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

Abstract

The peroxides from methylrhenium trioxide (MTO) and hydrogen peroxide, CH 3 ReO 2 (η 2 -O 2 ), A, and CH 3 Re(O)(η 2 -O 2 ) 2 (H 2 O), 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 [H 2 O 2 ] and [H 3 O + ]. The complex peroxide and pH dependences could be explained by one of two possible pathways: attack of either hydroxide on A or HO 2 - 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) × 10 9 and k MTO = (4.1 ± 0.2) × 10 8 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 O 2 and CH 3 ReO 3 . 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, CH 3 ReO 3 is stable below pH 7, but decomposes in alkaline aqueous solution to yield CH 4 and ReO 4 - . As a consequence, the decomposition rate rises sharply with [H 2 O 2 ], peaking at the concentration at which [ A ] is a maximum, and then falling to a much smaller value. Variable-temperature 1 H NMR experiments revealed the presence of a labile coordinated water in B, but supported the anhydride form for A .