO Schiff-Base Complexes: Synthesis, Structures and Catalytic Applications in Olefin Epoxidation

Several Schiff-base ligands readily form complexes with methyltrioxorhenium(VII) (MTO) by undergoing a hydrogen transfer from a ligand-bound OH group to a ligand N atom. The resulting complexes are stable at room temperature and can be handled and stored in air without problems. Due to the steric demands of the ligands they display distorted trigonal-bipyramidal structures in the solid state, as shown by X-ray crystallography, with the O(-) moiety binding to the Lewis acidic Re atom and the Re-bound methyl group being located either in cis or trans position to the Schiff base. In solution, however, the steric differences seem not to be maintained, as can be deduced from (17)O NMR spectroscopy. Furthermore, the Schiff-base ligands exchange with donor ligands. Nevertheless, the catalytic behaviour is influenced significantly by the Schiff bases coordinated to the MTO moiety, which lead either to high selectivities and good activities or to catalyst decomposition. A large excess of ligand, in contrast to the observations with aromatic N-donor ligands, is detrimental to the catalytic performance as it leads to catalyst decomposition.

Heterogenization of an organorhenium(vii) oxide on a modified mesoporous molecular sieve

A novel route to heterogenize an organorhenium(VII) oxide, derived from the well examined methyltrioxorhenium(VII) (MTO), on the surface of an iodosilane-modified MCM-41 is applied. The successful grafting of the -CH(2)-ReO3 moiety on the surface was evidenced by 1H CP MAS NMR, IR spectroscopy, TG-MS, and elemental analysis. XRD and TEM analyses confirm the retaining of long-range ordering throughout the grafting process. The rhenium loading of the mesoporous material after heterogenization of MTO is found to be 1.25 wt%. Despite containing formally a derivative of the very sensitive benzyltrioxorhenium(VII) the material is stable at room temperature and applicable as a heterogeneous catalyst for aldehyde olefination reactions.

Lewis Acidity of Methyltrioxorhenium(VII) (MTO) Based on the Relative Binding Strengths of N-Donors

This article presents a sigma acceptor strength scale for methyltrioxorhenium(VII) (MTO), one of the most versatile and useful high oxidation state organometallics ever described. The spectrophotometric titration of MTO with a series of N-donor bases in CCl(4) gives formation constants (K(f)) and enthalpies for the adduct formation reactions. An excellent linearity of log K(f) with respect to the Hammett sigma constants of the substituents on the ligands was observed. The resulting rho constant is proposed to be a good indication of the Lewis acidity of MTO. The enthalpies of adduct formation of N-donors with MTO also fit the ECW model to predict the values of E(A) and C(A) parameters for MTO. The parameters can be used to predict an acidity scale for MTO. These parameters also allow the chemists to predict and correlate quantitatively the enthalpies of MTO. Lewis base interactions. Significant chemical insights result from the fit of the data to the ECW model.