Bifunctional porphyrin-based nano-metal-organic frameworks: Catalytic and chemosensing studies

Article abstract of DOI:10.1021/acs.inorgchem.7b03214

The use of 5,10,15,20-tetrakis(p-phenylphosphonic acid)porphyrin (H₁₀TPPA) as a linker in the preparation of porphyrin-based metal-organic frameworks (Por-MOFs) through coordination to lanthanides cations is reported. The resulting unprecedented materials, formulated as [M(H₉TPPA)(H₂O)_x]Cl₂·yH₂O [x + y = 7; M³⁺ = La³⁺ (1), Yb³⁺ (2), and Y³⁺ (3)], prepared using hydrothermal synthesis, were extensively characterized in the solid-state, for both their structure and thermal robustness, using a myriad of solid-state advanced techniques. Materials were evaluated as heterogeneous catalysts in the oxidation of thioanisole by H₂O₂ and as chemosensors for detection of nitroaromatic compounds (NACs). Nano-Por-MOFs 1-3 proved to be effective as heterogeneous catalysts in the sulfoxidation of thioanisole, with Por-MOF 1 exhibiting the best catalytic performance with a conversion of thioanisole of 89% in the first cycle and with a high selectivity for the sulfoxide derivative (90%). The catalyst maintained its activity roughly constant in three consecutive runs. Por-MOFs 1-3 can be employed as chemosensors because of a measured fluorescence quenching up to 70% for nitrobenzene, 1,4-dinitrobenzene, 4-nitrophenol, and phenol, with 2,4,6-trinitrophenol exhibiting a peculiar fluorescence profile.

Full text of DOI:10.1021/acs.inorgchem.7b03214

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Bifunctional Porphyrin-Based Nano-Metal−Organic Frameworks:
Catalytic and Chemosensing Studies
Carla F. Pereira,†,‡ Flavio Figueira,†,‡ Ricardo F. Mendes,† Joao Rocha,† Joseph T. Hupp,§
́
̃
Omar K. Farha,§ Mario M. Q. Simoes,*,‡ Joao P. C. Tome,*,‡,∥ and Filipe A. Almeida Paz*,†
́
́
̃
̃
†Department of Chemistry & CICECO−Aveiro Institute of Materials and Department of Chemistry & QOPNA, University of
‡
Aveiro, 3810-193 Aveiro, Portugal
§Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States
∥CQE, Instituto Superior Tec
́
nico, Universidade de Lisboa, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal
S
* Supporting Information
ABSTRACT: The use of 5,10,15,20-tetrakis(p-phenylphosphonic acid)porphyrin
(H10TPPA) as a linker in the preparation of porphyrin-based metal−organic frameworks
(Por-MOFs) through coordination to lanthanides cations is reported. The resulting
unprecedented materials, formulated as [M(H9TPPA)(H2O)x]Cl2·yH2O [x + y = 7; M3+
=
La3+ (1), Yb3+ (2), and Y3+ (3)], prepared using hydrothermal synthesis, were extensively
characterized in the solid-state, for both their structure and thermal robustness, using a
myriad of solid-state advanced techniques. Materials were evaluated as heterogeneous
catalysts in the oxidation of thioanisole by H2O2 and as chemosensors for detection of
nitroaromatic compounds (NACs). Nano-Por-MOFs 1−3 proved to be effective as
heterogeneous catalysts in the sulfoxidation of thioanisole, with Por-MOF 1 exhibiting the
best catalytic performance with a conversion of thioanisole of 89% in the first cycle and
with a high selectivity for the sulfoxide derivative (90%). The catalyst maintained its activity
roughly constant in three consecutive runs. Por-MOFs 1−3 can be employed as
chemosensors because of a measured fluorescence quenching up to 70% for nitrobenzene,
1,4-dinitrobenzene, 4-nitrophenol, and phenol, with 2,4,6-trinitrophenol exhibiting a peculiar fluorescence profile.
materials with peculiar properties and architectures. We employ
a new synthetic strategy to obtain H10TPPA (5,10,15,20-
tetrakis(p-phenylphosphonic acid)porphyrin) which was used
for the hydrothermal preparation of the novel materials
INTRODUCTION
■
Metal−organic frameworks (MOFs) nowadays constitute an
outstanding class of crystalline materials.1−6 These materials are
prepared from metal ions or cluster nodes and organic linkers,
and because of the endless number of potential combinations
between these units, it is easy to imagine the large structural
diversity which can be attained. Examination of the various
organic linkers employed in the preparation of MOFs shows
that porphyrins (Pors) are a fascinating niche in the wide
spectrum of MOFs world: porphyrin-based metal−organic
frameworks (Por-MOFs).7−10 This niche has received
increased attention in the past decade because of two main
driving forces: Pors have intrinsically remarkable properties,11
and their pivotal role in nature in diverse biological functions is
also well-known.12−17
In this sense, the preparation of Por-MOFs to mimic
biological functions is of great interest.18−20 Por-MOFs have
already demonstrated their potential as promising materials for
a variety of applications which include, but are not limited to,
catalysis,7,18−21 gas separation and storage, and light harvest-
ing.7,9,22 Our research group has been focused on the
development of new MOFs based on linkers bearing
phosphonic acid groups coordinated to lanthanide cations.23−28
In this manuscript, we describe our most recent efforts to
extend our research to porphyrins in order to prepare novel
[M(H9TPPA)(H2O)x]Cl2·yH2O [where x + y = 7; M3+
=
La3+ (1), Yb3+ (2), and Y3+ (3)] through reaction with the
chloride salts of the corresponding lanthanides. We notice,
however, that this niche of MOFs remains highly dominated by
porphyrins bearing carboxylic/pyridyl moieties which mainly
coordinate to transition metals.10 Therefore, to the best of our
knowledge, the present report constitutes the very first study of
Por-MOFs comprising a porphyrin functionalized with
phosphonic acids as a building block. As in the past, we
observed that the isolated products are mainly poorly crystalline
materials, typically isolated as powders because of the fast rate
of crystal nucleation.29,30 We have successfully isolated and
characterized 1−3 as nanopowders which proved to simulta-
neously be effective heterogeneous catalysts in the oxidation of
thioanisole by H2O2 and chemosensors for detection of
nitroaromatic compounds (NACs) (NB, nitrobenzene; DNB,
1,4-dinitrobenzene; 4-NP, 4-nitrophenol; and TNP, 2,4,6-
trinitrophenol).
Received: December 21, 2017
© XXXX American Chemical Society
A
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unit. Intensity data were collected in the continuous mode (ca. 100 s
data acquisition) in the angular range ca. 3.5° ≤ 2θ ≤ 50°.
Absorption and fluorescence spectra were recorded using a
Shimadzu UV-2501-PC and FluoroMax3 (Horiba JovinYvon),
respectively.
EXPERIMENTAL SECTION
■
Chemicals. Pyrrole; boron trifluoride diethyl etherate at ≥46%
boron trifluoride basis; 4-bromobenzaldehyde dimethylacetal of
analytical grade; LaCl3·7H2O, YbCl3·6H2O, and YCl3·6H2O (99.9%);
hydrochloric acid (37%), thioanisole; and hydrogen peroxide (30% w/
w, Perdrogen aqueous solution) were obtained from Sigma-Aldrich.
Bromotrimethylsilane (TMS-Br) and Pd(PPh3)4 of analytical grade
were obtained from TCI. KBr for infrared spectroscopy was obtained
from BDH SpectrosoL. All chemicals were used as received without
further purification, except for the pyrrole which was subjected to
distillation prior to use.
Materials Preparation. Reactive mixtures composed of H10TPPA
(0.0125 g; 0.013 mmol; see Scheme 1 and the structural character-
a
Scheme 1. Synthetic Route for the Preparation of H10TPPA
1
Instrumentation. H, 13C, and 31P nuclear magnetic resonance
(NMR) spectra were recorded on a Bruker Avance III 300
spectrometer at 300.13, 125.77 or 75.47, and 121.49 MHz,
respectively. Deuterated chloroform or dimethyl sulfoxide-d6 were
used as solvents, and tetramethylsilane or H3PO4 (85%) were used as
internal references. Chemical shifts (δ) are expressed in ppm and the
coupling constants (J) in Hz. Mass spectra were collected on a
MALDI-TOF/TOF 4800 Applied Biosystems equipment for
5,10,15,20-tetrakis(p-(diethoxyphosphoryl)phenyl)porphyrin
(H2TPPE) and collected on a ESI-Q-TOF/TOF Micromass equip-
ment for 4-(diethoxyphosphoryl)benzaldehyde dimethylacetal.
31P {1H} CP MAS spectra were recorded at 9.4 T on a Bruker
Avance 400 wide-bore spectrometer (DSX model) on a 4 mm BL
cross-polarization magic-angle spinning (CPMAS) VTN probe. For
the 31P HPDEC spectra, a 90° single pulse excitation of 3.0 μs was
employed; recycle delay: 60 s; νR = 8 or 12 kHz. Chemical shifts are
quoted in parts per million (ppm) with respect to an 85% H3PO4
solution for 31P.
Fourier Transform Infrared (FT-IR) spectra in the range of 4000−
350 cm−1 were collected as KBr pellets (2 mg of each sample was
milled in a mortar with approximately 200 mg of KBr) using a Bruker
Tensor 27 spectrometer by averaging 256 scans at a maximum
resolution of 2 cm−1. ATR-FTIR spectra were collected using a FT-IR
Bruker Tensor 27 with a ATR accessory (Golden Gate ATR Diamond
45° with KRS-5 Lens, from SPECAC), averaging 256 scans at a
maximum resolution of 2 cm−1.
Thermogravimetric analyses (TGA) were performed using a
Shimadzu TGA 50 equipment from ambient temperature to ca. 350
°C, with a heating rate of 2 °C/min, under a continuous stream of air
at a flow rate of 20 cm3/min.
Elemental analyses for C, H and N were performed with a Truspec
Micro CHNS 630−200−200 elemental analyzer at the Department of
Chemistry, University of Aveiro. Analysis Parameters: sample amount
between 1 and 2 mg; combustion furnace temperature = 1075 °C;
after burner temperature = 850 °C. Detection methods: carbon,
infrared absorption; hydrogen, infrared absorption; nitrogen, thermal
conductivity. Analysis time = 4 min. Gases required: carrier, helium;
combustion, oxygen; pneumatic, compressed air.
a
(i) Diethylphosphite, Et3N/toluene, 7 mol% Pd(PPh3)4, mW (80 °C;
100 psi; 45 W; 45 min.). (ii) (a) pyrrole, BF3OEt2, CH2Cl2, r.t., 8 h;
(b) DDQ, 1 h. (iii) (a) TMS-Br, CH3CN, 70 °C, 7 days, (b) H2O.
ization of all compounds in Supporting Information section 1) and the
respective metal(III) chloride hydrate (LaCl3·7H2O, YbCl3·6H2O, or
YCl3·6H2O; 0.040 mmol) in ca 9.5 mL of distilled water and 500 μL of
hydrochloric acid at 6 M were kept under constant magnetic stirring in
open air and at ambient temperature for approximately 30 min. The
isolated homogeneous suspensions were transferred to Teflon-lined
Parr Instrument reaction vessels and then placed inside a MMM
Venticell oven. Reactions took place at 140 °C over a period of 72 h,
after which the oven was turned off and the reactors cooled slowly to
ambient temperature (while inside the oven). The resulting porphyrin-
based nano-MOFs were obtained as dark green powders and
recovered by centrifugation, washed with copious amounts of distilled
water, and then air-dried at ambient temperature.
[La(H9TPPA)(H2O)x]Cl2·yH2O (1) [where x + y = 7]. Elemental
CHN composition (%). Calcd: C 41.36; H 4.34; N 4.39. Found: C
42.04; H 4.23; N 4.67. Thermogravimetric analysis (TGA) data
(weight losses in %) and derivative thermogravimetric peaks (DTG; in
italics inside the parentheses): 25−179 °C, −10.6% (38 °C); 179−350
°C, −5.3% (322 °C). Selected FT-IR data (cm−1, from KBr pellets):
ν(H2O) = 3600−3158; ν(N−H) = 3158−2911w-m; ν(POH) =
2911−2688 w-m; δ(H2O) = 1633 m; δ(phenyl−P) = 1550 m, 1484 m,
and 1394m; ν(PO) = 1265−1093 m-vs; ν(P−O) = 1093−757 vs;
ν(phenyl) = 582 m, 570 m, and 522 m.
SEM (scanning electron microscopy) images were collected using
either a Hitachi S-4100 field-emission gun tungsten filament
instrument working at 25 kV or a scanning electron microscope SU-
70 working at 15 kV. EDS (energy-dispersive X-ray spectroscopy) data
was collected using a Esprit 1.9 EDS microanalysis systems. Samples
were prepared by deposition on aluminum holders followed by carbon
coating using an Emitech K950X carbon evaporator.
Powder X-ray diffraction (PXRD) data were collected at ambient
temperature on an Empyrean PANalytical diffractometer (Cu Kα1,2 X-
radiation, λ1 = 1.540598 Å; λ2 = 1.544426 Å), equipped with a PIXcel
1D detector and a flat-plate sample holder in a Bragg−Brentano para-
focusing optics configuration (45 kV, 40 mA). Intensity data were
collected by the step-counting method (step 0.01°), in continuous
mode, in the ca. 3.5° ≤ 2θ ≤ 50° range.
Variable-temperature powder X-ray diffraction data were collected
on an PANalytical X’Pert Powder diffractometer (Cu Kα1,2 X-radiation
λ1 = 1.540598 Å; λ2 = 1.544426 Å), equipped with an PIXcel 1D
detector, and a flat-plate sample holder in a Bragg−Brentano para-
focusing optics configuration (40 kV, 50 mA), and a high-temperature
Anton Paar HKL 16 chamber controlled by an Anton Paar 100 TCU
[Yb(H9TPPA)(H2O)x]Cl2·yH2O (2) [where x + y = 7]. Elemental
CHN composition (%). Calcd: C 40.29; H 4.23; N 4.27. Found: C
39.01; H 4.14; N 4.37. Thermogravimetric analysis (TGA) data
(weight losses in %) and derivative thermogravimetric peaks (DTG; in
italics inside the parentheses): 25−186 °C, −9.6% (44 °C); 186−350
°C, −5.2% (310 °C). Selected FT-IR data (cm−1, from KBr pellets):
ν(H2O) = 3600−3154; ν(N−H) = 3154−2917 w-m; ν(POH) =
2917−2678 w-m; δ(H2O) = 1633 m; δ(phenyl−P) = 1550 m, 1484 m,
and 1394 m; ν(PO) = 1261−1101 m-vs; ν(P−O) = 1101−779 vs;
ν(phenyl) = 584 m, 570 m, and 526 m.
[Y(H9TPPA)(H2O)x]Cl2·yH2O (3) [where x + y = 7]. Elemental CHN
composition (%). Calcd: C 43.04; H 4.52; N 4.56. Found: C 42.86; H
4.03; N 4.41. Thermogravimetric analysis (TGA) data (weight losses
in %) and derivative thermogravimetric peaks (DTG; in italics inside
the parentheses): 25−188 °C, −11% (38 °C); 188−350 °C, −5.6%
(301 °C). Selected FT-IR data (cm−1, from KBr pellets): ν(H2O) =
3600−3166; ν(N−H) = 3166−2921 w-m; ν(POH) = 2921−2663 w-
m; δ(H2O) = 1631 m; δ(phenyl−P) = 1550 m, 1484 m, and 1394 m;
ν(PO) = 1263−1097 m-vs; ν(P−O) = 1097−755 vs; ν(phenyl) =
584 m, 570 m, and 524 m.
B
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Catalytic Studies. In a typical experiment, CH3CN (2.0 mL) was
charged with thioanisole (0.3 mmol) and the catalyst (2 mol %);
hydrogen peroxide (30% w/w, Perdrogen aqueous solution) was then
added to the previous mixture (2 equiv). Reactions were carried out
under constant magnetic stirring and in the absence of light at 50 °C.
Prior to reuse, solid catalysts were separated from the reaction mixture
by centrifugation at 6000 rpm, washed with acetonitrile and ethanol,
and dried overnight at ambient temperature. Leaching tests were
carried out using the reaction conditions mentioned above. When a
conversion up to 40% was reached, the solid catalyst was filtered
through a 0.20 μm nylon GVS membrane. The filtrate was transferred
to a preheated (50 °C) flask and left to react for further 20 h at the
same temperature and in the absence of light. Reactions were followed
by GC-FID analyses performed on a Varian 3900 chromatograph
using helium as the carrier gas (30 cm/s). GC-MS analyses were
performed using an Agilent 6890 gas chromatograph equipped with a
mass selective detector (MSD 5973) with electron impact ionization
(EI) operating at 70 eV (Agilent Technologies, California, USA).
Fused silica DB-5 type capillary columns (30 m × 0.25 mm i.d.; 0.25
μm film thickness) were employed in both cases. The gas
chromatographic conditions were as follows: initial temperature of
40 °C during 4 min; final temperature of 250 °C with a temperature
rate of 25 °C/min; injector temperature of 250 °C; detector
temperature of 250 °C. Aliquots were withdrawn from the reaction
mixtures and injected directly into the GC injector.
conditions, namely, temperature and ratios between the metal
sources and H10TPPA did not produce any effect on the overall
crystallinity, purity, or even in the main structural features of
the compounds. All materials were extensively characterized
using a myriad of advanced techniques which include PXRD,
SEM, STEM, UV−vis, FT-IR, and elemental analysis. Please
see Figures S5−S13 for an extensive detailed characterization of
the materials.
Figure 1 collects the PXRD patterns of the Por-MOFs and
the corresponding SEM images. The particle size of Por-MOFs
Chemosensing Studies. A 1 mg sample of each Por-MOF (1−3)
was dispersed in 3 mL of distilled water. The solution was sonicated
for 10 min and used as a stock solution. Then, 100 μL of the stock
solution was taken in a 3 mL of quartz cuvette, and a magnetic bar was
placed inside. The mixture was stirred for 1 min to obtain a uniform
suspension. Different analytes such as nitroaromatic compounds and
phenols were prepared as 1 mM solution. The emission spectra of the
suspension were collected before and after the addition of the analyte
solution.
RESULTS AND DISCUSSION
■
Materials Preparation. Por-MOF synthesis started with
the design of a more efficient synthetic pathway for the
isolation of the organic linker H10TPPA (Scheme 1). The main
objective was to isolate the molecule in high yields and in short
periods of time.
Figure 1. (a) Optical photographs of bulk [M(H9TPPA)(H2O)x]Cl2·
yH2O materials; (b) powder X-ray diffraction data and SEM images of
[M(H9TPPA)(H2O)x]Cl2·yH2O materials [where x + y = 7; M3+
=
La3+ (1), Yb3+ (2), or Y3+ (3)]. Color code: red, 1; black, 2; green, 3.
The first synthetic stage of the new procedure involves the
isolation of 5,10,15,20-tetrakis(p-(diethoxyphosphoryl)
phenyl)porphyrin (H2TPPE) based on the Lindsey method-
ology,3 1 , 32 comprising the condensation of 4-
(diethoxyphosphoryl)benzaldehyde dimethylacetal obtained
by Hirao cross-coupling33 and pyrrole, in the presence of
BF3·(OEt)2, followed by oxidation performed by 2,3-dichloro-
5,6-dicyanobenzoquinone (DDQ). We stress that this new
procedure is significantly more efficient when compared to
those reported in the literature,34,35 with H2TPPE being here
obtained with a best overall yield (28%) and with a
concomitant drastic reduction in the overall reaction time
from 2 days (overall yield of 7%),34 or even 68 h (overall yield
of 7%),35 to just 9 h (see Scheme 1 and the Supporting
Information for further details).
H10TPPA was obtained in 94% yield from H2TPPE using the
Rabinowitz methodology,34,36 typically characterized by the
cleavage of the protecting groups through interaction with
chlorine or bromine-trialkylsilanes to yield the corresponding
trialkylsilyl esters which, in turn, can be readily hydrolyzed to
afford the phosphonic acid form.
materials 1−3 was determined by the Williamson−Hall method
(see Supporting Information section 2.3 for details).37,38
The average crystallite sizes were determined as ca. 13 ( 1),
17 ( 2), and 15 ( 2) nm for 1, 2, and 3, respectively. We note
that these average values are in good agreement with the crystal
shape and size observed from STEM studies as depicted in
Figure 2.
Catalytic Studies. Nano-Por-MOFs materials (1−3)
revealed to be effective catalysts in sulfoxidation through
green and sustainable procedure. We note that this type of
reaction is of great industrial and environmental importance
because of the negative impact induced by the presence of
sulfur-containing compounds in petroleum products.39−45
Remarkably, and to the best of our knowledge, there are only
two examples of Por-MOFs reported in the literature which are
efficient catalytic systems for the oxidation of organosulfur
compounds.46,47
The evaluation of the catalytic activity of materials 1−3 in
sulfoxidation was based on previous studies developed in our
laboratories.48,49 Acetonitrile was employed as the solvent and
H2O2 as the oxidant (see Supporting Information section 3.1
for the optimization procedure of the catalytic studies). We
note that H2O2 is the most common oxidant used in the
oxidation of sulfur-containing compounds, being considered a
The isolation of materials [M(H9TPPA)(H2O)x]Cl2·yH2O
[where x + y = 7; M3+ = La3+ (1), Yb3+ (2), and Y3+ (3)] was
achieved by employing hydrothermal synthesis using HCl (6
M) as modulator. Systematic modifications in the experimental
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Table 1. Catalytic Activity of the Nano-MOFs 1−3 and of the
Corresponding Building Units in the Oxidation of
a
Thioanisole with H2O2
a
Thioanisole (0.3 mmol) was dissolved in 2.0 mL of CH3CN and kept
under magnetic stirring at 50 °C for 24 h, in the presence of 2 mol %
of catalyst and 2 equiv of H2O2.
Figure 3. Kinetic profiles for the oxidation of thioanisole (0.3 mmol)
by H2O2 using Por-MOFs 1−3 as catalysts.
ratios (Figure S14). The presence of sulfoxide and sulfone
products were observed in all cases. We note that in the studies
involving the metal salts, another reaction product was also
detected and identified by GC-MS as a chlorine derivative of
the sulfoxide (Figure S15).
For the metal salts, the catalytic efficiency follows the order
YCl3·6H2O > YbCl3·6H2O > LaCl3·7H2O, with average
conversions of 86, 83, and 75%, respectively. The oxidation
profile is, however, strikingly different concerning selectivity,
with the results showing that these precursors are much less
selective to the sulfoxide when compared with the nano-MOFs
(1−3) (Table 1). When the ligand H10TPPA was evaluated as
catalyst, a conversion of 99% was achieved after 24 h, with the
sulfoxide being obtained in 97% selectivity.
Though apparently the precursors appear to be more
efficient than the nano-MOFs (1−3), these materials exhibit
clear advantages in comparison with the corresponding building
units: the metal salts act as homogeneous catalysts, which
ultimately requires significantly more demanding separation
and purification processes than for the pure heterogeneous
catalysis performed by the nano-MOFs, while although
H10TPPA was insoluble, the behavior of this molecule as
catalyst in two more runs (Figure S16) revealed a significant
decrease in conversion from the first to the third run (99−
56%). Data from 31P NMR (Figure S17) and FT-IR (Figure
S18) further suggested the decomposition of this ligand.
The recyclability of nano-Por-MOFs (1−3) was investigated
in consecutive batch runs for the oxidation reaction of
thioanisole by H2O2 at 50 °C (Figure 4). In all cases, the
Figure 2. STEM images of: (a) [La(H9TPPA)(H2O)x]Cl2·yH2O (1);
(b) [Yb(H9TPPA)(H2O)x]Cl2·yH2O (2), and (c) [Y(H9TPPA)-
(H2O)x]Cl2·yH2O (3) [where x + y = 7].
cheap and green oxidant with H2O being obtained as the sole
byproduct.50,51
Because the oxidation reaction of organosulfur compounds
proceeds via electrophilic addition of oxygen atoms, sulfides
with higher electron density on the sulfur should react more
readily, which motivated us to select thioanisole as substrate.
The obtained results for the sulfoxidation of thioanisole by
H2O2, using the nano-Por-MOFs (1−3) as catalysts, as well as
the corresponding building blocks, in acetonitrile at 50 °C are
summarized in Table 1.
The catalytic efficiency of the nano-Por-MOFs follows the
order 1 > 3 > 2, with average conversions of 89, 79, and 68%,
respectively (Table 1). In all cases, a selectivity up to or equal
to 90% with respect to the sulfoxide product was observed. The
kinetic profiles are depicted in Figure 3. This order of catalytic
efficiency agrees well with the observed average crystallite size:
Por-MOF (1) has smallest average crystallite size and,
consequently, the largest surface area with a concomitant
higher availability of catalytic sites to perform the reaction. In
the absence of catalyst, and under similar reaction conditions,
only 29% of conversion was reached after 24 h.
For comparative purposes, the catalytic activities of the
building units were also investigated (LaCl3·7H2O, YbCl3·
6H2O, YCl3·6H2O and H10TPPA), using equivalent molar
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fast detection methods.52−55 The fluorimetric method is, in this
context, an efficient and nondestructive approach with high
sensitivity and fast response time.54,55 Nitroaromatics, such as
NB, DNB, and TNP, are considered as models of the most
common nitro explosive molecules and were used to assess the
ability of Por-MOFs 1−3 to be used as chemosensors.52,55 The
absorption spectra of nano-Por-MOFs (1−3) suspended in
water exhibit a broad Soret band ca. 415 nm, and when
compared with the absorption spectrum of H10TPPA, an
alteration of the absorption pattern is observed (Figure S8).
The four Q-band pattern of H10TPPA changed to one or two
bands, with the new absorption band peaking at 662 nm. The
emission spectra (Figure S25) shows a broad emission at 678.7
nm upon excitation at 430 nm.
The variations in the absorption and emission spectral
patterns are in good agreement with the dicationic form of
H10TPPA,56 suggesting that the organic linker has the central
core protonated while in the composition of the Por-MOFs. As
evidenced from our previous work,56 protonated porphyrins
undergo additional intermolecular hydrogen bonding inter-
actions with nitroaromatic compounds (NACs) and show
strong enhancement in its sensitivity.
To explore the ability of the novel nano-Por-MOF materials
(1−3) to sense trace amounts of nitro explosive model
molecules, luminescence titrations were performed with an
incremental addition of 1 mM stock solutions of NB, DNB, and
4-NP (Figures S26−S28) and TNP (Figure 5) to dispersions of
each Por-MOF in water. Similar luminescence quenching
titrations were carried out with phenol (PhOH) (Figures S26−
S28).
Figure 4. Catalytic recycling tests for the oxidation of thioanisole using
[La(H9TPPA)(H2O)x]Cl2·yH2O (1), [Yb(H9TPPA)(H2O)x]Cl2·
yH2O (2), or [Y(H9TPPA)(H2O)x]Cl2·yH2O (3) [with x + y = 7].
Interestingly, most of the tested NACs showed, as expected,
fluorescence intensity quenching for this type of interaction in
materials 1−3. However, TNP showed a completely distinct
profile when compared with the remaining NACs. Figure 5
shows the changes in the luminescence intensity with the
incremental addition of TNP (up to 145.2 μM) to dispersions
of the three nano-Por-MOFs. The addition of 0−50.6 μM TNP
to dispersions of Por-MOFs 1 and 3 as well as 0−26.0 μM TNP
to the dispersion of 2 showed an increase in the emission
intensity, reaching a saturation point where further addition of
TNP (until 145.2 μM) leads to a fluorescence quenching.
The emission efficiencies were calculated for each material
(1−3). The negative tendency is due to the enhancement
profile observed in the emission (Figure 5) for concentrations
between 0 and 50.6 μM of TNP (Figure 6). The response of
the materials (1−3) varied with the lanthanide used, with Por-
MOF 1 being the most efficient in the initial step of recognition
(reactivity order: 1>3>2). This kind of response shows that the
acidic nature of the phenolic −OH groups in TNP seems to
interact with the porphyrin, providing an initial enhancement in
the emission intensity. The efficiency of this behavior was
ascribed to be −79, −69, and −76% for 1, 2, and 3, respectively.
After reaching the equilibrium, the emission intensity is
quenched with the incremental addition of TNP. The second
step, which seems to be dependent on the first, is associated
with a photoinduced electron transfer process between the
electron-deficient TNP and the electron-donating nano-Por-
MOFs. It is interesting to note that the initial enhancement
step is the more efficient one, whereas the less efficient one
corresponds to the quenching step.
solid catalysts were washed and dried under atmospheric
conditions prior to reuse. The recyclability of catalyst 1 was
investigated in four consecutive batch runs with the catalytic
activity remaining roughly constant. Sulfoxide and sulfone were
formed in 90 and 10% selectivity, respectively, in the first run,
while in the subsequent runs, sulfoxide was formed with a
selectivity of 96% and the sulfone product with 4%. In the case
of the ytterbium-based Por-MOF 2, a moderate decrease of
conversion between the first and second runs (11%) was
observed, though selectivity remained constant in both runs.
Recycling experiments of material 3 revealed a decrease of
conversion from 79% in the first run to 63% in the third run,
and a decrease of overall selectivity for the sulfoxide product
from 94 to 90% followed by 87% in the first, second, and third
runs, respectively. The structural integrity of all materials was
deeply evaluated by PXRD, SEM, and FT-IR at the end of each
run (Figures S19−S24). Among all Por-MOFs, material 1
showed to be the most promising catalyst keeping the catalytic
activity along four consecutive runs with no significant
modifications in the structure, even though a small reduction
of the overall crystallinity was observed.
In this sense, to ascertain its true heterogeneous nature, a
leaching test was performed. After separating the solid catalyst
from the reaction mixture (conversion of 42% after 4 h at 50
°C), the conversion increase during the period between 4 and
24 h was negligible (42−50%), thus indicating the absence of
soluble active species and, ultimately, that 1 is a truly
heterogeneous catalyst.
Chemosensing Studies. Nitro explosives constitute a huge
threat to homeland security and mankind health, which has
recently motivated the development of simple, inexpensive, and
To better understand the role of the acidic effect in the first
step, a control experiment was performed by adding trifluoro-
acetic acid (TFA) to the suspension of the nano-MOFs (Figure
E
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Figure 6. Fluorescence quenching efficiencies of TNP, when using the
nano-Por-MOFs [La(H9TPPA)(H2O)x]Cl2·yH2O (1), [Yb-
(H9TPPA)(H2O)x]Cl2·yH2O (2), and [Y(H9TPPA)(H2O)x]Cl2·
yH2O (3) [with x + y = 7].
can be used to calculate the association constants and sensitivity
of the interaction. However, the same cannot be considered for
lower concentrations (0−60 μM), where the curve shape seems
to be associated with an equilibrium when TNP interacts with
the porphyrin core. Furthermore, materials 1 and 3 show a
similar curve shape upon addition of TNP within the same
concentration range, while material 2 provides an interaction in
a shorter concentration range (0−26.0 μM) with a completely
different curve shape (Figure 7).
A first analysis plotting the emission intensity at 678.7 nm
against 1/[TNP]2 (Benesi−Hildebrand approach) fits in a 1:2
stoichiometry, providing a linear progression (Figure 7b). The
same fitting procedure to a 1:1 stoichiometry interaction shows
a slight curvature instead of the expected linear progression
(Figure 8 inset). In fact, porphyrins are well-known to interact
in a 1:2 stoichiometry with TNP, where a double protonation
of the porphyrin core occurs, maintaining the anionic TNP as
counterion in the final structure. The complexity inherent to
these equilibria with nano-MOF materials is effectively much
higher and provides a behavior which up to now and to the best
of our knowledge has not been reported.
The association constants show that 2 has the best
association constant (2.91 × 1010 M−2), followed by 1 and 3
with association constants around 1.43 × 109 and 1.78 × 108
M−2. These values are in good agreement with the
corresponding efficiencies of materials 1−3 among the
concentration ranges where it is possible to calculate the
association constants for each material. The curve shape
differences and association constants of materials 1−3 (Figure
7) are highly dependent on the turning point concentration
where, inevitably, both effects are present. As these provide
opposite responses in emission, the data closer to this
concentration does not fit a linear progression in any of the
approaches used to calculate the association constants. This
variation is even more prominent when plotting the I0/I versus
[TNP] (Figure 8) where it is possible to note that 3 seems to
provide a sharper turning point, where only one point is
neglected from both steps.
Figure 5. Emission spectra of [La(H9TPPA)(H2O)x]Cl2·yH2O (1),
[Yb(H9TPPA)(H2O)x]Cl2·yH2O (2), and [Y(H9TPPA)(H2O)x]Cl2·
yH2O (3) [with x + y = 7] dispersed in water upon increasing addition
of TNP (6.6 μM each addition) until a total concentration of 145.2
μM. Insets: variation in fluorescence upon addition of different
aliquots of TNP up to 52.8 μM.
S29). While TFA has a higher acidic nature than TNP, only a
40% enhancement in the emission intensity was observed. The
use of a higher concentration of TFA does not provide the
quenching effect. This clearly shows that the enhancement
profile observed is intimately associated with the pKa of the
host and rules out its participation in the second step
(quenching effect). Plots of the emission variation at 678.7
nm against the concentration of TNP provided a nonlinear
variation at lower concentrations of TNP. This kind of behavior
prompted us to further study both effects individually, whereas
for concentrations above 60 μM, a Stern−Volmer (SV) analysis
The turning point for Por-MOFs 1 and 3 is reached at higher
concentrations, while 2 has a broad turning point ranging from
26.0 to 50.6 μM. This behavior might be associated with the
crystallite size and shape, where the larger agglomerated
F
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To gain a better insight on the second step of interaction, a
SV analysis was performed to the emission data with
concentrations above 50.6 μM, where only the quenching
behavior is observed (Figures S30−S32). With increasing
concentrations, the SV plot showed a deviation from linearity
and began to bend upward for most of the analytes, at
concentrations higher than 50.6 μM. A linear SV relationship
may be observed if either a static or dynamic quenching process
is dominant. Thus, in the case of higher concentrations of
analytes, the two processes may be competitive, which results in
a nonlinear SV relationship. The only exceptions to this upward
curvature are 4-NP and TNP (Figure 8, S30, and S31). The
linear variation at lower concentration of the analytes in this
case can mainly be attributed to static quenching effects
(ground state interaction between the analytes and the Por-
MOFs). The Ksv constants calculated based in this assumption
are summarized in Table 2. Material 2, as before, systematically
Table 2. Stern−Volmer Constants for Nano-Por-MOFs 1−3
with Different Analytes
Stern−Volmer constant (M−1
)
analyte
1
2
3
NB
21740
15192
5600
34048
26605
10930
20813
34203
32189
17951
9904
DNB
TNP
4-NP
PhOH
21531
23583
18060
18581
provides the best association constants under the Volmer−
Stern equation for all the analytes (Table 2), corroborating the
quenching efficiencies showed in Figure 9. The SV rate
Figure 7. (a) Change in emission at 678.7 nm from materials 1−3 vs
[TNP]. (b) Plots of Benesi−Hildebrand equation for materials 1−3
showing the linear dependence in a 1:2 stoichiometry (inset: Benesi−
Hildebrand plot with a 1:1 stoichiometry).
Figure 9. Fluorescence quenching efficiencies of different analytes
when using the nano-Por-MOFs [La(H9TPPA)(H2O)x]Cl2·yH2O (1),
[Yb(H9TPPA)(H2O)x]Cl2·yH2O (2), and [Y(H9TPPA)(H2O)x]Cl2·
yH2O (3) [with x + y = 7].
Figure 8. I0/I versus [TNP] and the linear ranges, where Ksv can be
calculated for materials 1−3.
constants are of a similar order of magnitude when compared
to those of other materials reported in the literature for the
nitro-aromatic compounds NB and DNB, with TNP exhibiting
moderate SV constants and an unprecedented fluorescence
behavior.55
All nano-Por-MOFs showed a good sensing behavior toward
the studied analytes (Figure 6). Por-MOF 3 proved to be the
most efficient chemosensor by quenching the initial fluo-
rescence intensity up to 80% in all analytes, except for TNP
nanoparticle sizes might affect the TNP diffusion to the
material generating a wider concentration turning point
between the two steps. In this regard, 3 is highly effective in
both processes (emission enhancement and quenching), and a
closer analysis shows the dispersed needle shape and size of this
material seems to enhance the interaction between the material
and TNP.
G
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Article
Notes
where a peculiar profile of initial enhancement followed by
quenching demanded a special attention. Even in this special
case, Por-MOF 2 experienced a fluorescence quenching of 55%,
at concentrations of 145.2 μM, while Por-MOFs 1 and 3
showed fluorescence quenching of only 34 and 49%,
respectively. Furthermore, it was observed that detection of
TNP is possible with a detection limit as low as 0.82 ppm with
material 3. The remaining materials provided a LOD
significantly higher around 19.84 ppm (1) and 21.21 ppm
(2). The LODs calculated for the remaining analytes are shown
in Table S2. We note, however, that herein a “green” solvent
was employed for the detection process (water), contrasting
with the toxic nature of the solvents used in the vast majority of
the reported studies available in the literature.52,54,55
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Funding Agencies and Projects: We wish to thank Fundaca
para a Ciencia e a Tecnologia (FCT, Portugal), the European
̂
̧
o
̃
Union, QREN, FEDER through Programa Operacional
Factores de Competitividade (COMPETE), CICECO - Aveiro
Institute of Materials (POCI-01-0145-FEDER-007679; FCT
ref. UID/CTM/50011/2013), QOPNA (FCT UID/QUI/
00062/2013), and CQE (FCT UID/QUI/0100/2013) re-
search units, financed by national funds through the FCT/
MEC and when appropriate cofinanced by FEDER under the
PT2020 Partnership Agreement. We also thank FCT for
funding the R&D project FCOMP-01-0124-FEDER-041282
(ref. FCT EXPL/CTM-NAN/0013/2013). Individual Grants
and Scholarships: FCT is also gratefully acknowledged for the
Ph.D. grant no. SFRH/BD/86303/2012 (to C.F.P.), SFRH/
BD/84231/2012 (RFM) and FCT UID/QUI/00062/2013
(F.F.). F.F. and R.F.M. acknowledge the project “Smart Green
Homes − BOSCH” (POCI-010247-FEDER-007678) for the
postdoctoral and Masters scholarships (refs. BPD/CICECO/
5508/2017 and BIM-SGH/CICECO-UA/174-01/2017, re-
spectively). We also thank Dr. Venkatramaiah Nutalapati for
insightful discussions concerning the chemosensing studies.
CONCLUDING REMARKS
■
In short, we have successfully designed and tested a new and
more efficient route for the isolation of porphyrinic molecule
H10TPPA, which was then employed as a primary building unit
in the preparation of novel nanosized Por-MOFs [M-
(H9TPPA)(H2O)x]Cl2·yH2O [where x + y = 7; M3+ = La3+
(1), Yb3+ (2), or Y3+ (3)]. Nano-Por-MOFs 1−3 exhibit true
multifunctionality, acting both as efficient heterogeneous
catalysts in the sulfoxidation of thioanisole and as chemo-
sensors for the detection of nitroaromatic compounds.
ASSOCIATED CONTENT
■
REFERENCES
S
* Supporting Information
■
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b03214.
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: filipe.paz@ua.pt. Fax: +351 234 401470. Tel.: +351
234401418.
*E-mail: jtome@ua.pt. Fax: +351 234370084. Tel.: +351
234370342.
*E-mail: msimoes@ua.pt. Fax: +351 234370084. Tel.: +351
234370713.
ORCID
Joao Rocha: 0000-0002-0417-9402
Joseph T. Hupp: 0000-0003-3982-9812
Omar K. Farha: 0000-0002-9904-9845
̃
Mario M. Q. Simoes: 0000-0001-9071-2252
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Filipe A. Almeida Paz: 0000-0003-2051-5645
Author Contributions
C.F.P. and F.F. contributed equally to the present work.
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DOI: 10.1021/acs.inorgchem.7b03214
Inorg. Chem. XXXX, XXX, XXX−XXX