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Bimetallic Figure-Eight Octaphyrins Split into Four-Pyrrolic Macrocycles.

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Highlights
Porphyrinoid Complexes
Bimetallic Figure-Eight Octaphyrins Split into
Four-Pyrrolic Macrocycles
Lechosław Latos-Grażyński*
Keywords:
macrocyclic ligands · octaphyrins · porphyrinoids ·
rearrangement · spiro compounds
Expanded porphyrins can be considered as suitable and adjustable macrocyclic frameworks for the construction
of binuclear or polynuclear coordination
compounds.[1] It can be expected that a
multifunctional expanded porphyrin
structure will enforce structurally
unique coordination motifs and provide
models for bimetallic metalloenzymes.
Significantly, two adjacent, possibly cooperating, metal ions are in a position to
prefer some unknown reaction routes
that could be exploited in catalytic
processes. Representative examples of
such coordination chemistry have been
demonstrated by amethyrins,[2] accordion porphyrins,[3] N-confused hexaphyrins,[4] calix[4]pyrrole Schiff base
macrocycles,[5] rubyrins,[6] structural analogues of Pac-Man porphyrins,[7] and
other macrocycles containing a pyrrolic
unit.[8] Bimetallic complexes were also
obtained for octaphyrins,[9] and these are
the primary topic of this Highlight.[10–14]
Vogel and co-workers demonstrated
that an acid-catalyzed MacDonald condensation of a bipyrrole derivative and a
complementary, suitably functionalized
bipyrrole component resulted in the formation of octapyrrole 1 or the octaphyrins [36]octaphyrin(2.1.0.1.2.1.0.1) (2),[9, 15]
[32]octaphyrin(1.0.1.0.1.0.1.0)
(3),[16]
and [34]octaphyrin(1.1.1.0.1.1.1.0) (4;
Scheme 1).[15] The syntheses of [30]octaphyrin(0.0.0.0.0.0.0.0) (5)[17] and [32]octaphyrin(1.0.0.0.1.0.0.0) (6)[18] were re-
[*] Prof. Dr. L. Latos-Grażyński
Department of Chemistry
University of Wrocław
14 F. Joliot-Curie Street
Wrocław 50 383 (Poland)
Fax: (+ 48) 713-282-348
E-mail: llg@wchuwr.chem.uni.wroc.pl
5124
Scheme 1. Tetrahyhydroctaphyrin 1 and octaphyrins 2–7 (the b and meso substituents are omitted for clarity).
ported by Sessler and co-workers. These
expanded porphyrins contain eight pyrrolic fragments linked directly by Ca Ca
bonds or by unsubstituted meso-methine (CH)n units (n = 1,2).
A modification of the Rothemundtype synthesis resulted in the formation
of expanded porphyrins, which contain
more than six pyrrole moieties. Acidcatalyzed condensation of tetraalkylbipyrrole and ortho-substituted benzaldehydes yielded a series of giant porphyrins, including meso-substituted [32]octaphyrin(1.0.1.0.1.0.1.0) (3).[19] [36]Octaphyrin(1.1.1.1.1.1.1.1) (7), the fundamental macrocycle used in octaphyrin
splitting studies, is obtained by straight-
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200460645
forward condensation of pyrrole and
pentafluorobenzaldehyde as reported
by Osuka, Furuta, and co-workers.[20]
The fact that these expanded porphyrins
are synthesized from commonly accessible substrates by a relatively simple
methodology creates a foundation for
extensive studies, including of their
coordination chemistry and anion-binding properties.
This Highlight focuses on octaphyrins 2–4 and 7, which have skeletons with
a
helical
figure-eight
arrangement.[9, 11, 14–16, 20] Such octaphyrin molecules exist in a chiral figure-eight conformation of two equidirectional helices.
The crossing points in the octaphyrins
Angew. Chem. Int. Ed. 2004, 43, 5124 –5128
Angewandte
Chemie
discussed consist of chemically different
moieties which belong to the upper and
bottom part of the macrocyclic frame.
The nitrogen atoms of the pyrrole rings
adjacent to the crossing location are
oriented in opposite directions, thus
creating a “zigzag” turn.
The figure-eight molecules reveal
specific intramolecular mobility and
two mechanisms were considered to
account for the dynamic behavior of
octaphyrins.[21–23] The first mechanism
involves an equilibrium between two
enantiomeric forms of the double helical, figure-eight macrocycle and includes inversion of the helix. In selected
cases the restriction of the intramolecular mobility allowed the separation of
two enantiomers.[9] The second mechanism which seems to be operating for
the figure-eight molecules cyclooctapyrroles,[11, 15, 22]
41,43,45,47-tetrathia[36]octaphyrin(1.1.1.1.1.1.1.1),[23]
and
turcasarin[21] involves a conveyor-beltlike movement of the whole ring, but
excludes, however, racemization. Rearrangement of the whole figure-eight ring
of the molecule eventually takes place
to reconstruct the geometry around
“zigzag” spacers.
Figure-eight octaphyrins can contain
either 4n + 2 or 4n p electrons, thus
corresponding to the classical H?ckel
formulation for aromatic and antiaromatic rings, respectively. Consequently
the 1H NMR spectra of these molecules
exhibit the presence of either residual
diatropic or paratropic ring currents.[21, 23] As regards to this effect, one
has to be aware of the figure-eight
geometry compared to prototypical planar conjugated porphyrins or extended
porphyrins. The marked influence of the
size of the main conjugation pathway,
which involves either 4n p electrons for
2 and 3 or (4n+2) p electrons for 4, was
demonstrated by the electronic spectra:
a correlation between the wavelength of
the most intense band versus the number of conjugated p electrons was observed for a series of species formed in
the course of redox processes.[10] The
spectra of the (4n+2) p-electron species
in particular exhibit an intense and
sharp absorption band which reflects
an intense conjugation in the ligand.
The cyclooctapyrroles 1–4 and 7
appear to be suitably prearranged to
form binuclear metal complexes since
Angew. Chem. Int. Ed. 2004, 43, 5124 –5128
the remarkable conformation of these
macrocycles creates two structurally
identical, helical N4 pockets distinctly
separated by the figure-eight twist. For
example, the X-ray structure of 2 shows
the four dipyrrin units are almost planar
in each case, thus the helical conformation of the two tetrapyrrole subunits is
mainly attributable to the torsions of the
single bonds between the bipyrrole
units.[9] Significantly, the molecular symmetry remains unaffected after insertion of two PdII or CuII ions into the two
N4 pockets of 2. There are marked
conformational changes which are reflected by mutual orientation of the
bridging (CH)2 units. The CH=
CH bonds are orthogonal in the free
base 2 (Scheme 1), but they are parallel
in complex 8 (Scheme 2).
The ring skeleton of [32]octaphyrin(1.1.1.0.1.1.1.0) (3) was functionalized
to yield the dioxoderivative 9.[11] The
macrocycle binds two NiII ions with
preservation of the macrocyclic skeleton. Each NiII ion of 10 is coordinated by
two practically planar dipyrrin units.
Scheme 2. Conformation changes on insertion
of Cu2+ ions into 2.
Remarkably, the binuclear NiII spirodicorrole 12 was obtained in addition to
the expected insertion product 10
(Scheme 3). In compound 12 two orthogonally oriented corrolates (both as
isoforms) are linked through a common
spiro carbon atom without any spacer.
Directly linked porphyrins (meso-meso,
b-b, or meso-b) are, at present, the
subject of extensive investigation.[24, 25]
Scheme 3. Transformation of 9 during the insertion of NiII ions.[11]
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5125
Highlights
splitting into two cyclotetrapyrroles. Insertion of PdII ions into
4 yielded, apart from the expected binuclear PdII octaphyrin 14, its constitutional isomer
15 (Scheme 5).[11] Complex 15
contains a transannular C C
linkage between two pyrrolic
a a’ positions. The ligand consists of two identical tetrapyrrolic ring systems linked by the
newly formed C C bond. Remarkably the binuclear PdII
octaphyrin 14 and its spiro isomer 15 remain in reversible
thermal equilibrium, with 15
predominating at room temperature. The equilibrium can be
displaced photochemically towards 14. The thermal isomerization requires the presence of Scheme 5. Thermal equilibrium between binuclear PdII
coordinated metal ions, since complex 14 and its bis-spirodiporphin isomer 15. The
a’ reaction
the free ligand does not reveal black dots in structure 14 mark the Ca and C[11]
centers. Bottom: the postulated mechanism.
this type of rearrangement. The
incorporation of palladium substantially reduces the distance
between the reaction centers Ca and Ca’ 18-p-electron electrocyclization for each
from 4.10 to 2.99 E (as established by subunit was considered as an alternative
PM3 calculations) and induces strain mechanism.
The collaborating research groups of
that is relieved by formation of the Ca
Vogel and Houk observed unprecedentCa’ bond.
The transannular ring closure can be ed behaviors of bimetallic octaphyrins.
interpreted as a type of intramolecular The common feature is that the macroMichael addition in which the electro- cycles split into two cyclic, four-pyrrolic
philic azafulvene unit serves as the compartments which remain covalently
acceptor and a nucleophilic pyrrole linked by a spiro carbon atom.[11]
anion functions as a donor. Thermal
Recently, Osuka and co-workers
discovered a logical termination of the
sequence of the events observed in
intramolecular octaphyrin reactivity,
that is, a total cleavage of octaphyrin.[14]
Thus, their important contribution reveals an ultimate step in the sequence
of figure-eight octaphyrin transformations, namely thermal splitting of a
single binuclear CuII [36]octaphyrin(1.1.1.1.1.1.1.1) complex (16) into two
molecules of CuII [18]porphyrin(1.1.1.1)
(18, Scheme 6).[14] As discussed previously, [36]octaphyrin(1.1.1.1.1.1.1.1) (7)
acquires a figure-eight geometry in the
solid-state, with two identical porphyrinlike tetrapyrrolic ligands which are
available for step-wise metalation by
one or two CuII ions.
X-ray diffraction analysis of 16 revealed its effective C2 symmetry and the
presence of two copper ions centrally
Scheme 4. Rearrangement of 9 into the binuclear NiII spirodicorrole 12 via 11 and the binuclear
bound within the core of a severely
NiII lactone complex 13. Bottom: the postulated mechanism.[11]
Significantly, the spirodicorrole unit in
12 and the still to be synthesized spirodiporphyrin provide a unique mode of
covalently linking two porphyrin units.
The electronic spectra and electrochemically measured redox potentials of
12 reflect the spiro effect.[12] The homoconjugative interaction across the spiro
center (spiroconjugation), that is, a homoconjugative interaction between the
p systems of the two halves of the
molecule separated by the tetragonal
spiro center, is feasible because of the
structural constraints of 12.
The considered mechanism which
explains the formation of 12 includes
the conveyor-belt movement of 9
(marked by arrows in Scheme 3) which
causes the carbonyl groups to meet at
the crossing center of conformer 11. The
suggested skeletal rearrangement is presented in Scheme 4 (bottom) and involves an electrophilic attack of the
NiII ion (or of a proton) on one of the
carbonyl groups followed by its conversion into a carbenium ion and consecutive formation of the oxygen bridge.[11]
The carbenium ion thus generated undergoes a Wagner–Meerwein-type rearrangement to give the lactone 13, which
is expected to undergo a radical or ionic
cleavage of the Cspiro O bond followed
by subsequent loss of CO2 from the
diradical/zwiterrionic species.
To date only two other octaphyrins,
namely 4 and 7, revealed intramolecular
reactivity which led to their eventual
5126
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 5124 –5128
Angewandte
Chemie
Scheme 6. Thermal transformation of the binuclear CuII complex 16.[14]
distorted octaphyrin macrocycle. A tripyrrolic unit consisting of B–D pyrrole
rings is relatively flat while the plane of
pyrrole A is tilted by 638. The structure
of 16 reveals a severe distortion of the
coordination geometry compared to the
practically planar structure of 18. These
features suggest that the relief of strain
in 16 may be a main driving force for this
thermal splitting reaction. The splitting
reaction of 16 was found to be a unimolecular process with an enthalpy for the
formation of 18 of 135 kJ mol 1. One
possible mechanism may be a [2+2] cycloaddition to give the spirocyclobutane
intermediate 17, which divides into two
molecules of 18 by a cycloreversion
reaction. Significantly, a similar cleavage of octaphyrin 7 was detected for the
binuclear PdII, CoII, and NiII complexes
of 7 but in rather small yields.[14]
As demonstrated above, the
CuII-metalation of [36]octaphyrin(1.1.1.1.1.1.1.1) (7) gave rise to a facile
splitting which provided a rare example
of “molecular mitosis” for expanded
porphyrins.[14] In these terms, the molecular rearrangements, which result in
the formation of binuclear NiII spirodicorrole 12 and binuclear PdII bis-spirodiporphyrin 15 from appropriate bimetallic octaphyrins, illustrate some imaginable snapshots of intermediates preceding the final phase of the “octaphyrin
mitosis”.
The intriguing reaction which splits
octaphyrin 9 into two covalently spirolinked corrolates 12 seems to be facilitated by the proximity effect at the
figure-eight crossing center,[11] which is
in fact a common structural feature of
Angew. Chem. Int. Ed. 2004, 43, 5124 –5128
figure-eight bimetallic octaphyrins. Theoretically, this peculiar transannular reactivity is feasible for any figure-eight
metalloctaphyrin providing that a preorganization step (the metal-ion coordination) creates a geometry at the crossing center which resembles the transition state for splitting.
The feasible routes for further exploration can be directed toward molecular electronics and catalysis. The noteworthy reversibility of the binuclear PdII
[34]octaphyrin–spirodiporphyrin (14–
15) isomerization (Scheme 5) leads to
the conclusion that such a system behaves as a molecular switch containing
two nondegenerate quasi-stabile molecular states that are clearly distinguishable by electronic spectroscopy. Evidently the internal structural transformations, highlighted here for metallooctaphyrins, drastically modify the coordination cores surrounding two metal ions
and consequently may create reversibly
or irreversibly different chemical properties of metallic centers.
The coordination chemistry of expanded porphyrins is still in its infancy.
The transformation detected for figureeight octaphyrins revealed unusual coordination and transformation modes. It
remains to be seen how the most fundamental factors, that is, the nature of the
ribbon crossing motifs or the choice of
the metal ions influence the stability of
the transient species and eventually
favor the splitting into two porphyrinic
units over formation of a spirodiporphyrin.
Published Online: September 15, 2004
www.angewandte.org
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