Catalytic oxidations using ruthenium porphyrins - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 10

The 2nd-row transition metal porphyrin complexes of Rh(II) and Ru(II)
also react with but the resulting species are usually quite different in
character. Rhodium octaethyl- and tetraphenylporphyrin oxygen adducts are
like the Co analogues with an end-on superoxide; however, the
octaethylporphyrin (OEP) species is only stable below -80°C and on

warming to 20° C converts to a Rh(III) complex, analogous to the

Co systems 37,38 .
The interaction of oxidants, including with Ru porphyrins, and
subsequent oxygenation/oxidation catalysis, constitute the topics of the
remaining sections of this chapter; this review updates earlier 1992 and 1994
reviews from this laboratory on this topic 15 , while related reviews from the
group of Che 39 and Groves 40 appeared in 1999 and 2000, respectively. Under
certain conditions, “non-hindered” Ru(II) porphyrins can bind reversibly
to give species that contain either coordinated superoxide or peroxide,
although irreversible oxidation to a dinuclear species is more
common (Section 2). Ru(II) porphyrins, initially containing bulky
substituents at ortho -positions of meso -phenyl rings present in the porphyrin
ligand, were shown to have a remarkable and unique reactivity with
yielding trans -dioxo species which are stable in solutions at room
temperature; an example is shown in eq. 10, where TMP = the dianion of
5,10,15,20-tetramesitylporphyrin, see Figure 4 in Section 2). Some non-
hindered Ru(II)-porphyrins were later shown to form, in alcohol solutions,
less stable trans -dioxo species (Section 2).

Over the last 17 years or so, studies based on these trans -Ru(VI)-dioxo
complexes have led to the development of new, catalytic and selective
oxygenation and oxidation systems that generally operate via non-radical
pathways and that involve direct reaction between a metal complex and
as illustrated in eq. 10. The Ru(VI)-dioxo species, depending on
experimental conditions and substrates, can exhibit: (i) monooxygenase
activity in terms of effecting addition of one O-atom of to a molecule of
substrate, although both O-atoms of the are utilized for two molecules of
substrate (cf. eq. 1), and so the systems are in effect illustrating dioxygenase
activity based on the stoichiometric use of the molecule (Sections 3.1-
3.5). Perhaps the term bis(monooxygenase) activity is appropriate; (ii)
oxidase activity of the type illustrated in eq. 4 for certain dehydrogenation
reactions (Section 4); and (iii) free-radical type activation of (eqs. 5-7).
A very wide range of organic oxidation reactions is catalyzed by non-
porphyrin complexes of Ru, including a vast literature on the use of Ru-oxo