The Curious Case of a Parasitic Twin of the Corroles

Article abstract of DOI:10.1002/anie.201801555

An expanded porphyrinoid has been obtained by a simple ring expansion from a contracted porphyrinoid, namely corrole. Spectroscopic, structural, and computational investigations reveal peculiar π-conjugation and geometry. The effect of extended π-conjugat

Full text of DOI:10.1002/anie.201801555

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Accepted Article
Title: A Curious Case of Parasitic Twin of Corroles
Authors: Biju Basumatary, R V Ramana Reddy, Rahul Rahul, and
Jeyaraman Sankar
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10.1002/anie.201801555
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The Curious Case of a Parasitic Twin of the Corroles
Biju Basumatary,[a] R. V. Ramana Reddy,[a] Rahul,[a] and Jeyaraman Sankar[a]*
smaller porphyrinoid.[10] To this effect, herein we report the
synthesis and electronic structure of hexaphyrins (4 and 4-Rh,
Abstract: A new expanded porphyrinoid has been obtained by a
simple ring expansion from a contracted porphyrinoid, i.e.,
Figure 1) obtained by -expansion of
a ring contracted
corrole.
Spectroscopic,
structural
and
computational
porphyrinoid. Alternatively, this macrocycle can also be viewed
investigations reveal peculiar -conjugation and geometry. The
effect of extended -conjugation is evident from perturbed redox
behavior and photophysical properties. Due to strong diatropic
ring current of the corrole and cross-conjugation, the molecule
exhibits non aromatic nature for the expanded -circuit as
evident from NMR studies.
as a parasitic twin of a corrole[12] and an isomer of corrorin (V).[13]
Expanded porphyrinoids[1] have been gaining remarkable
attention in light of their versatile attributes such as diverse -
conjugation pathways due to flexible structures,[2] near infrared
(NIR) absorption/emission,[3] facile interconversion between
multiple redox-states,[4] and multi-metal coordination cavities for
various divalent and trivalent metal ions.[5] For example, the
regular
hexaphyrin(1.1.1.1.1.1)
exhibits
UV-Vis-NIR
absorption/emission[1,3] and capable of switching between a
Hückel aromatic and antiaromatic (or Möbius aromatic)
species.[6] From the perspective of coordination chemistry, the
hexaphyrin(1.1.1.1.1.1) displays two unique ‘NNCC’ coordination
cavities to afford bimetallic complex with various conformations
owing to their inherent topological flexibility.[4a,5a,7] Similarly, the
singly and doubly N-confused hexaphyrins (I and II, Figure 1)
are also very attractive candidates as NIR absorbing dyes,
interconversion between multiple redox-states and multi-metal
coordination ligands with unique cavities.[8]
Figure 1: Structure of singly and doubly N-confused hexaphyrin (I and II),
internally-aromatic-straped hexaphyrin (III), trans-vinylene-bridge hexaphyrin
(IV), corrole isomer (V) and internally C-linked doubly N-confused hexaphyrins
(4 and 4-Rh). The N-confused pyrroles are represented in blue colour.
The target molecule was synthesized by acid catalysed
condensation reaction of (TPFC)Ga(III)(Py)-2,17-biscarbinol (2)
via two routes (Scheme 1, see details in Supporting Information).
The Vilsmeier-Haack formylation of (TPFC)Ga(III)(Py) (1) gave
2,17-bisformyl(TPFC)Ga(III)(Py)2 (1b) in good yield.[12b] 1b on
reaction with phenylmagnesiumbromide in tetrahydrofuran
yielded 2. Succesively, the reaction of 2 with excess of pyrrole in
presence of trifluoroacetic acid (TFA) afforded the compound 3.
Notably, due to the decomposition of 2 and 3 during column
chromatography, these compounds were directly utilized without
purification in the subsequent reactions. To achieve the target 4,
On the other hand, the internally bridged expanded
porphyrinoids such as hexaphyrin,[9] octaphyrin[11] and
decaphyrin[10a] with various linkers are electronically very
different owing to their multiple conjugated -electronic pathways.
As a rare example, Osuka et. al., reported internally 5,20-
aromatic-bridged hexaphyrin (III), which exhibits dual -
conjugation consisting of [18]porphyrin and/or [26]hexaphyrin
pathway.[9a] A significant contribution of [18]porphyrin was
observed in case of 2,5-pyrrylene and 2,5-thienylene bridged
hexaphyrins due to the smaller dihedral angle with the
tripyrromethene fragment. Similarly, the electronic structure of
trans-vinylene-bridged hexaphyrin (IV) can also be considered
as a [16]Annuleno[16]Annulene fused conjugated -electronic
system, nevertheless the resonance contribution is minor.[9b]
In this direction, it will be interesting to see the effect of
lateral -extension of an existing -electronic pathway of a
3
was reacted with 2,3,4,5,6-pentafluorobenzaldehyde in
dichloromethane (CH2Cl2) followed by oxidation with 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Repeated
purification by silica gel and size exclusion gel permeation
column chromatography gave 4 in 5% yield as the only isolable
product. High resolution mass spectrometry-electron-spray
ionization (HRMS-ESI) showed parent ion peaks at m/z =
1426.1284 (calculated for C71H26F20GaN7: 1426.1259 [M+H]+)
and 1347.0834 (calculated for C66H21F20GaN6: 1347.0837 [M+H-
Py]+).
Alternatively, an improvement in the yield was observed
upon
direct
condensation
of
2
with
2,3,4,5,6-
pentafluorophenyldipyrromethane in presence of TFA. We could
achieve a yield of 18% with 1:1 molar ratio of 2 and 2,3,4,5,6-
pentafluorophenyldipyrromethane under dilute condition (0.05
mM) and TFA (0.3 equiv.) as catalyst for 3 hours. In fact, the
similar product conversion was also observed with
borontrifluoride diethyletherate (BF3.OEt2).
[a]
Mr. Biju Basumatary, Dr. R. V. Ramana Reddy, Mr. Rahul, Prof. Dr.
Jeyaraman Sankar*
Department of Chemistry
Indian Institute of Science Education and Research Bhopal, Bhopal
Bypass Road, Bhopal, M.P, India - 462066
E-mail: sankar@iiserb.ac.in
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Figure 2. Partial 1H NMR spectra of (a) 4 and (b) 4-Rh in CDCl3 at 25C.
Scheme 1: Synthesis of 4 and 4-Rh: (a) GaCl3 (50 equiv.), pyridine, 100C, Ar,
2 hrs, 82%; (b) POCl3/DMF (100 equiv.), 0C, Ar, 2 hrs, K2CO3 (aq), 12 hrs,
70 %; (c) PhMgBr (10 equiv.), THF, rt, Ar, 2 hrs, quantitative yield; (d) pyrrole
Residual
solvent
peaks
are
marked
with
asterisk
().
(100 equiv.), TFA (0.3 equiv.), CH2Cl2, rt, Ar,
2 hrs; (e) 2,3,4,5,6-
pentafluorobenzaldehyde (1 equiv.), TFA (0.3 equiv.), CH2Cl2, 0.05 mM, rt, Ar,
3 hrs, pyridine, DDQ (1.5 equiv.), 1 hour, 5%; (f) TFA (0.3 equiv.), CH2Cl2,
0.05 mM, rt, Ar, 3 hrs, pyridine, DDQ (1.5 equiv.), 1 h, 18%; (g) Rh(CO)2Cl2
(5 equiv.), CH3COONa (10 equiv.), CH2Cl2, Ar, 1 h, 50C, 90%.
However, the TFA-catalysed reaction was found to be consistent
in the reproducibility of the reaction. The dipyrromethene
fragment of corrorin isomer unit in 4 can offer a monoanionic
coordination environment. Thus, 4 was treated with reagents
such as BF3.OEt2/Et3N[14a] and POCl3/pyridine.[14b] However,
these attempts did not yield any metallated complexes. Rh(I)
had been shown earlier to exhibit out of plane coordination
mode along with two carbonyl ligands.[15] Interestingly, the
reaction of 4 with Rh(CO)2Cl2 in DCM at 50C for 2 hours gave
5 in 90% yield (Scheme 1), which decomposes gradually in
solution during crystallization. The HRMS-ESI revealed
molecular ion peak at m/z
=
1583.0075 (calculated for
Figure 3: Single crystal X-ray structure of 4, (a) top view and (b) side view.
Hydrogen atoms are omitted for clarity. DFT optimized structure of 4-Rh, (d)
top view and (e) side view. Interplanar angle between 24-atom mean-plane of
Ga(III)corrole unit (red) and 11-atom mean-plane of dipyrromethene fragment
(blue) in 4 (c) and 4-Rh (f) respectively.
C73H25F20GaRhO2N7: 1583.0056 [M]+).
1H NMR spectrum in CDCl3 indicated an unsymmetrical
structure for 4, which exhibits two singlets (훿 9.52 and 8.38 ppm,
bipyrrolic β-H) and four doublets (훿 8.81, 8.66, 8.53 and 8.43
ppm; 3JH-H ~4.6 hz, pyrrolic β-H) as assigned to the Ga(III)corrole
unit (Figure 2a). The β-Hs of dipyrromethene fragment of
corrorin unit resonate as four doublets (훿 7.54, 7.20, 6.83 and
support the absence of an aromatic ring current inside the
corrorin isomer unit while maintaining a strong local diatropic
current for the Ga(III)corrole ring.
3
The structure of 4 was confirmed by single crystal X-ray
diffraction analysis obtained from slow evaporation of a DCM
solution containing small amount of pyridine at low
temperature.[17] Unexpectedly, the geometry of hexaphyrin
framework has adopted a non-planar conformation owing to the
out of plane deviation of dipyrromethene fragment of corrorin
isomer unit (Figure 3). This is probably due to the smaller cavity
of corrorin isomer core to accommodate two inner hydrogens
(i.e., NH and CH each), and steric hinderance amongst the
meso-aryl substituents at 15C and 20C having centroid to
centroid distance of roughly 3.60 Å. Notably, this spatial
proximity was clearly evident in solution as inferred from the
broad aryl 1H and 19F resonances.
6.45 ppm; JH-H ~5.2 hz). The downfield resonance of β-Hs
reflected a diatropic ring current for Ga(III)corrole unit, which is
similar to that observed for [18] -electronic corrole or
porphyrin.[16] On the other hand, the corrorin unit resembled a
non-aromatic core (vide infra). A broad peak at 훿 11.86 ppm was
assigned to internal NH proton and was confirmed by deuterium
exchange experiment (Figure S14, see Supporting Information).
The relatively larger downfield resonance of NH proton can be
attributed to the presence of intramolecular hydrogen bonding.
In addition, the NH proton can easily be deprotonated with
tetrabutylammonium fluoride (TBAF) as observed by 1H NMR
titration experiment (Figure SI15). The compound 4-Rh exhibits
similar 1H NMR spectrum as compound 4 except for the
noticeable upfield resonance for inner β-H of corrorin isomer unit
(Figure 1b). The β-H of 4 and 4-Rh were correlated and
assigned on the basis of 1H-1H COSY and 1H-1H NOESY
spectra (Figure S10 and Figure S13). The NMR investigations
The electronic structure of 4 and 4-Rh were investigated
by density functional theory (DFT) calculation with Gaussian
09/D.01 at the B3LYP/6-31G* (C, H, N, F) + LANL2DZ (Ga/Rh)
level.[18] Despite the reduced symmetry for
4
and 4-Rh,
computations suggested the presence of near-degenerate pairs
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spectral window (λmax = 600 nm;  = 0.4x 105 M-1cm-1), which is
akin to those characteristic of Ga(III)corrole[12] but indicated
slightly extended -conjugation as evident from the red shifted
spectra. Furthermore, it exhibited a sharp emission at λmax ~ 605
nm along with a shoulder peak (λ ~ 660 nm). In fact, the
emission is corrole centered, which further supported the intact
aromatic character of Ga(III)corrole unit. The absolute quantum
yield of 4 was measured to be 1.3%. The electronic absorption
spectra of 4-Rh in DCM exhibited similar pattern as 4 but slightly
red shifted. The emission in 4-Rh was found to be quenched
completely due the internal heavy atom effect.
of HOMOs and LUMOs as commonly observed in the case of
symmetrical porphyrin (D4h) or corrole (C2v) macrocycles (Figure
S21).[19]
Figure 4: (a) Possible resonance ([18] -electron and [26] -electron) of 4 (4-
Rh) and conjugation in ref. Dipyrromethene. The conjugation pathways are
presented in bold black line and cross-conjugation is presented in red line. (b)
NICS(0) values at various point of 4 and ref. dipyrromethene calculated at
B3LYP/6-31G* (C, H, N, F) + LANL2DZ (Ga/Rh) level. (c) HOMA values of 4
and 4-Rh. aout of plane calculated NICS value owing to the non-planar
conformation.
Figure 5: (a) UV-Vis-NIR absorption spectra of 4 and 4-Rh in CH2Cl2, and
emission spectra of 4 in CH2Cl2. Excitation at max = 433 nm
Furthermore, in order to probe the aromaticity for 4 and 4-
Rh, nucleus-independent chemical shift (NICS),[20] anisotropy-of
the induced current density (ACID)[21] and harmonic oscillator
model for aromaticity (HOMA)[20] were calculated (Figure 4).
Inside Ga(III)corrole unit of 4, the calculated NICS(0) values
were all large and negative (-17.8, -15.1, -15.8 and -15.2)
indicating the intact aromatic character of Ga(III)corrole core.
The corrorin isomer core exhibited moderately negative NICS
values in all calculated regions (-3.9, -7.2, -2.3 and -7.4), which
is only slightly larger than the values obtained for ref.
dipyrromethene fragment (Figure 4b). The AICD plot showed a
strong clockwise current flow in Ga(III)corrole unit and absence
of any recognizable directional current flow on the
dipyrromethene fragment of the corrorin isomer unit (Figure S21).
This suggests a non-aromatic [26] -electronic hexaphyrin
contribution and a strongly localized aromatic character of [18] -
electronic Ga(III)corrole core. In addition, the [18]Ga(III)corrole
unit and [26]hexaphyrin -circuit exhibit HOMA value of 0.589
and 0.506 respectively in 4 (Figure 4c).[17] The computational
results further substantiated with the observed downfield signal
for inner -H in 4 ( = 9.52 ppm) and 4-Rh ( = 8.23 ppm).
Probably, the non-aromatic character of [26] -electronic
hexaphyrin is due to the cross-conjugation[22] at the bipyrrole unit
(Figure 4a) and non-planarity of the dipyrromethene fragment.
Notably, the inner -H for 4-Rh appeared more shielded roughly
by 1.25 ppm as compared to 4. This is most likely due to the
improved co-planarity of dipyrromethene fragment and
Ga(III)corrole core in 4-Rh having interplanar angle of roughly
12.7 than that of 4 with interplanar angle of roughly 28.9
(Figure 3c and Figure 3f). This induces an upfield resonance for
inner -H due to the slight enhancement of conjugation of [26] -
electronic circuit in 4-Rh. Nevertheless, we believe a non-
aromatic [26] -electronic hexaphyrin and a strongly aromatic
Ga(III)corrole unit.
Figure 6: (a) Molecular structure of chlorinated species [4-Cl] after oxidation
of 4 with Magic Blue. (b) UV-Vis-NIR spectral change of 4 with gradual
addition of tris(4-bromophenyl)ammoniumyl hexachloroantimonate in
acetonitrile. (c) X-band ESR spectra of in-situ generated radical [4-Cl] in
toluene/DCM (1:1, v/v) at 110 K. (d) Spin density of [4-Cl] calculated at
RB3LYP/6-31G*(C, H, N, F, Cl) + LANL2DZ (Ga).
Cyclic voltammetry (CV) and differential pulse voltammetry
(DPV) of 4 (Figure S23 and Figure S24) in DCM showed three
oxidation processes at the potential (vs SCE) 0.53 V (rev.), 0.82
V (quasi rev.) and 1.01 V (quasi rev.), and two reduction
processes at potential -1.10 V (irrev.) and -1.20 V (irrev.). The
first oxidation potential of 4 is shifted cathodically by about ~300
mV with respect to (TPFC)Ga(III)(Py),[12a] suggesting an
extended -electronic conjugation. Similarly, the 4-Rh (Figure
S23 and Figure S24) showed three oxidation potential waves at
0.43 V (rev), 0.77 V (quasi rev.) and 1.12 V (quasi rev.), and two
reduction potential waves at 1.10 V (irrev) and 1.27 V (irrev).
The 4-Rh has smaller electrochemical HOMO-LUMO gap of
1.53 eV than that of 4 (1.73 eV). The smaller energy gap of 4-Rh
was also evident from the moderately red shifted electronic
absorption spectra. This may stem from the enhanced
The electronic absorption spectrum (Figure 5) of 4 in DCM
showed a Soret-type band at 433 nm ( = 1.3x 105 M-1cm-1) and
a weaker broad Q-type band spanning roughly 500-800 nm
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10.1002/anie.201801555
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coplanarity of the dipyrromethene moiety to the Ga(III)corrole
unit.
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Soc. 2001, 123, 7190; (b) S. Saito, A. Osuka, Angew. Chem. Int. Ed.
2011, 50, 4342.
Furthermore, to verify the electronic nature of the intact -
circuit of Ga(III)corrole, 4 was treated with one electron oxidizing
[4]
[5]
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agent
tris(4-bromophenyl)ammoniumhexachloroantimonate
619.
(Magic Blue, E1/2= 1.1 V Vs SCE) in acetonitrile[12a,23] and
spectral change was monitored by UV-Vis-NIR spectroscopy
(Figure 6b). It exhibited a blue shifted Soret-type band (~400
nm), diminished Q-type band and new broad near-infrared
absorption (~1070 nm). The X-band ESR spectrum of 4 on
reaction with Magic Blue reveals the formation of a radical with
giso = 2.0057 in toluene/DCM (1:1, v/v) at 110 K (Figure 6c). As
expected, the HRMS-ESI analysis showed a molecular ion peak
at m/z = 1381.0441 (Figure S20), which can be attributed to a
chlorinated radical [4-Cl] having empirical formula of
C66H21F20ClN6Ga (1381.0447[M]+). The structure of chlorinated
radical [4-Cl] was assigned on the basis of mass-spectrometry
analysis and supported from the previous report,[23] where a
chlorine ion was found to be coordinated to the Ga(III) ion
(Figure 6a). The singly occupied molecular orbitals (SOMOs)
and spin density distribution plot of compound [4-Cl] showed
that the electron density of unpaired electron is distributed over
entire -system of hexaphyrin (Figure S25 and Figure 6d) with
no contribution from the Ga(III) centre, which is also consistent
with the isotropic ESR signal.
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In summary, a strategically new expanded porphyrinoid
having corrole and a corrorin isomer unit onto a single
framework has been achieved by -expansion of corrole. The
NMR investigations reveal an interesting behavior of diatropic
and cross-conjugated circuit within the same macrocycle.
Computational investigations supported the experimental and
structural observations. The ability of the molecules to metallate
Rh(I) into the corrorin isomer ring system is demonstrated. The
facile oxidation of the molecules has been exploited to observe
the formation of a radical on one electron chemical oxidation.
Due to the presence of a dominating diatropic current for corrole
and non aromatic nature of corrorin, it is named as a parasitic
twin as both corrole and corrorin are isomers. The current work
demonstrates that this simple -expansion strategy via
condensation could be beneficial to access many more such
interesting molecules in the future.
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[17] Crystallographic data for 4, see Supporting Information. Marginal
quality of the X-ray diffraction data did not allow discussing the bond
lengths and bond angles in detail. The HOMA values for 4 and 4-Rh
were calculated for the optimized geometries.
Acknowledgements
J.S. acknowledges DST-SERB-EMR/2016/005768 (New Delhi)
for research funding. B.B. acknowledges CREST-MHRD-FAST
(New Delhi) and UGC for SRF. R.V.R.R. thank IISER, Bhopal
and Rahul thank CSIR for fellowship.
[18] Gaussian 09, Revision D.01, see Supporting Information for full citation.
[19] A. Ghosh, T. Wondimagegn, A. B. J. Parusel, J. Am. Chem. Soc. 2000,
122, 5100.
Keywords: -expansion • corrole • hexaphyrin • corrorin
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10.1002/anie.201801555
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
-expansion of
yields a novel hexapyrrolic expanded
porphyrinoid with aromatic and cross-
a
Ga(III)[18]corrole
Biju Basumatary, R. V. Ramana Reddy,
Rahul, Jeyaraman Sankar*
Page No. Page No.
conjugated 휋-circuit in
a
single
molecular framework.
The Curious Case of a Parasitic Twin
of the Corroles
This article is protected by copyright. All rights reserved.