Synthesis, spectroscopic and electrochemical studies of phosphoryl and carbomethoxyphenyl substituted corroles, and their anion detection properties

Article abstract of DOI:10.1039/c4dt01853b

The synthesis, electrochemical studies and anion detection properties of triphosphoryl (2) and triester corroles (3) are reported and compared with triphenylcorrole (1). These corroles exhibited typical acid-base binding behaviour in CH₃CN and

Full text of DOI:10.1039/c4dt01853b

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
Synthesis, spectroscopic and electrochemical
studies of phosphoryl and carbomethoxyphenyl
substituted corroles, and their anion detection
properties†
Cite this: Dalton Trans., 2014, 43,
14680
Pinky Yadav and Muniappan Sankar*
The synthesis, electrochemical studies and anion detection properties of triphosphoryl (2) and triester
corroles (3) are reported and compared with triphenylcorrole (1). These corroles exhibited typical acid–
base binding behaviour in CH3CN and were converted to monoprotonated and dianionic species,
respectively. 2 has shown a ∼30 fold lower Keq value for monoprotonation than that of 1 in a TFA–CH3CN
medium. The detection ability of these corroles was also tested in acetonitrile towards various anions.
The observed spectral changes in free-base corroles (1–3) are due to anion-induced deprotonation
rather than the hydrogen bonding interaction between the imino protons of the corrole moiety with
anions. 2 and 3 have shown higher equilibrium constants with F− ions (4.7 × 103 fold higher for 2 and
9.7 × 103 fold higher for 3) as compared to 1 and are able to detect 0.06 μM of F− ions. The Cu(III) and
Ag(III) complexes of 2 and 3 exhibited an anodic shift of ∼250 mV in first ring oxidation and ∼100–150 mV
in metal centred reduction as compared to the Cu(III) and Ag(III) complexes of 1. The anodic shift in the
redox potentials, lower protonation constants and lower detection limit of anions have been explained in
terms of the electron-withdrawing nature of the diethylphosphite and carbomethoxy substituents at the
meso-phenyl positions of the corrole ring.
Received 20th June 2014,
Accepted 4th August 2014
DOI: 10.1039/c4dt01853b
www.rsc.org/dalton
dye sensitized solar cells,22,23 medicinal chemistry24, and non-
linear optical materials (NLO).25–27
Introduction
Johnson and Kay discovered the corrole macrocycle as a by-
product during the synthesis of vitamin B12 in the late 1960s.1
Corroles were widely explored after the successful synthesis of
triaryl corroles by Paolesse et al. and Gross et al. at the same
time.2,3 After that, the synthetic methodology about corroles
has improved significantly.4 The development of corroles has
led to the new version of a fully aromatic corrin ring.5–10 The
direct pyrrole–pyrrole linkage is responsible for their unique
physico-chemical properties. Unlike porphyrins, corroles can
stabilize the higher oxidation state of metal centers.10,11 Fur-
thermore, low valent metal complexes are extremely reactive
when chelated by corroles and the opposite holds true for
their high valent counter parts. The corroles and their metal
complexes have been extensively utilized in the areas of
catalysis,12–18 sensors,19,20 artificial light-harvesting antennas,21
Sulphonic, phosphonic and carboxylic acid substituted por-
phyrins show good efficiency in dye-sensitized solar cells.
Recently, β-substituted carboxy23 or sulphonic acid22 substi-
tuted corroles were used in dye-sensitized solar cells. Phos-
phorylporphyrins have been reported as structural building
blocks for supramolecular assemblies.28–31 Porphyrins substi-
tuted with diethoxyphosphoryl groups at the meso-position of
the ring or the para-position of the meso-phenyl ring show
aggregation tendencies in solution. Corroles exhibit interesting
physical and chemical properties when compared with por-
phyrins because of their lower symmetry (C2V).
Corroles can be deprotonated easily due to their high NH-
acidity as compared to porphyrins, which require vigorous con-
ditions. Corrole forms a stable dianion after abstraction of two
protons by a strong base such as TBAOH, which shows typical
characteristics of aromatic compounds.32 The UV-Visible spec-
trum obtained after monoprotonation of the macrocycle shows
aromatic nature, whereas in concentrated acids, it loses the
aromatic nature since the second proton is attached to the
meso-position of the macrocycle.32–37
Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee – 247667,
India. E-mail: sankafcy@iitr.ac.in
†Electronic supplementary information (ESI) available: Figures of optical
absorption and emission spectra, 1H NMR spectra, MALDI-TOF mass spectra,
UV-Visible spectral titrations for the protonation, deprotonation and anion
binding studies and cyclic voltammograms of free-base corroles. See DOI:
10.1039/c4dt01853b
In recent years, several studies have been carried out in
order to explore different fluorescent materials for the develop-
14680 | Dalton Trans., 2014, 43, 14680–14688
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
ment of chemical sensors. In general, corroles show intense employed was ∼1 mM. All measurements were performed in
absorption and emission in the visible region, good photo- triply distilled CH2Cl2, which was purged with Ar gas using 0.1
stability in various solvents and high NH-acidity. All these M TBAP as the supporting electrolyte. Protonation, deprotona-
characteristics of corroles are useful and quite interesting in tion and anion induced deprotonation of corrole 1–3 were
anion sensor19 and pH sensor39 applications. Anion sensing carried out in CH3CN at room temperature. The equilibrium
plays an important role in environmental science and bio- constants, Keq, were evaluated using a previously reported pro-
chemistry. As an example, the F− ion is an important anion in cedure.44 The equation for the evaluation of the Keq is as
the human body, which prevents dental caries and maintains follows:
solidity of bones. Anion sensing based on porphyrin and
1=Keq ¼ ½baseꢀf½corroleꢀLðεc ꢁ εcaÞ=ðAn ꢁ A0Þ ꢁ 1g
corrole receptors are of current interest.19,38,40–42 Recently,
Cavaleiro and co-workers have studied the anion sensing pro-
perties using the free-base of 5,10,15-tris(pentafluorophenyl)-
corrole (H3TF5PCor) and its derivatives.20 In this paper, we
report the synthesis, electronic spectral, electrochemical redox
properties and anion detection abilities of corroles bearing
diethoxyphosphoryl and carbomethoxy groups at the para-
position of the meso-phenyl ring. To the best of our knowledge,
this is the first report on corroles, which describes the correct
mechanism of anion detection (anion induced deprotonation
rather than H-bonding) and shows the lower detection limit
towards basic anions such as fluoride, acetate and dihydrogen-
phosphate ions.
where εc and εca are the molar extinction coefficients of the
free-base corrole and corrole-anion complex, respectively.
A0 and An are the absorbance values in the absence and in the
presence of added anion at a particular wavelength. L is the
path length of the cell employed. A plot of [anion]2 versus
[anion]2/An − A0 yielded a straight line. Using the above
equation, the Keq can be evaluated by taking the negative of
the 1/intercept. The stoichiometry of binding was analysed by
the Hill method.45 A Hill plot was constructed by plotting ln
[(A0 − An)/(An − Af)] against ln[anion], where A0 and An are the
absorbance values of the corrole employed and the corrole-
anion complex, respectively, at a given concentration of the
anion added. Note that Af denotes the absorbance of the com-
pletely bound corrole-anion complex at a particular wave-
length. The slope of the line was found to be two, which
indicates a 1 : 2 stoichiometry between the corrole and the
added anion. The above-mentioned method was employed for
protonation and deprotonation studies.
Experimental section
Chemicals
4-Bromobenzaldehyde, 4-carbomethoxybenzaldehyde and Pd-
(PPh3)4 were purchased from Alfa Aesar and used as received.
Et3N, hexane and CH3CN were dried and distilled over CaH2.
CHCl3 and toluene were dried and distilled over P2O5. 5,10,15-
Triphenylcorrole (1) and 5,10,15-tris(4′-carbomethyphenyl)
corrole (3) were synthesized according to a literature pro-
cedure.4 Tetrabutylammonium salts (TBAX, X = F, Cl, Br, I,
CH3COO, H2PO4, HSO4, PF6 and ClO4) were used as received
from Alfa Aesar. We have calculated the limit of detection
(LOD) and the limit of quantification (LOQ) for anions by
2 and 3 using the formula, LOD = 3.3 SD/S and LOQ = 10 SD/S,
where SD stands for standard deviation of blank, S stands for
slope.43
Synthesis of (4-diethoxyphosphoryl)benzaldehyde46
1 g of 4-bromobenzaldehyde (5.40 mmol) was taken in a
100 mL two-necked RB flask. To this solution, dry toluene
(8 mL), dry triethylamine (8 mL) and diethyl phosphite
(0.773 mL, 6 mmol) were added and purged with Ar gas for
2 minutes. Finally, Pd(PPh3)4 (0.31 g, 0.27 mmol) was added
and the reaction mixture heated to 90 °C for 24 h under
argon atmosphere. The reaction mixture was cooled to room
temperature and the solvent mixture evaporated to dryness
under vacuum, redissolved in CHCl3 (70 mL), washed with dis-
tilled water (3 × 100 mL), followed by brine solution (100 mL),
and finally dried over sodium sulphate. The crude product was
Instrumentation
UV-Visible and fluorescence spectra were recorded using a purified on a silica column using CHCl3–EtOAc (7 : 3, v/v) as
Cary 100 spectrophotometer and Hitachi F-4600 spectrofluo- the eluent. The yield of (4-diethoxyphosphoryl)benzaldehyde
rometer, respectively. All 1H NMR measurements were per- was found to be 0.8 g (61%).
formed using a Bruker AVANCE 500 MHz spectrometer in
1H NMR (CDCl3): δ (ppm) 10.05 (s, 1H, CHO), 7.62 (m, 2H,
CDCl3. MALDI-TOF mass spectra were measured using a o-ph-H), 7.42 (td, J = 8 Hz, J = 2.5 Hz, 2H, m-ph-H), 4.12 (m,
Bruker UltrafleXtreme-TN MALDI-TOF/TOF spectrometer using 4H, –OCH2), 1.30 (t, J = 7.1 Hz, 6H, –CH3).
dithranol as a matrix. The ESI Mass spectrum of 2 was
5,10,15-Tris(4′-diethoxyphosphorylphenyl)corrole (H3TPPC)
recorded using a Bruker daltanics-microTOF instrument in [2]. (4-Diethoxyphosphoryl)benzaldehyde (1 g, 4.13 mmol)
negative ion mode and elemental analyses were carried out on and pyrrole (0.570 mL, 8.26 mmol) were dissolved in 400 mL
a Elementar vario EL III instrument. Electrochemical measure- of a water and MeOH (1 : 1, v/v) mixture. Subsequently, 3.5 mL
ments were carried out with a BAS Epsilon electrochemical of conc. HCl (36%) was added dropwise and stirred for 3 h at
workstation. A three-electrode system, consisting of a GC room temperature. The reaction mixture was extracted with
Working electrode, Ag/AgCl reference electrode and a Pt-wire 100 mL of CHCl3, washed twice with water (2 × 100 mL), dried
counter electrode, was used. The concentration of all corroles over Na2SO4, and diluted with CHCl3 to 270 mL. To this solu-
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 14680–14688 | 14681

View Article Online
Paper
Dalton Transactions
tion, p-chloranil (1 g, 1 mmol) was added and then refluxed (m/z): 761.348 [M]+ (calcd, 761.273). The yield of 3a was found
for 1 h. The crude product was purified on a silica column to be 66% (36 mg, 0.047 mmol).
using 3% MeOH in CHCl3 as the eluent. Yield of 2 was found
to be 10% (0.130 g, 0.139 mmol).
5,10,15-Tris(4′-carboxymethylphenyl)corrolato Ag(III) [3b].
UV/Vis (CH2Cl2): λmax (nm) (ε × 10−3 L mol−1 cm−1) 428
UV/Vis (CH2Cl2): λmax (nm) (ε × 10−3 L mol−1 cm−1) 419 (64.5), 527 (4.5), 584 (17.4). 1H NMR in CDCl3 (500 MHz):
(66.0), 575 (11.2), 617 (8.1), 647 (6.6). 1H NMR in CDCl3 δ (ppm) 9.07 (d, 2H, J = 4.3 Hz, β-pyrrole-H), 8.92 (d, 2H, J =
(500 MHz): δ (ppm) 9.04 (d, 2H, J = 4.15 Hz, β-pyrrole-H), 8.89 4.7 Hz, β-pyrrole-H), 8.68 (d, 2H, J = 4.5 Hz, β-pyrrole-H), 8.66
(d, 2H J = 4.7 Hz, β-pyrrole-H), 8.63 (d, 2H, J = 4.1 Hz, (d, 2H, J = 4.5 Hz, β-pyrrole-H), 8.49 (d, 4H, J = 8.1 Hz, meso-o-
β-pyrrole-H), 8.56 (d, 2H, J = 4.7 Hz, β-pyrrole-H), 8.46–8.50 (m, phenyl-H), 8.45 (d, 2H, J = 8.1 Hz, meso-o-phenyl-H), 8.34 (d,
4H, meso-o-phenyl-H), 8.18–8.32 (m, 8H, meso-o and m-phenyl- 4H, J = 8.1 Hz, meso-m-phenyl-H), 8.23 (d, J = 8.1 Hz, 2H, meso-
H), 4.30–4.42 (m, 12H, –OCH2), 1.51 (t, 18H, J = 6.47 Hz, –CH3) m-phenyl-H), 4.12 (s, 6H, –OCH3), 4.11 (s, 3H, –OCH3). MALDI-
ESI-MS (m/z): found 971.272 [M + Cl−], calcd 971.268. Anal. TOF-MS (m/z): 805.373 [M]+ (calcd, 805.589). The yield of 3b
calcd for C49H53N4O9P3: C, 62.95; H, 5.71; N, 5.99%. Found: C, was found to be 56% (32 mg, 0.039 mmol).
62.70; H, 5.95; N, 6.15%.
Preparation of the Cu(III) and Ag(III) corrole complexes
Results and discussion
Synthesis and characterization
Free-base corrole (0.050 g, 0.0535 mmol) was dissolved in
30 mL of CHCl3. To this solution, 10 equiv. of copper(II)
acetate monohydrate or silver(I) acetate in 8 mL of MeOH was
added and stirred at room temperature for 1 h. Then, the reac-
tion mixture was evaporated to dryness, redissolved in the
minimum amount of CHCl3 (15 mL) and washed with distilled
water (50 mL). The organic layer was dried over Na2SO4 and
purified on silica column using 1% MeOH in CHCl3 as the
eluent. The yields of 2a, 2b, 3a and 3b are reported below
along with their characterisation data.
5,10,15-Tris(4′-diethoxyphosphorylphenyl))corrolato Cu(III)
[2a]. UV/Vis (CH2Cl2): λmax (nm) (ε × 10−3 L mol−1 cm−1) 412
(54.9), 536 (4.2), 617 (2.9). 1H NMR in CDCl3 (500 MHz):
δ (ppm) 7.90–7.96 (m, 8H, meso-o, m and p-phenyl-H),
7.84–7.86 (m, 4H, meso-o-phenyl-H), 7.74–7.75 (m, 2H,
β-pyrrole-H), 7.61 (d, 2H, J = 3.8 Hz, β-pyrrole-H), 7.32 (d, 2H,
J = 3.9 Hz, β-pyrrole-H), 7.20 (d, 2H, J = 4.4 Hz, β-pyrrole-H),
4.28–4.12 (m, 12H, –OCH2), 1.41 (at, 18H, J = 4.7 Hz, –CH3)
MALDI-TOF-MS (m/z): 995.420 [M]+ (calcd, 995.425). The yield
of 2a was found to be 56% (30 mg, 0.03 mmol).
We have synthesized different free-base corroles (1–3) and
their Cu(III) and Ag(III) complexes as shown in Chart 1. 1 and 3
were synthesized according to the procedure described by
Gryko in a H2O–MeOH mixture.4a Initially, we tried to
synthesize 5,10,15-tris(4′-diethoxyphosphorylphenyl)corrole via
Pd-catalysed coupling reaction using 5,10,15-tris(4′-bromophe-
nyl)corrole and diethylphosphite by refluxing in ethanol under
an argon atmosphere as reported in the literature for the syn-
thesis of phosphorylporphyrins.29 Unfortunately, the reaction
yielded an inseparable mixture of products in a very low yield
(<0.5%). Alternatively, we synthesized 4-diethoxyphosphoryl-
benzaldehyde46 and then condensed it with pyrrole in a water–
methanol mixture4a and the yield was found to be 10%
(Scheme 1).
The Ag(III) and Cu(III) metal complexes were prepared by
reacting the free-base corroles and the corresponding metal
acetates in a CHCl3–methanol mixture as shown in Scheme 1.
All the synthesized corroles were characterized by UV-Visible,
fluorescence, 1H NMR and mass spectroscopic techniques.
The UV-visible spectral features of 2 and 2a–b in CH2Cl2 are
shown in Fig. 1a. Table 1 shows the optical absorption spectral
data of the corroles synthesized. Moreover, the electronic
absorption spectra of 3 and 3a–b are shown in the ESI
5,10,15-Tris(4′-diethoxyphosphorylphenyl)corrolato
Ag(III)
[2b]. UV/Vis (CH2Cl2): λmax (nm) (ε × 10−3 L mol−1 cm−1) 429
(57.5), 561 (8.3), 583 (15.1). 1H NMR in CDCl3 (500 MHz):
δ (ppm) 9.26 (d, 2H, J = 4.0 Hz, β-pyrrole-H), 8.99 (d, 2H, J = 4.5
Hz, β-pyrrole-H), 8.79 (d, 2H, J = 4.0 Hz, β-pyrrole-H), 8.75 (d,
2H, J = 5.0 Hz, β-pyrrole-H), 8.41–8.44 (m, 4H, meso-o-phenyl-
H), 8.33–8.35 (m, 2H, meso-o-phenyl-H), 8.29–8.25 (m, 6H,
meso-m-phenyl-H), 4.43–4.36 (m, 12H, –OCH2), 1.52 (t, J = 6.5
Hz, 18H, –CH3) MALDI-TOF-MS (m/z): 1057.740 [M·H2O]+
(calcd, 1057.757). The yield of 2b was found to be 45% (25 mg,
0.024 mmol).
5,10,15-Tris(4′-carbomethoxyphenyl)corrolato Cu(III) [3a].
UV/Vis (CH2Cl2): λmax (nm) (ε × 10−3 L mol−1 cm−1) 414 (47.8),
541 (3.4), 626 (2.3). 1H NMR in CDCl3 (500 MHz): δ (ppm) 8.20
(d, 4H, J = 7.5 Hz, meso-o-phenyl-H), 8.17 (d, 2H, J = 8.0 Hz,
meso-o-phenyl-H), 7.96 (d, 2H, J = 3.2 Hz, β-pyrrole-H), 7.84 (d,
4H, J = 8.1 Hz, meso-m-phenyl-H), 7.74 (d, 2H, J = 8.1 Hz, meso-
m-phenyl-H), 7.62 (d, 2H, J = 4.3 Hz, β-pyrrole-H), 7.34 (d, 2H,
J = 4.3 Hz, β-pyrrole-H), 7.21 (d, 2H, J = 4.6 Hz, β-pyrrole-H),
4.02 (s, 6H, –OCH3), 4.01 (s, 3H, –OCH3). MALDI-TOF-MS Chart 1 Molecular structures of the corroles synthesized.
14682 | Dalton Trans., 2014, 43, 14680–14688
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
Scheme 1 Synthesis of triphosphorylphenylcorrole (2) and its metal
complexes (2a and 2b).
Fig. 2 UV-Visible spectral titration of 2 (2.13 × 10−5 M) with μM concen-
tration of TFA in acetonitrile. The inset shows a plot between [TFA] and
[TFA]/(An − A0).
emission spectra of 2 and 3 in CH2Cl2 showing maxima at 678
and 686 nm, respectively. 2 exhibited a 7 nm red shift and
8 nm blue-shift as compared to 1 and 3, respectively.
1H NMR spectrum of 2 shows the characteristic four doub-
lets corresponding to β-protons of the corrole macrocycle. A
multiplet at 4.30–4.42 ppm (12 H) and a triplet at 1.51 ppm
(18 H) confirm the diethoxyphosphoryl group at the meso-
phenyl position of the corrole. 1H NMR signals of 2b showed a
downfield shift (0.2–0.3 ppm) than 2 and 2a, possibly due to
1
the enhanced planarity in the Ag corrole complexes.47 The H
NMR spectra of 2, 2a, 2b and 3b are shown in the ESI (Fig. S2–
S5†). Aggregation behaviour of 2 and 3 were studied using a
UV-Visible spectrophotometer at various concentrations
(0.53–1.59 × 10−5 M) in CH2Cl2. These corroles did not show
any considerable changes, which indicates that 2 and 3 do not
aggregate in CH2Cl2 at a concentration of 10−5 M, whereas the
broad peaks observed in the 1H NMR spectra of 2 and 3 at
∼0.015 M in CDCl3 indicates the aggregation behaviour of
these corroles at higher concentrations. The MALDI-TOF mass
spectra of 2a, 2b, 3a and 3b are shown in the ESI (Fig. S6–S9†)
and the experimental mass values are in accordance with the
calculated mass values for [M+].
Fig. 1 (a) UV-Visible spectra of 2 and 2a–b in CH2Cl2. (b) Fluorescence
spectra of 2 and 3 in CH2Cl2.
Table 1 Optical absorption spectral data of compounds 1–3 and their
metal complexes (λmax (nm) (ε × 10−3 L mol−1 cm−1)) in CH2Cl2
Protonation studies
Corrole
B band (nm)
Q bands (nm)
Protonation studies of the free-base corroles (1–3) were studied
in CH3CN using trifluoroacetic acid (TFA) as the titrating
agent. Fig. 2 shows the UV-Visible spectral changes of 2 with
an increase in the concentration of TFA. The absorbance at
421 nm for 2 had concomitant decrement and a new shoulder
was observed at 429 nm with a 8 nm red-shift. Similarly,
2 showed a concomitant decrement in the absorbance at
582 nm and a rising new band at 680 nm accompanied with a
2
419 (66.0)
412 (54.9)
429 (57.5)
424 (52.5)
414 (47.8)
428 (64.5)
575 (11.2), 617 (8.1), 647 (6.6)
536 (4.1), 617 (2.9)
561 (8.3), 583 (15.1)
579 (9.3), 622 (6.6), 653 (4.9)
542 (3.4), 621 (2.3)
526 (4.5) , 584 (17.4)
2a
2b
3
3a
3b
(Fig. S1†). 2 shows a Soret band at 419 nm and three Q bands red-shift of 22 nm in the Q band as shown in Fig. 2. In all
at 575, 617 and 647 nm, which are marginally red shifted com- cases, we obtained a monoprotonated corrole species, which
pared to 1 and 5 nm blue-shifted than 3. Fig. 1b shows the was further confirmed by the Hill plot showing a slope value
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 14680–14688 | 14683

View Article Online
Paper
Dalton Transactions
of 1 (Fig. S10a in the ESI†). All the monocationic corroles and Scheme 2. Deprotonation spectral changes of 3 are shown
(Scheme 1) exhibited a stable aromatic nature as confirmed by in Fig. S12 in the ESI.† The deprotonation constants of 1–3 are
their UV-Visible spectral features. A representative plot for the summarized in Table 2. The log β2 value of 2 (9.27) is margin-
evaluation of Keq for the protonation of 2 is shown as an inset ally less than the free base β-octaethylcorrole (9.50) and close
in Fig. 2. Moreover, a similar behaviour was observed for 1 and to tris(p-methylphenyl)corrole (9.2).36 This suggests that the
3 as shown in Fig. S11 in the ESI.† Table 2 lists the protonation acidity of the imino protons of 2 and 3 are comparable to that
constants for 1–3 in CH3CN. The highest log K was observed of 1, which is different from the expected trend.
for 1 (4.93) and lowest for 2 (3.75), which suggests that the
Anion detection
electron-withdrawing nature of the phosphoryl groups makes
it difficult to protonate the inner core nitrogen of corrole 2. Anion detection by 1–3 was studied in CH3CN with various
Similar results were obtained for 3 as compared to 1. The pro- spherical halide ions such as F−, Cl−, Br− and I− and bulky
anions such as CH3COO−, H2PO4−, HSO4−, PF6 and ClO4
−
−
tonation constants (log K) follow the trend expected: 1 > 3 > 2.
using a UV-Visible spectrophotometer with the addition of an
aliquot of anionic TBA salts. The free-base corroles (1–3) selec-
Deprotonation studies
tively bind with F−, CH3COO− and H2PO4 ions and show a
−
A UV-Visible titration method was employed to determine the
deprotonation constants of 1–3 in CH3CN using tetrabutyl-
ammonium hydroxide (TBAOH) as a base and the spectral
changes of 2 during the course of titration shown in Fig. 3.
The absorbance decreased at 421 nm with the emergence of a
new B band at 442 nm accompanied by a 21 nm red-shift.
Similar behaviour was observed for the Q-bands (decrement in
the absorbance at 583 nm and a concomitant increase in
absorbance at 646 nm) as shown in Fig. 3. The inset shows a
straight line between [TBAOH]2/A0 − An versus [TBAOH]2 from
which the β2 value was calculated for the deprotonation of 2.
The Hill plot refers to a 1 : 2 stoichiometry or two proton
abstraction by the base (TBAOH) as shown in Fig. S10b (ESI†)
considerable red-shift (10–15 nm) in the UV-Visible spectra as
shown in Fig. 4 and S13a (ESI†), whereas no shifts were
observed for other anions. There are two possibilities of
corrole and anion interaction: (i) anions may have strong
hydrogen bonding with the NH moieties in the corrole core or
(ii) these basic anions can abstract the acidic proton of NH
group, a process called anion induced deprotonation. The UV-
visible spectral changes of receptor 2 with different anions
were the same as that of TBAOH (Fig. 4). These results indicate
that the spectral changes were most probably due to the depro-
tonation of the NH moiety by the basic anions (F−, CH3COO−
and H2PO4−) and not through hydrogen bonding. The spectro-
photometric titration of 2 with fluoride ions is shown in Fig. 5.
As we increased the concentration of F− ions, a decrement in
the absorbance was observed at 417 nm and 581 nm and a
concomitant increment in the absorbance at 432 nm and
642 nm, respectively with the isosbestic points at 425 nm and
618 nm. The equilibrium constant for F− ion binding was cal-
culated using a plot of [anion]2 versus [anion]2/An − A0 as
shown in the inset of Fig. 5. The Hill plot (Fig. S13b in the
ESI†) shows a straight line between log[F−] and log(A0 − An/
Af − An) with a slope value of 2, which indicates a 1 : 2 (corrole-
to-anion) stoichiometry as shown in Scheme 2. A similar
behavior was observed for 3 as shown in Fig. S14 in the ESI.†
Similar spectral changes were obtained for CH3COO− and
Table 2 Protonation and deprotonation constants of 1–3 in CH3CN
TFA
TBAOH
Corrole
K
Log K
n
β2
Log β2
n
1
2
3
8.52 × 104
0.32 × 104
0.94 × 104
4.93
3.51
3.97
1
1
1
5.76 × 109
1.89 × 109
1.91 × 109
9.76
9.27
9.28
2
2
2
−
H2PO4 with 1–3 (Fig. S15–S16 in the ESI†). The equilibrium
constants of 1–3 with different anions are summarized in
Table 3. The F−, CH3COO− and H2PO4− induced deprotonation
is fully reversible as shown upon the addition of CH3OH. The
addition of a polar protic solvent such as CH3OH results in a
gradual decrease in the absorbance as reflected in the
UV-Visible spectral studies (see Fig. S17 in the ESI†). This is
possibly due to competition between the protic solvent versus
the anions (F−, CH3COO− and H2PO4−) with NH moiety. More-
over, the presence of a high amount of protic solvent disfavors
the formation of the deprotonated receptor 2.
2 and 3 have shown higher equilibrium constants with F−
ions (4.7 × 103 fold higher for 2 and 9.7 × 103 fold higher for 3)
as compared to that of 1. The electron-withdrawing nature of
diethoxyphosphoryl and carbomethoxy groups make corroles
2 and 3 more acidic, which is responsible for the deprotonation
Fig. 3 UV-Visible spectral changes in 2 (2.59 × 10−5 M) during the
addition of TBAOH in acetonitrile (the inset shows a plot between
[TBAOH]2 and [TBAOH]2/(A0 − An)).
14684 | Dalton Trans., 2014, 43, 14680–14688
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
Scheme 2 Schematic representation of protonation, deprotonation and anion binding with 1–3.
Table 3 Equilibrium constantsa of 1–3 upon the addition of μM quan-
tities of anions in CH3CN
Compound
Anion
Log β2
A : L
LOD (μM)
LOQ (μM)
1
F−
6.06
10.53
9.39
1 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
0.50
0.18
0.17
0.06
0.05
0.05
0.06
0.04
0.04
2.43
6.48
6.90
0.17
0.14
0.14
0.19
0.12
0.11
CH3COO−
−
H2PO4
2
3
F−
9.74
CH3COO−
10.80
10.27
10.05
9.63
−
H2PO4
F−
CH3COO−
−
H2PO4
9.50
a Within the error 0.14.
of the inner core NH protons by basic anions as compared to
1. No changes were observed for 1–3 in the presence of Cl−,
Fig. 4 Absorption spectral changes of 2 with μM quantities of base and
anions.
Br−, I−, HSO4−, PF6 and ClO4−. This indicates that the recep-
−
−
tor provides selectivity towards F−, CH3COO− and H2PO4
,
which is solely based upon its deprotonation and is related to
the following factors: (a) the intrinsic acidity of the receptor,
(b) the basicity of the anion and (c) the polarity of the
solvent.48 The imino protons of the free-base corroles (2 and 3)
are highly acidic in nature; hence, they are sensitive for basic
anions such as F−, CH3COO− and H2PO4 ions. The small
−
ionic size (1.3 Å) and the high electronegativity (4.1) of fluoride
ions are responsible for the easy deprotonation of the NH
group as compared to other anions. Moreover, the size of the
corrole core (2.50 Å) is compatible with the fluoride ion size,
which is responsible for the higher equilibrium constants
observed.
The limit of detection (LOD) and limit of quantification
(LOQ) for F−, CH3COO− and H2PO4 were calculated in the
−
presence of 1–3 in CH3CN. 2 shows more sensitivity towards
F− ions, the LOD for 2 was found to be 0.06 μM and the LOQ
was found to be 0.17 μM. 3 shows higher sensitivity towards
the basic anions CH3COO− and H2PO4 with a very low detec-
−
Fig. 5 UV-Visible titration of 2 (2.13 × 10−5 M) with TBAF (the inset
shows a plot between [TBAF]2 and [TBAF]2/(A0 − An)).
tion limit of 0.04 μM. These results suggest that 2 and 3 are
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 14680–14688 | 14685

View Article Online
Paper
Dalton Transactions
very much sensitive for anion detection as compared to other
reported corroles.20a
Table 4 Electrochemical redox data for compounds 1–3 and their
metal complexes in CH2Cl2
Oxidation
Reduction
I
Electrochemistry
Corrole
3σ
I
II
III
II
To probe the influence of the meso-phenyl substituents on the
π-electronic properties of the corrole ring system, the electro-
chemical redox behaviours of these corroles were studied by
cyclic voltammetric analysis. 1–3 showed two reversible oxi-
dations and one irreversible reduction as shown in Fig. S18 in
the ESI.† 2 and 3 exhibited an anodic shift in their first oxi-
dation (∼120 mV) and reduction (160–400 mV) potentials as
compared to 1. Since the anionic radicals of 1–3 formed after
electrochemical reduction are unstable and undergo dispro-
portionation we used their metal complexes to probe further.
The CVs of the Ag(III) and Cu(III) complexes of 1–3 are shown in
Fig. 6. 1b shows three oxidations and two reductions as
reported in the literature,47 whereas 2b and 3b show two ring
centred oxidations and one metal centred reduction (Table 4).
The first oxidation potentials of 2a–b and 3a–b are more anodi-
cally shifted (∼250 mV) as compared to 1a–b. The metal
centred reductions of 2a and 3a are anodically shifted
(∼100–150 mV) as compared to 1a. The anodic shift of redox
potentials is ascribed to the strong electron-withdrawing
nature of the diethoxyphosphoryl and carbomethoxy groups
present.
1a
2a
3a
1b
2b
3b
0
648
904
882
580
884
856
1420
1385
—
744
1372
1295
—
—
—
1239
—
−223
−76
−105
−836
−752
−800
—
—
—
−1017
—
—
1.59
1.35
0
1.59
1.35
—
The higher reduction potentials of the Ag(III) corroles as
compared to the Cu(III) corroles is due to the more electroposi-
tive nature of silver metal. For the Hammet equation E1/2 = 3σρ,
the plot of first ring oxidation and metal centred reduction
versus the Hammet parameter (σp) of the substituents was
examined to delineate the role of the substituents on the
corrole π-system and exhibited a linear trend, as shown in
Fig. 7. The reaction constant, ρ, for the first oxidation versus σp
was found to be higher (195 mV) for the Ag corroles relative to
that of the Cu corroles (165 mV), which indicates the higher
reactivity of the Ag corrole towards oxidation. The reaction con-
stant obtained from the plot of reduction potential versus σp
was found to be higher for the Cu corroles (91 mV) as com-
Fig. 7 A plot of Hammet parameter (3σ) versus the first ring oxidation
and the metal centered reduction of the Cu(III) and Ag(III) complexes
derived from 1–3.
pared to the Ag corroles (44 mV), which indicates a higher ten-
dency towards reduction for the Cu corroles. These results
clearly indicate that the diethoxyphosphoryl and carbomethoxy
corroles are electron deficient in nature; therefore, they show
highest equilibrium constant towards the basic anions.
Conclusion
We have synthesized a new family of electron deficient corroles
and characterized them using spectroscopic techniques and
electrochemical studies. 2 and 3 exhibited red-shifted absorp-
1
tion spectra and a downfield shift in their H NMR spectra as
compared to 1. The protonation constant of 2 is ∼30 fold lower
as compared to 1, reflecting the electron-withdrawing nature
of the diethoxyphosphoryl groups. 2 and 3 have shown higher
equilibrium constants with F− ions (4.7 × 103 fold higher for 2
and 9.7 × 103 fold higher for 3) as compared to 1 and are able
to detect 0.06 μM of F− ions. 2 was also found to be more sen-
sitive towards CH3COO− and H2PO4 ions, and it allowed the
detection of 0.05 μM of these anions with a LOQ of 0.14 μM.
−
Fig. 6 Cyclic Voltammograms of 1a–3b in CH2Cl2 containing 0.1 M
TBAP (with a scan rate of 100 mV s−1). A GC working electrode, Ag/AgCl
reference electrode and Pt wire counter electrode were used.
Tricarbomethoxyphenyl and triphosphorylphenyl substituted
14686 | Dalton Trans., 2014, 43, 14680–14688
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
metallocorroles exhibited an anodic shift of ∼250 mV in the 13 H. Y. Liu, M. H. R. Mahmood, S. X. Qiu and C. K. Chang,
oxidation and ∼100–150 mV in their metal centered reduction Coord. Chem. Rev., 2013, 257, 1306–1333.
as compared to 1 due to the electron-withdrawing nature of 14 Y. Han, Y. M. Lee, M. Mariappan, S. Fukuzumi and
the substituents. The linear trend in the Hammet plot also W. Nam, Chem. Commun., 2010, 46, 8160–8162.
substantiates the effect of the substituents on the corrole 15 (a) Z. Gross, L. Simkhovich and N. Galili, Chem. Commun.,
π-system. These results indicate that the electron-deficient cor-
roles can be utilized as efficient basic anion detectors.
1999, 599–600; (b) K. M. Kadish, L. Fremond, F. Burdet,
J.-M. Barbe, C. P. Gros and R. Guilard, J. Inorg. Biochem.,
2006, 100, 858–868.
16 (a) G. Golubkov, J. Bendix, H. B. Gray, A. Mahammed,
I. Goldberg, A. J. DiBilio and Z. Gross, Angew. Chem., Int.
Ed., 2001, 40, 2132–2134; (b) H. Y. Liu, T. S. Lai, L. L. Yeung
and C. K. Chang, Org. Lett., 2003, 5, 617–620.
Acknowledgements
We are grateful for the financial support provided by the
Council of Scientific and Industrial Research (01(2694)/12/ 17 I. Luobeznova, M. Raizman, I. Goldberg and Z. Gross,
EMR-II), Science and Engineering Research Board (SB/FT/ Inorg. Chem., 2006, 45, 386–394.
CS-015/2012) and Board of Research in Nuclear Sciences 18 G. Golubkov and Z. Gross, J. Am. Chem. Soc., 2005, 127,
(2012/37C/61BRNS/253). PY thanks CSIR for the Junior
Research Fellowship (JRF).
3258–3259.
19 R. Paolesse, C. D. Natale, A. Macognano, F. Sagone,
M. A. Scarselli, P. Chiaradia, V. I. Troitsky, T. S. Berzina and
A. D’Amico, Langmuir, 1999, 15, 1268–1274.
20 (a) C. I. M. Santos, E. Oliveira, J. F. B. Barata,
M. A. F. Faustino, J. A. S. Cavaleiro, M. G. P. M. S. Neves
and C. Lodeiro, J. Mater. Chem., 2012, 22, 13811–13819;
(b) C. I. M. Santos, E. Oliveira, J. F. B. Barata,
M. A. F. Faustino, J. A. S. Cavaleiro, M. G. P. M. S. Neves
and C. Lodeiro, Inorg. Chim. Acta, 2014, 417, 148–154.
21 R. Paolesse, F. Sagone, A. Macagnano, T. Boschi, L. Prodi,
M. Montalti, N. Zaccheroni, F. Bolletta and K. M. Smith,
J. Porphyrins Phthalocyanines, 1999, 3, 364–370.
References
1 A. W. Johnson and I. T. Kay, J. Chem. Soc., 1965, 1620.
2 (a) R. Paolesse, L. Jaquinod, D. J. Nurco, S. Mini, F. Sagone,
T. Boschi and K. M. Smith, Chem. Commun., 1999, 1307–
1308; (b) R. Paolesse, S. Nardis, F. Sagone and
R. G. Khoury, J. Org. Chem., 2001, 66, 550–556.
3 (a) Z. Gross, N. Galili, L. Simkhovich, I. Saltsman,
M. Botoshansky, D. Blaser, R. Boese and I. Goldberg, Org.
Lett., 1999, 1, 599–602; (b) Z. Gross, N. Galili and 22 D. Walker, S. Chappel, A. Mahammed, J. J. Weaver,
I. Saltsman, Angew. Chem., Int. Ed., 1999, 38, 1427–1429.
4 (a) B. Koszarna and D. T. Gryko, J. Org. Chem., 2006, 71,
3707–3717; (b) C. Capar, L. K. Hansen, J. Conradie and
B. S. Brunschwig, J. R. Winkler, H. B. Gray, A. Zaban and
Z. Gross, J. Porphyrins Phthalocyanines, 2006, 10, 1259–
1262.
A. Ghosh, J. Porphyrins Phthalocyanines, 2010, 14, 509–512; 23 K. Sudhakar, V. Velkannan and L. Giribabu, Tetrahedron
(c) K. E. Thomas, I. H. Wasbotten and A. Ghosh, Inorg. Lett., 2012, 53, 991–993.
Chem., 2008, 47, 10469–10478; (d) A. Ghosh, Angew. Chem., 24 A. Haber, A. Mahammed, B. Fuhrman, N. Volkova,
Int. Ed., 2004, 43, 1918–1931; (e) E. Steene, A. Dey and
A. Ghosh, J. Am. Chem. Soc., 2003, 125, 16300–16309.
R. Coleman, T. Hayek, M. Aviram and Z. Gross, Angew.
Chem., Int. Ed., 2008, 47, 7896–7900.
5 C. Erlen, S. Will and K. M. Kadish, in The Porphyrin Hand- 25 (a) A. Rebane, M. Drobizhev, N. S. Makarov, B. Koszarna,
book, ed. K. M. Kadish, K. M. Smith and R. Guilard,
Academic Press, New York, 2000, vol. 2, pp. 233–300.
6 J. Bendix, I. J. Dmochowski, H. B. Gray, A. Mahammed,
L. Simkhovich and Z. Gross, Angew. Chem., Int. Ed., 2000,
39, 4048–4051.
M. Tasior and D. T. Gryko, Chem. Phys. Lett., 2008, 462,
246–250; (b) P. T. Anusha, D. Swain, S. Hamad, L. Giribabu,
T. S. Prashant, S. P. Tewari and S. V. Rao, J. Phys. Chem. C,
2012, 116, 17828–17837.
26 S. Cho, J. M. Lim, S. Hiroto, P. Kim, H. Shinokubo,
A. Osuka and D. Kim, J. Am. Chem. Soc., 2009, 131, 6412–
6420.
7 A. Mahammed and Z. Gross, J. Inorg. Biochem., 2002, 88,
305–309.
8 K. Sorasaenee, P. Taqavi, L. M. Heling, H. B. Gray, 27 X. Ying, X. Y. Long, M. H. R. Mahmood, Q. Y. Hu, H. Y. Liu
E. Tkachenko, A. Mahammed and Z. Gross, J. Porphyrins
Phthalocyanines, 2007, 11, 189–197.
and C. K. Chang, J. Porphyrins Phthalocyanines, 2012, 16,
1276–1284.
9 S. Nardis, D. Monti and R. Paolesse, Mini-Rev. Org. Chem., 28 K. E. Borbas, H. L. Kee, D. Holten and J. S. Lindsey, Org.
2005, 2, 355–372.
Biomol. Chem., 2008, 6, 187–194.
10 Z. Gross, J. Biol. Inorg. Chem., 2001, 6, 733–738.
11 Z. Gross and H. B. Gray, Comments Inorg. Chem., 2006, 27,
61–72.
29 K. M. Kadish, P. Chen, Y. Yu. Enakieva, S. E. Nefedov,
Y. G. Gorbunovab, A. Y. Tsivadze, A. Bessmertnykh-
Lemeune, C. Stern and R. Guilard, J. Electroanal. Chem.,
2011, 656, 61–71.
12 S. H. Kim, H. Park, M. S. Seo, M. Kubo, T. Ogura, J. Klajn,
D. T. Gryko, J. S. Valentine and W. Nam, J. Am. Chem. Soc., 30 A. A. Sinelshchikova, S. E. Nefedov, Y. Y. Enakieva,
2010, 132, 14030–14032.
Y. G. Gorbunova, A. Y. Tsivadze, K. M. Kadish, P. Chen,
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 14680–14688 | 14687

View Article Online
Paper
Dalton Transactions
A. Bessmertnykh-Lemeune, C. Stern and R. Guilard, Inorg. 40 Y. Zhang, M. X. Li, M. Y. Lu, R. H. Yang, F. Liu and K. A. Li,
Chem., 2013, 52, 999–1008. J. Phys. Chem. A., 2005, 109, 7442–7448.
31 D. L. Officer, F. Lodato and K. W. Jolley, Inorg. Chem., 2007, 41 (a) F. Zhi, X. Lu, J. Yang, X. Wang, H. Shang, S. Zhang and
46, 4781–4783.
Z. Xue, J. Phys. Chem. C, 2009, 113, 13166–13172;
(b) P. Kubat, K. Lang and P. Anzenbacher, Biochim. Biophys.
Acta, 2004, 1670, 40–48.
32 (a) Y. Fang, K. M. Kadish, P. Chen, Y. Gorbunova,
Y. Enakieva, A. Tsivadze, A. Bessmertnykh-Lemeune and
R. Guilard, J. Porphyrins Phthalocyanines, 2013, 17, 1035– 42 J. M. M. Rodrigues, A. S. F. Farinha, F. P. V. Muteto,
1045; (b) Z. Ou, J. Shen, J. Shao, M. Galezowski, D. T. Gryko
and K. M. Kadish, Inorg. Chem., 2007, 46, 2775–2786;
(c) J. Shen, Z. Ou, J. Shao, M. Galezowski, D. T. Gryko and
K. M. Kadish, J. Porphyrins Phthalocyanines, 2007, 11, 269–
S.
M.
Woranovicz-Barreira,
F.
A.
A.
Paz,
M. G. P. M. S. Neves, J. A. S. Cavaleiro, A. C. Tome, M. T. S.
R. Gomes, J. L. Sessler and J. P. C. Tome, Chem. Commun.,
2014, 50, 1359–1361.
276; (d) Z. Ou, H. Suna, H. Zhua, Z. Daa and K. M. Kadish, 43 V. M. Patel, J. A. Patel, S. S. Havele and S. R. Dhaneshwar,
J. Porphyrins Phthalocyanines, 2008, 12, 1–10. Int. J. ChemTech Res., 2010, 2, 756–761.
33 J. Shen, J. Shao, Z. Ou, B. Koszarna, D. T. Gryko and 44 (a) J. P. Collman, J. I. Brauman, K. M. Doxsee,
K. M. Kadish, Inorg. Chem., 2006, 45, 2251–2265.
34 J. P. Gisselbrecht, M. Gross, E. Vogel and S. Will, J. Electro-
anal. Chem., 2001, 205, 170–172.
35 A. Mahammed, J. J. Weaver, H. B. Gray, M. Abdelas and
Z. Gross, Tetrahedron Lett., 2003, 44, 2077–2079.
36 R. Grigg, R. J. Hamilton, M. L. Jozefowicz, C. H. Rochester,
R. J. Terrell and H. Wickwar, J. Chem. Soc., Perkin Trans. 2,
1973, 407–413.
T. R. Halbert, S. E. Hayes and K. S. Suslick, J. Am. Chem.
Soc., 1978, 100, 2761–2766; (b) K. S. Suslick, M. M. Fox
and T. J. Reinert, J. Am. Chem. Soc., 1984, 106, 4522–
4525.
45 (a) A. V. Hill, J. Physiol., 1910, 40, IV–VII; (b) C. A. Hunter,
M. N. Meah and J. K. M. Sanders, J. Am. Chem. Soc., 1990,
112, 5773–5780.
46 S. K. Hau, Y. J. Cheng, H. L. Yip, Y. Zhang, H. Ma and
A. K. Y. Jen, Appl. Mater. Interfaces, 2010, 2, 1892–1902.
47 (a) A. Alemayehu, J. Conradie and A. Ghosh, Eur. J. Inorg.
37 M. J. Broadhurst, R. Grigg and G. Shelton, J. Chem. Soc.,
Perkin Trans. 1, 1972, 143–151.
38 P. E. Ellis, J. E. Linard, T. Szymanski, R. D. Jones,
J. R. Budge and F. Basolo, J. Am. Chem. Soc., 1980, 102,
1889–1896.
Chem.,
2011,
1857–1864;
(b)
K.
E.
Thomas,
A. B. Alemayehu, J. Conradie, C. Beavers and A. Ghosh,
Inorg. Chem., 2011, 50, 12844–12851.
39 C. Y. Li, X. B. Zhang, Z. X. Han, B. A. Kermark, L. Sun, 48 A. K. Mahapatra, K. Maiti, P. Sahoo and P. K. Nandi,
G. L. Shena and R. Q. Yua, Analyst, 2006, 131, 388–393. J. Lumin., 2013, 143, 349–354.
14688 | Dalton Trans., 2014, 43, 14680–14688
This journal is © The Royal Society of Chemistry 2014