First examples of metal-organic frameworks with the novel 3,3′-(1,2,4,5-tetrazine-3,6-diyl)dibenzoic spacer. Luminescence and adsorption properties

Article abstract of DOI:10.1021/ic302318j

We report the synthesis of a novel ligand, 3,3′-(1,2,4,5-tetrazine-3, 6-diyl)dibenzoic acid (1). In this fragment, we have introduced two carboxylate groups with the aim of using this ligand as a linker to construct three-dimensional metal-organic frameworks (MOFs). We have been successful in the formation of zinc (2) and lanthanum (3) MOFs. The zinc compound is a two-dimensional structure, while the lanthanum material is a three-dimensional MOF with interesting channels. We include the luminescence and adsorption studies of these materials. Moreover, we have evaluated the in vitro toxicity of this novel ligand, concluding that it can be considered negligible.

Full text of DOI:10.1021/ic302318j

Communication
pubs.acs.org/IC
First Examples of Metal−Organic Frameworks with the Novel 3,3′-
(1,2,4,5-Tetrazine-3,6-diyl)dibenzoic Spacer. Luminescence and
Adsorption Properties
A. J. Calahorro,† Antonio Penas-Sanjuan,‡ Manuel Melguizo,‡ David Fairen-Jimenez,§
̃
Guillermo Zaragoza,⊥ Belen Fernandez,∥ Alfonso Salinas-Castillo,¶ and A. Rodríguez-Dieguez*,†
́
́
́
†Departamento de Química Inorgan
‡Departamento de Química Inorgan
́
́
ica, Universidad de Granada, Avenida Fuentenueva s/n, 18071 Granada, Spain
ica y Organica, Universidad de Jaen, Campus Las Lagunillas, 23071 Jaen, Spain
́
́
́
§Department of Chemical & Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3120,
United States
⊥Unidad de Rayos X, RIAIDT Edificio CACTUS, Universidad de Santiago de Compostela, 15782 Santiago de Compostela, Spain
∥Instituto de Parasitología y Biomedicina Lop
́
ez-Neyra (CSIC), Avenida del Conocimiento s/n, Armilla, Granada
¶Departamento de Química Analítica, Universidad de Granada, 18071 Granada, Spain
S
* Supporting Information
luminescence, and adsorption properties of the first examples of
ABSTRACT: We report the synthesis of a novel ligand,
3,3′-(1,2,4,5-tetrazine-3,6-diyl)dibenzoic acid (1). In this
fragment, we have introduced two carboxylate groups with
the aim of using this ligand as a linker to construct three-
dimensional metal−organic frameworks (MOFs). We have
been successful in the formation of zinc (2) and
lanthanum (3) MOFs. The zinc compound is a two-
dimensional structure, while the lanthanum material is a
three-dimensional MOF with interesting channels. We
include the luminescence and adsorption studies of these
materials. Moreover, we have evaluated the in vitro toxicity
of this novel ligand, concluding that it can be considered
negligible.
coordination polymers [Zn(dbtz)(H2O)]n (2) and
{[La2(dbtz)3(H2O)2](H2O)6}n (3) with the new multidentate
ligand H2dbtz, shown in Scheme S1, demonstrating the potential
of this new ligand to construct new MOFs with interesting
adsorption properties because of their similarity to other linkers,
e.g., NOTT-101 MOF.8 To the best of our knowledge, these are
the first examples of crystal structures of MOFs with ligands that
contain the H2dbtz ligand.
The synthesis (see SI file) of 1 was performed following a
variation of the classical Pinner-type scheme in two steps.9 First,
the reaction between 3-cianobenzoic acid and excess hydrazine
catalyzed by N-acetylcysteine under an inert atmosphere renders
the dihydrotetrazine derivative, which was oxidized to give the
desired fully aromatic derivative by simple stirring of a methanol
suspension of the dihydro derivative in an air atmosphere.
Although a similar air oxidation procedure has been reported to
aromatize some dihydrotetrazine derivatives,10 its scope seems to
be more general than initially recognized, as proven here.
1 was characterized by 1H and 13C NMR spectrometry
[Supporting Information (SI), Figures S1 and S2], electron-
impact high-resolution mass spectrometry (HRMS), and X-ray
diffraction.12 Compound 1 crystallizes in the monoclinic space
group P21/c; the asymmetric unit consisting of a medium H2dbtz
ligand that grows by symmetry generated an almost linear
alignment between the aromatic rings and terminal carboxylate
groups (dihedral angles 3−6°) and one crystallization dimethyl
sulfoxide (DMSO) molecule (Figure 1). In the H2dbtz unit, the
bond distances and angles are very similar to those expected in
comparison with the tetrazine and benzene rings.11 In the
structure, there is only one type of hydrogen bond (SI, Figure
S3), yielding a trinuclear unit formed by one central H2dbtz unit
and two DMSO molecules. In this strong hydrogen bond (O1−
H···O3 = 2.565 Å), the oxygen atom pertaining to DMSO and
one oxygen atom from the carboxylate group are involved. These
n the last 15 years, great attention has been paid to metal−
Iorganic frameworks (MOFs) as a consequence of their
functional properties and potential uses in many applications.1
Recently, the design and study of zinc- and lanthanum-based
MOF has evolved enormously2 because of their interesting
structures and potential applications in areas such as magnetism,3
luminescence,4 gas adsorption,5 sensing, and optical storage.6
These materials are commonly prepared through a bottom-up
approach, using solvothermal methods, connecting transition/
lanthanide metal ions with the appropriate bridging ligands. Still,
there is great interest in the design of new bridging ligands that
will allow the preparation of novel MOFs. Here, we have
designed a new symmetrically multidentate bridging ligand, 3,3′-
(1,2,4,5-tetrazine-3,6-diyl)dibenzoic acid (1; H2dbtz), which
contains two benzoic group donors, bonding to the metals, and a
central tetrazine p-acceptor function. Our idea stems from the
use of 3,6-disubstituted 1,2,4,5-tetrazine moieties, which have
become popular as efficient electronic spacers in dinuclear and
polynuclear systems.7 This is primarily due to the fact that the
tetrazine-based low-lying p* orbital conveys strong p-accepting
characteristics, leading to excellent electronic communication
between the metal termini. In this work, we report the structural,
Received: October 20, 2012
Published: January 3, 2013
© 2013 American Chemical Society
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Inorganic Chemistry
Communication
that propagate along the a crystallographic axis. Each LaIII ion
exhibits a LaO8 coordination environment, which is made of
seven oxygen atoms belonging to carboxylate groups of two
deprotonated dbtz2− bridging ligands and one coordination
water molecule. The La−Owater bond distance is 2.82(3) Å,
whereas the La−Ocarb distances are in the range of 2.394(19)−
2.564(12) Å. In the structure, lanthanum ions are connected by
carboxylate groups pertaining to dbtz2− ligands, generating
double chains along the a-axis direction. Some oxygen atoms
from the carboxylate groups unite two or three lanthanum
centers. The dihedral angles between the mean planes of the
benzene and tetrazine rings are in the range 7.28−22.67°. These
double chains are bridged with six other double chains by dbtz2−
ligands.
Figure 1. Crystalline structure of 1. Color code: carbon, gray; nitrogen,
blue; oxygen, red; sulfur, yellow; hydrogen, white. The DMSO molecule
has been omitted for clarity. Thermal ellipsoids are drawn at the 50%
probability level.
Each lanthanum ion is linked to 17 other metal ions by 7
dbtz2− ligands, generating a three-dimensional structure (Figure
3). Considering the dbtz2− ligands as spacers and the lanthanum
trinuclear units are packed through stacking interactions among
the tetrazine with benzene rings [3.531(5) Å] and benzene−
benzene rings of neighboring units [3.699(5) Å] (SI, Figure S3
and Table S1).
Hydrothermal reactions (see SI file) of the appropriate metal
salts (1 mmol) with H2dbtz (1 mmol) in water (10 mL) at 160
°C for 12 h followed by cooling to room temperature over 3 h
yield prismatic pink crystals of 2 (in 57% yield) and 3 (in 75%
yield). The crystal structures were determined by single-crystal
X-ray crystallography.12
Compound 2 consists of double sheets parallel to the ac plane.
These layers can be described as chains generated by the
carboxylate groups, pertaining to the ligand along the b directions
that are bridged by the dbtz2− spacer. The metal centers have
tetrahedral coordination environment ZnO4 formed by three
oxygen atoms, pertaining to three carboxylate groups of three
different spacers and one coordinated water molecule.
There are four units of formula [Zn(dbtz)(H2O)] per cell, and
in the crystal structure, each bridging dbtz2− ligand is bound to
three metal ions and each metal ion is linked to five other metal
ions by three dbtz2− ligands, thus generating stacked double
sheets parallel to the [101] direction. The ligand coordinates to
the metal center in monodentate and bidentate coordination
modes with each carboxylate group, respectively. The coordi-
nated water molecule generates hydrogen bonds with mono-
dentate mode, and infinite two-dimensional networks of the
formula [Zn(dbtz)(H2O)]n are thus formed; the values of these
hydrogen bonds are 2.744(6) and 2.676(6) Å. The association of
two such layers by the bidentate mode through one of the
carboxylate group bridges gives rise to double layers [Zn(dbtz)-
(H2O)]n. These double layers are separated by stacking
interactions [3.719(4) Å] (SI, Figure S4 and Table S2). Figure
2 shows a perspective view of the double sheet. The SI, Table S3,
gives the main bond lengths and angles.
Figure 3. Perspective view along the a direction of 3 with the channels in
which are located the water molecules. Crystallization water molecules
and hydrogen atoms have been omited for clarity.
atoms as nodes, the resulting network generates channels along
the a-axis direction (SI, Figure S5). These channels have a
diameter of 5.02 Å. The shortest and longest La···La distances
through dbtz2− spacers are 4.308 and 18.653 Å, respectively.
Thanks to its extended aromaticity and to the presence of poly-
heterosubstituted hexaatomic rings, H2dbtz is a good candidate
for enhanced emissive properties, tunable, in principle, by
coordination to different metals or environments. The emission
spectra of compounds 1−3 in the solid state at room temperature
are shown in (SI, Figure S8). The emission spectra of 1 and 2 at
room temperature in the solid state exhibit broad intense
emission bands centered at about 454 and 461 nm, respectively,
upon excitation at 310 nm. For 3, using 310 nm incident
radiation, intense emission bands at 453 and 470 nm and a
stronger one at 498 nm were observed. The emissions in
complexes 2 and 3 are assigned to intraligand p−p* transitions,
although a considerable red shift is observed with respect to the
ligand emission band. Moreover, the emission is more intense
than that of the free ligand, which may be explained in terms of
the rigidity. The rigidity of the coordinated ligand reduces the
loss of energy, thereby increasing the emission efficiency.13
We first studied the porous properties of 3 by means of their
pore-size distribution (PSD) in a Monte Carlo calculation.14
The crystal structure of 3 consists of a three-dimensional MOF
with channels occupied by disordered solvent water molecules
Figure 2. Representation, down [101], of the two-dimensional
[Zn(dbtz)(H2O)]n layer present in compound 2.
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Inorganic Chemistry
Communication
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isotherm revealed that the porous structure collapsed during
activation using conventional methods (heating at 393 K and
high vacuum and 10−4 bar).
We evaluated the in vitro cytotoxicity of cells exposed to
compound 1 at different concentrations and a wide range of
incubation times (SI, Figure S7). The results obtained show that
for long incubation times and at the highest concentrations
analyzed apparent mild signs of toxicity appeared that can be
considered negligible with cell viability greater than 80−90%.
These results show that new and interesting materials or
MOFs with biomedical applications can be assembled from metal
ions and the new H2dbtz ligand. Work along this line using other
paramagnetic/lanthanide metals and X-ray measurements with
pressure are in progress in our laboratory. We are also working on
the use of a supercritical activation process to allow the removal
of solvent molecules from the porosity, keeping the porous
texture of the as-prepared material unaffected.
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ASSOCIATED CONTENT
2090.
■
(12) Crystal data for 1 (C20H22N4O6S3), M = 478.56, triclinic, space
S
* Supporting Information
group P1, a = 6.3054(6) Å, b = 8.0715(9) Å, c = 11.4005(13) Å, α =
X-ray crystallographic data in CIF format, H and 13C NMR,
1
̅
73.397(7)°, β = 82.195(7)°, γ = 85.851(7)°, V = 550.53(10) Å3, Z = 1,
ρcalcd = 1.443 g·cm−3, μ(Mo Kα) = 0.287 mm−1, R1(Fo) = 0.0598, and
crystal structure pictures, computational details, cytotoxicity,
preparation of 2 and 3, luminescence and adsorption graphics,
and additional references. This material is available free of charge
via the Internet at http://pubs.acs.org. CCDC reference
numbers are 902753−902755. The atomic coordinates for
these structures have also been deposited with the Cambridge
Crystallographic Data Centre. The coordinates can be obtained,
upon request, from the Director, Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.
2
wR2(Fo ) = 0.1468 with a GOF on F2 = 1.015. Crystal data for 2
(C16H10N4O5Zn): M = 403.67, monoclinic, space group P21/n, a =
7.247(4) Å, b = 6.814(4) Å, c = 29.631(17) Å, β = 90.015(9)°, V =
1463.3(15) Å3, Z = 4, ρcalcd = 1.832 g·cm−3, μ(Mo Kα) = 1.72 mm−1,
2
R1(Fo) = 0.0495, and wR2(Fo ) = 0.1113 with a GOF on F2 = 1.02.
Crystal data for 3 (C24H12N6O10La): M = 683.31, monoclinic, space
group P21/c, a = 5.1474(10) Å, b = 18.232(4) Å, c = 26.783(5) Å, β =
90.08(3)°, V = 2513.5(9) Å3, Z = 4, ρcalcd = 1.806 g·cm−3, μ(Mo Kα) =
2
1.771 mm−1, R1(Fo) = 0.0727, and wR2(Fo ) = 0.1677 with a GOF on F2
= 1.034. Data were collected by ω and ψ scans on a Bruker APEXII
diffractometer with graphite-monochromated Mo Kα radiation (λ =
0.71073 Å). The structures were solved by direct methods and refined
on F2 by the SHELX-97 program.
AUTHOR INFORMATION
Corresponding Author
*E-mail: antonio5@ugr.es.
■
(13) (a) Aldridge, S.; Downs, A. J. The Group 13 Metals Aluminium,
Gallium, Indium and Thallium; John Wiley & Sons: New York, 2011.
Notes
The authors declare no competing financial interest.
́
(b) Rodríguez-Dieguez, A.; Salinas-Castillo, A.; Sironi, A.; Seco, J. M.;
Colacio, E. CrystEngComm 2010, 12, 1876−1879. (c) Tan, B.; Xie, Z.-L.;
Feng, M.-L.; Hu, B.; Wu, Z.-F.; Huang, X.-Y. Dalton Trans. 2012, 41,
10576−10584.
ACKNOWLEDGMENTS
This work was supported by the Junta de Andalucia (Predoctoral
■
́
Grant FQM-4228 to A.J.C.), the University of Granada, and the
Department of Energy’s Office of Energy Efficiency and
Renewable Energy, Fuel Cell Technologies Program under
Grant DE-FC36-08GO18137. B.F. thanks to the MEC for the
postdoctoral contract “Juan de la Cierva”.
(14) (a) Gelb, L. D.; Gubbins, K. E. Langmuir 1999, 15, 305.
(b) Duren, T.; Millange, F.; Fer
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