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metric, trimerization may lead a priori to two diastereomers, RRR/SSS or
RRS/SSR. Remarkably, the assembly process is diastereoselective, leading
only to the RRS/SSR diastereomer.
To continue the series, cyclic tetramers, based on Ru(II), were
described by Imamura and coworkers (Figure 25) 59.60 por example,
treatment of the TPP derivative 45 bearing one 4-pyridyl substituent,
with Ru3(CO)|2, produced a cyclic, oriented tetramer of [Ru(CO)]
porphyrins in which the pyridyl substituent of one porphyrin coordinates
the Ru(II) center of one porphyrin neighbor in a trans orientation with
respect to the carbonyl ligand. The tetrameric structure 46 was
established by FAB-MS and by the equivalence of the Ru porphyrins as
shown by 'H NMR. The tetramer was stable with increasing temperature but
was broken upon addition of a 100-fold excess of pyridine, which produced
the Ru(II)-porphyrin 47. The Soret band of the tetramer is relatively
broad, as compared to that of the model compound [Ru(TPP)(CO)Py], and its
molar extinction coefficient is smaller. Some perpendicularly linked
metalloporphyrin dimers (see below) do not exhibit a broadening of the
Soret band. Hence, the broadening of the Soret band observed in this case
must result from excitonic interactions between two [Ru(CO)] porphyrins
of the aggregate that are aligned in a parallel fashion. Finally, to
bring a definite answer to the problem of tetramerization vs
polymerization, Imamura and coworkers reinvestigated the system described
by Fleischer and Shachter.46 They showed by variable temperature 'H NMR,
that this system behaves like their [Ru(CO)] porphyrin-based tetramer at
low temperature; that is, low temperature is required for the cyclic
tetramer to predominate over the coordination polymer when the
metalloporphyrins bearing a 4-pyridyl substituent contain Zn(II).
high affinity form
Â = pyridyl ligand ; L = CO or CN'
Figure 23. The different equilibrium processes involving the iron
porphyrin 41 in the presence of CO (Fe(ll)) or CN (Fe(lll)) as the
external ligand, leading to the dimer 42.
40/ Noncovalent Multiporphyrin Assemblies
Figure 24. Trimerization of the Fe(lll) porphyrin 43, obtained by
treatment in basic conditions. The phenyl meso substituents have been
omitted in the trimer 44.
Figure 25. Tetramer 46 of the [Ru(CO)] complex of porphyrin 45. The
phenyl meso substituents have been omitted in the aggregate. In the
presence of an excess of pyridine, the tetramer breaks into the Ru(ll)
2. Porphyrins Gathered by External Bridging Ligands
The different topologies of the aggregates prepared according to (his
principle are represented in Figure 26. The bridging ligand can be a
porphyrin, but this case will be
discussed in a separate section. The simplest aggregate of (his series is
the Fe(III)-porphyrin //-oxo dimer described by Fleischer et alas early
as 1971, in which the bridging ligand is the Î - anion.
Similar dimers, in which the bridge is a molecule like pyrazine/'- are
well known. 4,4'-bipyridine was used to
Figure 26. Porphyrin dimer (a), trimer (b) and tetramer (c) made from
metalloporphyrins (thick line) gathered by an external bridging ligand
represented by arrows: bidentate (a), terdentate (b), and tetradentale
Chambron et al.
Figure 27. A ladder-like supramolecular assembly (48) made
from two rod-shaped Zn(ll) bis-porphyrins bridged by diazabicyclooctane
bridge two Zn(II)-porphyrins, placing them even further apart. More
complex systems were obtained when covalent dimers of porphyrins were
used to build the aggregates.
Ladder-like aggregates were obtained by Anderson,63 who synthesized rigid
rod molecules containing two Zn(II)-porphyrins bridged by diacetylenic
spacers (Figure 27). He showed that in the presence of diazabicyclooctane
(DABCO), the two rods formed the poles of a ladder with the DABCO bridges
playing the role of the rungs in the molecular assembly 48. Crossley,
Meijer and coworkers,64 designed a bis-porphyrin conjugate covalently
bridged by a molecular fragment derived from chiral Troger's base (49).
The bis-porphyrin forms a cleft which, after complexation with Zn(II),
acts as a receptor for a,w-diaminoalkanes. For example 1,12-
diaminododecane is bound with an association constant of A'a = 7.0 x
107mol- 'L. When a first-generation dendritic polyamine bearing four
terminal primary amine groups is used as a guest, two receptor molecules
assemble in such a way that they encapsulate the small "dendrimer"
molecule, as in 50 (Figure 28). This was proven by titration studies,
using 'H NMR and UV-vis absorption spectroscopy.
Anderson and Sanders65-66 used the bisacetylenic porphyrin 51 as a
monomeric precursor. The reactive functions are anchored to the meta