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whereas in the ease of 63. the binding constant Ãîã the assembly 81 had
decreased to 5 x I O''mol !L, that is, the value already observed lor the
5.!5-bis(3-pyridv!)porphyrin 64 (/rcHi.v-relative orientations of the
pyridyl arms). These differences were interpreted in terms of matching of
the distance between the Zn- 1 cations of the dimer and that between the
coordinating nitrogens. The be.sl match was observed for the 5,1()-bis(4-
pyridyl)porphyrin 58 guest (12.2 A, vs. 12.6 A for the receptor).
The same authors, using 5,!0.! 5.2()-telrnkis(4-pyri(iyl)-porphyrin 61
could observe the formation of the pentamerie structure 82, in which the
free-base porphyrin bridges the two covalently linked bis-porphyrins used
for making the assemblies 77 and 80 of Figures 37 and 41 respectively
(Figure 42)./x Absorption bands ol' the pentamerie complex show a
characteristic redsliiii of 500cm 1 due to coordination elTcets.
Nonplanar dislorsion of the free-base macrocycles leads to an additional
red shift of the Q((),()) absorption and fluorescence bands as compared
to the free-base in the triads.
B. PORPHYRINS ASSEMBLED BY COORDINATION TO NON PORPHYRINIC METALS
1. Open Structures
Figure 43 shows the variety of aggregates that can be formed according to
this construction principle. The porphyrins are anchored to ligands,
monodentate or chelating. The simplest systems are built from the 5-(4-
pyridyl)porphyrin 45. that was used for making some of the
metalloporphyrin aggregates seen before. They are usually coordinated to
metals having a square planar geometry like Pd(ll) or Pt(ll). The use of
ri,v-PtCb(dmsob or M(dppp)(OTfb, where dppp
- diphenvlphosphinopropane and M = Pd or Pt. by Woo and coworkers7'1
allowed them to prepare complexes where both porphyrins are in a d.v-
position with respect lo each other. The Pd complex 83 of Figure 44 was
crystallized and its structure determined by radioerystallography. As
shown in Figure 44, the pyridyl planes are nearly orthogonal to the Pd
square plane and the mean porphyrin planes are canted with respect to the
Pd square plane by about 30 .
Chambron et al.
Figure 42. The supramolecular system 82 results from the
coordination of porphyrin 61 to two units of the same p-phenylene-bridged
Zn(ll) porphyrin covalent dimer as in Figure 41.
Ar= (ÑÍ2)áÑÍç Figure 41. Supramolecular systems 80
and 81 result from the coordination of porphyrins 58 and 63 to p-
phenylene-bridged Zn(ll) porphyrin covalent dimers.
presumably to minimize steric interactions of the tolyl groups in the
positions 10 and 20 between porphyrins in the same molecule. The center-
to-center distance between the two porphyrins is 13.6 A. When frans-
PdCbldmsob was used as starting material, a linear array of two
porphyrins was obtained in which the center-to-center distance was
19.5 A. Nolte and coworkers80 showed that a similar system, made from
porphyrins bearing long alkyl chains, formed
Figure 43. Porphyrin dimer (a), (b) and (e), trimers (c) and (f) and
tetramer (d) based on a nonporphyrin metal as assembling unit. The empty
squares represent the porphyrins; the arrows, monodentate ligands; the
arcs of a circle are bidentate or terdentate coordinating subunits.
40 / Noncovalent Multiporphyrin Assemblies
Figure 44. Porphyrin dimer 83 results from the coordination of two
analogues of porphyrin 45 to a Pd(dppp)'" complex metal fragment.
ring-shaped assemblies oi porphyrins, different from more conventional
vesicles. By using a metal salt like M(ClbCN)j(OTfÜ (M = Pd, Pt) it is
possible to assemble four porphyrins at a square planar metal center, as
shown in Figure 45 lor the Pd complex 84
Alessio and coworkers'4 demonstrated that octahedral Rn(ll) complexes
could also be used as assembling units. For example, reacting /ããø,v-
RuCb(Me>SO)4 and (wo equivalents of porphyrin 45 afforded a disubstituled
adduct, in which the two porphyrins are in c/.v, being field by a irons-,
(7,v-RuCb(MeiSO)i metal complex fragment.
Metallodendrimers based on square-planar Pd(ll) pineer-type complexes
were developed by van Veggel, Reinhoudt and coworkers.Sl The chloride
ligands of the square-planar Pd complex 85 (Figure 46) can be removed by
treatment with Ag(I), leaving a vacant coordinating site, which can be
occupied by the 4-pyridyl arm of the 5-(4-pyridyl)porphyrin 45. The
simplest dendrimer built on 85 (i.e., 87) is represented in Figure 47.
The dendrimer 88 of Figure 48 is obtained by substituting the chloride
ligands of 85 with the building block 86 of Figure 46. Up to twelve
porphyrins could be assembled at the periphery of the most complex
dendrimer elaborated using the Pd(If) pincer connectors. As evidenced by
absorption spectroscopy studies, the porphyrins at the dendrimer surface