Aromatic and Antiaromatic Pathways in Triphyrin(2.1.1) Annelated with Benzo[b]heterocycles

Article abstract of DOI:10.1002/chem.201903863

Understanding of the aromatic properties and magnetically induced current densities of highly conjugated chromophores is important when designing molecules with strongly delocalized electronic structure. Linear extension of the triphyrin(2.1.1) skeleton with an annelated benzo[b]heterocycle fragment modifies the aromatic character by extending the electron delocalization pathway. Two-electron reduction leads to an antiaromatic triphyrin(2.1.1) ring and an aromatic benzo[b]heterocycle subunit. Current-density calculations provide detailed information about the observed pathways and their strengths.

Full text of DOI:10.1002/chem.201903863

A Journal of
Accepted Article
Title: Aromatic and Antiaromatic Pathways in Triphyrin(2.1.1)
Annelated with Benzo[b]heterocycles
Authors: Miłosz Pawlicki, Krzysztof Bartkowski, Maria Dimitrova, Piotr
Chmielewski, and Dage Sundholm
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To be cited as: Chem. Eur. J. 10.1002/chem.201903863
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COMMUNICATION
Aromatic and Antiaromatic Pathways in Triphyrin(2.1.1)
Annelated with Benzo[b]heterocycles
[
a]
[b]
[a]
[b]
Krzysztof Bartkowski, Maria Dimitrova, Piotr J. Chmielewski, Dage Sundholm and Miłosz
Pawlicki*[a]
Abstract: Understanding of the aromatic properties and magnetically
induced current densities of highly conjugated chromophores is
important when designing molecules with strongly delocalised
electronic structure. Linear extension of the triphyrin(2.1.1) skeleton
with an annelated benzo[b]heterocycle fragment modifies the
aromatic character by extending the electron delocalisation pathway.
Two-electron reduction leads to an antiaromatic triphyrin(2.1.1) ring
and an aromatic benzo[b]heterocycle subunit. Current-density
calculations provide detailed information about the observed
pathways and their strengths.
optical and magnetic properties as well as an unusual aromatic
character.[2d,2e]
We have synthesized novel molecular structures with
benzo[b]furan
and
benzo[b]thiophene
annelated
to
triphyrin(2.1.1) (2, Scheme 1). The annelated arene ring may
influence the global aromatic properties by incorporating the
external benzoic (C6) ring into the delocalisation pathway. The
synthesized macrocycles can also function as boron(III) ligands
forming antiaromatic complexes as shown in this work.
Extending the -electron system in conjugated molecules is
an important area of research, because the conjugation pathway
exhibits notable influence on the optical and magnetic
properties.[ Linear extensions of the acene  cloud by annelation
of molecular rings lead to different aromatic behaviour of five- and
six-membered rings.[3,4] A side fusion of a five-membered ring to
acene cores results in major differences in the electron
delocalisation with the bridging heteroatom separating the
aromatic and antiaromatic moieties.[5]
1]
Scheme 2. Synthetic approach. Conditions: a) 1. 2,5-bis(3,5-di-t-butyl-
hydroxymethylphenyl)furan, CH
2
Cl
2 3 2
, BF Et O, 2. DDQ, 3. Anion exchange; b)
Zn/Hg (excess), CH(D)Cl , argon; c) air; d) PhBCl
3
2
toluene, Et N, argon, reflux).
3
Green a and b dots present points at which the NICS(1) were calculated.
Triphyrins 2a and 2b were obtained by acid-catalysed
condensation of reagents 3a and 3b, respectively. They were
efficiently synthesized via Suzuki-Miyaura coupling from easily
obtainable and commercially available starting materials (2,3-
dibromobenzo[b]heterocycle and pyrrole-boronic acid),[6] followed
by a thermolytic deprotection.[2d,2e] 2a and 2b were isolated with a
yield of 10% and 12%, respectively. The small coordination core
of the macrocycle leads to a very efficient entrapment of a proton
making it possible to isolate the cationic form. As the counterion
of the condensation product was unknown, we replaced it with a
chloride anion by using a previously reported approach.[
Characterization and further studies were performed on the
chloride derivatives. The X-Ray analysis performed for 2b has
confirmed the postulated pattern (Figure S44).
Scheme 1. The molecular structure of triphyrins(2.1.1). The green box encloses
the benzo[b]heterocycle subunit.
Triphyrins(2.1.1) (Scheme 1) are triangularly shaped,
contracted
porphyrinoids
whose
structural
constraints
[
2]
significantly affect their electronic properties. The C2 bridge (1,
Scheme 1) makes it possible to merge an acene to the triphyrin
macrocycle potentially resulting in molecular structures with novel
2d,2e]
[
[
a]
b]
K. Bartkowski, Prof. P. J. Chmielewski, Dr M. Pawlicki
Department of Chemistry, University of Wrocław
F. Joliot-Curie 14, 50383 Wrocław, POLAND
E-mail: milosz.pawlicki@chem.uni.wroc.pl
www: http://mjplab.org
2a (2b) has a UV-Vis spectrum that is typical for aromatic
porphyrinoids with a Soret-like band at 415 nm (430 nm) and a
set of Q bands that reach 600 nm (620 nm). The bands are red
shifted by 40-50 nm as compared to triphyrins(2.1.1) such as
M. Dimitrova, Prof. D. Sundholm
University of Helsinki, Department of Chemistry, Faculty of Science,
P.O. Box 55, FIN-00014 University of Helsinki, Finland
email: dage.sundholm@helsinki.fi
1b[ and 1c , whose electron delocalisation pathway is limited
to 14  electrons. The red shift suggests that the annelated C6
fragment is part of the electron delocalisation. Triphyrin 2a (2b)
fluoresces at 615 nm (640 nm) with a Stokes shift of ~20 nm. The
2b]
[2f]
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
fluorescence quantum yield is 0.12 (0.01). The S
1
1
lifetime ( ) of
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7
-1
8
-1
7
-1
antiaromatic molecules.[8,9] 5a (5b) is chiral, belonging to the non-
symmetric C point group. The chirality was documented by
1
2
1
.6 ns (0.6 ns) contributes 4.6‧10 s /3.4‧10 s (1.7‧10 ‧s /17‧
8
-1
0 s ) to the radiative/non-radiative (k
r
/knr) rate constants. The
excitation spectrum recorded at 615 nm (640 nm) (Figure S36-
S37) confirms the origin of the observed fluorescence.
measuring the Cotton effect for 5a (5b) enantiomers with identical
UV-Vis and NMR spectra using a chiral column (Figure 1C). The
absolute configurations of the 5a (5b) enantiomers were assigned
by comparing experimental CD spectra with spectra calculated at
the time-dependent density functional theory (TDDFT) level. The
calculated and measured spectra qualitatively agree (see SI).
Figure 1. The absorption and emission spectra of 2a (green) and 2b (blue) (A)
and the absorption spectra of 5a (green) and 5b (blue) (B) measured in CH
at 295 K. Insets in traces A and B present zoomed-in Q region. CD experiments
C) performed for 5a (green) and 5b (blue) (CH Cl , 295 K).
2 2
Cl
(
2
2
Triphyrin 2a (2b) can be reversibly reduced. The electrochemical
experiments (Figure S38-39) yielded two well-resolved reductions
at –664 mV (–668 mV) and –1136 mV (–1123 mV) relatively to
the ferrocene (Fc/Fc+) pair in dichloromethane. No oxidation was
registered for the whole tested region. The redox modifications
significantly change the aromatic character. 2a (2b) is reduced to
Figure 2. The 1H NMR spectra of 2a (A), 2b (B), 5a (C) and 5b (D) measured
in chloroform-d, (600 MHz, 300 K).
4
a (4b) by treating it with an excess of a zinc amalgam in an inert
atmosphere (Scheme 1, path b). The absorption spectrum of 4a
4b) is significantly different from that of 2a (2b). 4a (4b) is air
The aromatic character is analysed based on the measured
1H NMR spectra. The cations appear to be aromatic (Figure
(
sensitive and exposure to ambient conditions quantitatively
converts it to the starting reagents. 4a (4b) can entrap boron(III)
cation yielding 5a (5b) complexes, which was synthesized by a
two-step reaction of 2a (2b) with phenylboron(III) dichloride in
boiling toluene in the presence of triethyl amine as reducing agent
2
A, B), since all resonances assigned to the perimeter of the
macrocycle are significantly downfield shifted (2a δ = 9.31 ppm
4), δ = 8.78 ppm (5), δ =8.95 ppm (9), δ = 8.98 ppm (10),
δ = 8.61 ppm (14), δ = 9.04 ppm (15) and for 2b δ = 9.08 ppm (4),
δ = 8.54 ppm (5), δ = 8.84 ppm (9,10), δ = 8.43 ppm (14),
(
(
Scheme 2, d). The complexes can also be obtained via a
consecutive reduction and insertion process.[2d,e,7] The character
δ = 8.73 ppm (15)) suggesting that the macrocycle sustains a
global diatropic ring current, which is also expected to shift the
1H NMR signal of the inner entrapped hydrogen upfield.[1,14]
of 5a (5b) differs from that of 2a (2b), since the shape of the
absorption spectra of 5a (5b) (Figure 1B) is characteristic for
antiaromatic porphyrinoids with 4n  electrons.[1,2d,e,7] 5a (5b)
have intense Soret-like bands at 370 nm (365 nm) accompanied
bands are very broad red-shifted transitions with small oscillator
strengths. 5a (5b) does not fluoresce, which is typical for
However, the inner hydrogen of 2a (2b) has a significant
1
downfield shift of δ = 9.98 ppm (δ = 11.35 ppm) in the H NMR
1
spectrum as shown in Figure 2. The H NMR spectrum for 4a (4b)
shows upfield relocation of the perimeter lines to the region of
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δ = 5.2-6.2 ppm (Figure S18 and Figure S22) with the two NH
resonances shifted to δ = 19.1 ppm (17.3 ppm) and δ = 17.1 ppm
shifted, suggesting an extension of delocalisation over the
annelated fragment and a substantial contribution from the 22 -
electron resonance structure. Two electron reduction and
formation of 4a (4b) brings the C6 resonances at 7.1-7.0 ppm
(7.5-7.1 ppm) close to the signal positions of 7.5-7.2 ppm (7.7-7.3
ppm) for 3a (3b), which suggests that the delocalisation of the C6
fragment is isolated from the current density pathway of the
macroring, leading to the presence of both aromatic and
antiaromatic character in the same molecule. The annelated C6
ring with 6 electrons is aromatic and the triphyrin ring is
antiaromatic with 16  electrons in the antiaromatic pathway
(Scheme 3). The aromatic and antiaromatic character is even
more prominent in the boron(III) complexes 5a (5b).
(
14.5 ppm) for 4a (4b) suggesting a change of the aromatic
character from aromatic to antiaromatic.[1] Significantly more
upfield shifted signals were recorded for the boron(III) complexes
(
5a δ = 5.29 ppm (4), δ = 4.98 ppm (5), δ = 4.45 ppm (9), δ = 4.39
ppm (10), δ = 5.26 ppm (14), δ = 4.91 ppm (15) and for 5b
δ = 5.69 ppm (4), δ = 5.33 ppm (5), δ = 4.78 ppm (9), δ = 4.71
ppm (10), δ = 5.15 ppm (14), δ = 5.47 ppm (15)), which is typical
for antiaromatic porphyrinoids.[1,7] The chemical shifts of the axial
aryl group attached to the central boron inside the macrocycle are
sensitive probes for assessing the aromatic character of the
macrocycle.[1,10] The significantly downfield shifted resonances of
the axial phenyl group (Figure 2C,D; 5a δ = 10.09 ppm ortho,
δ = 8.08 ppm meta, δ = 7.93 ppm para; 5b δ = 9.66 ppm ortho,
δ = 7.92 ppm meta, δ = 7.78 ppm para) confirm that the
macrocycle is antiaromatic. The antiaromatic character is also
supported by the measured UV-Vis spectra (Figure 1).
Molecular structure optimisations and calculations of the
magnetically induced current densities (MICD),[
16]
nucleus
independent chemical shifts (NICS),[12] the anisotropy of the
induced current density (AICD),[15] were performed to bring
additional light on the observed behaviour. The calculations show
that the positively charged 2a (2b) is almost planar, whereas the
molecular structure of 4a (4b) is significantly distorted from
planarity. The insertion of phenylboron(III) yielding 5a (5b) leads
to a more planar structure than for 4a (4b) as also suggested by
the spectroscopic studies (Figure 3). The calculated NICS(1)
value of –11.80 ppm (–11.60 ppm) in point a of 2a (2b) shows that
it is aromatic. The two-electron reduction yields the antiaromatic
4a (4b) with a NICS value of +13.4 ppm, (+11.87 ppm) in point a.
The boron(III) complex 5a (5b) is more antiaromatic than 4a with
a NICS value of +18.94 ppm (+11.60 ppm) in point a. The NICS(1)
value of +11.60 ppm for 4b is practically the same as for 2b. The
annelated C6 ring of 2a and 4a (2b and 4b) are aromatic with
NICS(1) values of –11.18 ppm and –11.17 ppm (–10.69 ppm and–
11.05 ppm) in point b. The AICD plots in the SI show that the
diatropic current in 2a (2b) splits into two pathways with the
stronger ring current along the 22 electron pathway (Figure S42).
The AICD analysis of the reduced 4a (4b) and 5a (5b) molecules
(Figure S43-44) suggests that there is an antiaromatic 16
electron pathway in the main macrocycle and that the 6 electrons
of the annelated C6 ring sustain a diatropic ring current. The AICD
plots suggest that 4a (4b) and 5a (5b) sustain diatropic and
paratropic ring currents. The paratropic ring current of the 16
delocalisation pathway passes the inner pyrrole nitrogens,
whereas the C=C bridges are not part of the delocalisation circuit
of 4a (4b) and 5a (5b).
Scheme 3. Resonance structures for oxidized (top) and reduced (bottom) forms
of triphyrin(2.1.1) annelated with benzo[b]heterocycles.
The spectroscopic analysis suggests that the electron
delocalisation of the oxidized (aromatic) and reduced
(
antiaromatic) triphyrins cannot be described with a single
aromatic pathway. The aromatic pathway may consist of several
routes (Scheme 3) including the annelated fragment (18/22 for
aromatic and 20/24 for antiaromatic) or solely following the
internal circuit (14+6  or 16+6  respectively). The absorption
and emission spectra suggest a significant contribution to the
aromatic character from an extended delocalisation in the
aromatic structures, and from only the inner pathway in the
The more detailed GIMIC[16] analyses of the MICD confirm
that 2a (2b) sustains net diatropic ring current of 23.4 nA/T (21.8
nA/T) around the macroring, which is comparable to the ring-
current strength of 27 nA/T for the free-base porphyrin.[ The
main part of the global ring current of 2a (2b) passes around the
C6 ring. The strength of the current density flowing via the outer
pathway at the C6 ring is 13.8 nA/T (14.2 nA/T). The C6 ring in 2a
sustains a weak local ring current of 2.1 nA/T, whereas the C6
ring of 2b does not sustain any local ring current. The pathway of
the global ring current of 2b splits into a stronger branch passing
on the outside of the C6 ring, and a weak branch of 1.6 nA/T
passing between the thiophene and C6 rings. The current
strength of the pathway between the benzofuran (benzothiophene)
moiety and the triphyrin(2.1.1) ring is 7.3 nA/T (5.8 nA/T).
17]
1
antiaromatic triphyrins. H NMR chemical shifts are very sensitive
[
1]
probes for assessing the degree of aromaticity. The chemical
3
4
2
3
shifts for the protons at positions 1 , 1 , 2 and 2 (Scheme 2) in
the acyclic 3a (3b) → aromatic 2a (2b) → antiaromatic 4a (4b)
compounds suggest that the annelated C6 ring plays different
roles for the current-density pathways in the aromatic and
antiaromatic triphyrins. With the starting structures 3a (3b) as a
1
reference, significant differences in the H NMR chemical shifts of
the C6 hydrogens are observed. In the aromatic 2a (2b), the C6
resonances at 8.2-9.2 ppm (Figure 2) are strongly downfield
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COMMUNICATION
Figure 3. The optimized molecular structures, the strengths of current pathways, and MICD plot for 2a (A), 4a (B) and 5a (C). The integrated current strengths (in
nA/T) passing selected chemical bonds of 2a (2b), 4a (4b), and 5a (5b) are given in red (blue). Current strengths of the most important current pathways are
reported in Table S1 in the SI. The molecular pictures to the right show the most relevant current pathways of 2a (2b), 4a (4b), and 5a (5b).
The MICD calculations show that 4a (4b) is antiaromatic with the
macroring sustaining a net paratropic ring currents of -20.8 nA/T
forces the global paratropic ring current on the inner side of the
heteroatom. In contrast, at the furan ring of the macroring, the
current density splits via the inner and outer pathway between the
nitrogen atom and the C–C bond with about the same strength.
Benzofuran in 4a and benzothiophene in 4b sustain a weak
diatropic ring current of 4.2 nA/T (4.7 nA/T). A local diatropic ring
current of 6.7 nA/T (5.2 nA/T) is sustained by the C6 ring. For 4a
and 4b, the strength of the ring current passing the outer part of
the C6 ring is 10.3 nA/T, which is close to the net ring-current
strength of 11.8 nA/T for benzene.[20] The neutral boron-
containing complex 5a (5b) is antiaromatic sustaining a strong net
paratropic ring current of -26.5 nA/T (-20.8 nA/T). The ring-current
pathways of 5a (5b) and 4a (4b) are very similar.
(
2
-16.3 nA/T), which is comparable to the ring-current strength of -
5 nA/T for doubly oxidized octaethylporphyrinate zinc(II).[18] The
current density in 4a (4b) is more complex than for 2a (2b) and
consists of diatropic and paratropic contributions with a global
diatropic ring current of about 3.4 nA/T (3.8 nA/T) that flows along
the outer contour of the entire molecule. The observed
antiaromatic character originates from the strong paratropic ring
current flowing mainly along the inner pathway of the macroring.
Figure 3 shows that the paratropic ring current splits in two
branches at the furan ring, which is typical for porphyrinoids.[17,19]
The atomic current at the nitrogen moiety of the pyrrolic rings
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COMMUNICATION
However, the detailed analysis of the current pathways is more
difficult for 5a (5b) than for 4a (4b) due to the interference of the
phenyl substituent of the boron. The current strength of the two
branches at the pyrrolic rings were estimated by rotating the
phenyl group by 90 degrees before integration, since the global
ring-current strength was found to be almost independent of the
orientation of the phenyl substituent. The local diatropic current
density at the nitrogen moiety of the pyrrolic ring forces the
paratropic ring current inwards as in 4a (4b). The net strength of
the ring current passing the outer part of the C6 ring of 5a (5b) is
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bond between the C6 and the triphyrin rings, where it turns back
to the common C–C bond of the C6 ring and the furan (thiophene)
ring. (Figure 3).
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We have shown that molecules consisting of benzo[b]furan and
benzo[b]thiophene annelated to triphyrin(2.1.1) skeletons lead to
two strongly aromatic molecules with about two thirds of the ring
current passing along the 22electron pathway on the outer side
of the annelated C6 motif. The rest of the diatropic ring current
takes the 14electron pathway. The current pathways have been
determined spectroscopically as well as by employing NICS,
AICD and MICD calculations. Quantitative values for the ring-
current strength along possible ring-current pathways have been
obtained by integrating the MICD passing selected bonds. The
aromatic character can be modified by reduction, which leads to
molecules that sustain diatropic ring currents in the benzo[b]furan
and benzo[b]thiophene moieties and paratropic ring currents
around the triphyrin(2.1.1) ring. The paratropic ring current of the
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16-electron inner pathway is slightly stronger for the boron(III)
1
complexes, where the phenylboron unit preserves the
antiaromatic character of the triphyrin ring and the aromaticity of
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the boron(III) complexes results in a rare planar chirality of the
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Keywords: triphyrin • aromaticity • antiaromaticity • chirality •
acenes
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Aromaticity Fusion! Annelation of a
benzo[b]heterocycle subunit to the C2
bridge of triphyrin(2.1.1) significantly
modifies its aromatic character. Two-
electron reduction changes the
aromaticity from global aromatic to
antiaromatic delocalisation in the
triphyrin(2.1.1) ring, whereas the
benzo[b]heterocycle moiety remains
aromatic.
Krzysztof Bartkowski, Maria Dimitrova,
Piotr J. Chmielewski, Dage Sundholm
and Miłosz Pawlicki*
Page No. – Page No.
Aromatic and Antiaromatic Pathways
in Triphyrin(2.1.1) Annelated with
Benzo[b]heterocycles
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