Metal Ion-Linked Porphyrin Dimers
A R T I C L E S
the simple sum of the properties of each porphyrin: these
additional properties might lead to molecular materials for new
applications.7 However, the syntheses of multiply linked por-
phyrin oligomers becomes even more tedious than those of
singly connected porphyrin oligomers.
On the other hand, one might use noncovalent bonds to build
greater assemblies. Hydrogen bonds,8 catenation of rings,1a
metal-linked sandwich structures,9 or coordination of porphyrin
peripheral groups to metals have been used to build finite or
infinite assemblies.10 Linking the macrocycles through coordina-
tion bonds offers additional variations, with each metal center
presenting specific properties: oxidation levels, coordination
numbers, and geometries. However, most examples deal with
porphyrins bearing meso-pyridinyl groups, since the wide
coordination spectrum of the pyridine group allows large
assemblies to be built. Due to noncoplanarity between the meso-
pyridines and the porphyrin ring, one does not expect much
information to be transmitted through the coordinated metal.
A porphyrin bearing a peripheral coordinating site, preferably
with chelating properties and coplanar with the macrocycle,
would combine the efficiency of the covalent approach in terms
of porphyrin-porphyrin interaction and the modularity of the
connection of porphyrins via metal coordination. In addition,
since the formation of coordination bonds might be considered
as a reversible reaction in many cases, if the different compo-
nents of the assembly (metalloporphyrin as ligands and metal
ions as connectors) are carefully chosen, one might reach the
ideal case of a purely thermodynamically driven reaction whose
yields should be very high. In these systems, the possibility of
modulating the properties of the macrocycle through the internal
coordination site still remains.
This approach has been successfully illustrated in the por-
phyrazin series: one or two chelating sites positioned on phenyl
groups of benzoporphyrazin molecules have been used to obtain
di- or trinuclear complexes with significant magnetic interactions
between the different metal ions through the aromatic rings.11
We already briefly described the use of metal coordination
to connect two porphyrins, leading to dimers with strong
interactions between the two macrocycles as demonstrated by
UV-visible spectroscopy and preliminary electrochemical
experiments.12 In this article, we present the synthesis of
porphyrins bearing a peripheral enaminoketone moiety fully
conjugated with the macrocyclic π-system, a set of related
reactions, and the coordination properties of enaminoketo-
porphyrin ligands and demonstrate that strong porphyrin-
porphyrin interactions can be detected spectroscopically and
electrochemically when two such ligands are connected by a
metal.
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Preparation of Enaminoketones
Preparation of Ketones 1 and 2. The usual route to ketone
1 requires first the formylation of a metalloporphyrin under
Vilsmeier-Haack conditions followed by acid-catalyzed cy-
clization of the aldehyde and demetalation of the porphyrin.13
The cyclization reaction was usually run under the oxidative
conditions described by Ishkov and Zhilina,14 to optimize the
conversion to the ketone. Demetalation had to be done carefully
to avoid a second cyclization previously described by Dolphin
and co-workers for meso-tetraanisylporphyrins.15 The overall
yield of this sequence is fair (∼50-60%; Scheme 1).
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