Synthesis and switching the aromatic character of oxatriphyrins(2.1.1)

Article abstract of DOI:10.1002/anie.201310129

Triangularly shaped, contracted porphyrinoids belong to a group of molecules where the geometry significantly modifies the observed electronic properties. The need for a controllable, effective, and widely applicable approach to triphyrins drives extensive research towards macrocyclic materials that act as potential controlling motifs by switching their aromaticity. Two isomeric thiophene-fused triphyrins(2.1.1) were synthesized by applying an innovative approach. Spectroscopic techniques (NMR, UV/Vis) show that both macrocycles are aromatic and quantitatively convert into anti-aromatic structures after reduction with a zinc amalgam. The reduced forms were stabilized through boron(III) coordination, thereby allowing the observation of anti-aromatic 16 π delocalization within a contracted porphyrin. Going for a spin: The precisely crafted thiophene-fused oxatriphyrins(2.1.1) show an aromatic character consistent with 14 π or 18 π delocalization pathways. The aromaticity is strongly dependent on a thiophene incorporation mode and can be switched to anti-aromatic (16 π electrons) or non-aromatic by reduction with a zinc amalgam. Both reduced structures are stable as boron(III) complexes.

Full text of DOI:10.1002/anie.201310129

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DOI: 10.1002/anie.201310129
Triphyrins(n.1.1)
Synthesis and Switching the Aromatic Character of
Oxatriphyrins(2.1.1)**
´
Miłosz Pawlicki,* Karolina Hurej, Ludmiła Szterenberg, and Lechosław Latos-Graz˙ynski*
Abstract: Triangularly shaped, contracted porphyrinoids
belong to a group of molecules where the geometry signifi-
cantly modifies the observed electronic properties. The need for
a controllable, effective, and widely applicable approach to
triphyrins drives extensive research towards macrocyclic
materials that act as potential controlling motifs by switching
their aromaticity. Two isomeric thiophene-fused triphyrins-
(2.1.1) were synthesized by applying an innovative approach.
Spectroscopic techniques (NMR, UV/Vis) show that both
macrocycles are aromatic and quantitatively convert into anti-
aromatic structures after reduction with a zinc amalgam. The
reduced forms were stabilized through boron(III) coordina-
tion, thereby allowing the observation of anti-aromatic 16p
delocalization within a contracted porphyrin.
P
orphyrins, or more broadly porphyrinoids, are regarded as
an elementary motif for fundamental studies in several
scientific fields,[1] including for medicinal applications[2] and
modern optoelectronics.[3] Attention focuses on the search for
organic alternatives of currently, widely explored inorganic
molecular devices.
Scheme 1. Triphyrins(n.1.1).
Triphyrins(n.1.1) (Scheme 1) are a new class of porphyr-
inic macrocycles with a reduced number of donors compared
to porphyrins and are created by the removal of at least one
pyrrole subunit. The rational approach for the synthesis of
boron(III) triphyrin(1.1.1) 1-B,[4] boron(III) subporphyra-
zines,[5] and boron(III) subphthalocyanines,[6] involves a tem-
plating effect. The formation of free-base subpyriporphyrin[7]
and several triphyrins(n.1.1) (Scheme 1), where n varies from
2 to 6,[8] required a stepwise strategy. The triphyrin(n.1.1)
subgroup is rather small and there are only a few examples of
such macrocycles (Scheme 1). Nevertheless, they reveal
intriguing properties, including the topology-dependent aro-
maticity switching of triphyrin(6.1.1) 3 on coordination.[9] The
boron(III) triphyrin(1.1.1) complexes 1-B have demonstrated
a variety of optoelectronic properties, such as nonlinear
optical absorption and high emission quantum yields, that are
directly related to the 14p aromatic delocalization path.[10] An
alternative route to tune the properties of triphyrins is
a modification of their coordination unit by replacing nitro-
gen atom(s) with other heteroatoms to yield aromatic
thiatriphyrin(2.1.1)
4,[8d]
oxatriphyrin(4.1.1),[8i]
and
thiatriphyrins(4.1.1) 6,[8h] as well as non-aromatic
oxatriphyrin(3.1.1) 5.[8g] On the other hand, the aromatic
character of triphyrins(n.1.1) has not been explored to date,
and research concentrates on an unmodified 14p electron
circuit. All the above-mentioned aspects of triphyrin chemis-
try stimulate the intensive search for nontrivial synthetic
routes that lead to controllable triphyrin scaffolds with
appropriately defined functionality.
Here we report on the synthesis of two isomeric thio-
phene-fused oxatriphyrins(2.1.1) by a coherent method
involving the use of precisely crafted synthons. Both macro-
cycles are aromatic with alternative delocalization and
undergo reduction to afford anti-aromatic structures that
can be stabilized as boron(III) complexes.
[*] Dr. M. Pawlicki, K. Hurej, Dr. L. Szterenberg,
´
Prof. L. Latos-Graz˙ynski
Department of Chemistry, University of Wrocław
F. Joliot-Curie 14, 50383 Wrocław (Poland)
E-mail: milosz.pawlicki@chem.uni.wroc.pl
lechoslaw.latos-grazynski@chem.uni.wroc.pl
Homepage: http://llg.chem.uni.wroc.pl/
The reported rational approach to form meso-substituted
triphyrins(2.1.1) 11 and 13 requires an effective synthesis of
the synthons 8 and 10. Suzuki–Miyaura coupling was applied
to achieve this demanding task (Scheme 2) under conditions
adapted from those previously reported for heterocycles.[11]
The formation of 7 and 9 has been accomplished in yields of
80 and 85%, respectively, starting from commercially avail-
able substrates. Both substituted thiophenes 7 and 9 are stable
and can be stored without any degredation for several weeks.
The deprotection step required the thermal removal of the
[**] Financial support from the Ministry of Science and Higher
Education (Grant N N204 021939) and National Science Centre
(2012/04/A/ST5/00593 K.H.) is kindly acknowledged. The DFT
calculations were carried out at the Poznan Super-computer Centre.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310129.
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Scheme 2. Formation of thiophene synthons. Conditions: a) N-Boc-2-
pyrroleboronic acid (3 equiv), Pd(OAc)2 (2% per halogen), SPhos (4%
per halogen), K3PO4 (4 equiv), nBuOH, 1008C, 4 h; b) HOCH2CH2OH,
reflux, inert atmosphere, 1 h).
Scheme 3. Formation and reduction of oxatriphyrins(2.1.1). Condi-
tions: a) 2,5-bis(p-tolylhydroxymethyl)furan (1 equiv) CH2Cl2, BF3:Et2O,
DDQ (3 equiv); b) Zn/Hg, CDCl3; c) MeOH/Et3N; d) CH2Cl2/HCl.
DDQ=2,3-dichloro-5,6-dicyano-1,4-benzoquinone.
Figure 1. X-ray structures of 8 (A) and 10 (B).
tert-butyloxocarbonyl (Boc) group in refluxing ethylene
glycol and led to the quantitative formation of 8 and 10.
Formation of the desired products was confirmed by spec-
troscopic analysis (see the Supporting Information) as well as
increases the solubility and prevents aggregation (see the
Supporting Information).
The lack of symmetry in 11-HCl is reflected by the
presence of eight b-H heterocyclic signals in the aromatic
region (d = 9.2–8.3 ppm). The full assignment given in Fig-
ure 2A was made on the basis of 2D experiments (NOESY,
COSY), taking an NOE contact between H(21) and H(4) as
the starting point. In contrast to other aromatic porphyr-
inoids, the resonance of the inner hydrogen atom in 11-HCl is
strongly downfield shifted (d = 11.5 ppm). This peculiar shift
reflects a deshielding contribution from a strong intramolec-
ular N···HN hydrogen bond acting within the cavity, which
dominates the typical shielding influence of a diatropic ring
current. The effect resembles one reported previously for an
N-fused porphyrin[13] and other triphyrins(n.1.1).[7,8f,g] The
aromaticity of 11-HCl can be accounted for by the 14p path of
112, which is typical for triphyrins(2.1.1). This interplays,
however, with the alternative 18p delocalization route of 113,
which is imposed by 2,3-thiophene fusion (Figure 3). To shed
additional light on the control of the electronic structure,
a thiophene ring was fused to the oxatriphyrin(2.1.1) back-
bone at its b positions, with the aim of limiting the 14p
electron route in favor of the “extended” 18p electron one,
albeit with involvement of the sulfur atom on the perimeter.
By following the synthetic approach applied for 11
(Scheme 3), synthon 10 was condensed with 2,5-bis(p-tolyl-
hydroxymethyl)furan to eventually form monoprotonated
oxatriphyrin(2.1.1) 13-HCl in 15% yield. An unidentified
counterion obtained during condensation was replaced with
X-ray analysis (Figure 1).[12] The substitution affects the C S
À
bond lengths in 8 and 10. The unsymmetrical substitution of
À
thiophene in 8 causes a difference within the C S bonds
À
(1.704(3) ꢀ and 1.678(1) ꢀ), while the lengths of the C S
bonds in the fully symmetric 10 are equal (1.705(2) ꢀ and
1.701(2) ꢀ). The deprotected reagents are less stable and
need to be condensed without any extended storage.
The formation of target oxatriphyrin(2.1.1) 11 was ach-
ieved by condensation of 8 and 2,5-bis(p-tolylhydroxyme-
thyl)furan[8g] in an equimolar ratio under standard Lindsey
conditions (Scheme 3). The macrocycle was isolated solely as
the monoprotonated form 11-H. An unidentified counterion
obtained during condensation was subsequently replaced by
a chloride ion (see the Supporting Information). The anion
x exchange process involves a quantitative conversion of the
isolated macrocycle into a phlorin-like skeleton followed by
acidification with HCl (Scheme 3, paths c and d), thereby
resulting in the final formation of 11-HCl in 20% overall
yield. 11-HCl has aromatic character, as proven by the
features in the 1H NMR spectrum. The rather large and
planar part of the final macrocycle—a thiophene and two
pyrrole moieties—results in the high degree of aggregation
observed in the NMR spectra. A nicely resolved spectrum was
obtained after the addition of 1% TFA (Figure 2A), which
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a chloride by using the same procedure as reported for 11-
HCl (see the Supporting Information). 13-HCl behaves
similarly to 11-HCl and requires the use of trifluoroacetic
acid to minimize the aggregation. The 1H NMR spectrum
(Figure 2B) reveals the diminished macrocyclic aromaticity
compared to 11-HCl. The a-H thiophene resonance is
observed at d = 9.32 ppm, whereas the pyrrole and furan
resonances are located at d = 8.0–7.8 ppm. The resonance
contributors for 11-H and 13-H are presented in Figure 3. 113
and 133 include the sulfur atom in the aromatic conjugation.
The difference in the aromatic character between 11-H
and 13-H is related to the relative participation of the
principal aromatic contributors, which implies 14p (112) and
18p (113, 133) delocalization pathways. The 6p involvement
(111 and 131) in both compounds is insignificant (Figure 3).
Thus, the 14p electron path is essential in 11-H, but negligible
in 13-H. The 18p route (133) is solely responsible for the
aromatic character of 13-H.
DFT optimization of oxatriphyrin(2.1.1) monocations
showed a planar structure stabilized by an N···HN hydrogen
bond within the internal cavity (Figure 4). The N-N distances
Figure 4. DFT-optimized structures of 11-H (A) and 13-H (B).
Figure 2. 1H NMR spectra. A) 11-HCl (CDCl3 +1% TFA, 300 K), B) 13-
HCl (CDCl3 +1% TFA, 300 K), C) 17 (CDCl3, 300 K), D) 18 (CDCl3,
300 K). The insets show a downfield region of the NH protons (A, B)
and axial phenyl group (C, D).
(2.57 ꢀ 11-H 2.60 ꢀ 13-H) significantly increased in the
neutral forms (2.70 for 11 and 2.75 ꢀ for 13; see Figure S26 in
the Supporting Information). The bond lengths within the
frameworks of 11-H and 13-H are consistent with those
expected for aromatic macrocycles. In particular, the aroma-
ticity of 11-H and 13-H is clearly demonstrated by the
À
equalization of the Ca Cmeso distances (11-H: C(6)-C(7)
1.406 ꢀ, C(7)-C(8) 1.431 ꢀ; 13-H: (C(6)-C(7) 1.404 ꢀ, C(7)-
C(8) 1.432 ꢀ). The macrocyclic aromaticity has a relatively
À
small effect on the thiophene moieties. The Cb Cb bonds are
À
characteristically longer than the Ca Cb bonds, closely
resembling the pattern found in thiophene or tetrathiapor-
phyrinogen. The reverse is true for thiaporphyrinoids,[16]
where the thiophene is built into the macrocyclic ring at two
À
Ca positions. An elongation of the C(1) C(2) bond of 11-H is
evident. The observed aromaticity is consistent with NICS
values calculated for the cation 11-H (d = À11.4 ppm) and the
neutral form 11 (d = À9.4 ppm), and are similar to that of
simple oxatriphyrin(2.1.1) (d = À13.8 ppm). The lower aro-
matic character of 13-H was also confirmed by NICS (d =
À3.7 ppm).
The presence of“porphyrinic” patterns in the UV/Vis
electronic spectra of 11-H and 13-H confirm the aromaticity.
Figure 3. Resonance contributors for 11-H (A) and 13-H (B).
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Scheme 4. Formation of boron(III) complexes. Conditions: a) PhBCl2,
Et3N, toluene, reflux). Ar=p-Tol.
previously observed for a 16p path in porphyrin.[17b] Evidently
the fused thiophene ring of 17 is not involved in 20p
delocalization, and the final complex prefers the 16p anti-
aromatic path proven by the NMR spectra and NICS value of
d =+ 17.9 ppm. A strongly downfield shift of the ortho
protons of the axial phenyl group (d = 9.85 ppm, Figure 2C
(inset), compared to phenylboronic acid, d = 7.8 ppm) proves
a deshielding influence of the paratropic current observed
above (below) the macrocycle plane. Thus, the presence of the
axial phenyl group creates another spectroscopic probe that
can quantify the anti-aromatic character not shown before.
The opposite direction of the observed effect is a complement
to the well-established upfield shift recorded for substituents
located above (below) the plane of an aromatic compound (a
strong shielding effect). The UV/Vis spectrum of 17 confirms
its anti-aromaticity (Figure 5A).[18] In addition, 17 remain
absolutely silent in fluorescence experiments.
In contrast to 17, the paratropicity of 18 (NICS =
+ 1.4 ppm) is less noticeable (Figure 2D), which indicates
that the 20p contributor involving the external sulfur atom is
less essential. Nevertheless, the ortho protons of the axial
phenyl group resonate at d = 8.11 ppm, which is slightly
downfield shifted. 18 does not show any fluorescence over the
whole tested region.
In conclusion, an efficient synthetic approach leading to
thiophene-fused oxatriphyrins(2.1.1) has been developed.
The generality of the method has been proven by the
formation of two feasible isomers. Both macrocycles are
aromatic and extend the aromatic delocalization over the
external sulfur atom of the rigid C2 bridge derived from the
specifically linked-in thiophene ring. Both fused macrocycles
can be reduced, and the reduced forms were entrapped as
stable boron(III) complexes, thus making them easier to
manipulate. A paratropic current (16p) was observed for the
very first time in a triphyrin skeleton.
The thiophene-fused oxatriphyrins(2.1.1) represent
unique and stable heterotriphyrins and are part of the group
of triphyrins(2.1.1)—a promising material for effective
absorbers and emitters through modification of the electronic
structure by the triangular geometry significantly changing
the dipole distribution. On the other hand the observed
ability to switch between aromatic/non-aromatic/anti-aro-
matic structures opens another aspect of exploration for
compounds capable of acting as controlling motifs.
Figure 5. Absorption (CH2Cl2, 298 K) and emission (CH2Cl2, 298 K
dotted, insets) spectra for: A) 11-HCl (solid line) and 17 (dashed line)
and B) 13-HCl (solid line) and 18 (dashed line).
The spectra were measured in a solution of methanol/1%
trifluoroacetic acid to avoid the possible aggregation
observed in dichloromethane solutions (see the Supporting
Information). The Soret-like band (B-band) detected at 411
(11-H) and 409 nm (13-H) is accompanied by a set of four Q-
bands (480–650 nm; Figure 5). The macrocycles show a photo-
luminescence with a maximum at 602 nm (11-HCl, F = 0.038)
and 660 nm (13-HCl, F = 0.012) and a Stokes shift of 30–
40 nm, as observed previously for triphyrins.[4,8]
Cyclic voltammetric studies (see Figures S24 and S25 in
the Supporting Information) demonstrate that 11-HCl and
13-HCl undergo two consecutive, semireversible one-electron
reductions with half-wave potentials for 11-H of (1) À718 mV
and (2) À1186 mV and for 13-H (1) À698 mV and
(2) À1128 mV (versus Fc/Fc+ in CH2Cl2). Reduction with
a zinc amalgam (Zn/Hg) has been used to test the possibility
of transforming diatropic 11-H and 13-H into paratropic
forms. The two samples were prepared in an inert atmosphere
in deuterated chloroform and treated with an excess of zinc
amalgam to give two-electron reduced compounds 12 and 14,
respectively, with a changed aromatic character (see Fig-
ures S23 and S27 in the Supporting Information). The NICS
values (d =+ 14.9 ppm and d =+ 4.9 ppm, respectively)
obtained for the two macrocycles confirmed the formation
of anti-aromatic circuits.
The reaction of 11-H or 13-H with an excess of
phenylboron(III) dichloride gave complexes 17 or 18, where
both triphyrins act as dianionic ligands (Scheme 4). Reduc-
tion of both ligands during the insertion (Et3N used as
a reducing agent) significantly modifies the observed proper-
ties. All the b-resonances of 17 are located in the d = 4.5–
5.1 ppm range (Figure 2C), noticeably upfield compared to
those of 11-H, but also to those of relevant porphodime-
thenes[14] resembling weakly anti-aromatic 22-oxybenzipor-
phyrin[15] and [16]thiaethyneporphyrin,[16] and in the range
Received: November 21, 2013
Published online: February 7, 2014
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Keywords: anti-aromaticity · aromaticity · subporphyrin ·
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