Dramatic effect of the porphyrinic metal on the conformation of a two- ring threaded system

Article abstract of DOI:10.1039/a907284e

Porphyrin homodimers have been assembled by transition-metal-directed threading of porphyrin-incorporating macrocycles onto a rigid-rod bis- chelate, and the relative orientations of the porphyrins shown to be controlled by the nature of the metal that th

Full text of DOI:10.1039/a907284e

Dramatic effect of the porphyrinic metal on the conformation of a two-ring
threaded system
Myriam Linke, Jean-Claude Chambron,* Valérie Heitz and Jean-Pierre Sauvage*
Laboratoire de Chimie Organo-Minérale, U.M.R. 7513 au C.N.R.S.,Université Louis Pasteur, Institut Le Bel, 4, rue
Blaise Pascal, 67000 Strasbourg, France. E-mail: sauvage@chimie.u-strasbg.fr
Received (in Basel, Switzerland) 6th September 1999, Accepted 25th October 1999
Porphyrin homodimers have been assembled by transition-
metal-directed threading of porphyrin-incorporating macro-
cycles onto a rigid-rod bis-chelate, and the relative orienta-
tions of the porphyrins shown to be controlled by the nature
of the metal that they contain.
Assemblies of naturally-occurring porphyrins or analogues held
by noncovalent interactions are important components of
photosynthetic systems, as light antennas, energy funnels, and
electron transfer chains.1 These facts, recently highlighted by
beautiful X-ray crystal structures,2 have stimulated the design
and synthesis of multifarious porphyrin aggregates mainly
based on coordination bonds, some of them showing relevant
electron transfer properties.3 In particular, the preparation and
study of simple dimers of stacked porphyrins is highly
significant,4 as far as mimicking the unusual spectroscopic
properties of the Special Pair of bacterial reaction centers is
concerned.5
We have recently described the efficient assembly of
porphyrin dimers by the transition-metal-controlled threading
of macrocycles bearing pendant porphyrins and incorporating a
2,9-diphenyl-1,10-phenanthroline (dpp) chelate onto molecular
threads containing two such dpp fragments.6 Here we show that
when a rod-like, phenanthroline-based bis-chelate and macro-
cycles incorporating metalloporphyrins in their backbone are
used, the relative orientations of the porphyrins can be
controlled by the nature of the metal that they contain.†
The bis-phenanthroline rigid-rod molecule 4 and the dpp-
based macrocycles 9 and 10 incorporating a ZnII and a AuIII
porphyrin, respectively, as well as their precursors, are
represented below. Suzuki coupling of 3-bromophenanthroline
17 and diboronic acid 28 in stoichiometric amounts [Pd(PPh3)4,
aq. Na2CO3, EtOH–PhCH3, reflux] afforded bis-phenanthroline
3 in 55% yield after chromatography.9 This compound was
further reacted with lithioanisole (3 equiv.) in THF at 25 °C.10
After hydrolysis, MnO2 aromatization and chromatographic
separation from substitution isomers and homologues, bis-
phenanthroline 4 was isolated in 32% yield. Direct precursors of
macrocycle 8, which incorporates a free-base porphyrin, are the
phenanthroline derivative 5 and porphyrin 7. This latter
compound was obtained in quantitative yield by treatment of
porphyrin 611 with an excess of BBr3 in CH2Cl2 at 278 °C.
High-dilution condensation of 5 and 7 in DMF at 55 °C
containing Cs2CO3 (5 equiv.) provided 8 in 28% yield after
chromatography.12 Insertion of ZnII was carried out by reaction
with Zn(OAc)2·2H2O (1 equiv.) in a 2+1 refluxing mixture of
CHCl3 and CH3OH, to produce the ZnII porphyrin-incorporat-
ing macrocycle 9 in 85% yield. The preparation of the AuIII
porphyrin-incorporating macrocycle 10, which could not be
obtained by direct metallation of 8, has been described
elsewhere.11
of the solvents under reduced pressure, the crude material was
flash-chromatographed on silica gel, affording the threaded
assembly. The ZnII porphyrin dimer 11 was obtained in 85%
yield and its AuIII porphyrin analogue 12 in 50% yield. Since the
threading experiments proceeded more or less quantitatively,
these isolated yields show that partial dethreading takes place
during chromatographic purification, especially in the case of
the AuIIIporphyrin dimer 12.
Diagnostic of macrocycle threading onto bis-phenanthroline
4 are the large upfield shifts of protons mA of 4 both for 11
(21.28 ppm) and 12 (20.98 ppm), due to ring current effects of
the phenanthroline fragment included in the macrocycles on the
anisyl extremities of 4.13 Methyl protons of these groups are
also affected, albeit to a lesser extent. As expected, threading
also breaks off the symmetry plane of the macrocycle, which
contains the C2 symmetry axis. The largest resulting splittings
are observed for 11. oB and mB protons of the porphyrins
produce two pairs of signals, indicating that the rotation of the
Copper(i)-directed threading of either macrocycle 9 or 10
onto bis-phenanthroline rod 4 was carried out as follows
(Scheme 1).6,10 Equimolar amounts of Cu(CH3CN)4··PF6 in
CH3CN and the desired macrocycle in CH2Cl2 were combined
under argon. Compound 4 (0.5 equiv.) in CH2Cl2 was
subsequently transferred to the reaction mixture. After removal
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the case of 12. For the latter, a symmetrical correlation between
py1 of the AuIII porphyrins and the pairs (5A,6A) of protons of 4
could be observed, which, in the case of 11, involved protons 4A
and 5A instead.
All of these data suggest that, in the threaded system 11, the
ZnII porphyrin components are roughly antiparallel, as shown in
Scheme 1, each being tilted towards the phenanthroline chelate
of the macrocycle maybe thanks to attractive interactions
between the electron-rich ZnII porphyrin of one macrocycle and
the electron-deficient CuI-complexed phenanthroline included
1
in the other macrocycle. The significant differences in the H
NMR spectra of 11 and 12 suggest that the AuIII porphyrin
dimer has a conformation dramatically different from that of the
ZnII porphyrin dimer. The AuIII porphyrins being cationic, they
are no longer able to stack with the CuI-complexed phenanthro-
lines, due to electrostatic repulsion. A likely conformation
would be that represented in Scheme 1, in which the two AuIII
porphyrins are roughly parallel. In conclusion, the threaded
complexes 11 and 12 represent new members of the family of
porphyrin dimers assembled with coordination bonds. The
present study also shows that, by modifying the nature of the
central metal in threaded porphyrin-containing systems, dra-
matic geometrical changes are induced.
We warmly thank J.-D. Sauer for the 400 MHz NMR spectra
and R. Huber for the FAB mass spectra. We are grateful to P.
Staub for the preparation of 3-bromophenanthroline. M. L.
thanks the Ministère de l’Enseignement Supérieur et de la
Recherche for a fellowship.
Notes and references
† All of the new compounds were characterized by 1H NMR and mass
spectrometry or elemental analysis, and the data are in agreement with the
structures.
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Scheme 1 CuI-directed threading of macrocycle 9 or 10 onto bis-
phenanthroline 4. Protons highlighted in the text are indicated, as well as
intercomponent NOE effects.
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corresponding phenylene groups is slow on the NMR timescale,
and a and b protons of 11 clearly form two sets of diastereotopic
pairs of atoms.
1
Despite these common features, the H NMR spectra of the
threaded complexes 11 and 12 showed dramatic differences,
which could not be attributed solely to the different nature of the
metal cation of the porphyrins. Upon threading, protons 2A of 4
and (4,7) of the macrocycles are 22.54 and 20.59 ppm more
shielded, respectively, when 11 is compared to 12. 2D 1H NMR
ROESY experiments on 11 showed a remarkable inter-
component NOE crosspeak between the ZnII porphyrin But
groups and the pair (5,6) of protons of the phenanthroline
fragment belonging to a different macrocycle, that was absent in
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