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Figure 56. a, Metalloporphyrins 99 have meso aryl substituents bearing
crown ether subunits; b, Addition of 4 equiv of K+ produces the porphyrin
dimers 100; c, Schematic view of the dimer: the squares are the porphyrin
macrocycles and the arrows represent the coordinating units. M is the
and - rotaxanes that we have prepared and whose electron transfer
properties have been extensively studied by Harri-man and his
coworkers.102-106 More recently, porphyrin-incorporating rotaxanes,
constructed via hydrogen-bond sets used as templates, have also been
reported.107 However, as for the other porphyrin-stoppered rotaxanes,
they will not be discussed here. Monoporphyrinic -catenanes have also
been reported, which incorporate a strong aromatic electron acceptor in
one of the two rings.108
Very few examples have been reported of -catenanes.109_ 111 The
compounds consist of two interlocking rings, each incorporating a
porphyrin moiety either as an integral part of the ring109 or as a
Momenteau and his group109 were the first to describe such a system,
the synthesis of which is represented in Figure 64.
Figure 57. A porphyrin pentamer (101) made of a molecular box based on
coordination of the Zn(ll) complex of porphyrin 95 to fac-Re(CO)3CI
complex metal fragments, which hosts porphyrin 61.
Figure 58. A covalent porphyrin cyclotetramer hosting porphyrin 61. The
/i-substituents of the porphyrinic subunits of the cyclotetramer have
been omitted for simplicity.
The strategy is similar to the one used extensively to make various
catenanes.112 It is based on the threading of an open chain fragment into
a coordinating ring, a process governed by a transition metal [copper(I)]
which gathers the two organic fragments (string and ring) and forces the
open-chain component to thread through the cycle. As seen in Figure 64,
the first ring 115 already incorporates a zinc porphyrin. From the
threaded copper(I) complex intermediate 116, the two-porphyrin catenane
is obtained in acceptable yield (31 %), to afford a symmetrical compound
incorporating two identical zinc porphyrins.
40 / Noncovalent Multiporphyi in Assemblies
Figure 59. Crystal structure of the letrameric zinc porphyrin complex
102. a, Side view, b, A view perpendicular to the central porphyrin plane
showing the crystallographic S4 symmetry and the orthogonal orientation
of the four peripheral zinc porphyrins of the covalent cyclotetramer.
Reprinted with permission from Anderson, S., Anderson, H. L., Bashall,
A., McPartlin, Ì., Sanders, J. Ê. M. Angew. Chem. Int. Ed. Engl. 1995,
Interestingly, the average planes of both porphyrins in 117 are
parallel to one another. However, little or no electronic interaction
exists between the porphyrins, as shown by their absorption spectroscopy
properties, which are consistent with a large spatial separation. Space-
Figure 60. A porphyrin trimer (103) made of a covalent molecular box
incorporating Zn(ll) porphyrin subunits hosting a third, free-base
models suggest a center-to-center (Zn---Zn) distance of ~ 18 A for the
catenane of Figure 64.
A different porphyrinic catenane has been made, in view of studying
long-range photoinduced electron transfer.110111 The two porphyrin
components must now be different or differently inetalated since one of
them will have to act as an electron donor (D), the other as an electron
acceptor (A). The system is schematically depicted in Figure 65.
An obvious choice for D and A is to use a zinc(II) and a gold([II)
porphyrin respectively since these components have proven particularly
useful for the study of photo-chemically induced electron transfer.*7-
The -catenate was prepared according to the route shown in Figure
66. The gold(lll) porphyrin-appended macrocycle 118 is a key compound,
obtained in several steps from appropriate starting materials.
Treatment of the niaerocycie 118 with Cu(CH;CN)4-(PF,,) followed by
threading of the diiodide 119 results in the formation of the precatenate
120 in quantitative yield. This complex was combined with the zincfll)
Figure 61. In structure 108, three porphyrins are assembled by a
combination of H bonds and rnetal-ligand bonds. The sub-assembly formed
by bis-pyridine 105 and two [Rh(lll)CI] porphyrins 106 is hosted by
Chambron et al.
and the resulting mixture was reacted with Cs2CO;, under high dilution
conditions. The heteroporphyrinic -catenate 122 was obtained in 13%
As for Momenteau's catenate (Fig. 64), there is no significant
interaction between the two porphyrins. However, emission measurements
show strong quenching of luminescence of the zinc(II) poiphyrin. The
mechanism of the quenching reaction is as yet uncertain; further