Nitrous Oxide Activation by a Ruthenium Porphyrin
John T. Groves* and J. Scott Roman
Department of Chemistry
Princeton University
Princeton, New Jersey 08544
J. Am. Chem. Soc., 1995, 117, 5594.
Ruthenium porphyrins have received considerable attention due to
their potential as catalysts for the oxidation of a variety of organic
functional groups.1 Pathways to high-valent oxoruthenium complexes
include the use of organic1b,2 and inorganic1d,g,3 oxidants, electrochemical
techniques,1h and aerobic oxidation.1a,c,f,2 The latter could be
of considerable practical importance for the large-scale catalytic oxygenation
of hydrocarbons using ruthenium porphyrins.1k We have recently considered
nitrous oxide as an alternative oxidant due to its vast availability as
a by-product of the industrial manufacture of nylon and its wider range
of process safety vis-a-vis oxygen. Further, N2O has been implicated
as a greenhouse gas and ozone antagonist which may make current emission
practice untenable.4
Nitrous oxide is an extremely inert molecule5 and a very poor
ligand.6 This is surprising given its free energy of formation (DG*=
+24.9 kcal/mol) and potential oxidizing power.7 To our knowledge
[Ru(NH3)5(N2O)]2+ is the only well-characterized complex containing nitrous
oxide.8 Of the few known reactions of this gas are its deoxygenation
by certain transition-metal complexes,5 and the catalytic oxidation of
PPh3 using a cobalt(I) species.9 Dinitrogen oxide has also been observed
to function as a bridging ligand in the preparation of oxo-bridged molecular
clusters of Ti, V, and Cr.6,10 Insertions of N2O and oxidation at
a ligand site are also known.11 The reduction of nitrous oxide over
cobalt porphyrins12 , polyamine complexes of nickel,13 transition-metal
oxides14 and group VIII carbonyl anion15 catalysts has been reported, as
well as the oxidation of simple hydrocarbons over supported metals16 or
FeZSM-5 zeolites17 at elevated temperatures. Our interest in the
catalytic oxidation of organic substrates using trans-dioxoruthenium(VI)
porphyrins1a-c has prompted us to investigate the reactivity of nitrous
oxide towards these catalysts. We describe here the first example
of direct oxygen atom transfer from N2O to a metalloporphyrin.
The oxidation of RuII(TMP)(THF)2 [TMP= dianion of tetramesitylporphyrin]
to RuVI(TMP)(O)2 using nitrous oxide under mild conditions is described.
The extent of oxidation was found to be dependent on porphyrin concentration.
Formation of two reaction intermediates, RuIV(TMP)(O)(THF) and RuII(TMP)(THF)(N2),
was observed at higher concentrations (~10-3 M) of Ru(TMP)(THF)2, while
only Ru(TMP)(O)(THF) was produced at ~10-4 M porphyrin. The independent
formation of Ru(TMP)(O)(THF) from the reduction of Ru(TMP)(O)2 by trans-b-methylstyrene
in the presence of free THF was used to help assign the formulation of
this new, paramagnetic species. The mechanism of oxygen atom transfer
to ruthenium and the relevance of N2O as a potential oxidant in the ruthenium-promoted
catalytic oxidation of a variety of organic substrates are discussed.
