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.
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