Bipyricorrole: A corrole homologue with a monoanionic core as a fluorescence ZnII sensor

Article abstract of DOI:10.1002/anie.201508829

In the corrole homologue, 6,11,16-triarylbipyricorrole, the bipyrrole unit is replaced by a 2,2′-bipyridine unit. This modification effectively alters the corrole N4 coordination sphere from the trianionic [(NH)₃N] to the monoanionic [N₃NH] state. The newly formed monoanionic core stabilizes Zn^II ions with enhanced emission properties. The enhanced emission was further utilized for metal ion sensing studies and exploited for the selective detection of Zn^II ions. Ion-stabilizing corroles: The nonaromatic meso-triarylbipyricorrole with a monoanionic core was achieved by the combination of aromatic bipyridyl and π-conjugated dipyrromethene units. Upon coordination with a Zn^II ion, the bipyricorrole was found to exhibit chelation-induced emission enhancement (CIEE) characteristics and was further explored for selective detection of Zn^II ions.

Full text of DOI:10.1002/anie.201508829

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Communications
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International Edition: DOI: 10.1002/anie.201508829
German Edition: DOI: 10.1002/ange.201508829
Corrole Chemistry
Bipyricorrole: A Corrole Homologue with a Monoanionic Core as
a Fluorescence ZnII Sensor
B. Adinarayana, Ajesh P. Thomas, Prerna Yadav, Arun Kumar, and A. Srinivasan*
Dedicated to Professor Vadapalli Chandrasekhar
Abstract: In the corrole homologue, 6,11,16-triarylbipyricor-
role, the bipyrrole unit is replaced by a 2,2’-bipyridine unit.
This modification effectively alters the corrole N4 coordination
sphere from the trianionic [(NH)3N] to the monoanionic
[N3NH] state. The newly formed monoanionic core stabilizes
ZnII ions with enhanced emission properties. The enhanced
emission was further utilized for metal ion sensing studies and
exploited for the selective detection of ZnII ions.
N-confused derivative,[3b] norrole,[3c] benzonorrole,[3d] oxacor-
role,[3e,f] diaoxacorrole,[3g] and thiacorrole,[3h] were introduced.
Recently, we reported a meso-aryl biphenylcorrole, where the
bipyrrole moiety is replaced by biphenyl unit and the
trianionic core stabilizes CuIII ions.[4] In a continuation of
our ongoing research, the next target in this series is to
introduce a pyridyl/bipyridyl unit in the molecular framework
and explore its properties.
The pyridine-based porphyrinoid, pyriporphyrinone, was
reported by Berlin and Breitmaier, where the carbonyl group
was inserted in the meso-position.[5a] Since then, several
pyridyl derivatives have been reported, including pyripor-
phyrin (2) and its ZnII (2a) and FeIII complexes,[5b] confused
pyriporphyrin[5c–f] and its FeII, FeIII,[5c–e] and PdII complexes,[5f]
oxypyriporphyin[6a–d] and its NiII[6e] and FeIII[6f] complexes, and
a dipyridinoid-substituted porphyrin[6g] derivative. Similarly,
a variety of pyridine-based derivatives have been described
and characterized: (i) expanded porphyrins,[7] such as 12-
hydroxypyrisapphyrin,[7a] dipyrihexaphyrin,[7b–d] tetrapyrioc-
taphyrin,[7e] bipyrioctaphyrin,[7f] and cyclo[m]pyridine-
[n]pyrroles;[7g,h] (ii) phathalocyanine derivatives such as hemi-
porphyrazines;[8] (iii) calixpyrrole derivatives such as meso-
octaethyl calix[4]pyridine;[9] and (iv) cryptand-like pyripor-
phyrinoid macrocycles.[10] Recently, a series of porphyrin-
related macrocycles with N4 coordination spheres were
reported, including: 1,10-phenanthroline-embedded porphyr-
ins (3) as MgII sensors;[11] carbazole and pyridine building
blocks with high-spin CoII complexes;[12a] and cobalt(II)
phenanthroline-indole macrocycles as electrocatalysts for
oxygen reduction.[12b] However, the pyridine-based con-
tracted porphyrinoids are scarcely reported. The subpyripor-
C
orroles (1) are contracted porphyrinoids with 2,2’-bipyr-
role units in the macrocyclic framework, and thus constitute
a bridge between corrin and porphyrin units (Figure 1).[1] The
Figure 1. Structures of porphyrin and contracted homologues.
´
phyrin (4) was synthesized by Latos–Graz˙ynski and co-
workers, who described its reaction with a boron complex.[13]
Recently, the nickel(II) pyricorrole (5) was reported by Neya
and co-workers.[14] Overall, the corrole and its modified
derivatives are in the di- or trianionic form in the neutral
state. The monoanionic form,[3g] however, is scarcely reported.
It is important to note that the basic framework of biologically
important cob(I)alamin is constituted by corrin units, which
are in the monoanionic state and stabilize the CoIII ion.[15]
Herein, we report the synthesis of monoanionic corrole,
6,11,16-triarylbipyricorrole (6), and its ZnII complex (7 and 8).
Introduction of a 2,2’-bipyridyl unit in the macrocycle is
unprecedented in the corrole chemistry and provides a stable
tetra-nitrogen (NNNN) core with monoanionic charge. The
zinc complex is stabilized by the monoanionic core with axial
coordination, and exhibits Chelation-Induced Emission En-
hancement (CIEE)[16] upon metal ion insertion.
trianionic core is known to stabilize higher oxidation state
metal complexes, and are widely applied in catalysis, sensors,
and dye-sensitized solar cells.[2] Structural modification in the
framework alters the electronic structure, thereby leading to
unusual optical, photophysical, and coordination properties.[3]
A series of core-modified corroles, such as iso-carbacorrole,[3a]
[*] B. Adinarayana, Dr. A. P. Thomas, P. Yadav, Dr. A. Kumar,
Dr. A. Srinivasan
School of Chemical Sciences
National Institute of Science Education and Research (NISER)
Bhubaneswar - 751005, Odisha (India)
E-mail: srini@niser.ac.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201508829.
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The synthesis of compound 6 and its coordination
chemistry is described in Scheme 1. The target macrocycle
was achieved in four steps. The first step was the synthesis of
2,2’-bipyridine-6,6’-diylbis(phenylmethanone) (10) from 2,2’-
bipyridine-6,6’-dicarbonitrile (9)[17] by using freshly prepared
phenylmagnesium bromide in THF in 75% yield. Sodium
[H2, H20] ppm, while the pyrrolic b-CH protons were
observed at 7.16 [H8, H14] and 6.87 [H9, H13] ppm. These
signals were further confirmed by 2D homonuclear correla-
tion spectroscopy (Supporting Information, Figure S18). The
meso-phenyl protons appeared at 7.58 ppm. Furthermore, the
singlet peak at 1.62 ppm was assigned to the methyl group
from the axially coordinated acetate ion, suggesting that the
ZnII ion is stabilized by the monoanionic core.
The structures of 6 and 7 were confirmed by single-crystal
X-ray analysis (Figure 2; Supporting Information, Table S5).
Scheme 1. Synthesis of 6 and 7.
borohydride reduction of 10 in THF:CH3OH (7:3) formed
6,6’-bis(phenylhydroxymethyl)-2,2’-bipyridine (11) in quanti-
tative yield in the second step. The key precursor (12) was
synthesized in the third step, where the direct conversion of 11
in the presence of excess pyrrole and BF3·Et2O as an acid
catalyst was not successful. Instead, we adopted a strategy as
[5b,c]
´
similar to that reported by Latos–Graz˙ynski,
where
compound 11 was reacted with methanesulphonyl chloride,
TEA, and DMAP, followed by condensation with 100 equiv
of pyrrole, afforded 12 in 40% yield. The target macrocycle
was achieved in the final step by an acid-catalyzed condensa-
tion reaction of 12 with pentafluorobenzaldehyde in the
presence of trifluoroacetic acid (TFA) in CH2Cl2, followed by
oxidation with 2,3-dichloro-5,6-dicyano-p-benzoquinone
(DDQ). The crude mixture was purified by column chroma-
tographic separation, where a blue color fraction, eluted with
CH2Cl2 and CH3OH (99:1), was identified as 6 in 12% yield.
The coordination chemistry of 6 was further examined by
using Zn(OAc)2 in CH2Cl2/CH3OH mixture, where the green
fraction was eluted from a silica gel column, to afford 7 in
quantitative yield. The ESI-Q-TOF analysis showed the
molecular ion signals of 6 and ZnII complex (7) at m/z
641.1664 [M + 1] and 703.0741 [M-OAc], and confirmed the
exact composition.
Figure 2. Single-crystal X-ray structures of 6 and 7. Top view (a,c) and
side view (b,d). The meso-aryl groups are omitted for clarity in the side
view.
As predicted from the spectral analysis, the nonaromatic
character in 6 is further reflected from the crystal analysis,
where i) the bipyridyl unit and dipyrromethene moieties are
connected by sp2-sp2 single bond character, with the bond
À
À
lengths of 1.455 Š (C5 C6) and 1.472 Š (C16 C17); ii) the
sp2-sp2 double bond character (bond lengths between 1.337
and 1.399 Š), and the average bond angle (119.9968) within
the bipyridyl unit[18] maintains the individual aromatic
character; and iii) the sp2-sp2 single and double bond charac-
ter (bond lengths between 1.336 and 1.472 Š) within the
dipyrromethene moiety support the effective p-delocalization
(Figure S30, Table S1). Thus, the individual aromaticity in the
bipyridyl unit and the p-conjugation within the dipyrrome-
thene moiety remain isolated from the overall macrocyclic
aromatization. Furthermore, the saddling dihedral angles (c1–
c4) are between 3.908 and 7.358 (Table S4), which are far less
than those found in 1 and its derivatives.[19a] The pyrrole units
are hardly deviated from the mean N4 plane, with a maximum
deviation of 5.9138, suggesting that the macrocycle 6 adopts
planar conformation (Figure 2b). The amine hydrogen (N3-
H3) in the pyrrolic unit is engaged in intramolecular hydrogen
bonding with the neighboring imine nitrogens of pyrrole (N2),
as well as the pyridine unit (N4). The bond lengths and angles
of N3-H3··N2 and N3-H3··N4 are 2.23 Š and 122.318, and
2.34 Š and 118.278, respectively (Figure 2a).
The 1H NMR spectrum of 6 and 7 was recorded in CDCl3
at 298 K. The bipyridyl protons in 6 were detected as two
doublets at 8.16 [H4 and H18] and 7.36 [H2, H20] ppm, and
a triplet at 7.87 [H3, H19] ppm. The pyrrolic b-CH protons
appeared as a doublet at 6.91 [H8, H14] and 6.46 [H9,
H13] ppm, respectively. The pyrrolic NH [H24] is observed as
a broad singlet at 11.77 ppm, which was further confirmed by
a CDCl3/D2O exchange experiment. The meso-phenyl pro-
tons appeared at 7.50 ppm. Overall, the peak positions of 6
are consistent with nonaromatic character.[4,11a] On the other
hand, the absence of an inner NH signal, and slightly
deshielded bipyridyl, pyrrolic b-CH protons confirm the
metal ion insertion in 6 to form 7. The bipyridyl protons in 7
were observed at 8.81 [H4, H18], 8.33 [H3, H19], and 8.02
970
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Metal ion insertion and axial coordination is also reflected
and iii) the absence of the Soret band and a reduction in the
in the single crystal X-ray structure of 7 (Figure 2c), where
the ZnII ion is 0.54 Š above the mean N4 plane (Figure 2d).
The geometry around the metal center is square pyramid,
with four nitrogens in the equatorial plane and the fifth
position occupied by an axially coordinated acetate ion. The
molar absorption coefficient of the intense band as compared
to 1,[2a] indicating that 6 is not aromatic. Upon ZnII ion
insertion (7), the color of the solution changes from blue to
green. The intense band and weak Q-bands in 6 are redshifted
by 8 and 40–50 nm, and observed at 373 and 628–682 nm,
respectively, with a moderate increase in the e value. The
results are further compared with 2a,[5b] where the Q-band is
166 nm blue shifted, suggesting that 7 has less p-delocaliza-
tion and maintains nonaromaticity.
The emission spectrum of 6 shows a weaker band at
699 nm with the fluorescence quantum yield (FF) of 0.011.
Upon chelation, the emission color changes from light
brickred to intense red and the band is redshifted by 10 nm
and appears at 709 nm with eight-fold increase in emission
intensity and the quantum yield (FF) is 0.089. It is pertinent to
point out here that there is no emission intensity observed in
the case of ZnII complex of N-substituted derivative 1[20] and
2a.[5b] The Chelation-Induced Emission Enhancement
(CIEE) character shown by 6 upon ZnII metalation may
find potential application in biological imaging.[21]
À
À
bond lengths of Zn N1 and Zn N4 are 2.120 and 2.114 Š,
À
À
which are longer than Zn N2 and Zn N3 with values of 2.016
and 1.997 Š, (Figure S31, Table S2). The bond lengths are
comparable with 2a [Zn-Npyridine: 2.35 Š and Zn-Npyrrole: 2.05–
2.11 Š][5b] and 3a [Zn-Npyridine: 2.16–2.17 Š and Zn-Npyrrole
:
2.03–2.04 Š].[11b] Furthermore, the saddling dihedral angle
values (c1–c4) in 7 are between 2.52688 and 11.8098[19b]
(Table S4); the pyrrole units are slightly tilted from the
mean N4 plane, with a maximum deviation of 12.4688. The
presence of fluorine atoms in the pentafluoro unit generates
intermolecular hydrogen bonding interactions, where one of
À
the meso-phenyl-CHs (C23 H23) interacts with the neigh-
boring molecule fluorine atom (F3) to generate a hexameric
structure in the solid state. The units are arranged in
alternating fashion, where the distance between the adjacent,
alternate, and the opposite ZnII ions are 13.84 Š, 20.25 Š, and
The enhanced emission upon ZnII ion insertion prompted
us to perform sensing studies of 6 to determine whether 6 is
selective towards ZnII ion or any other metal ions. A
preliminary qualitative experiment was performed by using
dilute CH3OH solution of 6 with 10 equiv of various metal
ions, such as AgI, CaII, CdII, CoII, CrIII, CuII, FeIII, HgII, MgII,
3
24.53 Š, respectively, with the cavity size of 1563 Š (Fig-
ure S26).
The electronic absorption and emission spectrum of both
6 and 7 in CH3OH is shown in Figure 3, along with visible and
À
MnII, NiII, and ZnII, in the form of perchlorate (ClO4 ) salts.
Among the tested metal ions, only the ZnII ion exhibited turn-
on emission (Figure 4a). To have a quantitative picture,
various equivalents of ZnII ions were gradually added to the
10 mm CH3OH solution of 6. Upon increasing the concen-
tration of ZnII ions, the intensity of the emission band
increased gradually up to 1 equiv. Further increasing the
Figure 3. The electronic absorption and emission spectrum of 6 and 7
in CH3OH along with absorption (left) and emission (right) color
changes.
emission color changes. The absorption spectrum of 6 shows
an intense band at 365 nm and the weak Q-type bands
between 588 and 632 nm, respectively, with the molar
extinction coefficient of 105. The spectral results of 6 were
compared with biphenylcorrole,[4] 2,[5b] and 1,[2a] and the
results are summarized as follows: i) the molar absorption
coefficient and the spectral pattern are similar to biphenyl-
corrole[4] and 2,[5b] reflecting the nonaromatic character in 6;
ii) the intense band and weak Q-bands in 6 are blue shifted by
46 and 53 nm, as compared to 2,[5b] suggesting a reduction in
the p-delocalization (dipyrromethene vs. tripyrromethene);
Figure 4. Sensing properties of 6. a) The emission spectral changes
upon addition of ZnII in CH3OH solution. b) The Job’s plot for the
complexation. c,d) Single-crystal X-ray structure of 8. Top view (c) and
side view (d). The meso-aryl groups are omitted for clarity in the side
view.
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concentration of ZnII ion failed to produce an appreciable
change in the emission intensity. The Jobꢀs plot showing the
change in the emission spectral data suggests formation of 1:1
binding mode (Figure 4b), and the association constant is
5.47 ” 104 mÀ1 from the Benesi–Hildebrand plot. The compet-
itive recognition studies of the ZnII ion over other metal ions
found that 6 selectively senses ZnII ions, even in the presence
of 100 equiv of other ions (Figures S43–S44), and the
detection limit was found to be 1.5 ppm. The experiments
were also conducted in 0.1m HEPES buffer (pH 7.4) aqueous/
CH3OH solution (CH3OH/H2O; 4:6 v/v), and similar trends
were observed (Figures S39–S40).
´
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In summary, we have successfully synthesized 6,11,16-
triarylbipyricorrole and its ZnII complex. Their nonaromatic
character was demonstrated by spectral studies and structural
analysis. To the best of our knowledge, the monoanionic core
stabilizes the ZnII ion without altering the internal coordina-
tion sphere, which was previously unprecedented in corrole
chemistry. The enhanced emission upon metal ion insertion
was further exploited for sensing studies, where the ligand was
effectively utilized for selective detection of ZnII over other
metal ions by retaining the nonaromaticity. The flexible axial
coordination in 6 provides an advantage for the development
of catalytic and biomimetic models, and efforts exploring this
chemistry is currently the direction of our research group.
Acknowledgements
Dr. A.S. thanks NISER, Department of Atomic Energy for
financial support. B.A. thanks CSIR for the fellowship.
Keywords: bipyridyl units · corroles · monoanionic ·
nonaromaticity · sensing studies
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[16] Chelation Enhanced Fluorescence (CHEF) is the emission
phenomenon commonly used in chelation studies. See: R. D.
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references cited therein. The term AIE (Aggregation Induced
Emission) and AIEE (Aggregation Induced Emission Enhance-
ment) are two different emission phenomena widely used in
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cited therein. Based on this, we have introduced the term
Chelation-Induced Emission Enhancement(CIEE), where the
weak emission property of 6 is further enhanced upon chelation.
Received: September 21, 2015
Revised: November 5, 2015
Published online: December 2, 2015
Angew. Chem. Int. Ed. 2016, 55, 969 –973
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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