.GIF)
Generation of OxoMn(V)
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Oxomanganese(V) porphyrin complexes have never
been observed, in marked contrast to the corresponding well-characterized
oxo-species of Cr, Fe and Ru. The reaction of tetra-N-methylpyridylporphyrinatomanganese(III)
[Mn(III)TMPyP] with a variety of oxidants, m-CPBA, HSO5-,
and ClO-, have been shown by stopped-flow spectrophotometry
to produce the same, short-lived intermediate, 1. The Soret maximum
of 1 was found at 443 nm, intermediate between that of oxo-Mn(IV)
(428 nm) and Mn(III)TMPyP (462 nm). The rate of formation of 1 from
Mn(III)TMPyP followed second order kinetics, first order in porphyrin and
first order in oxidant. The rate constants have the following order: m-CPBA
( 2.7 X 107 M-1s-1) > HSO5-
( 6.9 X 105 M-1s-1) ~ ClO-
( 6.3 X 105 M-1 s-1) . Once formed, the
intermediate species 1 was rapidly converted to oxoMn(IV), 2,
by one-electron reduction with a first order rate constant of 5.7 s-1.
The oxoMn(IV) species was relatively stable under the reaction conditions,
decaying slowly to Mn(III)TMPyP with a first order rate constant of 0.027
s-1. The identity of 1 as an oxomanganese(V) complex
was indicated by its reactivity. The one-electron reduction of 1
to oxoMn(IV) was greatly accelerated by nitrite ion (k = 1.5 X 107
M-1s-1). However, the reaction between nitrite and
oxoMn(IV) is much slower (k = 1.4 X 102 M-1s-1).
The oxoMn(V) intermediate 1 was shown to be highly reactive toward
olefins, affording epoxide products. By contrast, oxoMn(IV) was ineffective
in performing the same reaction under these conditions. In the presence
of carbamazepine, 3, efficient oxygen transfer from the highly reactive
oxoMn(V) to the olefin (second order rate constant 6.5 X 105
M-1s-1) resulted in the conversion of oxoMn(V) directly
back to Mn(III)TMPyP without the appearance of the stable oxoMn(IV) intermediate.
With m-CPBA as the oxidant in the presence of H218O,
the product epoxide was shown to contain 35% 18O, consistent
with an O-exchange-labile oxoMn(V) intermediate. Nitrite ion inhibited
the epoxidation reaction competitively by one electron reduction of the
oxoMn(V) intermediate to the non-reactive oxoMn(IV). |