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substrate (the thermodynamic affinity of a tyrosyl radical for a hydrogen atom is high).
56
The metal ion then
reduces the transient ketyl radical in a single electron transfer mechanism.
57,58
Large non-classical kinetic
isotope effects have been reported, as expected for the tunneling of a hydrogen atom in the rate limiting
step.
50
In the second half-reaction, the aldehyde is released into the medium and the copper(I) - tyrosine
site binds oxygen and reduces it very quickly to H
2
O
2
, thus regenerating the initial copper(II) - tyrosyl
radical state.
8.4 Complexes with coordinated phenoxyl radicals
To create stable metal - phenoxyl complexes, each of the phenol(ate)s has either to be incorporated into
elaborate polydentate ligands, or the metal:ligand stoichiometry has to be strictly controlled. These are
important aspects to consider as each metal ion exhibits specific preferences in the terms of geometry
and/or kind of coordinating atom. Solvent molecules can also be involved in coordination if the donor set
of the ligand is not sufficient for a given metal ion. Most of the ligands studied belong to the four main
classes briefly described in the next section. As all the ligands having at least one coordinating phenol,
that is a huge number of molecules, are a priori precursors of phenoxyl radicals, the following is limited to
ligands specifically designed to stabilize coordinated phenoxyl radicals, that is for which the phenols are
ortho
and
para
protected by electron donating and/or bulky groups. The nomenclature used to present the
complexes is: H
x
L
y
for a ligand y possessing x acido-basic sites and [M(H
x
L
y
)
(X)
z
] for the M complex
of H
x
L
y
bearing z coordinated exogenous ligands X. Following the general discussion on the architecture
of the ligands, the radical complexes will be presented in distinct parts, each being devoted to a specific
first row d transition metal. Reported electrochemical potentials for complexes of each metal are tabulated
at the beginning of each metal-specific section.
8.4.1 General ligand structures
The macrocyle 1,4,7-triazacyclononane (TACN) is a modulable scaffold with high complexing ability.
TACN coordinates to a metal ion by three amine groups, and can be further derivatized to incorporate one,
two or three additional coordinating groups such as phenols. This modularity makes them good chelators
for metals requiring four, five or six ligands, that is metals of the first row. Some representative structures
are shown in Figure 8.6.
Tripodal ligands possess a pivotal nitrogen on which one phenol and two pyridines, or two phenols
and one pyridine, can be introduced (Figure 8.7). Their copper complexes are excellent structural and
sometime functional models of the GO active site. The phenols have been diversely substituted, as well
as the pyridine, and the length of the linker can also be modulated. The solution chemistry of their copper
complexes is the most extensively studied of the four classes of ligands.
Tetradentate Schiff bases are famous catalysts for oxidation (Jacobsen's catalyst for example) that involve
two salicylidene moieties joined together with a linker that modulates the geometry around the metal
ion (Figure 8.8): binaphthyl linkers constrain the metal ion in a tetrahedral environment whereas
o
-
phenylenediamines make the ligand almost planar and highly conjugated, thus favoring a square planar
environment around the metal ion. As will be shown, the phenoxyl/phenolate redox potentials in this series
are usually high, due to the conjugation of the imine with the phenolate, but the radical species are rather
stable again due to conjugation. Phenol-benzimidazole are not strictly speaking Schiff bases but they will
be included in this section for clarity, as they coordinate in a six-membered chelation ring with one oxygen
and one sp2 nitrogen.
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