Kinetics of the interconversion of parahydrogen and orthohydrogen catalyzed by paramagnetic complex ions

The rate constants of para-/orthohydrogen (p-/o-H2) nuclear spin isomerization have been measured by means of 1H NMR in deuterated solvents at 298.2 K. The indicated reaction is catalyzed by paramagnetic complex ions giving rate constants that are proportional to the concentrations of the catalysts. The second-order rate constants are directly proportional to the squares of the magnetic moments for the solvated metal complexes for two classifications: M(solv)m2+, M = 3d transition metals; Ln(solv)n3+, where in 1:9 D2O-CD3CN the aqua complexes are the predominant species, Ln = lanthanides. The other 3d transition metal complexes with different ligands show rate constants that also depend on the sizes of ligands. Whereas the correlation between the second-order rate constants and magnetic moments is consistent with Wigner's theory, the size of catalyst shows a more modest effect on the rate constants than expected. The effective collision radii of the complexes, calculated from the rate constants, proved to be approximately constant for each series of solvated metal complexes.     

Oxorhenium(V) dithiolates catalyze the oxidation by tert-butyl hydroperoxide of sulfoxides and sulfides, including 4,6-dimethyldibenzothiophene

tert-Butyl hydroperoxide (TBHP) efficiently converts a wide variety of sulfides to sulfoxides and sulfones. The method offers the advantage that one product or the other can be obtained in high purity by a modest variation of conditions. The reactions occur smoothly at 25minus sign50 C in chloroform and, to the extent studied, in toluene and methylene chloride. A catalyst is required; the most extensively studied was MeReO(mtp)PPh(3), 1, where mtpH(2) is 2-(mercaptomethyl)thiophenol. Other chelating dithiolate ligands can be used with comparable results. These oxidations were tested for dialkyl, alkylminus signaryl, and diaryl sulfides; thiophenes; and thianthrene. Even the "hard" sulfide, 4,6-dimethyldibenzothiophene (DMDBT) was quantitatively oxidized to the dioxide with TBHP:DMDBT 3.0-3.5 and 0.05-3.8 mol % 1. The mechanism was explored in kinetics studies carried out only for methyl tolyl sulfide. The product buildup curve was complex, with an induction period followed by a rapid growth phase. The kinetic data could be modeled adequately but not perfectly by allowing five rate constants to refine. Their values are consistent with the chemical sense of the mechanism.     

Kinetics and mechanism of oxygen transfer to methyloxo(dithiolato)rhenium(V) complexes

The kinetics and mechanism of the reactions of the dimeric and monomeric methyloxo(dithiolato)rhenium(V) complexes [(o-SC6H4CH2S)Re(O)CH3]2 and [(o-SC6H4CH2S)PyRe(O)CH3] (Py = pyridine) with XO, sulfoxides, and pyridine N-oxides are studied. In these reactions, an oxygen atom from XO is transferred to rhenium, from which it later removed. A reaction scheme is proposed to interpret the kinetic data. This scheme features the formation of a monomeric (sulfoxide)- or (pyridine N-oxide)(dithiolato)methyloxorhenium(V) complex followed by its bimolecular oxidation in a rate-controlling step. Several sulfoxides (methyl, methyl phenyl, and substituted diphenyl) all react at similar rates. Activation parameters are determined for dimethyl sulfoxide and di-4-tolyl sulfoxide from temperature-dependent studies. The reactions with pyridine N-oxides show autocatalysis in which the catalyst is confirmed to be pyridine formed in the reactions.     

Pairwise exchanges of oxo and imido groups in rhenium(VII) compounds

The kinetic and thermodynamic parameters for the oxo and imido exchange reactions among MeReO(3), MeReO(2)(NR), MeReO(NR)(2), and MeRe(NR)(3) (R = 1-adamantyl, Ad; or 2,6-diisopropylphenyl, Ar) have been measured. The rate constant for the NAr series decreases from 0.27 to 0.0024 L mol(-1) s(-1) at 25 degrees C in benzene as the total number of participating imido groups increases from 2 to 4, indicating that steric effects play an important role in the kinetics of the ligand exchange reactions. But, with NAd, the values of k/L mol(-1) s(-1) are 0.2 (4 NAd), 100 (3 NAd), and 0.74 (2 NAd). The equilibrium constants, also subject to steric effects, are much larger than those predicted by ligand combination statistics and greatly favor the mixed oxo-imido compounds. The different steric demands by imido and oxo ligands are believed to be the main factor for the larger equilibrium constants because the equilibrium constant shows minimal dependence on temperature. The large negative activation entropies for the ligand exchange reactions are consistent with a metathesis mechanism featuring nearly concurrent interchange of oxo and imido groups.     

Catalysis by methyltrioxorhenium(VII): reduction of hydronium ions by europium(II) and reduction of perchlorate ions by europium(II) and chromium(II)

The title reactions occur stepwise, the first and fastest being MeReO3 + Eu2+ --> Re(VI) + Eu3+ (k298 = 2.7 x 10(4) L mol(-1) s(-1)), followed by rapid reduction of Re(VI) by Eu2+ to MeReO2. The latter species is reduced by a third Eu2+ to Re(IV), a metastable species characterized by an intense charge transfer band, epsilon410 = 910 L mol(-1) cm(-1) at pH 1; the rate constant for its formation is 61.3 L mol(-1) s(-1), independent of [H+]. Yet another reduction step occurs, during which hydrogen is evolved at a rate v = k[Re(IV)][Eu2+][H+](-1), with k = 2.56 s(-1) at mu = 0.33 mol L(-1). The 410 nm Re(IV) species bears no ionic charge on the basis of the kinetic salt effect. We attribute hydrogen evolution to a reaction between H-ReVO and H3O+, where the hydrido complex arises from the unimolecular rearrangement of Re(III)-OH in a reaction that cannot be detected directly. Chromium(II) ions do not evolve H2, despite E(Cr) degrees approximately E(EU) degrees. We attribute this lack of reactivity to the Re(IV) intermediate being captured as [Re(IV)-O-Cr(III)]2+, with both metals having substitutionally inert d3 electronic configurations. Hydrogen evolution occurs in chloride or triflate media; with perchlorate present, MeReO2 reduces perchlorate to chloride, as reported previously [Abu-Omar, M. M.; Espenson, J. H. Inorg. Chem. 1995, 34, 6239-6240].     

Oxidation of vanadium(III) by hydrogen peroxide and the oxomonoperoxo vanadium(V) ion in acidic aqueous solutions: a kinetics and simulation study

The reaction between vanadium(III) and hydrogen peroxide in aqueous acidic solutions was investigated. The rate law shows first-order dependences on both vanadium(III) and hydrogen peroxide concentrations, with a rate constant, defined in terms of -d[H(2)O(2)]/dt, of 2.06 +/- 0.03 L mol(-)(1) s(-)(1) at 25 degrees C; the rate is independent of hydrogen ion concentration. The varying reaction stoichiometry, the appreciable evolution of dioxygen, the oxidation of 2-PrOH to acetone, and the inhibition of acetone formation by the hydroxyl radical scavengers, dimethyl sulfoxide and sodium benzoate, point to a Fenton mechanism as the predominant pathway in the reaction. Methyltrioxorhenium(VII) does not appear to catalyze this reaction. A second-order rate constant for the oxidation of V(3+) by OV(O(2))(+) was determined to be 11.3 +/- 0.3 L mol(-)(1) s(-)(1) at 25 degrees C. An overall reaction scheme consisting of over 20 reactions, in agreement with the experimental results and literature reports, was established by kinetic simulation studies.     

Kinetics of self-decomposition and hydrogen atom transfer reactions of substituted phthalimide N-oxyl radicals in acetic acid

Kinetic data have been obtained for three distinct types of reactions of phthalimide N-oxyl radicals (PINO(.)) and N-hydroxyphthalimide (NHPI) derivatives. The first is the self-decomposition of PINO(.) which was found to follow second-order kinetics. In the self-decomposition of 4-methyl-N-hydroxyphthalimide (4-Me-NHPI), H-atom abstraction competes with self-decomposition in the presence of excess 4-Me-NHPI. The second set of reactions studied is hydrogen atom transfer from NHPI to PINO(.), e.g., PINO(.) + 4-Me-NHPI <=> NHPI + 4-Me-PINO(.). The substantial KIE, k(H)/k(D) = 11 for both forward and reverse reactions, supports the assignment of H-atom transfer rather than stepwise electron-proton transfer. These data were correlated with the Marcus cross relation for hydrogen-atom transfer, and good agreement between the experimental and the calculated rate constants was obtained. The third reaction studied is hydrogen abstraction by PINO(.) from p-xylene and toluene. The reaction becomes regularly slower as the ring substituent on PINO(.) is more electron donating. Analysis by the Hammett equation gave rho = 1.1 and 1.8 for the reactions of PINO(.) with p-xylene and toluene, respectively.     

Kinetics of oxidation and comproportionation reactions involving an oxammonium ion,benzyl alcohol and methyltrioxorhenium(VII)

The reactions of 4‐hydroxy‐2,2,6,6‐tetramethylpiperidinium N‐oxide, an oxammonium ion abbreviated R₂NO⁺, have been studied. The previously unreported triflate salt was used in this study because the anions of the usual chloride and bromide salts can themselves be oxidized. Reactions between R₂NO⁺ and alcohols produce ketones and aldehydes; the rate constant for PhCH₂OH is 4.4 × 10⁻³ L mol⁻¹ s⁻¹ in acetonitrile at 298 K. The immediate product is the hydroxylamine, R₂NOH, but its further comproportionation reaction with R₂NO⁺ yields the stable piperidinyl oxyl radical, R₂NO^·. The rate constant of this reaction is 1.78 × 10³ L mol⁻¹ s⁻¹ at 298 K. The possibility of using R₂NO⁺ and MTO as co‐catalysts for the oxidation of alcohols was explored, but the competitive rates are such that the resultant is not particularly attractive. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 381–385, 1999     

Syntheses and oxidation of methyloxorhenium(V) complexes with tridentate ligands

Four new methyloxorhenium(V) compounds were synthesized with these tridentate chelating ligands: 2-mercaptoethyl sulfide (abbreviated HSSSH), 2-mercaptoethyl ether (HSOSH), thioldiglycolic acid (HOSOH), and 2-(salicylideneamino)benzoic acid (HONOH). Their reactions with MeReO(3) under suitable conditions led to these products: MeReO(SSS), 1, MeReO(SOS), 2, MeReO(OSO)(PAr(3)), 3, and MeReO(ONO)(PPh(3)), 4. These compounds were characterized spectroscopically and crystallographically. Compounds 1 and 2 have a five-coordinate distorted square pyramidal geometry about rhenium, whereas 3 and 4 are six-coordinate compounds with distorted octahedral structures. The kinetics of oxidation of 2 and 3 in chloroform with pyridine N-oxides follow different patterns. The oxidation of 2 shows first-order dependences on the concentrations of 2 and the ring-substituted pyridine N-oxide. The Hammett analysis of the rate constants gives a remarkably large and negative reaction constant, rho = -4.6. The rate of oxidation of 3 does not depend on the concentration or the identity of the pyridine N-oxide, but it is directly proportional to the concentration of water, both an accidental and then a deliberate cosolvent. The mechanistic differences have been interpreted as reflecting the different steric demands of five- and six-coordinate rhenium compounds.