Synthesis of Meso-Substituted Subphthalocyanine-Subporphyrin Hybrids: Boron Subtribenzodiazaporphyrins

Article abstract of DOI:10.1002/anie.201502662

The first syntheses of hybrid structures that lie between subphthalocyanines and subporphyrins are reported. The versatile single-step synthetic method uses a preformed aminoisoindolene to provide the bridging methine unit and its substituent while trialkoxyborates simultaneously act as Lewis acid, template, and provider of the apical substituent. The selection of each component therefore allows for the controlled formation of diverse, differentially functionalized systems. The new hybrids are isolated as robust, pure materials that display intense absorption and emission in the mid-visible region. The new compounds are further characterized in solution and solid state by variable-temperature NMR spectroscopy and X-ray crystallography, respectively.

Full text of DOI:10.1002/anie.201502662

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International Edition: DOI: 10.1002/anie.201502662
German Edition: DOI: 10.1002/ange.201502662
Porphyrinoids
Synthesis of Meso-Substituted Subphthalocyanine–
Subporphyrin Hybrids: Boron Subtribenzo-
diazaporphyrins**
Sonia Remiro-BuenamaÇana, Alejandro Díaz-Moscoso, David L. Hughes,
Manfred Bochmann, Graham J. Tizzard, Simon J. Coles, and
Andrew N. Cammidge*
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Abstract: The first syntheses of hybrid structures that lie
between subphthalocyanines and subporphyrins are reported.
The versatile single-step synthetic method uses a preformed
aminoisoindolene to provide the bridging methine unit and its
substituent while trialkoxyborates simultaneously act as Lewis
acid, template, and provider of the apical substituent. The
selection of each component therefore allows for the controlled
formation of diverse, differentially functionalized systems. The
new hybrids are isolated as robust, pure materials that display
intense absorption and emission in the mid-visible region. The
new compounds are further characterized in solution and solid
state by variable-temperature NMR spectroscopy and X-ray
crystallography, respectively.
M
acrocyclic oligopyrrole structures are ubiquitous in both
nature and everyday life. The general class is exemplified by
the two symmetric frameworks of porphyrin 1 (4 pyrrole units
linked by methine bridges) and phthalocyanine 2 (4 isoindole
units linked by nitrogen bridges; Figure 1). Many thousands
of studies have been published covering their design, syn-
thesis, properties, and applications, and research effort
Figure 1. The parent structures porphyrin 1, phthalocyanine 2,
expanded phthalocyanine 3, and subphthalocyanine 4 (M=metal or
H,H).
[
1]
continues to increase.
Of particular importance are recent synthetic efforts to
prepare modified structures where the macrocyclic core
reported by Osuka and Kobayashi leading to the first
[
14]
[15]
(
usually aromatic) itself is perturbed, yielding derivatives
syntheses of boron tribenzosubporphyrins 5
and 6
[2–5]
with fundamentally different properties and uses.
Studies
(Figure 2).
[
6]
[7]
have focused separately on ring-expanded and -contracted
analogues. The manipulation of porphyrin-like structures has
proved most straightforward for ring-expanded systems, with
synthetic strategies building on established polypyrrole con-
struction methods. Expanded phthalocyanine structures
The most challenging core modification of the oligopyr-
role/indole macrocycles is arguably the generation of hybrid
[16]
structures
that lie between the classic porphyrin and
phthalocyanine parent structures. The challenge posed by
these systems is perhaps best illustrated by the limited
[8]
[16]
include the uranyl derivative 3, which comprises five
number of studies to date, despite their first identification
[
7]
[17]
indole-type units. Ring-contracted phthalocyanines are
especially intriguing and have been the subject of growing
research efforts since the serendipitous formation of boron
being reported in the late 1930s by Dent and the Linstead
[18]
group. The greatest drawback to the investigation of the
hybrid structures is the limited synthetic possibilities available
to access modified and bespoke derivatives. Nevertheless,
interest in the hybrid structures has accelerated rapidly over
the last decade even though most syntheses have relied on the
[
9]
subphthalocyanine (SubPc, 4) by Meller and Ossko in 1972.
[
7,10]
SubPcs
adopt a bowl-like conformation. The central
boron atom bears a further apical group (X in 4), which can be
[
7]
[16,19,20]
interchanged under appropriate conditions. The optical
properties of SubPcs are distinctive as they show intense
absorption and emission around 550 nm, leading to a partic-
ular focus on their application as components in photovoltaic
original procedures, albeit with some improvements.
We recently reported a significant breakthrough in this
[21]
area,
controlled access to functionalized TBTAPs (TBTAP = tet-
rabenzotriazaporphyrin, phthalocyanine–tetrabenzopor-
disclosing a new, versatile procedure that gives
[
7,11–13]
cells.
A small number of subporphyrins are also known,
a
the most notable examples being the synthetic breakthroughs
phyrin hybrid in which a single aza bridge of the Pc ring is
replaced by carbon). Importantly, the synthesis allows for the
introduction of substituents at the “new” meso site. Chepra-
kov and co-workers have subsequently reported a comple-
mentary approach to a second member of the hybrid series,
[*] S. Remiro-BuenamaÇana, Dr. A. Díaz-Moscoso, Dr. D. L. Hughes,
[22]
Prof. M. Bochmann, Prof. A. N. Cammidge
School of Chemistry, University of East Anglia
Norwich Research Park, Norwich NR4 7TJ (UK)
E-mail: a.cammidge@uea.ac.uk
Dr. G. J. Tizzard, Dr. S. J. Coles
UK National Crystallography Service, School of Chemistry
University of Southampton, Southampton SO17 1BJ (UK)
[
**] Support from the UEA (S.R.-B.), the EU (A.D.-M.), and the EPSRC
the National Crystallography and Mass Spectrometry Services) is
(
gratefully acknowledged. We thank Dr. C. MacDonald and Prof. P.
Ballester for help with NMR measurements and calculations,
respectively.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201502662.
Figure 2. Tribenzosubporphyrins reported by the Osuka (5) and
Kobayashi (6) groups.
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the trans-TBDAPs (Scheme 1). Until now, hybrid structures
based on the ring-contracted systems, SubPc and tribenzo-
SubPn, are unknown. Herein, we report a versatile method
that provides access to the first examples of such hybrids,
alongside a preliminary examination of their properties.
Scheme 2. Synthesis of a boron subphthalocyanine–subporphyrin
hybrid, a so-called SubTBDAP (13).
chromatography, allowing for the isolation of the first pure
SubTBDAP 13 as a bright pink material with intense yellow
fluorescence (fꢀ 0.5, Stokes shift ca. 15 nm). The yield
(
15%) is impressive, especially when compared to typical
Scheme 1. Synthetic breakthroughs giving straightforward access to
functionalized porphyrin–phthalocyanine hybrids.
yields of both simple SubPcs and unsymmetric phthalocya-
nines. The absorption and emission spectra are compared to
those of the parent SubPc (4, X = OPh) in Figure 3. The
The optimized synthesis of SubPc 4 (X = Cl) typically
involves the reaction of phthalonitrile (8) with BCl at
3
[7,23]
elevated temperature (xylene at reflux).
In these reac-
tions, the boron reagent acts as a Lewis acid and as a template.
In our recently developed synthesis of TBTAP hybrids 9, we
employed aminoisoindolene precursor 7 to initiate the
macrocyclization process around a magnesium template ion,
and we therefore reasoned that the same precursor could
potentially initiate macrocyclization in the presence of
a boron template leading to a hybrid in which a (functional-
ized) meso carbon atom formally replaces one of the bridging
nitrogen atoms present in the SubPc parent. The target
product is a boron subtribenzodiazaporphyrin (SubTBDAP).
Consequently, in the initial set of successful reactions, amino-
Figure 3. Absorption (c) and emission (b) spectra of hybrid
SubTBDAP 13 (red) and SubPc-OPh (blue).
[
24]
isoindolene 12
was reacted with phthalonitrile in the
presence of BCl in xylene at reflux. Three distinct, highly
absorbance and fluorescence maxima are both blue-shifted by
approximately 10–20 nm compared to those of SubPc,
reflecting the change towards a more porphyrin-like character
in the hybrid 13. The absorption spectrum of SubTBDAP also
shows the main band to be split—a consequence of the
reduced symmetry in the macrocyclic chromophore core. The
blue shift also indicates that as expected, the appended meso
phenyl substituent does not contribute to the main p-system,
lying essentially perpendicular to the SubTBDAP core. This
parallels the arrangement in both the well-known meso-
phenyl porphyrins and the meso-aryl TBTAPs.
3
colored products were formed. The first product was identi-
fied as the simple, symmetric SubPc-Cl, formed by the
expected homomacrocyclization of phthalonitrile. MALDI-
MS studies strongly implied that the other two new com-
pounds were the required SubTBDAP-Cl and the azaBO-
[
25]
DIPY-type compound formed from self-condensation
of
the aminoisoindolene precursor 12 followed by complexation
with boron. However, attempts to isolate and characterize the
new products were impeded by the reactivity of the labile
BÀCl bond present in all components of the mixture. There-
1
fore, in a subsequent reaction (Scheme 2), macrocyclization
was performed again, but followed directly by treatment of
the crude product mixture with an excess amount of phenol to
convert all BÀCl fragments into stable BÀOPh moieties.
The H NMR spectrum for hybrid 13 at room temperature
shows the expected signals for the macrocyclic core and the
apical OÀPh group (signals for the OÀPh group are shielded
by the core ring current). However, the signals for the ortho-
and meta-hydrogen atoms of the meso phenyl substituent are
Separation of the mixture could then be achieved by
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very broad and featureless, indicating slow rotation of the
group. Lowering the temperature leads to sharpening and
resolution of the signals (Figure 4). A large chemical shift
difference is observed for the two doublets corresponding to
the ortho-hydrogen atoms, and they were assigned based on
their sensitivity to interchange of the apical alkoxide sub-
The synthesis outlined above allowed for the isolation of
a SubTBDAP hybrid. However, the versatility of the proce-
dure suffers from the practical drawback of the simultaneous
formation of a symmetric SubPc. The formation of the SubPc
is of course expected because these initial conditions are
known to lead to the macrocyclization of phthalonitrile alone.
We therefore turned our attention to the development of
reaction conditions that would lead to the formation of the
hybrid structures (macrocyclization initiated by aminoisoin-
dolenes 7) without competing formation of the SubPc. This
was achieved by employing less reactive boronate esters as
the Lewis acid/template, recognizing that phthalonitrile itself
requires a more reactive boron source for SubPc formation.
stituent. H , which lies under the conical macrocycle, appears
o
at approximately 6.9 ppm in all cases, whereas the resonance
The use of boronate esters (B(OR) ) has the added advantage
3
of leading directly to the stable SubTBDAP-OR variants in
a single operation (Scheme 3). Diglyme was found to be
1
Figure 4. Variable-temperature H NMR spectra of hybrid 13 and its
crystal structure.
for H , the proton lying on the same side as the alkoxide, is
o’
sensitive to its nature and is seen in the range of 8.2–8.6 ppm.
The assignment was further supported by calculations (see the
Supporting Information). Crystals suitable for X-ray diffrac-
tion were eventually grown from a mixture of acetone and
hexane allowing for the determination of the solid-state
[
26]
structure. In the crystal, the boron atom is bonded to the
three pyrrole nitrogen atoms and the phenoxy oxygen atom
(
which is in the apical site of the pyramidal arrangement) in
Scheme 3. Versatile syntheses of TBDAP hybrids enabling the selective
a tetrahedral (or trigonal pyramidal) arrangement (Figure 4).
The boron atom is displaced 0.614(5) Š from the plane of the
three N atoms, and the SubTBDAP ring system is wrapped
around the base; each benzene ring plane is tilted by
approximately 208 with respect to the plane of the three
pyrrole N atoms away from the phenoxy group. The meso
phenyl group was confirmed to be lying essentially perpen-
dicular to the SubTBDAP core.
The SubTBDAP molecules are arranged in pairs with the
overlapping, centrosymmetrically related N21/C29 isoindole
rings approximately 3.62 Š apart; these groups are also
bridged by a pair of solvent water molecules, which appear to
form hydrogen bonds to the nitrogen atoms N20 and N30.
functionalization at the meso aryl and apical sites.
a convenient and suitable solvent, and the reaction between
the aminoisoindolene precursor and phthalonitrile (1:3)
proceeded smoothly at 2008C in three hours. SubPc was not
formed, and the hybrid SubTBDAPs were easily isolated.
Yields were marginally lower than for the stepwise procedure
(typically around 10%) but compensated by synthetic con-
venience and versatility. Excess phthalonitrile can be recov-
ered from the reaction alongside the hydrolysis products
(phthalimide). The main side products are the expected
azadipyrromethene (from self-condensation) and polar/oligo-
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meric baseline material. The reactions can also be performed
in xylene or indeed in the absence of solvent.
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ability to engineer the new “meso” substituents at will—
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structures uniquely attractive. Aminoisoindolenes 7 are
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In conclusion, we have reported the first examples of
a new class of hybrid macrocyclic systems whose core
structure lies between those of subphthalocyanines and
subporphyrins. The synthesis is controlled, allowing for the
manipulation of the functional groups at both the central
boron atom and importantly at the new meso site. Of
particular significance is the synthetic ease and versatility
that will permit imaginative design of bespoke materials to
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Keywords: dyes/pigments · heterocycles · phthalocyanines ·
porphyrinoids · synthetic methods
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Received: March 23, 2015
Published online: May 15, 2015
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