A 20π-electron heteroporphyrin containing a thienopyrrole unit

Article abstract of DOI:10.1002/anie.201204954

Easy as π: A stable nonaromatic heteroporphyrin bearing a thienopyrrole moiety has been prepared. The nonaromatic nature of this compound is clearly evident based on UV/Vis absorption, ¹H NMR spectroscopy, and theoretical calculations. Single-c

Full text of DOI:10.1002/anie.201204954

Angewandte
Chemie
DOI: 10.1002/anie.201204954
Porphyrinoids
A 20p-Electron Heteroporphyrin Containing a Thienopyrrole Unit**
Yi Chang, Huachao Chen, Zhikuan Zhou, Yue Zhang, Christian Schꢀtt, Rainer Herges, and
Zhen Shen*
Dedicated to Professor Noboru Ono on the occasion of his 70th birthday
Porphyrins have been the focus of intense research interest
owing to their pivotal role in many important biochemical
processes and their suitability for diverse applications in
medicine, material science, and catalysis, based on the
properties associated with their heteroaromatic 18p-electron
conjugation systems.[1] The synthesis of many structural
analogues, such as core-modified porphyrins,[2] N-confused
porphyrins,[3] corroles,[4] porphycenes,[5] contracted porphyr-
ins,[6] and expanded porphyrins,[7] have been investigated in
conjunction with their diverse applications and the wider
study of macroaromaticity.[8] Typically, synthetic work has
focused on increasing or decreasing the number of the
pyrrolic (or other heteocyclic) subunits within the macrocyclic
ring and on varying the relative connectivity between the
pyrrolic constituents or adjusting the number of bridging
meso-carbon atoms. The synthesis of functionalized pyrrole
subunits is challenging, and there has been comparatively
little research on core-modified porphyrins, in which pyrrolic
nitrogens are replaced by other heteroatoms such as O, S, Se,
and Te.[9] Herein, we report the first example of the synthesis
and characterization of a core-modified porphyrinoid con-
taining a thienopyrrole moiety (1; Scheme 1).
Scheme 1. Heteroporphyrin containing one thienopyrrole unit.
anticipated for tautomers with two inner protonated pyrroles,
the p system is nearly planar. Highly distorted structures have
recently been reported for (4n)p-electron porphyrinoids such
as oxidized 16p-electron and reduced 20p-electron (isophlor-
ins) porphyrins, which have inner perimeters consisting of 16
rather than 17 atoms.[10] The p system can be readily trans-
formed into a transient radical species, through chemical and
electrochemical methods.
The incorporation of the fused heteropyrrole ring of
thienopyrrole creates a very stable nonaromatic macrocycle
with a 27 atom 30p-electron system. The optical and redox
properties differ markedly from those of conventional 18p-
The thienopyrrole-containing porphyrin 1 was prepared
from thienopyrrole dialdehyde 2 and tripyrrane diacid 3b[12]
by using the acid-catalyzed “3 + 1” MacDonald method[11]
(40% yield, the experimental details are provided in the
Supporting Information). Compound 2 was prepared from 2-
formylthiophene and ethylazidacetate. The initial reaction
provided ethyl 2-azido-3-(thiophen-2-yl)acrylate, which upon
cyclization gave ethyl 4H-thieno[3,2-b]pyrrole-5-carboxylate
electron
heteroaromatic
tetrapyrrolic
porphyrinoids.
Although a 17 atom 20p-electron inner perimeter would be
4
[13] and then subsequent formylation provided 2 (Scheme 2).
The structure of 1 was unambiguously elucidated by
[*] Y. Chang, H. Chen, Z. Zhou, Y. Zhang, Prof. Z. Shen
State Key Laboratory of Coordination Chemistry
Nanjing National Laboratory of Microstructures
School of Chemistry and Chemical Engineering
Nanjing University, Nanjing, 210093 (China)
E-mail: zshen@nju.edu.cn
single-crystal X-ray diffraction analysis (Figure 1).[14] Suitable
crystals were obtained by slow diffusion of hexane into
a dichloromethane solution. The macrocycle adopts a near
planar conformation, which differs markedly from the
distorted structures that have been reported for porphyrins
with (4n)p-electron systems.[10] Planar conformations are
normally expected to lead to a destabilization of the p system
for porphyrins with (4n)p-electron systems. The average
displacement of the b-carbon atoms from the least-square
plane of the meso-carbon atoms was 0.093 ꢀ, which is in
marked contrast with the 1.36 ꢀ value for [20]isophlorin.[13a] It
is also noteworthy that the thienopyrrole ring is tilted out of
the least-square plane by only 2.6958, whereas the other three
pyrrole rings are tilted by 3.0128, 3.0928, and 7.0408. In
a similar manner to the [20]isophlorins, however, clear
C. Schꢀtt, Prof. R. Herges
Otto-Diels-Institute for Organic Chemistry
University of Kiel, 24118 Kiel (Germany)
[**] We gratefully acknowledge the financial support provided by the
Major State Basic Research Development Program of China (Grant
Nos. 2013CB922101 & 2011CB808704) and the National Natural
Science Foundation of China (Nos. 20971066 and 21021062). We
thank Dr. Atsuya Muranaka from RIKEN-ASI (Japan) and Dr. John
Mack of Rhodes University, South Africa for help with the molecular
orbital calculations, and Prof. H. Yamada, Dr. Z.-L. Xue, and Dr. D.
Kuzuhara from the Nara Institute of Science and Technology (Japan)
for help in structure characterization.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201204954.
ꢀ
alternation is observed in the C C bond lengths around the
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circuit available so the molecule can be viewed as being
nonaromatic.
The NMR signals of protons attached to porphyrinoid
rings provide a precise indication of the aromatic character of
the molecule, and are of particular value when comparisons
can be made between structurally related systems.[15] There-
fore, the 1H NMR spectra of 1 and an aromatic 18p-electron
thiaporphyrin analogue (5) were measured in CDCl3
(Figure 2), so that a comparison could be made. The
peripheral proton signals of heteroaromatic porphyrinoids
Scheme 2. Synthesis of heteroporphyrin 1.
Figure 2. The 1H NMR spectra of a) N3S-thiaporphyrin (5) and b) 1 in
CDCl3.
experience marked downfield shifts as a result of the shielding
effect of the macrocyclic ring current, whereas the inner
protons are deshielded, and are often found at negative d
values. In the spectrum of the meso-unsubstituted thiapor-
phyrin (5) the meso-C–H signals are located at 10.14 and
10.13 ppm. In contrast, the signals of the four meso-protons of
1 appeared at higher field as four sharp singlets (Figure 2b,
signals b–e) at 6.57, 6.20, 4.97 and 4.82 ppm, thus reflecting
not only the lower symmetry, but also the nonaromaticity of
this macrocycle. The singlet at 7.16 ppm (signal a) can be
assigned to the thiophene ring proton, which lies significantly
upfield relative to the analogous signal (10.72 ppm) in the
[18]thiaporphyrin (5) spectrum. It is noteworthy that three
Figure 1. The X-ray crystal structure of 1: a) Top view, solvent mole-
cules have been omitted for clarity; b) side view, the solvent
molecules, hydrogen atoms, and peripheral groups are omitted for
clarity. c) Displacement (in 0.01 ꢁ units) of the core atoms from the
least-square plane of the meso-carbon atoms (black values) and torsion
angles between heterocyclic ring and the mean plane (values in
italics). d) Bond lengths (in ꢁ units) in the core.
periphery of the macrocycle. Half of the Cmeso–Ca bonds are
elongated, with an average bond distance of 1.427 ꢀ, whereas
the others have an average value of 1.363 ꢀ. Compound
1 therefore has a localized bond structure with discrete single
and double bonds, in marked contrast with the p-electron
delocalization on the inner perimeter of heteroaromatic
porphyrinoid p systems. B3LYP geometry optimizations car-
ried out on an unsubstituted model structure with 6-31G*
basis set (see the Supporting Information, Table S1) predict
that the X-ray structure corresponds to the most stable of six
possible tautomers for the p system. When the nitrogen on
the outer perimeter is protonated there is no conjugation
ꢀ
separate signals corresponding to nonequivalent N H pro-
tons have been confirmed by deuteroxide exchange experi-
ments to lie in the range of 9–17 ppm (see the Supporting
Information, Figure S12). These signals represent a significant
downfield shift relative to the negative d value for the signal
ꢀ
corresponding to the N H of [18]thiaporphyrin. In contrast to
ꢀ
the signals of the peripheral protons, the signals for N H have
been observed to exchange rapidly between measurements
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ꢀ
made at room temperature. This phenomenon can be
possible tautomers that has two inner N H protons contains
counterclockwise currents (Figure 3b and the Supporting
Information Figure S17); this result suggests that a stronger
antiaromatic ring current is generated in the tautomers that
have a 20p-electron conjugation system. The nucleus-inde-
pendent chemical shift (NICS) method[18] was also used to
evaluate the aromatic properties of 1. A relatively small
positive NICS(1) value was calculated for the most stable
tautomer of 1 (see the Supporting Information, Table S2),
compared with the large negative NICS(1) value for an 18p-
electron porphyrin. Larger positive values are calculated for
the tautomers having two inner NH protons.
ꢀ
interpreted as being due to N H tautomerism (Scheme 3)
and as evidence that other more-antiaromatic tautomers
coexist in solution.[16] Tautomers with two inner NH protons
Molecular orbital calculations have been carried out for
1 (Figure 4a). Notably, the nodal patterns of the HOMO and
LUMO of 1 are similar to what would be anticipated for the
doubly degenerate LUMO of a 20p-electron porphyrinoid, as
Scheme 3. The three most stable prototropic tautomers of 1 based on
B3LYP geometry optimizations with 6-31G(d) basis sets for an
unsubstituted model compound.
arranged in cis and trans alignments are predicted to be 3–
6 kcalmolꢀ1 less stable than the tautomer that has a proto-
nation pattern corresponding to that of the X-ray structure
(see the Supporting Information, Table S1).
The anisotropy of the induced current density (ACID) of
1 was calculated (Figure 3). ACID calculations have been
used in the analysis of various planar and nonplanar
porphyrinoids (in closed-shell forms),[17] because they
reveal, through numerous point vectors, the induced ring
current density expected when an external magnetic field is
applied. The conjugation pathways can be estimated by
adjusting the isosurface values. The ACID isosurface plot for
the most stable tautomer of 1 predicts relatively poor electron
conjugation, as there is a disrupted pattern within the
electronic current. This finding is consistent with a weak
ring current and a poorly delocalized topology, as would be
expected for a nonaromatic p system. The current density
vectors mapped onto the ACID isosurface plot exhibit
a paramagnetic (counterclockwise) current around the mac-
rocycle and diamagnetic ring currents within the pyrrole and
thienopyrrole rings. In addition, the critical isosurface value
(CIV) is very low at several bonds and high within the
heterocyclic ring. Interestingly, the ACID plot of one of the
Figure 4. a) Frontier p-MO energies and nodal patterns for 1. b) The
observed electronic absorption spectrum of 1 and the calculated TD-
DFT (black bars) oscillator strengths (right axis) at the B3LYP/6-31G*
level of theory.
the number of angular nodes is the same. Michl and co-
workers have demonstrated that this is the pattern that would
normally be anticipated for what they termed an antiaromatic
(4n)p-electron system.[19] This (4n)p-electron system lacks
biradical character as a result of a large splitting between
the HOMO and the LUMO. MO calculations carried out for
the six tautomers of an unsubstituted model compound (see
the Supporting Information, Figure S15) clearly demonstrate
that the angular nodal patterns of the HOMO and LUMO are
retained in each case. The UV/Vis absorption spectrum of
1 contains two intense bands at 356 nm and 455 nm (Fig-
ure 4b). The optical properties of 1 can be readily interpreted
based on Michlꢁs 4n perimeter model.[19] As the HOMO and
LUMO of 1 have an equal number of angular nodes, the
HOMO!LUMO transition is predicted to be electric-dipole
forbidden. The weak absorption bands observed between
700–1000 nm can be assigned to the HOMO!LUMO
transition, which is referred to as the S transition in Michlꢁs
4n perimeter model.[19] Thus, the spectrum is similar to those
reported recently for nonaromatic porphyrinoids with (4n)p-
electron systems.[10,20]
Spectroelectrochemistry has been used to explore the
redox properties of 1. During reduction at a controlled
potential of ꢀ1.35 V (the first reduction peak in CV is shown
in the Supporting Information, Figure S10) in a thin-layer cell,
the intensity of the two main peaks at 356 and 455 nm
decreases, while a new band appears at 400 nm together with
Figure 3. ACID plots of a) 1 and b) a tautomer with two inner NH
protons at an isosurface value of 0.03. The external magnetic field is
applied orthogonal to the macrocycle plane with its vector pointing
towards the viewer. Current density vectors (lines and conics) are
plotted onto the ACID isosurface. The critical isosurface values (CIV)
for compound 1 are provided.
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species (a radical anion) is being formed. When 1 was treated
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solution containing 0.1m of TBAPF6. b) The absorption spectrum of
1 (solid line) and of 1 in the presence of NaBH4 (dotted line).
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green. The absorption spectrum changed to show an intense
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the long-wavelength region between 500–800 nm (Figure 5b),
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active with a g factor of 2.002 (see the Supporting Informa-
1
tion, Figure S13).[21,22] The H NMR signals corresponding to
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Supporting Information, Figure S14), as would be expected
upon the generation of a radical species. The MALDI-TOF
mass spectrum is consistent with the proposed structure (see
the Supporting Information, Figure S15).
In summary, a nonaromatic heteroporphyrin bearing one
thienopyrrole moiety has been successfully synthesized and
characterized for the first time. The stable nonaromatic
system can be transformed into a radical species by a chemical
or spectroelectrochemical reduction. We believe that the use
of thienopyrrole as a building block offers a new perspective
for the design of porphyrins with novel photophysical proper-
ties. Efforts to introduce additional thienopyrrole moieties
into the porphyrin macrocycle are currently underway.
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Received: June 25, 2012
Revised: August 1, 2012
Published online: November 9, 2012
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Keywords: ACID plot · nonaromatic compounds ·
.
porphyrinoids · radicals · thienopyrrole
[14] Crystallographic data for 1: C71H78Cl2N8 S2, Mr = 1242.47, crystal
3
ꢀ
dimensions: 0.200 > 0.040 > 0.010 mm , triclinic P1 (# 2), a =
8.5350(5), b = 13.3574(9), c = 14.685(1) ꢀ, V= 1567.0(2) ꢀ3,
Z = 1, Dcalcd = 1.317 gcmꢀ3, T= 100 K, 12901 measured reflec-
tions, 5735 reflections with[I>2s(I)]. R = 0.0723, RW= 0.2109,
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