Deactivation of Methylrhenium Trioxide−Peroxide Catalysts by Diverse and Competing Pathways

Abstract

The peroxides from methylrhenium trioxide (MTO) and hydrogen peroxide, CH3ReO2(η2-O2), A, and CH3Re(O)(η2-O2)2(H2O), B, have been fully characterized in both organic and aqueous media by spectroscopic means (NMR and UV−vis). In aqueous solution, the equilibrium constants for their formation are K 1 = 16.1 ± 0.2 L mol-1 and K 2 = 132 ± 2 L mol-1 at pH 0, μ = 2.0 M, and 25 °C. In the presence of hydrogen peroxide the catalyst decomposes to methanol and perrhenate ions with a rate that is dependent on [H2O2] and [H3O+]. The complex peroxide and pH dependences could be explained by one of two possible pathways: attack of either hydroxide on A or HO2 - on MTO. The respective second-order rate constants for these reactions which were deduced from comprehensive kinetic treatments are k A = (6.2 ± 0.3) × 109 and k MTO = (4.1 ± 0.2) × 108 L mol-1 s-1 at μ = 0.01 M and 25 °C. The plot of log k ψ versus pH for the decomposition reaction is linear with a unit slope in the pH range 1.77−6.50. The diperoxide B decomposes much more slowly to yield O2 and CH3ReO3. This is a minor pathway, however, amounting to <1% of the methanol and perrhenate ions produced from the irreversible deactivation at any given pH. Within the limited precision for this rate constant, it appears to vary linearly with [OH-] with k = 3 × 10-4 s-1 at pH 3.21, μ = 0.10 M, and 25 °C. Without peroxide, CH3ReO3 is stable below pH 7, but decomposes in alkaline aqueous solution to yield CH4 and ReO4 -. As a consequence, the decomposition rate rises sharply with [H2O2], peaking at the concentration at which [A] is a maximum, and then falling to a much smaller value. Variable-temperature 1H NMR experiments revealed the presence of a labile coordinated water in B, but supported the anhydride form for A.

Carbon–Oxygen Bond Formation via Organometallic Baeyer–Villiger Transformations: A Computational Study on the Impact of Metal Identity

“…39 However, in the present case for a diversity of metal ions and coordination environments, examining the geometries of the four-and six-coordinate species revealed no correlation between the computed barriers and the active-site bond lengths (see Supporting Information for more details). To identify correlations between properties of the precursor complexes and the calculated ΔG …”

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

“…A large excess of H 2 O 2 (10 equivalents) leads to the formation of a signal at d = 2.89 ppm, which originates from a bisperoxo complex. [4,10,21] In the case of complex 5, after the addition of one equivalent of H 2 O 2 the same spectral changes are observed as in the case of complex 3, with the signal of the monoperoxo complex occurring at d = 2.83 ppm. However, when 10 equivalents of H 2 O 2 are added no signal assignable to a bisperoxo complex is found.…”

Facile Functionalization of a Metal Carbon Bond by O-Atom Transfer

“…Anticipating that the known MeOReO 3 complex 14 should be generated but not hydrolyzed in aprotic media at moderate temperature, we examined the reaction of MTO with 1 equiv of PyO in THF-d 8 at 125°C in the presence of excess pyridine-d 5 by 1 H NMR. It is known 11 and we observe that MTO is quantitatively converted to the MTO-Py-d 5 adduct 15 (s, 1.70 ppm) at room temperature. Upon heating, clean conversion of this adduct to MeOReO 3 -Py-d 5 (s, 4.48 ppm) and free pyridine-h 5 is observed based on comparison to the chemical shift of the known MeOReO 3 -amine adduct.…”

“…10 A reported observation that attracted our attention was that an undesirable side reaction is the decomposition of a MTO catalyst to methanol at room temperature. 11 We were intrigued because, despite the high Re(VII) oxidation state, unlike Pt(IV) alkyls, 1b,4 treatment of MTO in basic or acidic water does not generate Re(V) and methanol. Consistent with the observations of the initial investigators, 11 we find that the formation of methanol from MTO in water requires added H 2 O 2 as the oxidant.…”