Rational Syntheses of CdIIand PbIIMetal-Organic Frameworks for Luminescence Sensing of Nitroaromatics, Ferric and Chromate Ions

Article abstract of DOI:10.1002/zaac.201600344

The metal-organic frameworks (MOFs), {[Cd(L)(H₂O)]·CH₃OH·DMF} (1) and [Pb(L)(DMF)] (2) (L = 2,4-dihydroxybenzaldehyde nicotinoylhydrazone), were synthesized and characterized using microanalyses, IR spectroscopy, and single-crystal X-ray diffraction. Single crystal X-ray analyses revealed that both 1 and 2 exhibit 2D grid networks. The photoluminescence investigation indicates that 1 and 2 could be a prospective candidate for developing luminescencent sensors for the selective sensing of nitrobenzene, which is used as a precursor for explosives. The luminescent properties of 1 and 2 in different solvents were investigated and corroborated by theoretical calculations. Additionally compounds 1 and 2 also show selective sensing ability for Fe³⁺as well as for CrO₄^2–ions.

Full text of DOI:10.1002/zaac.201600344

Journal of Inorganic and General Chemistry
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DOI: 10.1002/zaac.201600344
Rational Syntheses of CdII and PbII Metal-Organic Frameworks for
Luminescence Sensing of Nitroaromatics, Ferric and Chromate Ions
Yu Wu,*[a] Yulong Li,[a] Like Zou,[a] Jianshen Feng,[a] Jianqiang Liu,*[b]
Miaomiao Luo,[b] Jingwen Xu,[b] Reena Yadav,[c] and Abhinav Kumar*[c]
Keywords: Coordination polymer; Photoluminescence; Sensing; Topology; Density functional calculations
Abstract.
The
metal-organic
frameworks
(MOFs), indicates that 1 and 2 could be a prospective candidate for developing
luminescencent sensors for the selective sensing of nitrobenzene,
{[Cd(L)(H2O)]·CH3OH·DMF} (1) and [Pb(L)(DMF)] (2) (L = 2,4-di-
hydroxybenzaldehyde nicotinoylhydrazone), were synthesized and which is used as a precursor for explosives. The luminescent properties
characterized using microanalyses, IR spectroscopy, and single-crystal of 1 and 2 in different solvents were investigated and corroborated by
X-ray diffraction. Single crystal X-ray analyses revealed that both 1
theoretical calculations. Additionally compounds 1 and 2 also show
2–
and 2 exhibit 2D grid networks. The photoluminescence investigation selective sensing ability for Fe3+ as well as for CrO4 ions.
ating coordinatively unsaturated metal sites, and (3) con-
Introduction
structing the mesoporous channels in the MOFs. The effective-
Self-assembled metal organic frameworks (MOFs) compris-
ness of LMOFs can largely be improved by incorporating
ing of organic linkers and metal ions or clusters provides impe-
active functional groups (such as –NH2 and –COOH) in the
tus for developing new functional materials, e.g. development
ligands. From the aforementioned viewpoints one can say that
of porous materials.[1–3] Also, because of the considerable spe-
the development of dual functional materials with high chemi-
cific surface area and recognition capability, the microporous
cal stability, which can act as a fluorescent probe for different
MOFs have drawn a great deal of attention as an excellent
ions as well as can also detect nitrobenzene is a challenging
candidate for the rapid recognition and sensing of cations,
task. To develop such type of LMOF, a salen type ligand was
anions, small molecules.[4–6] The MOF-based fluorescence
selected in the presented investigation due to the following
sensor was firstly reported in 2009.[7] After this investigation
reasons:[13,14] (1) Salen type ligands form chemically stable
MOF-based fluorescence sensor represents a new sub-field of
MOF research especially as explosive detectors.[8–10] Some of
these MOFs are used for the recognition and detection of elec-
materials; (2) this type of ligands comprises of electron pair
rich nitrogen linkers and aromatic π ring, which could adjust
the position of ligands to provide interactions and in turn mod-
tron-deficient nitroaromatic molecules.[8–10] However, in com-
ulate the luminescent character (Scheme 1); and (3) they can
parison with the other applications of MOFs, this area of re-
interact with electron deficient compounds such as nitroben-
search still under progress.
zene and its derivatives. Keeping these aspects in mind, herein,
To develop the luminescent metal-organic frameworks
we wish to report the syntheses, characterization, and sensing
[11,12]
(LMOFs), three strategies
(1) Introducing functional organic sites in the ligand, (2) gener-
can be deployed at present:
capabilities of two new LMOFs comprising of central CdII and
PbII atoms. The synthesized MOFs are capable of sensing not
* Dr. Y. Wu
E-Mail: wuyuhlj@163.com
* Dr. J. Liu
E-Mail: jianqiangliu2010@126.com
* Dr. A. Kumar
E-Mail: abhinavmarshal@gmail.com
[a] College of Chemistry and Environmental Engineering
Sichuan University of Science & Engineering
Zigong, 643000, P. R. China
[b] School of Pharmacy
Guangdong Medical University
Dongguan, 523808, P. R. China
[c] Department of Chemistry
Faculty of Science
University of Lucknow
Lucknow 226 007, India
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201600344 or from the au-
thor.
Scheme 1. Illustration of rational design for stable MOFs based on
CdII/PbII ions and the ligand involving different functional groups.
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only the nitrobenzene but also having capability to sense Fe3+ In that case the overall planarity of the two ligands was more
2–
and CrO4 ions. The results of these investigations are pre- pronounced, while the chelate rings were relatively less planar.
sented herein.
Like for complex 1, a 2D grid layer coordination polymer
of 2 was obtained. The asymmetric unit of 2 consists of a PbII
ion on a crystallographic inversion center, having L ligand and
one coordinated DMF molecule. The PbII ion is coordinated in
a trans {PbN2O4} octahedral fashion (Figure 2a). Two trans
coordination sites are taken up by a carbonyl oxygen atom and
a pyridyl nitrogen atom. The remaining sites are occupied by
azomethine nitrogen atom, phenolic oxygen atom, as well as
DMF. Selected bond lengths and angles are given in Table
S1 (Supporting Information). Pairs of exobidentate L ligands,
binding two PbII ions via pyridyl and carbonyl donor, construct
a 2D layer (Figure 2b).
Results and Discussion
Molecular Structures
Single-crystal X-ray analysis revealed that the asymmetric
unit of 1 is comprising of one CdII ion, one L ligand, one
coordinated water molecule, one free CH3OH, and one DMF
molecule. The central CdII atom adopts a distorted octahedral
coordination environment with O1, N1, N3, O1W, O2, and
O2A (A: –x, –1–y, –1–z) (Figure 1a). The structure contains a
crystallographically imposed inversion center in the middle of
a M2(μ2-O)2 core, where M refers to Cd1 and the oxygen atoms
(O2) belong to the hydroxyl group. The doubly deprotonated
H2L ligand behaves as a tetradentate fashion coordinated to
CdII via carbonyl, azomethine nitrogen atom, phenolic oxygen,
as well as pyridyl nitrogen atoms. The CdII ion is found to be
displaced by 0.22 Å from the plane constituting atoms N3, N1,
O1, and O2. Based on the above connection, adjacent CdII ions
are connected by L ligand into 2D layer along the ab plane
(Figure 1b). As observed for the neutral complex 1, in this case
the ONO coordination generates two chelation rings, all planar
within 0.09 Å, but not perfectly coplanar to each other, having
a dihedral angle of ca. 8°. Accordingly, the overall skeletons
of the two ligands are slightly bent and deviate from planarity
by 0.12 Å. It is interesting to compare the reported complex of
[Fe(HPNO)2]3+, which differs only in the substituent at the
ester terminus and shows an identical coordination mode.[14b]
Figure 2. (a) Local structure of the Pb2(O2C)2 cluster in the structure
of 2. (b) Extended view of 2D layer constructed by the cluster and the
complete attached ligands.
Li et al. have reported four Zn polymers based on aroylhyd-
razone Schiff base ligand (H3L). These polymers displayed
Zn4O4 boat-shaped core and discrete linear tetranuclear central
ZnII atom, mononuclear and dinuclear units, respectively. They
explored the effect of the hydroxyl group substituent in salicyl-
aldehyde on the fluorescent properties.[15a,15b] Using an un-
symmetrical bis-pyridylhydrazone ligand, series of solid
materials were synthesized by Mahmoudi and his co-
workers.[15c,15g] The Schiff base acted as a tridentate N2O-do-
nor ligand through the oxygen, the imine and pyridine nitrogen
Figure 1. (a) Local structure of the Cd2(O2C)2 cluster in the structure
of 1. (b) Extended view of 2D layer constructed by the cluster and the
complete attached ligands.
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atoms in all the complexes. Also, the central metal atom is cant quenching behavior due to the solute and solvent interac-
hemidirectionally coordinated and consequently sterically ideal tion.
for establishing tetral bonding interactions. Furthermore, theo-
retical calculations (DFT) were employed to analyze some rel-
evant noncovalent interactions observed in the solid state.
In order to gain insight into the sensing process, 1 and 2
were titrated with various NB in DMF at room temperature
(Figure 3). This facilitated possible host-guest interactions as
a result of electron transfer from the electron-donating frame-
work to the highly electron-deficient NB molecule upon exci-
tation, resulting in fluorescence quenching.[21–24] Excellent lin-
ear dependencies of luminescence intensities on the NB con-
centration were obtained (R2 = 0.9973 for 1 and R2 = 0.9972
for 2). The luminescence intensity decreased to 50% at
15ϫ10–4 and 24ϫ10–4 m for 1 and 2, respectively, and com-
plete quenching in the luminescence was observed at
48ϫ10–4 and 75ϫ10–4 m (Figure 3), suggesting that the two
luminescent MOFs sensor can be used for detecting the con-
centration of NB.[25] The absorption spectrum for NB exhibits
a massive overlap with the emission of 1 and 2 (Figure S4,
Supporting Information), which suggests that both electron-
and energy-transfer mechanisms are present for the fluores-
cence quenching by NB.[22]
Thermogravimetric Analyses
Thermogravimetric analysis (TGA) of complexes 1 and 2
indicated two step weight loss (Figure S2, Supporting Infor-
mation). The first major weight loss of complex 1 is in the
range between 25 and 256 °C. The observed weight loss of
25.6% corresponds to the loss of the free DMF, CH3OH and
coordinated H2O molecules (calcd. 25.1%). The second
weight loss at 400 °C may be due to the elimination of coordi-
nated L (calcd. 52.4%; found 52.0%). The final residual mass
of 25.8% is nearly indicating the formation of CdO (calcd.
26.1%, found 24.8%). Complex 2 has the similar decomposi-
tion behavior. The observed weight loss of 14.2% corresponds
to the loss of the coordinated DMF molecule (calcd. 13.6%).
To substantiate this point and to gain insight into the
quenching mechanism associated with the analytes mentioned
in the presented investigation, the HOMO-LUMO energies of
the nitrobenzene along with the other analytes were calculated
using density functional theory (DFT) at the B3LYP/6-31G*
level (Table S2, Supporting Information). The most probable
reason behind quenching may be the charge transfer from the
1 and 2 framework to the LUMO of the analytes and this
charge transfer will occur when the LUMO of the donor (1
and 2) lie at the higher energy scale than the LUMO of the
acceptor in the analyte. Maximum fluorescence intensity
quenching observed in the case of nitrobenzene is due to easy
electron transfer from the excited metal-organic framework to
the LUMO of nitrobenzene.[29,30] Also, the LUMOs of 1 and
2 are at the lower energy level than that of LUMO of other
analytes, which rules out the effective charge transfer from the
1 and 2 to these analytes and hence the efficient quenching.
However, the observed order of quenching with other analytes
is not in full agreement with the corresponding LUMO ener-
Photoluminescence Measurements and Sensing
Experiments
Complexes 1 and 2 having central CdII and PbII atoms with
d10 configuration are coordinated to the luminescent ligand and
hence show strong luminescence with emission peaks at 499
and 503 nm (λex = 350 nm) for 1 and 2, respectively. This
emission band can be assigned to H2L ligand-centered emis-
sion, because the emission for the free H2L was observed at
555 nm (λex = 360 nm) (Figure S3, Supporting Information).
The hypochromatic shift in the emission may arise from the
aggregation induced emission for 1 and 2, where the coordina-
tion of the ligand to the metal ions can reduce the freedom of
the ligands and enhance their non-radiative transitions.[16–18]
To investigate the fluorescence sensing ability of 1 and 2,
the emission spectra of 1 and 2 crystal powder dispersed in
DMF with different solvent emulsions were measured (Figure
S5, Supporting Information). It has been reported that the pho-
toluminescence (PL) intensities are solvent dependent, mainly gies of the other analytes, which indicates that electron transfer
in the case of nitrobenzene (NB),[22–23] which exhibits signifi- is not the sole mechanism for the intensity quenching.
Figure 3. Emission spectra of (a) 1 and (b) 2 dispersed in DMF with the titration of NB, and the quenching of the original emission.
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Figure 4. Comparison of the luminescence intensity of different concentrations of Fe3+ ions added to (a) 1 and (b) 2 in 10–2 m aqueous solution.
The luminescence spectra of 1 and 2 dispersed in DMF solu- 2 after interacting with CrO42–, powder XRD were performed
tion containing the same concentrations of M(NO3)x (M = Li+, after soaking the LMOFs 1 and 2 in Na2CrO4 for 24 h. As
Na+, K+, Mg2+, Ca2+, Pb2+, Al3+, Ca2+, Ni2+, Fe3+, Co2+) were evidenced by powder XRD, the samples soaked in Na2CrO4
studied. The luminescent properties of Mn+@1 and Mn+@2 solution for 24 h still maintains their crystallinity (Figure S11,
were recorded and compared (Figure S6, Supporting Infor- Supporting Information). As shown in Figure S12, the absorp-
mation). On addition of Mg2+ to MOFs 1 or 2 the intensity of tion band of 1 and 2 is located from 350 to 450 nm, which
Mn+@1 and Mn+@2 gets slightly enhanced, whereas the emis- is largely overlapped by the absorbing band of CrO42–. Upon
sion intensities declined on addition of other metal ions. Par- excitation, there is a competition of absorption of the light
ticularly, the Fe3+ ion shows a significant quenching effect on source energy between CrO42– ions and 1 or 2. Combined with
the luminescence intensity to a degree. According to the re- the absorption and luminescent spectra, it can be expected that
ported literature, Lewis basic sites, such as pyridyl, amide, the energy absorbed by 1 or 2 is transferred to CrO42–, re-
hydroxyl sites,[28–31] containing porous MOFs could have an sulting in a decrease in the luminescence intensity. This
effect on metal ions. As the possible reason of the quenching quenching mechanism is in agreement with that proposed pre-
by Fe3+, we consider that it might be related to the interaction viously by Chen et al.[34]
between the Fe3+ ions and hydroxyl sites on the Mn+@1 and
Mn+@2.[32] The energy transfer efficiency from L to the
CdII/PbII ions within Mn+@1 and Mn+@2 gets minimized by
the interaction between the Fe3+ ions and the L ligand, which
may lead to the decrease in the luminescent intensity. As illus-
trated in Figure 4, the luminescent intensity is almost com-
pletely quenched at a Fe(NO3)3 concentration of 21ϫ10–4 and
87ϫ10–4 m for 1 and 2, respectively. Therefore, 1 and 2 can
be used as a highly sensitive luminescent probe for the quanti-
tative detection of Fe3+ ions.
Conclusions
The 2,4-dihydroxybenzaldehyde nicotinoylhydrazone ligand
H2L with free recognition sites was coordinated with CdII and
PbII ions to yield two robust 2D coordination polymers 1 and
2. The resulting material emits blue luminescence, which
exhibits quenching effect in presence of nitro aromatics. It was
shown that observed quenching is due to the efficient electron
transfer from the conduction band of 1 and 2 to the LUMO of
the nitrobenzene. Also, dispersed suspensions of 1 and 2 in
DMF solution display good selectivity and sensitivity toward
The luminescence investigations were also carried out to ex-
plore the influence of various anions on the luminescence of 1
and 2. Using ultrasonic method the suspensions of MOFs 1
and 2 were prepared in 4 mL of DMF, and 0.4 mL of NanX
2–
Fe3+ and CrO4 ions.
–
2–
(1ϫ10–2 m) (X = PO43–, F–, Cl–, Br–, I–, NO3 , OAc–, SO4
,
CO32–, C2O42–, or CrO42–) were added in dropwise manner
into the suspensions of 1 and 2 (1ϫ10–3 m). The resultant sus-
pensions were monitored under the perturbation of various
anions, using a fluorescence spectrophotometer, and only
dominant emission peaks were recorded. Interestingly, most of
the anions have insignificant effect on the luminescence of 1
Experimental Section
Materials and Methods: All chemicals were purchased and used
without further purification except for the L ligand. Powder X-ray dif-
fraction (PXRD) was collected with a Bruker D8 ADVANCE X-ray
diffractometer with Cu-Kα radiation (λ = 1.5418 Å) at 50 kV, 20 mA
with a scanning rate of 6°·min–1 and a step size of 0.02°. The simulated
powder patterns were calculated using Mercury 2.0. The purity and
homogeneity of the bulk products were determined by comparison of
the simulated and experimental X-ray powder diffraction patterns.
Fourier transform infrared (FT-IR) spectra as KBr pellet were
2–
and 2, while CrO4 exhibits a drastic quenching effect on
emission (Figure S7, Supporting Information). The quenching
effects on the emission of CrO42– are obviously observed, indi-
cating that 1 and 2 can be considered as a promising lumines-
2–
cent probe for detecting CrO4 among various colorless
measured with
a Nicolet Impact 750 FTIR in the range of
anions.[33] To gain information about the fate of LMOFs 1 and 400–4000 cm–1. Thermogravimetric analysis was performed in a nitro-
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gen atmosphere from room temperature to 800 °C at a heating rate of based on Pb). C16H16N4O4Pb: calcd. C 35.88; H 3.01; N 10.46%;
10 K·min–1, with a SDT Q600 thermogravimetric analyzer.
found C 35.73.; H 3.06; N 10.29%. IR (KBr): ν˜ = 3070 (s); 1646 (vs);
1599 (v); 1516 (v); 1422 (m); 1339 (m); 1221 (m); 983 (m); 842 (v);
720 (v); 594 (m) cm–1.
X-ray Crystallography: Room-temperature single-crystal X-ray dif-
fraction data collection were carried out with a Bruker SMART APEX
diffractometer that was equipped with a graphite monochromated Mo-
Kα radiation (λ = 0.71073 Å) by using an ω-scan technique. The inten-
sities were corrected absorption effects by using SADABS. The struc-
tures were solved by direct methods (SHLEXS-97)[35a] and refined by
a full-matrix least-squares procedure based on F2 (Shelxl-97).[35b] All
the hydrogen atoms were generated geometrically and refined iso-
tropically using the riding model. All non-hydrogen atoms were re-
fined with anisotropic displacement parameters. The relatively high
wR2 value for 2 are really small measurable crystals and the relatively
bad quality of data (the effects of weak high angle data), which cause
the high wR2 value.
Computational Details: In order to ascertain the nature of highest
occupied molecular orbitals (HOMO) and lowest unoccupied molecu-
lar orbital (LUMO) of different analytes, the ligand H2L as well as 1
and 2 density functional theory (DFT) calculations were performed.
Optimized molecular geometries were calculated using the B3LYP ex-
change-correlation functional.[36] The 6-31G** basis set for C, H, N
and O atoms were used, whereas for Cd MWB28 and for Pb MWB78
basis sets were used for geometry optimization. All the calculations
were performed using the Gaussian 09 program.[37]
Supporting Information (see footnote on the first page of this article):
IR, UV-Vis, and fluorescence spectra; TGA plots, pXRD patterns, and
HOMO-LUMO plots.
Crystal Data for 1: C17H22CdN4O6, M = 490.79, monoclinic, P21/n,
a = 11.698(9) Å, b = 13.324(10) Å, c = 14.837(11) Å, β = 92.552(13)°,
V = 2310(3) Å3, Z = 4, Dcalc = 1.411 mg·m–3, F(000) = 992, reflections
collected 11119, independent reflections 4010, Final indices
Acknowledgements
[I Ͼ 2σ(I)] R1 = 0.0739, wR2 = 0.1767, R indices (all data) R1
0.1043, wR2 = 0.1996, gof 1.146.
=
The authors acknowledge financial assistance from National Natural
Science Foundation of China (21501124), the Opening Project of Key
Laboratory of Green Catalysis of Sichuan Institutes of High Education
(No: LYJ1301, LYJ1306, LZJ1203, LZJ14201), the Education Com-
mittee of Sichuan Province (Nos. 12ZA090, 13ZB0131, 14ZB0212,
14ZB0220, 15ZB0222, 15ZB0214), the Start-up Foundation of
Sichuan University of Science & Engineering (Nos. 2014RC05,
2014RC34, 2015RC23, 2015RC26), the Institute of Functionalized
Materials (Nos. 2014PY01, 2015PY03), and the Project of Zigong Sci-
ence & Technology and Intellectual Property Bureau (No. 2014HX02,
2015HX16, 2015HX18).
Crystal Data for 2: C16H16PbN4O4, M = 535.52, monoclinic, P21/n,
a = 11.132(10) Å, b = 13.901(11) Å, c = 13.431(9) Å, β = 122.43(5)°,
V = 1754(3) Å3, Z = 4, Dcalc = 2.028 mg·m–3, F(000) = 1016, reflec-
tions collected 10104, independent reflections 4240, Final indices
[I Ͼ 2σ(I)] R1 = 0.0879, wR2 = 0.2314, R indices (all data) R1
0.1196, wR2 = 0.2712, gof 1.098.
=
Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK.
Copies of the data can be obtained free of charge on quoting the de-
pository numbers CCDC-1479617 for 1 and CCDC-1479619 for 2
(Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://
www.ccdc.cam.ac.uk).
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Synthesis of H2L: An ethanol solution (30 mL) of 2,4-dihydroxy-
benzaldehyde (1.38 g, 10 mmol) and one drop of acetic acid were
added with stirring to an ethanol solution (30 mL) of nicotinoylhydraz-
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1
ether. H NMR ([D6]DMSO): δ = 12.13 (s, 1 H, OH); 11.27 (s, 1 H,
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found C 41.49; H 4.46; N 11.37%. IR (KBr): ν˜ = 3432 (vs); 2971
(m); 1616 (vs); 1528 (v); 1489 (v); 1321 (m), 1214 (m); 1161 (m);
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Received: September 20, 2016
Published Online: ᭿
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© 0000 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Journal of Inorganic and General Chemistry
www.zaac.wiley-vch.de
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Y. Wu,* Y. Li, L. Zou, J. Feng, J. Liu,* M. Luo, J. Xu,
R. Yadav, A. Kumar* ......................................................... 1–7
Rational Syntheses of CdII and PbII Metal-Organic Frame-
works for Luminescence Sensing of Nitroaromatics, Ferric and
Chromate Ions
Z. Anorg. Allg. Chem. 0000, 0–0
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