Angewandte
Chemie
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
Angew. Chem. Int. Ed. 2014, 53, 2992 –2996
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim