Synthesis and structural, photophysical, electrochemical redox and axial ligation properties of highly electron deficient perchlorometalloporphyrins and selective CN- sensing by Co(ii) complexes

Article abstract of DOI:10.1039/c7nj04418f

A straightforward synthetic route has been adopted to synthesize highly nonplanar electron deficient perchlorometallo-porphyrins. Herein, we report the synthesis and characterization of MTPP(NO₂)Cl₇ where M = Co^II, Ni^II, Cu^II and Zn^II. Further, we examined their optical and electrochemical redox properties and the results are compared with MTPPCl₈. MTPP(NO₂)Cl₇ exhibited red-shifted (～10-15 nm) absorption spectra relative to MTPPCl₈ due to the strong electron withdrawing nature of the nitro group. Mixed β-substitution alters the electrochemical redox properties to such an extent that an appreciable increase in the anodic shift in reduction potential (200-300 mV) is observed for MTPP(NO₂)Cl₇ relative to MTPPCl₈ whereas only a minimal shift (15-50 mV) in the oxidation potential is observed. Nonplanarity of the macrocyclic core was investigated by single crystal X-ray analysis and DFT calculations. A higher ΔC_β (0.706 ?) for 1d as compared to 2d (0.642 ?) undoubtedly signifies nonplanarity induced by the nitro group. To substantiate the effect of mixed substitution, we performed axial ligation studies of Zn(ii) complexes with nitrogenous bases and basic anions and found higher log:β₂ values as well as a linear relation between log:β₂ and pK_a as compared to perbromoporphyrins. Highly electron deficient β-substituted Co(ii) porphyrins (1a and 2a) were utilized as novel sensors for selective rapid visual detection of CN^- ions.

Full text of DOI:10.1039/c7nj04418f

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Synthesis, Structural, Photophysical, Electrochemical Redox and
Axial Ligation Properties of Highly Electron Deficient
Perchlorometalloporphyrins and Selective CN‒ Sensing by Co(II)
Complexes
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Nivedita Chaudhria, Ray J. Butcherb and Muniappan Sankara*
www.rsc.org/
A straightforward synthetic route has been adopted to synthesize highly nonplanar electron deficient perchlorometallo-
porphyrins. Herein, we report the synthesis and characterization of MTPP(NO2)Cl7 where (M = CoII, NiII, CuII and ZnII).
Further, we examined their electronic spectral and electrochemical redox properties and the results are compared with
MTPPCl8. MTPP(NO2)Cl7 exhibited red-shifted ( ̴ 10-15 nm) absorption spectra relative to MTPPCl8 due to strong electron
withdrawing nature of nitro group. Mixed β-substitution alters the electrochemical redox properties to such an extent that
an appreciable increment in anodic shift in reduction potential (200-300 mV) is observed for MTPP(NO2)Cl7 relative to
MTPPCl8 whereas only a minimal shift (15-50 mV) in the oxidation potential is observed. Nonplanarity of macrocyclic core
was investigated by Single crystal X-ray analysis and DFT calculations. Higher ΔCβ (0.706 Å) for 1d as compared to 2d (0.642
Å) is undoubtedly signifying nonplanarity induced by nitro group. To substantiate the effect of mixed substitution, we
performed axial ligation studies of Zn(II) complexes with nitrogenous bases and basic anions and found higher logβ2 values
as well as a linear relation between logβ2 and pKa as compared to perbromoporphyrins. Highly electron deficient β-
substituted Co(II) porphyrins (1a and 2a) were utilized as novel sensors for selective rapid visual detection of CN− ions.
efficiency to the metal complexes.8,9 The β-substitution by
halogens results into π-electron density redistribution over the
macrocyclic conjugation pathway as well as on nitrogen atoms
Introduction
Porphyrins with exhaustive substitution are of considerable
interest due to their critical role in modulating physico-
chemical properties1, nonplanar conformation2 and acid base
properties of central coordination entity H2N4 of the
macrocycle.3 The distortion in tetrapyrrole macrocycles caused
by steric interactions involving the peripheral substituents
have been investigated in order to recognize the functional
consequences of similar distortions observed in various
proteins.2,4 Introduction of bulky groups at the β-pyrrolic
positions distorts the macrocyclic ring and this effect is more
when more than four halogens or electron withdrawing groups
are present at the β-pyrrolic positions.5 In particular,
substitution with halogens at β-positions of porphyrins seems
to confer some unusual optical properties,6 enhanced
electrochemical redox stability7 and increased catalytic
involving in conjugation and hence alters the acid-base and
axial ligation properties of the metalloporphyrins.10 Axial
ligation dynamics are strongly metal centre dependent based
on the metal electronic configuration. As the porphyrin
macrocycle can accommodate wide variety of metal ions, the
ligand binding and dissociation dynamics can vary according to
the electronic structure and ionic radius of the metal ion as
well as substitution on porphyrin macrocycle. Although β-
substituted, mainly halogenated porphyrins had appeared by
the mid-1970s11, later high-valent metalloporphyrins have
been employed as robust catalyst for the oxidation of organic
substances in presence of good oxygen donors, and the
effectiveness of these catalysts has been traced to the stereo-
chemical features of the porphyrins.12
The degree of nonplanarity is totally built upon the nature of
substituents. The fully β-substituted (dodecasubstituted)
porphyrins exhibited saddle confirmation of the porphyrin
core with displacement of Cβ in the range of 1-1.2 Å from the
porphyrin mean plane.13 Dodecasubstituted porphyrins fall
into two classes namely symmetrically β-substituted and
asymmetrically β-substituted porphyrins. Symmetrically β-
octasubstituted porphyrins have been widely explored due to
their ease of synthesis and exhibited interesting electronic
spectral, electrochemical redox and structural features in
aDepartment of Chemistry, Indian Institute of Technology Roorkee, Roorkee
247667, India. Tel: +91-1332-284753; E-mail: sankafcy@iitr.ac.in.
bDepartment of Chemistry, Howard University, Washington, DC 20059, USA.
Electronic Supplementary Information (ESI) available: 1H NMR, Cyclic
voltammograms, UV-Vis spectral titrations with pyridine and cyanide, DFT Studies
and Single Crystal Structures with selected bond length and bond angles. See
DOI: 10.1039/x0xx00000x
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solution as well as solid state.14 Contrary to the well-studied 1H NMR, ESI-MS and cyclic voltammetric studies and elemental
symmetrically β-substituted octaalkyl15 and octahalo- analyses. The structural aspects were justified by single crystal
porphyrins16 due to their rich spin state and catalytic studies. X-ray analysis and DFT calculations.
However, the chemistry of the unsymmetrically dodeca-
substituted porphyrins having two or more β-substituents
largely unexplored.10a,13b,17
The mixed β-substitution triggered by nitro group is also of
particular interest as a consequence of its versatility and its
feasible conversion into several other functional groups and
wide range applications.18 Metalloporphyrin based sensors
with different metal center have been reported to exhibit
excellent selectivity for specific ions.19 Cyanide is an important
target anion due to its toxic nature. Cyanide anion is known to
inhibit the process of cellular respiration by interacting with
cytochrome c and inhibits the mitochondrial electron transport
chain and causes hypoxia.20 Thus to design an anion receptor
with high selectivity has been the area of intensive research.
Metalloporphyrins were utilized for the detection of CN− ions
due to intrinsic affinity of cyanide for many metal ions such as
zinc, rhodium, copper coordinated to porphyrins.21 Although
the fluorescence or visual detection of cyanide ion by Co(II) or
Zn(II) complexes of organic ligands is known in literature22
whereas this is the first report on selective naked-eye
detection of cyanide ions by Co(II)-porphyrin with low
detection limit (< 1.5 µM).
Metal
Co(II) NO2
Ni(II) NO2
R
Porphyrin
Scheme 1. Facile synthetic route to mixed β-substituted perchlorometalloporphyrins
which are inaccessible by conventional synthesis.
CoTPP(NO2)Cl7 (1a)
NiTPP(NO2)Cl7 (1b)
CuTPP(NO2)Cl7 (1c)
ZnTPP(NO2)Cl7 (1d)
CoTPPCl8 (2a)
Crystal Structure Discussion
Cu(II) NO2
Zn(II) NO2
Co(II) Cl
To determine the correct conformational features and the
extend of sterics offered by NO2 substituent as compared to
Ni(II)
Cl
NiTPPCl8 (2b)
chloro at β-positions; we have examined the crystal structures
of MTPP(NO2)Cl7 (M = Co(II) & Zn(II)), and MTPPCl8 (M = Ni(II)
Cu(II) Cl
Zn(II) Cl
CuTPPCl8 (2c)
ZnTPPCl8 (2d)
and Zn(II)). The crystallographic data of CoTPP(NO2)Cl7(CH3OH)
Chart 1. Molecular structures of synthesized porphyrins.
(
1aꢀ
ZnTPP(NO2)Cl7(Py) (1d.Py) and ZnTPPCl8(CH3OH) (2d
CH3OH), NiTPPCl8 (2b), ZnTPP(NO2)Cl7(CH3OH)(1d
ꢀ
ꢀ
CH3OH),
CH3OH)
are listed in Table S1, electronic supplementary Information
(ESI).
Results and Discussion
Synthesis and Characterization
The ORTEP top views and displacement of macrocyclic core
Herein, a straightforward synthetic route has been adopted to
prepare synthetically nonviable mixed dodecasubstituted
porphyrins through direct functionalization instead of taking β-
substituted pyrrole, which is economically challengeable
from mean plane for ZnTPP(NO2)Cl7(CH3OH) (1dꢀCH3OH) and
ZnTPPCl8(CH3OH) (2d CH3OH) are shown in Fig. 1. The top
ꢀ
ORTEP views for MTPP(NO2)Cl7 (M = Co(II) and Zn(II)) and
NiTPPCl8 (1b) are shown in Fig. S1, ESI. Selected average bond
lengths and bond angles of these five porphyrins are listed in
Table S2 in the ESI. All the crystallized porphyrins have shown
the saddle shape conformation of macrocyclic core with very
high ΔCβ (±0.642-±1.028 Å) and Δ24 values (Fig. S3 in the ESI),
which clearly describing severe nonplanar nature of
macrocyclic core. This is further supported by the increment in
(Scheme1). 1b was synthesized by direct chlorination of
23a
NiTPPNO2
using N-chlorosuccinimide (NCS).23b Along with
desired product, we also obtained 2b as the side product, this
is possibly due to replacement of nitro group by chloro group
at high temperature.
MTPP(NO2)Cl7 and MTPPCl8 (where M = CoII, CuII, ZnII) were
prepared by acid demetallation of respective Ni(II) complexes
followed by conventional metallation with different metal
acetates in CHCl3/MeOH mixture in good yields. These
Cβ-Cα-Cm angle (
Cα-Cm angle ( 124
length ( 1.345 Å) as compared to reported planar porphyrins.
~
127
°
) with simultaneous decrement in the N-
~
°
) accompanied with larger Cβ–Cβ bond
~
metalloporphyrins
were
characterized
by
various
The nonplanarity of the macrocyclic core is enhanced by the
steric repulsion among the peripheral substituents, which
spectroscopic techniques including UV-Visible, Fluorescence,
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implements the relief of the strain through bond lengths and 1H NMR spectra of Ni(II) and Zn(II) complexes were recorded in
angles. The decreased N-M-N values (171 -162
) also indicates CDCl3 at 298K. The 1H NMR spectra of Ni(II) complexes are
°
the saddle shape conformation of the macrocyclic core. The shown in Figs. S4 and S5 in the ESI. The spectral data for
°
higher ΔCβ for 1d as compared to 2d reflecting the impact of MTPPCl8 where M = 2H, Co(II), Ni(II), Cu(II) and Zn(II) are
NO2 group over Cl group (effect of mixed β-substitution).
exactly matching with the reported literature.1a,16d,19d The
integrated intensities of the protons are in accordance with
the proposed structure. MTPP(NO2)Cl7 shown an asymmetric
multiplet for ortho-phenyl proton relative to their
corresponding homosubstituted MTPPCl8 (Fig. S4 in the ESI)
which has indicated the reduced symmetry offered by mixed
substitution. The ESI mass spectra of MTPP(NO2)Cl7 were
recorded in CH3CN and the observed mass values are matching
with the proposed structures (Figs. S6-S9 in the ESI).
The UV-Visible spectra of all metal derivatives were recorded
in CH2Cl2 at 298 K. Table 1 lists the electronic spectral data of
synthesized porphyrins in CH2Cl2 at 298 K. MTPP(NO2)Cl7
exhibited similar spectral features to MTPPCl8 and showed red
shift in Soret band (
Δλmax = 4-12 nm) as well as in Qx(0,0) band
(
Δλmax = 8–14 nm) relative to MTPPCl8 (Fig. S10), this
attributes to the strong electron withdrawing nature of nitro
group which pulls the electron density from the porphyrin ring
which stabilize the LUMO results in reduced HOMO-LUMO
gap, and also from induced nonplanarity due to more
electrostatic repulsion between mixed β-substituents. The
comparative optical absorption spectra of 1d and 2d
complexes is shown in Fig. 3a.
HOMOs of normal porphyrins usually situated at β-pyrrole
positions and substitution at this position with more electron
withdrawing substituents will further lift the degeneracy of a1u
and a2u orbitals by stabilization of a1u relative to a2u which
resulted into the depletion of intensity gap ratio between
QX(0,0) and QX(1,0) for 1d as compared to 2d as shown in inset of
Fig. 3a. Fig. 3b represents the comparative emission spectra of
1d and 2d complexes in CH2Cl2 at 298K. Due to heavy atom
effect, these perchlorinated-2-nitrotetraphenylporphyrins also
exhibited very feeble emission intensity with very low
quantum yields like bromo derivatives.10a 1d exhibited
relatively more intense and red-shifted emission spectra
relative to 2d.
Fig. 1 ORTEP diagrams showing top views of (a) ZnTPP(NO2)Cl7 (1d) and (b) ZnTPPCl8
(2d). Displacement of porphyrin core atoms (in Angstroms) from the porphyrin mean
plane for ZnTPP(NO2)Cl7 (1d) and ZnTPPCl8 (2d), respectively
Table 1 optical absorption and emission spectral dataa of MTPP(NO2)Cl7 and MTPPCl8.
porphyrin
B band
Q (band)s
λem
ϕf
1a
2a
1b
2b
1c
2c
1d
2d
447(186.2) 562(16.59), 605(9.77)
437(147.9) 556(12.30)
448(128.8) 562(10.96), 604(6.31)
439(190.5) 554(15.13)
Fig. 2 Packing diagram of CoTPP(NO2)Cl7 (1a) in a unit cell.
449(117.5) 576(12.88), 624(6.46)
437(158.4) 571(17.78), 610(6.16)
459(158.5) 593(9.12), 647(8.31)
455(204.3) 592(10.2), 646(9.55)
The packing diagram of 1a is showing that the NO2 group of
729
708
0.0009
0.0019
one porphyrin unit is interacting with the hydrogen of another
unit. Fig. S2 shows the packing diagram of ZnTPP(NO2)Cl7 (1d
)
a λ/nm. The values in parentheses refer to ε × 10-3 Lmol-1 cm-1
.
and ZnTPPCl8
(
2d), respectively. Both the porphyrins have
shown two molecules in a unit cell.
NMR and Electronic Spectral Studies
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homosubstituted porphyrins (MTPPCl8), whereas a minimal
anodic shift (12-86 mV) was observed in the oxidation
potentials which ascribed the impact of the strong electron
withdrawing nature of nitro group on porphyrin π-system of
macrocyclic core. MTPP(NO2)Cl7 have shown the reduced
HOMO-LUMO gap as compared to MTPPCl8 (Table 2).
Fig. 4 Comparative Cyclic voltammograms of 1a and 1b in CH2Cl2 under argon at room
temperature with 0.1 M TBAF6 as the supporting electrolyte at a scan rate of 100 mV/s.
Table. 2 Electrochemical redox dataa of MTPP(NO2)Cl7 and MTPPCl8 in CH2Cl2 at 298K.
Oxidation
II
Reduction
I
Por.
ΔE
Fig. 3 (a) Electronic absorption spectra of 1d (8.45×10-6 M) and 2d (7.09×10-6 M) in
CH2Cl2, inset shows expended region for Q bands. (b) Electronic emission spectra of 1d
and 2d in CH2Cl2 at 298 K.
I
CoII/III CoII/I
II
1a
2a
1b
2b
1c
1.34
1.13
1.27
1.22
1.13
1.04
0.99
0.98
1.51
1.25
-
1.00
-0.27
-1.12
-1.44
-0.64
-0.85
-0.66
-0.85
-0.92
-1.12
-1.60d
-
2.46
2.57
1.91
2.07
1.79
1.89
1.91
2.10
0.91
-0.37
-
-
-
-
-
-
-
-
-
-
-
-
-0.89
-1.16
-0.89
-1.11
-1.18
-1.47d
Electrochemical Studies
-
A wide variety of highly substituted halogenated porphyrins
have been examined in non-aqueous media. The
electrochemical behaviour of perhalogenated porphyrins have
been well documented in the literature.7b,17b,24 To analyse the
influence of nitro groups at β-position on the redox potentials
of porphyrin core, the electrochemical redox properties of
MTPP(NO2)Cl7 were examined by cyclic voltammetric studies in
distilled CH2Cl2 at 298 K under argon using 0.1 M TBAPF6 as the
supporting electrolyte. For comparison, the MTPPCl8 were also
investigated under similar conditions and the electrochemical
redox data was found in good agreement with reported value
in literature.14c,16d Comparative cyclic voltammograms of 1a
and 2a are shown in Fig. 4. Fig. S11 in the ESI represents the
comparative cyclic voltammograms of MTPP(NO2)Cl7 (M =
Cu(II) and Ni(II)) and MTPPCl8 (M = Cu(II) and Ni(II)). The
observed behaviour of 1a is quite similar to 2a containing two
reversible reduction and three reversible oxidations but there
is a remarkable shift in redox potentials. Table 2 lists the
electrochemical redox data of metal complexes of
MTPP(NO2)Cl7 and MTPPCl8 in CH2Cl2. The β-nitro substituted
porphyrins are easier to reduce and harder to oxidize as
compared to homosubstituted porphyrins. Interestingly, nitro
bearing porphyrins exhibited a significant anodic shift (190-325
mV) in the first ring reduction potentials as compared to
1.63
-
2c
1d
2dc
1.23
1.22
a In V, vs Ag/Agcl electrode, ddata obtained from differential pulse voltammetry c
in 5% THF in CH2Cl2
Axial Ligation Studies
To demonstrate the effect of basicity of axial ligands and
electron withdrawing substituents (chloro, nitro) on the acidity
of Zn(II) center of the porphyrin ring, we have examined the
axial ligation studies of ZnTPP(NO2)Cl7 (1d) and ZnTPPCl8 (2d
)
with various nitrogenous bases (pyridine, N-methylimidazole,
4-dimethylaminopyridine and piperidine) and basic anions
such as CN‒, OAc‒, F‒ and H2PO4 in the form of their
respective tetrabutylammonium salts in toluene at 298 K.
Fig. 5a represents the concomitant decrement in the
absorbance of ZnTPP(NO2)Cl7 (
1d) (8.29 × 10-6 M) at 460 nm
‒
with emergence of a new B-Band at 465 nm accompanied by 5
nm red shift upon sequential addition of pyridine (4-150 µM).
As axial ligation proceeds, the decrement in the Q(1,0) band
with 16 nm red-shift with simultaneous increment in Q(0,0)
band accompanied with 29 nm red-shift was observed. A plot
of [base] vs. [base]/(ΔA) show a straight line which in turn
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indicates the 1:1 complexation of porphyrin:base as shown in Similar UV-Vis spectral changes have been perceived for
inset of Fig. 5a.
The equilibrium constants (Keq) were calculated using literature
method.10a,25 The equation for the analysis is as follow:
ZnTPPCl8 (2d) with pyridine (Fig. S12 in the ESI), which also
shown 1:1 stoichiometry (porphyrin to pyridine) indicated by
slope of regression line. The axial ligation of other bases also
performed with 1d and 2d and found to be axially ligated with
porphyrins in 1:1 stoichiometric ratio. Table 3 lists the
equilibrium constant data, which clearly reveals that 1d having
higher equilibrium constant values as compared to 2d, this
again attributing to the enhanced nonplanarity by mixed
substitution and increased acidic nature of Zn(II) core of
porphyrin due to substitution with electron withdrawing nitro
group.
1/Keq = [base]/{[por.]L(εp-εpa)/(Ao-An)-1} Eq.1
Where εp is molar extinction coefficient for the Zn(II)
complexes and εpa is molar extinction coefficient for
porphyrin:anion/base complex, respectively. A0 is the initial
absorbance and An is the absorbance in presence of added
base or anion at a particular wavelength. L is the path length of
the cell.
Table. 3 Equilibrium constantsa (Keq) for the ligation of nitrogenous bases with
Zn(II) complexes of perchloroporphyrins in toluene at 298 K.
Base
1d
2d
pKa
K
P:B
1:1
K
P:B
1:1
Piperidine
4-DMAP
2076.97
212.12
1710.31
163.00
11.22
9.70
1:1
1:1
N-Methylimidazole
Pyridine
219.68
57.01
1:1
1:1
159.37
46.85
1:1
1:1
7.33
5.23
aThe Keq values refers to (×103 Keq/M-1) within the error range of ± 6%, P refers
to porphyrin and B refers to base.
To gain insight into the electronic effect of chloro vs. bromo on
the ligation properties, we compared the data of
perchlorinated porphyrins with perbrominated porphyrins
reported in litrature.10a Data listed in table 3 revealing high
binding constant for chloro complexes as compared to bromo
derivatives is straightforward indication of higher electron
deficient nature of porphyrin. The plot constructed for 1d and
2d between logKeq and the pKa values of used bases is shown
in Fig. 6 which displayed a linear relation with Keq. 1d showing
comparative impartial straight line as compared to
ZnTPP(NO2)Br7 with higher correlation coefficient. Among all
used bases, the trend in Keq values was found to be in
accordance with their pKa values, higher the pKa value greater
is the equilibrium constant.
a
Table. 4 The overall binding constants (β2
)
for the ligation of basic anions with Zn(II)
Fig. 5 (a) Axial ligation of pyridine to 1d (8.29×10-6 M) in toluene at 298K. (b)
Axial ligation of CN- ion to 1d (8.29×10-6 M) in toluene at 298K. Main plots show
the spectral changes in Soret region and insets show plot [Pyridine] vs.
[Pyridine]/ΔA.
complexes of perchloroporphyrins in toluene at 298K.
1d
2d
log β2
Anions
β2
log β2
P:A
β2
A:P
CN-
OAc-
H2PO4
F-
5.75×1010
2.45×1010
2.33×1010
9.22×108
10.75
10.38
10.36
8.96
1:2
1:2
1:2
1:2
3.53×1010
2.01×1010
1.12×1010
7.78×108
10.54
10.30
10.04
8.89
2:1
2:1
2:1
2:1
-
a
within the error range of ±5% for
anion.
β2, P refers to porphyrin and A refers to
The effect of electron withdrawing groups on the acidity of ZnII
core of the porphyrin π-system further proved by performing
again axial ligation studies with highly basic anions such as CN-,
Fig. 6 (a) comparative plot of log Keq Vs pKa is given for 1d and 2d. (b) Comparative plot
of log Keq Vs pKa is given for ZnTPP(NO2)Br7 and ZnTPPBr8 (values are taken from ref10a).
-
OAc-, F- and H2PO4 in the form of their tetrabutylammonium
salts in toluene at 298 K. Fig. 5b represents the changes in
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Soret Band region upon successive addition of CN- (1-18 µM) The higher binding constant for 1a is clearly reflecting the
to 1d in toluene at 298 K. The UV-vis. spectral features of highly electron deficient nature of nitro group.
anion binding are almost similar to that of base binding. While
increasing the concentration of cyanide ion to 1d (8.29 × 10-6
M), the decrement in absorbance at 459, 589, 644 nm and the
concomitant increments in the absorbance at 475 and 690 nm
was observed with multiple isosbestic points. The overall
binding constant and stoichiometry were analyzed by Benesi-
Hildebrand method. 1d and 2d both were found to be axially
ligated with basic anions in 1:2 (Porphyrin:Anions)
stoichiometric ratio (Figs. S13 and S14 in the ESI). Similar
spectral changes were observed during titration of 2d
(8.38×10-6 M) with CN‒ (Fig. S14b). A standard Benesi−
Hildebrand plots were also constructed in 1:1 stoichiometric
ratio from the titration data of 1d and 2d with cyanide ion as
shown in Figure S15 in the ESI. The cyanide binding was not
accurately modeled by these plots clearly indicating that the
binding of anions with 1d and 2d cannot be fit in 1:1
stoichiometric fashion. Other anions were also tested with
both Zn(II) complexes and the data for equilibrium constant is
listed in Table 4. Data revealed that as the basicity of anion
increases the equilibrium constant also increases.
Selective Cyanide Ion Sensing by Cobalt(II) Complexes
The use of 3d transition metal complexes of porphyrins for
Fig. 7 (Top) colorimetric changes of 1a with tested anions in toluene at 298 K. (Bottom)
anion sensing is cost effective and environmental friendly. The
UV-Visible spectral changes of 1a upon addition of excess of anions in the form of TBA
use of free base complexes of perhaloporphyrins for detection
salts in toluene at 298K.
of highly basic anions through anion induced deprotonation
process23b and Ni(II) complexes of perchloroporphyrins for
selective detection of cyanide ions through axial coordination
of CN- ion are reported by us.19d The anion recognition
properties of Co(II) complexes of these porphyrin were studied
in toluene with various anions by UV-Visible spectroscopy with
the sequential addition of their TBA salts. Interesting, their
remarkable feature was identified to specially recognize
cyanide ion in presence of all other anions. 1a and 2a showed
remarkable red shifted absorption spectra with cyanide ion
with distinct colour change whereas no observable shift was
found with other tested anion as shown in Figs. 7 and S16 in
the ESI.
UV-Visible spectral titration of 1a (1.04×10-5 M) with cyanide
ion is shown in Fig. 8. As we increase the concentration of
cyanide ion (0.13 × 10-5 - 1.58 × 10-5 M), the decrement in the
absorbance of 1a was observed at 447 and 561 nm with the
concomitant increment at 397, 489 and 682 nm with multiple
isosbestic points. The same behavior was observed for 2a
(1.05×10-5 M) with sequential addition of CN- ion as shown in
Fig. 8 UV-Visible spectral titration of 1a (1.04×10-5 M) upon sequential addition of
Figure S17 in the ESI. Both the Soret (43-45 nm) and Q bands cyanide ion in toluene at 298K. insight shows Benesi-Hildebrand plot.
(113-120 nm) were highly red-shifted upon addition of aliquots
of CN- ions to 1a and 2a. The stoichiometry and binding
The cyanide detection behaviour of these porphyrins (1a and
constant were calculated by Benesi-Hildebrand method by
plotting a graph between 1/[ΔA] and 1/[CN-] which shows a
straight line with r2 = 0.99. The representative plot of Keq for
2a) was also revealed by differential pulse voltammetric (DPV)
studies. The DPV changes of 1a and 2a upon addition of
cyanide ion were carried out in CH2Cl2 at 298 K using 0.1 M
cyanide ion sensing is shown as insets of Figs. 8 and S17 (ESI).
TBAPF6 as supporting electrolytes. Interestingly, the successive
logK values were found to be 4.89 and 4.59 with 1:1
addition of cyanide ions into 1a results in a cathodic shift of
(porphyrin:cyanide) stoichiometry for 1a and 2a, respectively.
310 mV in the first oxidation potential (Fig. 9) and a dramatic
cathodic shift of 700 mV in the first reduction potential
6 | J. Name., 2012, 00, 1-3
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possibly due to cyanide ion induced metal centered oxidation The addition of TFA to 1a-CN- adduct does not restore the
(CoII to CoIII) and leads to cyanide coordination to the metal absorption spectra of 1a which clearly indicate that the
center, 1a·CN−. This cathodic shift in the first redox potentials behaviour of cyanide ion binding is not reversible as indicated
for 2a is also pronounced (Fig. S18 in the ESI). Fig. S19 in the by the different spectral behavior of 1a-CN‒ with TFA and
ESI represents the DPV titration of 1a and 2a after successive without TFA. Similar behaviour has been found for 2a as
addition of cyanide ion.
shown in Fig. S24, ESI.
Fig. 9 DPV (in V vs Ag/ AgCl) traces recorded for 1a (blue) and 1a.2CN- (red) in CH2Cl2
containing 0.1M TBAPF6 with a scan rate of 0.1 Vs-1 at 298 K.
Interestingly, the most common interfering anions especially
basic anions such as fluoride, acetate and dihydrogen
phosphate were inactive towards these sensors. The
ratiometric experiments were performed to ascertain the
selectivity of synthesized Co(II) porphyrins using 10 equivalents
of interfering anions followed by two equivalents of cyanide
ions in toluene. The representative bar graph observed for 1a
is shown in Fig. 10 which clearly demonstrates the tolerance of
1a towards these potentially interfering anions and thus can
be used quantitatively and selectively to detect cyanide ions.
Similar results were observed for 2a as shown in Fig. S20 (ESI).
Fig. 11 Reversible studies of 1a using 1 mM solution of TFA in toluene at 298 K.
Scheme 2. The plausible mechanism for cyanide ion sensing.
DFT Studies
The geometry optimization of Zn(II) and Co(II) complexes has
been performed on Gaussian 09 using B3LYP functional with
LANL2DZ basis set in gas phase. Fig. S22 in ESI shows the top
and side views of the fully optimized geometries of 1d and 2d
respectively. To get insight into the electronic modulation of
porphyrin core after cyanide binding and their impact onto the
molecular orbitals, further we performed the geometry
optimization of Co(II) complexes (low spin; S = ½) with cyanide
ion i.e Co(III) complex, as well as TD-DFT has also been
evaluated to obtain the theoretical electronic spectra. Fig. 12
Fig. 10 Ratiometric absorbance changes (A490/A447) of 1a (1.04 × 10-5 M) on addition of
1-2 equiv. of CN− and 10 equiv. of other anions. Green bars indicate the blank and in
presence of other interfering anions, and purple bars indicate the addition of CN- to the
interfering anions
shows the fully optimized geometries of CoTPP(NO2)Cl7 (1a)
and CoTPP(NO2)Cl7.CN‒ 1a-CN-), whereas the optimized
(
geometries of CoTPPCl8 (2a) complex with and without cyanide
ion is given in Fig. S23 in the ESI. The repulsive interaction
between β-substituents leads to the saddle shape
conformation of the porphyrins core. As expected from
Gouterman’s four-orbital model the Soret transitions of these
metalloperhaloporphyrins were found to involve substantial
a1u to eg character. Fig. S24 in the ESI represents the frontier
To recognize the range of applicability of 1a and 2a the
reversibility test has been performed in toluene. 1a was
treated with 2 equivalent of cyanide ion resulted in a red-shift
of 43 nm in Soret band (447-490 nm) which corresponds to
Co(II) to Co(III) due to cyanide ion binding (scheme 2). The
reversibility test was attempted using TFA as shown in Fig. 11.
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molecular orbitals of Zn(II) complexes, 1d and 2d, respectively 2, 2-tetrachloroethane was purchased from Rankem and dried
which were exactly matching with FMOs for ZnBr8 and ZnI8 before use over molecular sieves (4Å). Silica gel (100-200
complexes given by Ghosh et al.26 The HOMO were found as b2 mesh) was purchased from Rankem used as received. TBAPF6
(a2u type) and HOMO-1 as b1(a1u type).
received from HiMedia and recrystallized twice with ethanol
and dried at 50 ºC under vacuum for 2 days. Pyridine was
purchased from Thomas Baker and piperidine purchased from
SDFCL was of analytical grade and used without further
purification. N-Methylimidazole and dimethylaminopyridine
were purchased from HiMedia and used as received. The
-
tetrabutylammonium salts (TBAX, X = CN-, F-, OAc- and H2PO4 )
were purchased from Alfa Aesar and used as received. Dry
CH2Cl2 for CV analysis distilled twice with P2O5 and third time
from CaH2 whereas dry toluene (for UV-Visible spectral
studies) was dried and distilled from sodium benzophenone
mixture.
Instrumentation and Methods:
Optical absorption and emission spectra were recorded on
Agilent Cary 100 UV-Vis. spectrophotometers and Hitachi F-
4600 spectrofluorometer, respectively using a pair of quartz
cells of 10 mm path length and 3.5 ml volume. 1H NMR spectra
were recorded on Bruker AVANCE 500 MHz spectrometer in
CDCl3. The ESI mass spectra were recorded on Bruker Daltonics
microTOF mass spectrometer in the negative ion mode using
CH3CN as solvent. Elemental analyses were performed on
Elementar vario EL III instrument. The X-ray quality single
Fig. 12 Fully optimized geometries of (a and c) top and side views of CoTPP(NO2)Cl7
(1a), (b and d) top and side views of CoTPP(NO2)Cl7•CN- (1a•CN-) .H atoms in top views
and phenyl rings in side views are omitted for clarity
In 2002 Scheiner and his coworker explained that for a simple
1
CoTPP, the porphyrin MOs (a2u and a1u) lie in between eg (dπ)
and dz2 of the metal due to the stabilization of MOs of
porphyrin by metal (Co) and Porphyin orbital interaction.27
However, the introduction of electron withdrawing
substituents at porphyrin periphery switches the HOMO from
metal 1eg to a2u type. The open shell molecular orbitals and
spin density profile of CoTPP(NO2)Cl7 is clearly indicating the
significant orbital interaction of the metal (dx2-y2) and a2u of
porphyrin. SUMOs (b2 and b1) of electron deficient saddle
shaped porphyrins are in good agreement with the reported in
literature.26,28,29 The axial coordination of the cyanide ion
changed the spin and multiplicity of system which again
perturbed the FMOs. Fig. S27 represents the theoretical UV-
Vis. spectra of 1a and 2a obtained by TD-DFT in gas phase
which are fairly matching with the experimental UV-vis.
spectra. The broader absorption for 1a as compared to 2a due
to the presence of nitro group is also confirmed by TD spectra.
The obtained UV-Vis spectra of 1a and 2a with cyanide ion are
given in Figs. S28 and S29, respectively. The spectral features
and absorption spectral data are also matching with the
experimental UV-Visible spectra. Red-shifts in the Soret band
of cobalt complexes and charge transfer band upon axial
coordination of cyanide ion were also observed in TD-DFT
spectra.
crystals of 1a, 1d, and 2d were obtained by vapour diffusion of
methanol onto a CHCl3 solution of respective porphyrins. 1d
was also crystalized with vapour diffusion of hexane onto the
CHCl3 solution of 1d with few drops of pyridine, and the single
crystals of 2b were obtained by vapour diffusion of methanol
onto the toluene solution of 2b. The single crystal XRD data
were analysed on
a Bruker Apex-II CCD diffractometer
equipped with graphite-monochromated Mo Kα (λ = 0.7107 Å)
by the ω-2θ scan. Crystallographic data are summarized in the
table S1 in the ESI. ORTEP views for ZnTPP(NO2)Cl7(CH3OH)
(
1d
ꢀ
= Co(II) and Zn(II)) and NiTPPCl8
CH3OH), ZnTPPCl8(CH3OH) (2dꢀCH3OH), MTPP(NO2)Cl7 (M
1b) are shown in Figures 1
(
and S1 in the ESI. Selected average bond lengths and bond
angles of these five porphyrins are listed in Table S2 in the ESI.
The octachlorometalloporphyrins were solved in a straight
forward manner whereas mononitroheptachloroporphyrins
were tough to solve since nitro group disordered over chloro
group(s). The correct occupancies of nitro and chloro groups
were in the corresponding crystallographic information files.
The ground state geometry optimization for Co(II) and Zn(II)
complexes was performed using B3LYP as functional and
LANL2DZ as the basis sets. Electrochemical redox studies were
carried out with a BAS Epsilon electrochemical workstation
using a three electrode assembly consists of GC working
electrode, Ag/AgCl as a reference electrode and Pt-wire as a
counter electrode. All measurements were carried out in triple
distilled CH2Cl2 which was degassed by Argon, containing 0.1
M TBAPF6 as the supporting electrolyte with scan rate of 100
Experimental Section
Materials:
Pyrrole,
C6H5CHO,
Co(OAc)2•4H2O,
Ni(OAc)2•4H2O,
Cu(NO3)2•3H2O, Zn(OAc)2•2H2O , Na2S2O4, NaHCO3 and N-
Chlorosuccinamide were purchased from HiMedia INDIA and
used as received. All solvents employed in presented work
were of analytical grade and distilled or dried before use. 1, 1,
µA. The concentration of all metal complexes was maintained
1 µM throughout the whole experiment. Axial ligation studied
were carried out in distilled toluene at room temperature,
̴
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equilibrium constant were calculated using already given spectroscopic techniques and single crystal X-ray analysis. The
equation in axial ligation studies. Using the above equation the results were compared with homosubstituted MTPPCl8 to
Keq can be evaluated by taking the negative of the 1/intercept.
highlight the effect of nitro group over their macrocyclic
conformation as well as their spectral properties. Nitro bearing
porphyrins exhibited 10-15 nm red-shift in the electronic
spectra and dramatic anodic shift in the reduction potentials.
The lower Cβ'-Cα'-Cm and higher Δ24 values for 1d as compared
to 2d indicate the effect of mixed β-substitution induced
nonplanarity by the nitro group. The axial ligation studies of
Zn(II) perchloroporphyrins with nitrogenous bases and basic
anions revealed the higher Lewis acidity of the metal centre
compared to perbromoporphyrins which is further evidenced
by higher logβ2 values. The electron deficient Co(II) porphyrins
Synthetic procedures
1b and 2b were synthesized by chlorination of NiTPPNO2 with
15 equivalents of N-chlorosuccinimide (NCS) in 1,1,2,2-
Tetrachloroethane.23b Free base porphyrins, H2TPP(NO2)Cl7
and H2TPPCl8 were prepared by acid demetalltion of 1b and 2b
using con. H2SO4.23b
General Procedure for the Synthesis of MTPP(NO2)Cl7 and MTPPCl8
(where M = CoII, CuII and ZnII):
(
1a and 2a) were utilized as sensors for the selective rapid
Free base porphyrins, H2TPP(NO2)Cl7 and H2TPPCl8 (25-30 mg)
were dissolved in 20-25 ml of distilled CHCl3. To this, 10 equiv.
of the corresponding metal acetate hydrate in 2 mL of
visual detection of cyanide ions for the first time in porphyrin
chemistry.
°
methanol was added and refluxed for 30 minutes at 75 C on
Acknowledgements
water bath. The solvent was removed by rotary evaporation
and the crude porphyrin was washed with water to remove
excess of metal salt and purified by silica-gel column
chromatography using chloroform as eluent. Further, these
metalloporphyrins were recrystallized by chloroform and
methanol mixture (1:5, v/v) and dried in vacuum oven for
We are grateful for the financial support provided by
Engineering Research Board (EMR/2016/4016). NC thanks
Council of Scientific and Industrial Research (CSIR), India for
the SRF fellowship.
0
overnight at 75-80 C. Yields were found to be in the range of
80-90%.
References
The spectroscopic data of MTPPCl8 was exactly matches with
the reported literature.
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CoTPP(NO2)Cl7
: UV/Vis (CH2Cl2): λmax (nm) (ε ×
10-3
L mol-1 cm-1) 447(186.2), 562(16.59), 605(9.77). ESI-MS (m/z):
found 992.821 [M+K]+, calcd 992.840. Anal. Calcd for
C44H20N5O2Cl7Co: C, 55.18; H, 2.10; N, 7.31, Found: C, 54.98; H,
2.12; N, 7.40. NiTPP(NO2)Cl7: UV/Vis (CH2Cl2): λmax (nm) (ε × 10-
2
3
L mol-1 cm-1) 448(128.8), 562(10.96), 604(6.31). 1H NMR in
CDCl3 (500 MHZ): δH (ppm), 7.94-7.87 (m, 8H, meso-o-Ph-H),
7.73-7.60(m, 12H, meso-m,p-Ph-H). ESI-MS (m/z): found
991.80
[M+K]+,
calcd
991.84.
Anal.
Calcd
for
C44H20N5O2Cl7Ni•(0.5CH2Cl2): C, 53.45; H, 2.12; N, 7.00, Found:
C, 53.19; H, 2.09; N, 7.34. CuTPP(NO2)Cl7: UV/Vis (CH2Cl2): λmax
(nm) (ε × 10-3 L mol-1 cm-1) 449(117.5), 575(12.88), 605(6.46).
ESI-MS (m/z): found 996.90 [M+K]+, calcd 996.84. Anal. Calcd
for C44H20N5O2Cl7Cu: C, 54.91; H, 2.09; N, 7.28. Found: C,
54.48; H, 2.10; N, 7.50. ZnTPP(NO2)Cl7: UV/Vis (CH2Cl2): λmax
(nm) (ε × 10-3 L mol-1 cm-1) 457(158.5), 590(9.12), 647(8.31). 1H
3
4
5
NMR in CDCl3 (500 MHZ): δH (ppm) 7.51-7.47 (m, 8H, meso-o-
3
Ph-H), 7.20-7.13 (m 10H, meso-m,p-Ph-H), 7.04-7.01 (t, JH,H
8Hz, 2H, meso-m,p-Ph-H). ESI-MS (m/z): found 997.190 [M+K]+,
calcd 997.836. Anal. Calcd for C44H21N5O2.5Cl7Zn (0.5CH3OH): C,
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New Journal of Chemistry
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DOI: 10.1039/C7NJ04418F
Graphical Abstract
Selective Naked Eye CN‒ Sensing by Co(II) porphyrins
447
1a.CN-
1a
489
CN-
CoII/CoIII
561
679
We report the facile synthesis of perchlorinated-2-nitro-meso-tetraphenylporphyrin metal complexes
and their characterization by single crystal X-ray, spectroscopic and electrochemical analysis. The axial
ligation studies of Zn(II) perchloroporphyrins with nitrogenous bases and basic anions revealed the
higher Lewis acidity of the metal centre as compared to known porphyrins. Co(II) porphyrins were
utilized as selective rapid visual cyanide ion sensors for the first time in porphyrin chemistry.