Polyfunctional lewis acids: Intriguing solid-state structure and selective detection and discrimination of nitroaromatic explosives

Article abstract of DOI:10.1002/chem.201500727

Synthesis and crystal structures of three porphyrin-based polyfunctional Lewis acids 1-3 are reported. Intermolecular HgCl...HgCl (linear and μ-type) interactions in the solid state of the peripherally ArHgCl-decorated compound 3 lead to a fascinating 3D supramolecular architecture. Compound 3 shows a selective fluorescence quenching response to picric acid and discriminates other nitroaromatic-based explosives. For the first time, an electron-deficient polyfunctional Lewis acid is shown to be useful for the selective detection and discrimination of nitroaromatic explosives. The Stern-Volmer quenching constant and detection limits of compound 3 for picric acid are the best among the reported small-molecular receptors for nitroaromatic explosives. The electronic structure, Lewis acidity, and selective sensing characteristics of 3 are well corroborated by DFT calculations.

Full text of DOI:10.1002/chem.201500727

DOI: 10.1002/chem.201500727
Full Paper
&
Porphyrins
Polyfunctional Lewis Acids: Intriguing Solid-State Structure and
Selective Detection and Discrimination of Nitroaromatic
Explosives
Chinna Ayya Swamy P and Pakkirisamy Thilagar *[a]
Abstract: Synthesis and crystal structures of three porphy-
rin-based polyfunctional Lewis acids 1–3 are reported. Inter-
molecular HgCl···HgCl (linear and m-type) interactions in the
solid state of the peripherally ArHgCl-decorated compound
3 lead to a fascinating 3D supramolecular architecture. Com-
pound 3 shows a selective fluorescence quenching response
to picric acid and discriminates other nitroaromatic-based
explosives. For the first time, an electron-deficient polyfunc-
tional Lewis acid is shown to be useful for the selective de-
tection and discrimination of nitroaromatic explosives. The
Stern–Volmer quenching constant and detection limits of
compound 3 for picric acid are the best among the reported
small-molecular receptors for nitroaromatic explosives. The
electronic structure, Lewis acidity, and selective sensing char-
acteristics of 3 are well corroborated by DFT calculations.
Introduction
Aryl mercuryl compounds have been used as precursors to
synthesize other organometallic compounds through transme-
talation reactions.[1] Recently, multifunctional organomercury
compounds have been used as Lewis acid catalysts, receptors
for the recognition of anions and neutral Lewis basic spe-
cies,[2,3] and as building blocks for supramolecular structures.[4]
In this context, seminal contributions from Kochi and co-work-
ers,[5a] Dean and Damude[5b,c] and Olah and co-workers[5d] are
notable. Recently, Gabba¨ı and co-workers elegantly utilized tri-
meric perfluoro-o-phenylene mercury as a supramolecular host
for several p-conjugated guest molecules.[6] The work of Jꢀkle
and co-workers on metallocene-based mercuryl compounds is
also noteworthy in this context.[7]
Among various organic p systems, porphyrins[8g,h] and ex-
panded porphyrins[8c,k] occupy a special position and have
found wide applications in organic light-emitting diodes,[8d,e,p,q]
Scheme 1. Synthesis of compounds 1–3. i) HC(OEt)3/EtOH/HCl (cat.). ii) 1) n-
BuLi/Et2O (À788C); 2) Me3SnCl; 3) 1% KHSO4/THF. iii) Pyrrole/BF3·Et2O/CHCl3,
DDQ/benzene. iv) Zn(OAc)2·2H2O/MeOH; v) HgCl2/THF/Et2O.
photodynamic
cancer
therapy,[8j]
photovoltaics,[8b,n]
catalysis,[8a,f,i,o] and chemosensing.[8l,m] The meso-functionalized
porphyrins exhibit different intramolecular electronic coupling
and coordination chemistry beyond classical coordination com-
pounds. Despite the vast developments in porphyrin chemistry,
surprisingly, the chemistry of peripheral organomercuryl
porphyrins has not been reported to date. This may be
attributable to difficulties in synthesizing mercury-substituted
porphyrins in good yields. Herein, we report a facile synthetic
route to peripherally Hg decorated porphyrin 3 (Scheme 1),
which can be regarded as a polyfunctional Lewis acid. At-
tempts were made to understand the fundamental electronic
and solid-state structure of 3 and to utilize of its properties for
potential applications.
Recently, fluorescent organic
p
systems have been
[a] Dr. C. A. Swamy P, Prof. Dr. P. Thilagar
Department of Inorganic and Physical Chemistry
Indian Institute of Science (IISc)
Bangalore 560012 (India)
comprehensively studied for detection of nitroaromatics.[9] Ni-
troaromatics detection is highly relevant to applications related
to environmental pollution control, forensic analysis, and for
national security (defense and antiterrorism).[10] Among chemi-
cal explosives, trinitrotoluene (TNT) and picric acid (PA) have
attracted a lot of attention due to their high energy content.
Fax: (+91)80-23601552
E-mail: thilagar@ipc.iisc.ernet.in
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500727.
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Particularly in vapor form, they are very hazardous and can
cause headache, weakness, anemia, and liver damage.[11] In
general electron-rich conjugated p systems that enable effec-
tive p–p stacking with electron-poor quenchers (TNT or PA)
have been employed for sensing these explosives.[12] However,
poor selectivity has been observed in general. Both TNT and
PA are electron-deficient nitroaromatics and they differ only
marginally in their chemical structures. Thus, designing a supe-
rior sensor for selective detection of TNT or PA is a topical and
challenging area of research.[13]
Figure 1. Molecular structures of 2 (left) and 3 (right), as determined by
single crystal X-ray diffraction studies. Atoms are shown with 50% proba-
bility thermal ellipsoids. Hydrogen atoms are omitted for clarity.
It occurred to us that the electron density of a large p
system such as a porphyrin could be modulated by decoration
with suitable Lewis acid (LA) groups. We anticipated that the
modified p cloud of the conjugated molecule may show selec-
tivity towards TNT or PA. We also envisioned that the presence
of LAs on the organic p system may facilitate intermolecular
hydrogen-bonding interaction with the hydroxyl group of PA
and thereby discriminate it from TNT, which is has no hydroxyl
group. Our expectation was met by the new chloromercuryl-
phenyl metalloporphyrin 3, which showed a highly selective
fluorescence-quenching response towards PA. To the best of
our knowledge, this is the first example of a polyfunctional
LA-based fluorescence sensor for nitroaromatics.
The molecular structures of 1–3 were confirmed by single-
crystal X-ray analysis. The molecular structures of 2 and 3 are
shown in Figure 1, and that of 1 is shown in the Supporting in-
formation. Compound 2 crystallizes with four H2O molecules;
two of them are bound to Zn and the remaining two are
trapped in the crystal lattice. The asymmetric unit of 3 contains
four DMF molecules, of which one is attached to Zn center,
two to HgCl moieties, while the fourth is encapsulated within
the crystal lattice (see Figure 2).
Results and Discussion
Synthesis and structural characterization of 1–3
The synthetic protocol for ZnII 5,10,15,20-tetra(4’-chloromercur-
iophenyl)porphyrin (3) is shown in Scheme 1. First, 5,10,15,20-
tetra(4’-trimethylstannylphenyl)porphyrin (1) was prepared by
acid-catalyzed condensation reaction between pyrrole and
4-trimethylstannylbenzaldehyde. Treatment of
1
with
Zn(OAc)2·2H2O in CHCl3/MeOH (1/1) gave ZnII 5,10,15,20-
tetra(4’-trimethylstannylphenyl)porphyrin (2) as a purple solid
in good yields. Compound 2 was treated with four equivalent
of HgCl2 in THF to give ZnII 5,10,15,20-tetra(4’-chloromercurio-
phenyl)porphyrin (3) as a dark brown solid. Compounds 1–3
were characterized by NMR spectroscopy (1H, 13C, 119Sn, 199Hg)
and ESI-MS. The molecular structures of 1–3 were confirmed
Figure 2. Supramolecular 3D network structure of 3 (bottom left) and 3 with
encapsulated DMF (bottom right). Bonding modes of 3 showing m-type (top
right) and linear-type (top left) HgÀl bonding.
1
by single crystal X-ray diffraction studies. The H NMR spectra
of 1–3 showed a singlet at about 8.86 ppm corresponding to
the b-pyrrole proton of porphyrin. The meso-phenyl moiety
gave rise to two doublets at about 8.20 and about 7.87 ppm,
respectively. The 119Sn resonances of 1 and 2 were observed at
À25.5 and À25.1 ppm, respectively. The 199Hg signal of 3 was
observed at À1181 ppm. ESI-MS analysis, which showed peaks
for 1 at 1272.07 [M+H]+, 1103.27 [MÀSnMe3]+ and 941.40
[MÀ2SnMe3]+, for 2 at 1330.92 [M]+, 1168.87 [MÀSnMe3]+,
1024.57 [MÀ2SnMe3+Na]+ 843.47 [M+À3SnMe3], and 679.67
[MÀ4SnMe3]+, and for 3 at 1619 [M+], 1383 [M+ÀHgCl], 1150
[M+À2HgCl], and 679 [M+À4HgCl] confirmed their chemical
composition. The experimentally observed and calculated iso-
topic distributions of the peaks corresponding to fragments
were consistent with their m/z values (see the Supporting In-
formation).
The metric parameters of 3 (1.49(3) ꢂ between tetrapyrrole
unit and meso-phenyl group and 2.08(2) ꢂ between HgCl unit
and meso-phenyl group) are shorter those of both 1 [1.50(2)
and 2.14(2)] and 2 [1.51(1) and 2.146(9)]. The CÀC bonds of
the porphyrin meso-carbon atoms and meso-phenyl substitu-
ents in 3 are significantly shorter than those observed for
1 and 2. The dihedral angle between meso-phenyl and tetra-
pyrrole units in 3 (568) is significantly smaller than those of
1 (908) and 2 (668). On the basis of the dihedral angles, one
can tentatively conclude that the electronic communication
between the porphyrin core and terminal Lewis acid moieties
should be stronger in 3 than in 1 and 2. The Zn center in com-
pound 2 is in octahedral geometry (four pyrrole nitrogen
atoms occupy the square plane, and two water molecules the
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axial positions). In 3, Zn is in square-pyramidal geometry (four
pyrrole nitrogen atoms in the square plane and one DMF mol-
ecule in an axial position). The Zn atom in 3 lies above the
mean plane of the porphyrin ring, in contrast to 2 (see the
Supporting Information, Table S1).This is possibly due to the
difference in the coordination environment around the Zn
center in 2 and 3.
In the solid state, compound 3 exhibits an interesting supra-
molecular 3D structure formed by intermolecular Hg-Cl-Hg in-
teractions between adjacent molecules. A closer look at the 3D
structure reveals the presence of two different types of Hg-Cl-
Hg interactions, that is, linear and m-type coordinative covalent
interaction between neighboring molecules (Figure 2, top). The
linear Hg-Cl-Hg bonds appear once after every two adjacent m-
type bonds. The chloride ions involved in the m-type bonding
also interact with the meso-phenyl hydrogen atoms, whereas
those in linear-type bonding interact with b-pyrrole protons of
the porphyrin. These multiple interactions generate two differ-
ent types of infinite tapes (see the Supporting Information, Fig-
ure S3). These tapes form a sandwich-herringbone pattern
along the a axis[8] (Figure 3). These infinite tapes enclose voids,
Figure 4. Normalized absorption spectra of 1–3 compared with those of TPP
and TPP+Zn [Soret bands (a) and Q-bands (b) recorded for 10À6 m solutions].
Normalized fluorescence spectra of 1–3 compared to those of TPP and
TPP+Zn [(c), lex =420 nm, 10À6 m DMF as solution].
Figure 3. Sandwich-herringbone pattern of 3 with DMF solvent molecules.
the largest redshift (ca. 17 nm). The Q bands of compounds 1–
3 are also redshifted with respect to the model compounds.
The significantly larger redshift of absorption features observed
for 3 can be tentatively explained in terms of the dihedral
angle between the tetrapyrrole unit and the heavy-metal-con-
taining meso-phenyl unit. This dihedral angle in compound 3
(568) is smaller than that of 2, as a result of which the interac-
tion between the tetrapyrrole unit and the heavy metal is
more favored in the former compared to the latter. On excita-
tion at 420 nm, compounds 1–3 exhibited typical porphyrin
and metalloporphryin (TPP and Zn–TPP) PL features. However,
the emission peaks of 1–3 are redshifted compared to TPP and
Zn–TPP. As in the absorption spectra, the redshift of 3 is larger
than those of 1 and 2. The quantum yields of 1 (5.8%), 2
(3.4%), and 3 (2.1%) are significantly lower than those of the
respective model compounds TPP and Zn–TPP. These results
are in line with values reported for heavy metal substituted
porphyrins and metalloporphyrins[15] (Table 1).
which are occupied by DMF molecules, which are not bound
to Hg or Zn atoms). The guest DMF molecules are non-inno-
cent and form weak C-H-Cl-Hg interactions with meso-
C6H4HgCl units of neighboring molecules and augment the 3D
supramolecular structure. The bond lengths and bond angles
of coordinative covalent interactions are in line with the
literature reports.[7a,14]
Photophysical properties
The absorption spectra of compounds 1–3 show Soret and Q
bands (Figure 4) typical of a porphyrin core. The spectral fea-
tures are compared with those of the model tetraphenylpor-
phyrin compounds TPP and TPP+Zn in Figure 4, top).
Interestingly, the absorption maxima (Soret bands) of 1–3 are
redshifted compared to TPP and Zn–TPP. Compound 3 shows
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Table 1. Photophysical properties of 1–3, TPP, and Zn–TPP.
Compound Absorption [nm] (e)
Emission [nm] Quantum
(lex =420 nm) yield[b] [%]
(lex =420
nm)
1
422 (0.65ꢃ106); 554
651, 714
5.8
(2.2ꢃ104); 592 (1.3ꢃ104); 646
(0.4ꢃ104).
2
424 (0.58ꢃ106); 550
601, 647
608, 658
651, 711
3.4
2.1
(4.0ꢃ104); 588 (1.0ꢃ104).
435, (0.61ꢃ106); 566
(1.3ꢃ104); 604 (9.1ꢃ103).
418 (0.71ꢃ106); 512
3
TPP
11.3
(4.0ꢃ105); 551 (3.2ꢃ104); 592
(2.0ꢃ104); 647 (1.4ꢃ104).
420 (0.82ꢃ106); 549
TPP+Zn
591, 638
6.4
(3.8ꢃ105), 588 (1.0ꢃ104).
[a] All given data are for 10 mm DMF solutions. [b] Quantum yields were
calculated by using quinine sulfate (0.1m H2SO4, lex =420 nm,
FF =57.7%) solution as reference (R) and using the following formula:
F=(FFI/IR)(AR/A)(h2/hR2), where F=quantum yield, I=emission intensity,
A=absorbance at lex, and h=refractive index of solvent.
Titration studies
The nitroaromatics-sensing ability of 3 was evaluated by titrat-
ing it against various nitroaromatics. As hypothesized above,
compound 3 discriminates TNT and shows very high selectivity
towards PA. This result is remarkable because the redox poten-
tials of TNT and PA are similar[9] and in general organic p sys-
tems such as porphyrin[16] interact with both TNT and PA and
cannot distinguish between them. The selective discrimination
in the present instance is possibly due to the presence of
Lewis acidic HgCl groups at the porphyrin p system. The Lewis
acidic functional groups in 3 can distinguish the minor
difference in functional groups between TNT and PA.
Figure 5. a) The absorption bands of 3 are redshifted in the presence of PA
(lex =425 nm, 10À6 m solution in DMF). The inset shows the changes associ-
ated with the Q bands of 3 in the presence of PA. b) The fluorescence inten-
sity of 3 decreased linearly with increasing equivalents of PA. The changes
saturate at 15 equiv of PA. Concentrations of solutions of 3 and PA were
10À6 m and 10À4 m respectively. For fluorescence studies, 3 was excited at
425 nm.
tensity of 3 on the gradual addition of PA is consistent with
the formation of a ground-state complex between 3 and the
PA quencher. Fluorescence lifetime studies on 3 with variable
concentrations of PA showed no significant change in lifetime,
which strongly suggested that the complex (3+PA) was
formed in the ground state.[18]
On gradual addition of PA to a DMF solution of 3, the initial
fluorescence intensity of the
3 was quenched rapidly
(Figure 5), but there was no change in the shape of the emis-
sion spectra. We believe that the observed quenching of fluo-
rescence intensity is due to complex formation between 3 and
PA (Figure 5). A linear Stern–Volmer plot was obtained from
the fluorescence-quenching titration profile, and Stern–Volmer
quenching constants KSV were determined from the slope of
the plot (see the Supporting Information). The KSV value of
1.6ꢃ107 mÀ1 is higher than those obtained for other PA chemo-
sensors.[13c,17] A linear Stern–Volmer relationship can be ob-
tained for a static or dynamic quenching process. The static
quenching mechanism involves the formation of a ground-
state nonfluorescent charge-transfer complex, whereas dynam-
ic quenching involves excited-state electron transfer from the
fluorophore to the quencher. The electronic absorption spec-
troscopic studies and excited-state lifetime measurement in re-
sponse to PA concentrations clearly indicate the formation of
We also investigated the effect of other electron-deficient ar-
omatic compounds on the fluorescence profile of 3 (Figure 5).
Clearly 3 shows very high selectivity towards PA over other
tested analytes such as dinitrobenzene, dinitrotoluene (DNT),
benzoquinone, tetrachlorobenzoquinone (chloranil), and 2,3-di-
chloro-5,6-dicyano-1,4-benzoquinone (DDQ). The reduction po-
tential of these and PA do not differ significantly, the observed
higher quenching response for PA is presumably due to the in-
teraction between Cl in 3 and OH of PA. To validate the above
hypothesis, the picric-acid binding event was monitored by
1H NMR spectroscopy. On addition of PA, three major chemical
1
shift changes were observed in the H NMR spectrum: 1) The
phenolic proton of PA shifted downfield from d=5.1 to
5.6 ppm, 2) the b-pyrrole protons of 3 shifted downfield from
d=8.76 to 8.95 pm, and 3) the meso-phenyl protons of com-
pound 3 shifted downfield from d=7.81 and 8.10 ppm to 7.93
and 8.25 ppm, respectively. There were no significant chemical
a
ground-state complex between Lewis acid 3 and PA
(Figure 5 and Supporting Information). On addition of PA, the
intensity of Soret band at 435 nm decreased with concomitant
redshift by 10 nm. The considerable change in absorption in-
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shift changes on addition of other nitroaromatics such as TNT,
DNT, and nitrobenzene or benzoquinone to compound 3 (see
the Supporting Information). The binding event was also moni-
tored by observing the 199Hg NMR chemical shift of compound
3 in the presence of various nitroaromatics. Only on addition
of PA did the 199Hg signal at d=À1186 ppm shift downfield to
À1160 ppm. Very little change in the chemical shift was ob-
served in the presence of dinitrophenol. These results directly
corroborated that the interaction between the phenolic OH
and C6H4HgCl groups could be a possible reason for the
selective detection of PA.
Figure 7. Digital photographs of 3 with various nitroaromatics. From left to
right: 3 (1) 4-aminophenol (2), 4-tert-butylphenol (3), 4-cresol (4), 3-cresol (5)
4-fluorophenol (6), benzoquenone (7), nitrobenzene (8), 1, 4-dinitrobenzene
(9), 1, 3-dinitrobenzene (10), 1, 2-dinitrobenzene (11), nitrotoluene (12), 2, 4-
dinitrotoluene (13), TNT (14), nitrophenol (15), dinitrophenol (16), and PA
(17)]. The photographs clearly indicate that, on addition of 2,4-dinitrophenol
or PA to compounds, the fluorescence intensity decreased, whereas for
other nitroaromatics there was no change.
Furthermore, compound 3 was titrated against the PA ana-
logue dinitrophenol (DNP), and the changes in emission profile
were monitored by fluorescence spectroscopy. As observed in
the case of titrations with PA, noticeable fluorescence quench-
ing of 3 was observed on gradual addition of DNP (KSV =4.16ꢃ
104 mÀ1), but the magnitude of quenching was less for DNP
(71% decrease in fluorescence intensity) compared to PA (94%
decrease in fluorescence intensity). Under similar conditions, 3
showed negligible fluorescence-quenching response to other
phenols such as 4-tert-butylphenol, 3-methoxyphenol, 4-nitro-
phenol, and 4-methoxyphenol. This clearly suggests that 3 can
be used as a selective sensor for PA-based explosives (Figures 6
The detection limits of compound 3 with PA and dinitro-
phenol were determined from the fluorescence-quenching
titration data. The x-axis intercept (here [X], X=PA or DNP)
was obtained by plotting (ImaxÀI)/(ImaxÀImin) versus lg[PA],
where Imax, I, and Imin are the initial fluorescence intensity, inten-
sity at a particular concentration, and intensity at saturation
point, respectively. Detection limits were calculated by the for-
mula ([X]ꢃMWX)/1000 (multiplied by 109 to give values in
ppm), where MWX is the molecular weight of PA or DNP and
the related spectrum shown in the Supporting Information.
The detection limits for compound 3 were found to be 18 and
140 ppb for PA and DNP, respectively. The molecular probe 3
showed much lower detection limits compared to reported
small-molecular probes for PA (Table 2).[9e,19]
Table 2. Quenching efficiencies, quantum yields, and detection limits of
1–3, TPP, and Zn–TPP with PA.
Compound Quenching efficiency [%] (I0ÀI/
I0)ꢃ100
Quantum
yield [%]
DNP
Detection
limit [ppb]
PA
DNP
PA
7.2
PA
DNP
1
2
3
TPP
12
15
94
9
7
6
71
4
7.2
n.d.[c] n.d.
n.d. n.d.
3.4
0.01
11.2
5.4
3.4
0.25
11.2
5.4
18
140
n.d. n.d.
n.d. n.d.
Zn–TPP
11
5
Figure 6. Fluorescence quenching efficiency of 3 (10–5m) for various
nitroaromatics (lex =425 nm, 10À6 m solution in DMF). From left to right:
4-aminophenol (1), 4-tert-butylphenol (2), 4-cresol (3), 3-cresol (4) 4-fluoro-
phenol (5), benzoquinone (6), naphthalene monohydrate (7) nitrobenzene
(8), 1,4-dinitrobenzene (9), 1,3-dinitrobenzene (10), 1,2-dinitrobenzene (11),
nitrotoluene (12), 2,4-dinitrotoluene (13), TNT (14), nitrophenol (15), 2,4-
dinitrophenol (16), and PA (17) (10À3 m). From this data one can conclude
that compound 3 is a selective fluorogenic sensor for trinitrophenol and
dinitrophenol.
[a] All given data are for 10 mm DMF solutions. [b] Quantum yields were
calculated by using quinine sulfate (0.1m H2SO4, lex =420 nm,
FF =57.7%) solution as reference (R) and using the following formula:
F=(FFI/IR)(AR/A)(h2/hR2), where F=quantum yield, I=emission intensity,
A=absorbance at lex, and h=refractive index of solvent. [c] Not detecta-
ble.
Computational studies
To gain further insight into the photophysical behavior of 1–3,
DFT calculations were performed. The hybrid B3LYP functional
was used in all calculations, as implemented in the Gaussian 09
package,[20] mixing the exact Hartree–Fock-type exchange with
Becke’s expression for the exchange functional and that pro-
posed by Lee–Yang–Parr for the correlation contribution. We
used the LANL2DZ basis set for Sn, Hg, and Zn and 6-31G for
the rest of the atoms (C, N, H, Cl). All of the energy calculations
were performed with 10À8 density-based convergence criteri-
on. The MOs were generated from corresponding cube files by
and 7). To probe the role of the Lewis acidity of HgII in 3, we
performed fluorescence titrations for model compounds TPP
and Zn–TPP and complexes 1 and 2 with nitroaromatics under
similar titration conditions. In all these cases, the changes in
the emission feature were insignificant. Despite the structural
similarity of 1 and 2 to 3, changes in emission spectra on addi-
tion of nitroaromatics are very small, which may be due to the
high Lewis acidity of 3 compared to 1 and 2 and model
compounds (TPP and Zn–TPP).
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Figure 8. Selected MOs of 3 and Zn–TPP (left). In 3, HOMO and LUMO are mainly centered on the porphyrin p system. The LUMO+1 and LUMO+2 are
centered only on the HgCl unit. In Zn–TPP, HOMO, LUMO, LUMO+1, and LUMO+2 are localized on the porphyrin unit. Right: MOs of 3 and Zn–TPP. For 3,
LUMO and LUMO+2 are doubly degenerate orbitals, whereas in Zn–TPP LUMO and LUMO+1 have similar energy but are not degenerate due to its C2h
symmetry.
using Gaussview 5.0 with isovalue=0.02. The structural param-
eters of DFT optimized structures closely resemble those ob-
tained from single-crystal X-ray analysis, with slight differences
in the values of the dihedral angles. The optimized structure of
compound 3 shows an C-Hg-Cl bond angle of about 1808 and
CÀHg bond length of 2.195 ꢂ. The phenyl rings are bent by
708 with respect to the porphyrin plane. The MOs of com-
pound 3 and Zn–TPP are completely different from each other.
As shown in Figure 8, the HOMO (À4.95 eV) of Zn–TPP is com-
pletely localized on the porphyrin, and LUMO (À2.17 eV) and
LUMO+1 (À2.17 eV) are also concentrated on the porphyrin
unit, which has same energy but nondegenerate orbitals due
Figure 9. ESP surfaces of 3 and 2.The potentials clearly indicate that 3 is
to its C2h symmetry. However, LUMO+2 (À0.64 eV) is localized
a stronger Lewis acid than 2.
more on the porphyrin and moderately on the meso-phenyl
units, whereas the HOMO (À5.59 eV) of 3 is comparatively con-
centrated on the porphyrin unit, the LUMO (À2.78 eV), which
is doubly degenerate, is followed by LUMO+1 (À2.04 eV) and
LUMO+2 (À2.04 eV), three molecular orbitals having same
energy, which are mostly spread over the mercury and chlorine
atoms. The participation of the atomic orbitals of the peripher-
al organometallic moiety in electronically important energy
levels may be responsible for the redshift observed in the
Soret band of compound 3 (Figure 4, top). It also indicates re-
distribution of electrons from the core towards the periphery
on electronic excitation, which can be useful if explored further
for photonic applications. These calculations show that there
should be a considerable redshift in absorption bands on
going from Zn–TPP to 3, which was clearly observed experi-
mentally, and also possible participation of peripheral organo-
metallic moieties in extending the conjugation in excited
states, which results in the stabilization of LUMO+1 and
LUMO+2 compared to Zn–TPP. Whereas compound 2 is not
much different from Zn–TPP in the localization of orbitals
(HOMO, LUMO, LUMO+1 and LUMO+2), compound 2 has
lower-energy frontier MOs than Zn–TPP (see the Supporting
Information).
The stronger Lewis acidity of the divalent organometallic Hg
moieties compared to SnMe3 groups can be well understood
from their electrostatic potential (ESP) surface maps (Figure 9).
The high charge separation near the (meso-phenyl)C-Hg-Cl
motifs in compound 3 demonstrates the Lewis acidity of the
Hg centers. Due to the ionic nature of the HgÀCl bonds, the
terminal chlorine atoms are electron-rich centers that may be
the driving force in controlling the interactions of the com-
pounds with electron-deficient nitroaromatics. The charge sep-
aration in the C-Hg-Cl motifs can also explain the nature of the
bonding interactions observed in the solid-state structure of 3.
The Cl···Hg···Cl and Hg···Cl···Hg bonding patterns in the solid-
state structure of compound 3 presumably result from the in-
termolecular Hg···Cl electrostatic interactions. In 3, the periph-
eral Lewis acidic units are also electronically important and
strongly participate in forming the low-lying vacant orbitals
(see the Supporting Information). As discussed above, LUMO
and LUMO+2 forms two degenerate energy levels in com-
pound 3, which are mostly concentrated on the terminal HgCl
units (Figure 9), and thus they are expected to participate
actively in controlling the optical properties of compound 3.
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proton signals of deuterated solvents as internal reference. Mass
spectral data were recorded on a Bruker Esquire 6000 ESI-MS spec-
trometer in standard spectroscopic-grade solvents. IR spectra were
recorded on a Bruker ALPHA FTIR spectrometer. Electronic absorp-
tion spectra were recorded on a PerkinElmer LAMBDA 750 UV/Vis
spectrophotometer. Solutions were prepared by using a microba-
lance (Æ0.1 mg) and volumetric glassware and then charged in
quartz cuvettes with sealing screw caps. Fluorescence emission
studies were carried out on a Horiba JOBIN YVON Fluoromax-4
spectrometer. DFT calculations were performed with the B3LYP
functional, as implemented in the Gaussian 09[20] software package.
For C, H, N, and Cl atoms the 6-31G(d) basis set was used, whereas
for the Zn, Sn, and Hg centers, the LANL2DZ pseudopotential
(pseudo=read) basis was used to generate high-quality results on
moderately shorter timescales.
In addition, to get further insight into the nature of the in-
teraction of PA with 3 and possible modes of interactions, de-
tailed computational investigations were performed. Due to
the large size of compound 3, which was originally kept in D2d
symmetry, the optimizations of its interactions with picric acid
in C1 symmetry did not converge. To overcome this problem,
optimizations were performed by considering fragment
models, that is, ZnII–porphyrin core and Ph-Hg-Cl (see the Sup-
porting Information). Optimizations of the individual systems
and their PA complex showed that the terminal C-Hg-Cl···aro-
matic (PA) interactions are more favorable (43.87 kJmolÀ1) than
p(porphyrin)···p(PA) interactions (15.87 kJmolÀ1). The terminal
C-Hg-Cl···aromatic (PA) interactions are partially electrostatic in
nature and enhance the intermolecular interaction between PA
and 3 (see the Supporting Information). The terminal interac-
tions can be also corroborated by the downfield shift of the
1H NMR signal of OH (PA) observed for 3+PA complexes in
X-ray diffraction studies
Single crystal X-ray data were collected on a Bruker SMART APEX
CCD diffractometer by using the SMART/SAINT software2. Intensity
data were collected with graphite-monochromatized MoKa
radiation (0.71073 ꢂ) at 90 K. The structures were solved by direct
methods by using the SHELX-972 program incorporated in WinGX2.
1
the H NMR titrations.
Conclusion
For the first time, we have synthesized and structurally charac-
terized a series of heavy-metal-decorated metalloporphyrins 1–
3. Solid-state structural analysis revealed that compound 3
shows a very exciting 3D supramolecular structure through in-
termolecular coordinative covalent interactions between Hg
and Cl. For the first time, we have shown that a polyfunctional
Lewis acid can be used for selective detection of PA. The fluo-
rescence studies demonstrated that 3 is a highly selective and
sensitive chemosensor for phenolic nitroaromatic explosives
such as PA and DNP. The manifestation of Lewis acidity in 3
(charge separation in HgÀCl motifs) and thus its selective fluo-
rescent sensing of electron-deficient nitrophenols (PA and
DNP) was validated by DFT calculations. The presence of LA
groups on the periphery of the metalloporphyrin modulated
the electron density of the porphyrin p system and endowed
compound 3 with a special ability to discriminate PA over TNT.
Such discrimination has not been reported for LA-based sen-
sors and has seldom been realized with other fluorescence
chemosensors.
Synthesis
(4-Bromophenyl)-1,3-dioxolane (a): 4-Bromobenzaldehyde (9.5 g,
51.4 mmol) was dissolved in ethanol (100 mL) followed by subse-
quent addition of triethyl orthoformate (16.75 g, 113 mmol) and
a catalytic amount of conc. HCl. The reaction mixture was heated
under reflux for 4 h and cooled. The reaction mixture was extract-
ed with EtOAc. The organic layers were combined, washed with
brine and dried over anhydrous Na2SO4 followed by evaporation
under vacuum to give a as a colorless oil. Yield: 98%.1H NMR
(400 MHz, CDCl3): d=7.49 (d, J=6.8 Hz 2H), 7.35 (d, J=8 Hz 2H),
5.46(s, 1H), 3.63–3.49(m, 4H), 1.24 ppm (t, J=6.8 Hz, 7.2 Hz 6H).
4-Trimethylstannylphenylbenzaldehyde
(b):
n-Butyllithium
(19.28 mL, 1.6m in hexane) 30.86 mmol) was added to a solution
of a (7.23 g, 28.05 mmol) in Et2O (to give 100) mL) over 30 min at
À788C. After 1 h, a THF solution of trimethyltin chloride (5 mL,
28.05 mmol) was added over 25 min. The reaction mixture was al-
lowed to warm to room temperature and stirred overnight. A mix-
ture of 30 mL THF and 30 mL of 1% aqueous KHSO4 was added
and the reaction mixture was stirred for a further 4 h. The resulting
mixture was partitioned between Et2O and water. The organic
layers were combined, washed with brine, and dried over anhy-
drous sodium sulfate (Na2SO4). Subsequent evaporation and purifi-
cation (neutral alumina column) gave b as a colorless liquid. Yield:
Experimental Section
1
81%. H NMR (400 MHz, CDCl3) d 9.96 (s, 1H), 7.78 (d, J=8 Hz 2H),
7.65 (d, J=8 Hz 2H), À2.75 (br, 2H) 0.30 ppm [satellite 0.58
Caution! The nitroaromatic compounds used in this study,
especially TNT and PA, are very powerful explosives. They must be
handled with care and also in very small quantities.
(J=26.4 Hz), 0.45 ppm (J=26.4 Hz)].
5,10,15,20-Tetrakis(4’-trimethylstannylphenyl)porphyrin
(1):
Freshly distilled pyrrole (0.27 mL, 3.70 mmol) was added to
a chloroform (30 mL) solution of b (1.0 g, 3.70 mmol)and the result-
ing solution was degassed by purging N2 for 30 min. Then,
BF3·Et2O (0.05 mL, 0.37 mmol) was added dropwise and the reac-
tion mixture was stirred until b was completely consumed (as ob-
served by TLC). Then, a solution of DDQ (0.84 g 3.70 mmol) in ben-
zene (40 mL) was added to the reaction mixture over 30 min. The
solution was stirred at room temperature overnight. The resulting
mixture was filtered, concentrated, and purified on a neutral alumi-
na column (petroleum ether/chloroform). The product 1 was ob-
tained as a purple solid. Yield: 21%. UV/Vis (in dichloromethane):
lmax [nm] (e [mÀ1 cmÀ1]): 420 (1.0ꢃ106); 451 (2ꢃ105); 517 (3.9ꢃ104);
General information
nBuLi (1.6m in hexane), 4-bromobenzaldehyde, and DDQ were pur-
chased from Sigma-Aldrich and used as received without any fur-
ther purification. Triethyl orthoformate and pyrrole were purchased
from Merck. All reactions were carried under an atmosphere of
pure N2 by using standard Schlenk techniques. Solvents were redis-
tilled by known laboratory procedures prior to use. All 400 MHz
1H NMR, 100 MHz 13C NMR, 149 MHz 119Sn NMR, 71 MHz 199Hg NMR
were recorded on a Bruker Advance 400 MHz NMR spectrometer.
Solution 1H and 13C NMR spectra were recorded with residual
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554 (2.2ꢃ104); 592 (1.3ꢃ104); 666 (2.4ꢃ104); 1H NMR (400 MHz,
CDCl3): d=8.86 (s, 8H), 8.20 (d, J=8 Hz 8H), 7.87 (d, J=7.6 Hz 8H),
0.51 ppm [satellites: 0.58 (J=26.4H), 0.45 (J=26.4 Hz)]. 13C NMR
(100 MHz, CDCl3): d=142.1, 141.4, 134.6, 134.0, 131.1, 127.7, 126.7,
120.2, À9.2 ppm [satellites: À7.6 (d, J=167 Hz), À11.0 (d, J=
174 Hz)]; 119Sn NMR (149 MHz, CDCl3): d=À26 ppm; ESI-MS:
m/z=1272.07 [M+2H]+, 1103.27 [MÀSnMe3]+ and 941.40
[MÀ2SnMe3]+.
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Zinc(II)
5,10,15,20-tetrakis(4’-trimethylstanylphenyl)porphyrin
(2): A solution of Zn(OAc)2·2H2O (0.17 g, 0.78 mmol) in MeOH
(20 mL) was added to a solution of 1 (0.20 g, 0.16 mmol) in 100 mL
of chloroform, and the reaction mixture was stirred for 6 h at room
temperature. The resulting reaction mixture was concentrated in
vacuum and further purified on a basic alumina column (petroleum
ether/dichloromethane). The desired product 2 was obtained as
a purple solid. Yield: 95%. UV/Vis (in dichloromethane): lmax [nm]
(e [mÀ1 cmÀ1]): 422 (1.0ꢃ106); 549 (4.0ꢃ104); 1H NMR (400 MHz,
CDCl3): d=8.89 (s, 8H), 8.14 (d, J=8 Hz 8H), 7.85 (d, J=8 Hz 8H),
0.44 ppm (satellites: 0.51 ( J=26 Hz,) 0.37 (J=23 Hz); 13C NMR
(100 MHz, CDCl3): d=150.3, 142.5, 141.8, 140.1, 134.8, 132.1, 126.6,
121.6, 7.6 (d J=166 Hz), À9.30, À10.96 ppm (d, J=166 Hz); 119Sn
NMR (149 MHz, CDCl3): d=À25 ppm; ESI-MS: m/z=1330.92 [M]+,
1168.87 [MÀSnMe3]+, 1024.57 [MÀ2SnMe3+Na]+, 843.47
[MÀ3SnMe3]+, 679.67 [MÀ4SnMe3]+.
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Zinc(II)
5,10,15,20-Tetrakis(4’-chloromercuriophenyl)porphyrin
(3): HgCl2 (36 mg, 0.13 mmol) was dissolved in THF (10 mL) and
the solution added to a solution of 2 (20 mg, 0.015 mmol) in Et2O
(20 mL). The reaction mixture was stirred at room temperature for
4 h. The solvent was removed under reduced pressure and the res-
idue was washed with water (4ꢃ10 mL) and dried. Subsequent
washing with Et2O (4ꢃ5 mL) and drying gave 3 as a purple solid.
Yield: 66%. UV/Vis (in dichloromethane): lmax [nm] (e [mÀ1 cmÀ1]):
428, (1.9ꢃ105); 557 (1.3ꢃ104); 600 (9.1ꢃ103); 662 (1.0ꢃ
104).1H NMR (400 MHz, [D6]DMSO): d=8.76 (s 8H), 8.10 (d, J=8 Hz,
8H), 7.85 ppm (d, J=7.6 Hz 8H); 13C NMR (100 MHz, [D6]DMSO):
d=151.17,
149.71,
142.84,
135.31,
134.50,
132.01,
120.57 ppm;199Hg NMR (71 MHz, [D6]DMSO): d=À1181 ppm; ESI-
MS: m/z=1619 [M]+, 1383 [MÀHgCl]+, 1150 [MÀ2HgCl]+ 679
[MÀ4HgCl]+.
CCDC 996313, 996314 and 996315 contain the supplementary crys-
tallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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P.T. thanks DST and CSIR, New Delhi for financial Support.
C.A.S.P. thanks IISc Bangalore for a fellowship.
Keywords: Lewis acids · mercury · porphyrinoids · sensors ·
supramolecular chemistry
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Received: February 21, 2015
Published online on && &&, 0000
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FULL PAPER
Mercurial porphyrin: The synthesis and
crystal structure of a polyfunctional
Lewis acid based on mercury-decorated
porphyrin are reported. The presence of
Lewis acidic groups on the periphery of
the metalloporphyrin modulated the
electron density of the porphyrin p
system and endowed the compound
with the ability to discriminate picric
acid from trinitrotoluene (see figure).
&
Porphyrins
C. A. Swamy P, P. Thilagar *
&& – &&
Polyfunctional Lewis Acids: Intriguing
Solid-State Structure and Selective
Detection and Discrimination of
Nitroaromatic Explosives
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