Catalytic oxidations using ruthenium porphyrins - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 13

(Im = imidazole), to the so-called met form via
the outer-sphere process exemplified in eq. 12 (one L = histidine, the other L
being unknown); the oxidation rate is faster than that of an axial ligand
dissociation, which, for example, is the initial step in formation of
Ru(porp)L(CO) via reaction with CO 51 .

A complex derived from has been stabilized within the
protected cap on one side of a “picnic basket” porphyrin 55 , and at low
temperature within a species 56 .
An unusual demetallation reaction of a Ru(II) porphyrin complex with
to give the insoluble, black and the free-base porphyrin is
illustrated in eq. 13, trace again being the key ingredient 57 . The
phosphine ligands were considered to be “burned off as phosphine oxides,

with a Ru(IV)=O species acting as oxidant the remaining

'Ru(OEP)', known in the form of the dimer then giving the

known that was shown to react with to give

(eq. 15). In the presence of excess the phosphine dissociation required

to generate the vacant site for coordination of (eq. 14) is prevented, and
superoxide is formed via an outer-sphere electron transfer, according to eq.
12.

Introduction of substituents at the ortho positions of the phenyl rings of
creates steric hindrance against 'dimerization'of the Ru derivatives
(cf. eq. 15) and allows for an initial binding of that finally yields readily

isolable, stable trans -Ru(VI)-dioxo species; e.g. in benzene, (L

= MeCN 15,59 or THF 60,61 ) reacts with (or air) to yield
(eq. 10, Figure 4). Its formation is thought to occur via steps of the type
shown in eq. 14, followed by disproportionation of the Ru(IV)-monooxo