Azoporphyrin: The porphyrin analogue of azobenzene

Article abstract of DOI:10.1002/anie.200604658

(Figure Presented) Excellent conjugation: The novel azoporphyrins (1,2-bis(porphyrinyl)diazenes) have been prepared by using copper-catalyzed coupling of primary amines. The structure of the azo(triphenylporphyrin) was determined by X-ray crystallography

Full text of DOI:10.1002/anie.200604658

Communications
DOI: 10.1002/anie.200604658
Porphyrinoids
Azoporphyrin: The Porphyrin Analogue of Azobenzene**
Louisa J. Esdaile, Paul Jensen, John C. McMurtrie, and Dennis P. Arnold*
Azobenzenes (1,2-diaryldiazenes) are very important organic
pigments and have a unique place in the field of photo-
responsive conjugated molecules because of their (usually)
reversible E/Z photoisomerization.[1] The current intense
interest in molecular analogues of mechanical components
and information storage and processing elements has stimu-
lated research into conjugated molecules whose shape and/or
optical properties can be switched electro- or photochemi-
cally.[2] Among the classes of conjugated pigments being
explored in these contexts are the porphyrinoids, which offer
Scheme 1. An example of an azoporphyrin 1 and bromoporphyrin
advantages of intense light absorption, a variety of accessible
oxidation states, and synthetic control of properties through
peripheral or central substitution.[3] Extension of porphy-
rinoid conjugation can be achieved by linking the peripheral
carbon atoms either by three direct bonds (as in the porphyrin
tapes of Osuka and co-workers)[4] or through potentially
conjugating bridges such as alkenes[5] or, even better,
alkynes.[6]
Anderson and co-workers compared both experimentally
and theoretically the strength of porphyrin–aryl electronic
coupling through alkene, alkyne, imino, and azo (diazeno)
bridges, and found the last of these to be the most efficient, in
that good orbital matching through continuous sp2 conjuga-
tion was combined with less steric interference than observed
with the alkene linker.[7] Thus, 1,2-di(porphyrinyl)diazenes
(or azoporphyrins; for example, 1, Scheme 1) are desirable
synthetic targets, in that they combine the extended porphyr-
inoid conjugation with the potential photoreactivity of the azo
linkage.[8] Herein, we describe the convenient synthesis and
crystallographic and spectroscopic characterizations of the
first (E)-azoporphyrins.
Ni-2a.
butoxycarbonyl-protected aryl hydrazines and aryl halides.[10]
Anderson and co-workers reported that the azo coupling
route was unsuccessful for the formation of Zn2-1b.[7] Our
initial approach involved the Pd-catalyzed coupling of
monoprotected hydrazines (carbazates) with bromoporphyr-
ins, but this approach generated a variety of products which
included the novel diiminoporphodimethenes.[11] When
unsubstituted hydrazine was used with bromoporphyrin
Ni-2a, we isolated the primary amine Ni-3a and the NiII
complexes of the corresponding bis(porphyrinyl) secondary
amine and 5-hydroxyporphyrin, in proportions that varied
with the initial stoichiometry.[12]
While trying to improve the yield of the secondary amine
from the coupling reaction of Ni-2a with Ni-3a using the
catalyst/base system Pd(OAc)2/rac-binap/Cs2CO3 (binap =
2,2’-bis(diphenylphosphanyl)-1,1’-binaphthyl) in toluene, we
found that a very small amount of a golden brown product
was always apparent on TLC plates.The visible absorption
spectrum of this compound corresponded with that expected
for the elusive target Ni2-1a.This product could arise simply
from oxidative dimerization of the primary amine induced by
the metal salt, so we omitted Ni-2a and changed the oxidant
system to copper(II) acetate/pyridine in air.We were pleased
to find that this proved to be a very clean method of coupling
Ni-3a to form Ni2-1a in high yield, with no side products
(Scheme 2).Indeed, with longer heating, even 2 mol% Cu
catalyst was suitable, but for convenient reaction times, we
chose the conditions given in the Supporting Information.
The copper-promoted formation of azoarenes from aryl
amines has been largely neglected since the earliest inves-
tigations in the 1950s.[13] The reaction was extended to the
corresponding zinc(II) complexes, which are favored by many
research groups because of their emissivity and ease of
demetalation.The preparation of the precursor amines Ni- 3a
and Zn-3a from the free base triphenylporphyrin (H2-4a) has
been streamlined to give respective yields of 68 and 58% for
the sequence nitration, metalation, and reduction, with
purification only after the final step (see the Supporting
Information).Alternatively, Ni- 3a can be obtained by the
Azobenzenes can be prepared by a variety of well-known
reactions,[9] such as the partial reduction of nitro-, nitroso-, or
azoxyarenes, coupling of aryl amines with nitrosoarenes, azo
coupling of aryl diazonium salts with electron-rich arenes, and
one-pot CuI-mediated coupling/oxidation reactions of bis-tert-
[*] L. J. Esdaile, Dr. J. C. McMurtrie, Dr. D. P. Arnold
School of Physical and Chemical Sciences
Queensland University of Technology
G.P.O. Box 2434, Brisbane 4001 (Australia)
Fax: (+61)7-3138-1804
E-mail: d.arnold@qut.edu.au
Dr. P. Jensen
Crystal Structure Analysis Facility
School of Chemistry
University of Sydney
N.S.W. 2006 (Australia)
[**] We thank the Australian Research Council for Discovery Grant
DP0663774, and the Faculty of Science, Queensland University of
Technology for a Postgraduate Scholarship for L.J.E.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Figure 1. The crystal structure of Ni2-1a shown from two different
Scheme 2. Synthesis of azoporphyrins 1a. a) Nitration: I2, AgNO2,
CH2Cl2, MeCN, argon, RT; b) metalation: M=Ni: [Ni(acac)2], toluene,
reflux; M=Zn: Zn(OAc)2, CHCl3, MeOH, reflux; c) reduction: 10%
Pd/C, NaBH4, CH2Cl2, MeOH, argon, RT; d) Cu(OAc)2, Py, toluene,
2.5–3.5 h, 808C; e) TFA, toluene, 20 min, RT. TFA=trifluoroacetic acid,
acac=acetylacetonate, Py=pyridine.
perspectives: a) orthogonal to the mean planes of the porphyrin rings;
b) parallel the to the mean planes. The side view in (b) illustrates the
extent of the distortion of the porphyrin rings and the perpendicular
displacement of the planes of the rings with respect to each other
(3.189 Š). The pyridine solvent molecule has been omitted for clarity.
azobridge.The dihedral angles between the C5–N5–N5 ’–C5’
mean plane and the mean porphyrin planes are 378.This out-
of-plane twist of the azo bridge is significantly smaller than
those of the alkene bridges in (E)-ethenediyl-linked Ni2
(508)[6e] and Zn2 (478)[5c] dimers.
hydrazine route.[12] Demetalation of Zn2-1a with TFA in
toluene gave the free base azoporphyrin H4-1a. All three
examples are very insoluble in common solvents, although the
metal complexes are more soluble in the presence of pyridine.
Single crystals of Ni2-1a·Py were grown by diffusion of
methanol into a pyridine solution, and the crystal structure
was determined by X-ray analysis.[14] The molecule (Figure 1)
has crystallographic inversion symmetry with the inversion
center on the N5–N5’ bridge.The pyridine solvent has twofold
symmetry, with the N atom disordered equally across the six
pyridyl atom positions.The asymmetric porphyrin ring is
considerably distorted from planarity, as is typical for NiII-
bound porphyrins.The maximum deviations of atoms from
the mean plane of the C20N4Ni porphyrin component occur
for the meso carbon atoms C5 and C15 (0.682 Š and 0.546 Š,
respectively) and C10 and C20 (0.555 Š and 0.609 Š,
respectively).The ring distortion projects C5 and C15 on
one side of the mean plane of the ring and C10 and C20 on the
other, in the manner described in the literature as ruffled.[15]
The mean plane of the N4Ni coordination sphere is almost
coplanar with that of the mean plane of the porphyrin ring
(dihedral angle 0.558 with a perpendicular distance for the
Ni···C20N4Ni plane of 0.010 Š). The mean C20N4Ni planes of
the porphyrin rings are exactly parallel and offset by a
perpendicular distance of 3.189 Š. This displacement is a
consequence of the ring distortion (which projects C5, and
therefore the azo-bridging nitrogen atom N5, away from the
plane of the ring) and the twisted conformation of the
The extension of the porphyrin conjugation and electronic
interaction with unsaturated substituents give rise to well-
known features in the electronic spectra, namely red-shifting
of the absorption and emission bands, increase in the relative
intensity of the Q bands against the B (Soret) bands, and
splitting of the Soret band.These features have been
intensively studied.[6,7] The visible absorption spectra of
azoporphyrins possess all these characteristics, as shown in
Figure 2.The Ni II complex displays a large complexation shift
(Q-band shift from 749 to 830 nm) when excess pyridine is
present—presumably the NiII centers are then six coordinate
(Figure 2a).The extent of interporphyrin communication can
be assessed qualitatively by means of the energy gap between
the two main components of the Soret band.Splittings
resulting from different bridges for similar dinuclear Zn
complexes are 2430 cmÀ1 for (E)-ethenediyl,[6e] 3320 cmÀ1 for
ethynediyl,[5c] and 3670 cmÀ1 for the azo bridge in Zn2-1a.[16]
The HOMO–LUMO gap (lowest-energy Q band) is another
measure of the expansion of the porphyrin p system.This
comparison between the C2H2/C2 and the azo linkers is
complicated by the effects on the HOMO–LUMO gaps of the
differences in the electronegativity of the carbon and nitrogen
groups as substituents, as well as by the steric demands of the
ethene linker.As there is no simple way to separate these
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butylphenyl)porphyrin instead of 5,10,15-triphenylporphyrin,
to leave an open meso position for further coupling
(Scheme 3).However, treatment of amine Ni- 3b under our
coupling conditions gave less than 10% yield of the desired
azoporphyrin Ni2-1b, the major product being the remarkable
head-to-tail dimer Ni2-5, which contained one iminoporpho-
dimethene ring.[18] A second similar product Ni2-6, the
corresponding oxo species, was also isolated.Clearly, another
strategy will be required to prepare larger arrays, as we have
so far been unable to prepare the required 5,15-diaminopor-
phyrins.
The properties of the azoporphyrins imply a stronger
interaction between the porphyrin rings across the azo bridge
than across ethene and ethyne linkers.In the solid state, the
lone pairs of electrons on the bridging nitrogen atoms still
have an effect which results in a 378 out-of-plane rotation of
[7]
the linker.Nevertheless, the prediction that the azo linker
would be an attractive molecular wire for connecting
porphyrins has been fulfilled.Our recent studies into the
coupling of nitrogen nucleophiles with porphyrins,[11,12] and
our preparation of the first azoporphyrins reported herein,
indicate that further studies in this field will be rewarding.In
particular, photophysical and nonlinear optical studies of
azoporphyrins should prove interesting.
Figure 2. Electronic absorption spectra of azoporphyrins: a) Ni2-1a in
CH2Cl2 (dotted line) and pyridine (solid line); b) Zn2-1a in CH2Cl2/
pyridine 99:1 (solid line), H4-1a in CHCl3 (dotted line).
Received: November 16, 2006
Published online: February 5, 2007
factors, we state that the azo dimer has significantly more red-
shifted Q bands than the C2H2/C2-linked dimers, that is,
841 nm for Zn2-1a, 713 nm for the ethynediyl dimer,[5c] and
661 nm for the (E)-ethenediyl dimer.[6e,16] This strong red shift
is reflected also in the HOMO–LUMO gap of Zn2-1a
measured by cyclic voltammetry, Eo1xÀ Er1ed = 1.34 V,[17]
which is markedly smaller than those for the ethynediyl
(1.98 V) and (E)-ethenediyl (1.92 V) dimers.[5c,17]
The photoisomerization of many (E)-azoarenes is nor-
mally readily achievable.However, for azo linkers in por-
phyrin/phthalocyanine-containing constructs, results are var-
iable.In some cases, the photoexcited states are quenched so
rapidly that no isomerization can be observed,[8a,d] whereas in
others, the isomerization is successful.[3a,c,8b,c] We irradiated
solutions of both Zn2-1a and Ni2-1a (see the Supporting
Information), but have not seen evidence for isomerization to
the (Z)-azoporphyrin.Evidently, the extended conjugation
promotes deactivation of the photoexcited states.
Keywords: absorption spectroscopy · azo compounds ·
conjugation · macrocycles · porphyrinoids
.
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