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Inorganic Chemistry
Article
lanthanide metal organic frameworks, most of which display
interesting topologies and outstanding properties. In addition
to the aforementioned ligands, 4,4′,4″-(phenylsilanetriyl)-
tribenzoic acid (H3L in Scheme 1), which possesses a
identical singlet networks, which results in a rare polythreading
network with (3,4)-connected topology. It also represents the
first interlocked polythreading network in a uranyl organic
system. They have been structurally determined by single-
crystal X-ray diffraction and characterized by IR, UV−vis,
photoluminescent spectroscopy, and density functional theory
(DFT) calculations.
Scheme 1. Rigid Tripodal Polycarboxylate Ligand with
Tetrahedral Silicon-Centered Linker H3L
EXPERIMENTAL SECTION
Caution! Standard procedures for handling radioactive material should be
followed, although the uranyl compounds used in the lab contained
depleted uranium.
■
Materials, Syntheses, and Characterization. All chemicals were
purchased commercially and used without further purification. 4,4′,4′-
(phenylsilanetriyl) tribenzoic acid (H3L) was synthesized according to
a literature procedure.32 X-ray powder diffraction (XRD) data were
collected on a D8 Focus (Bruker) diffractometer at 40 kV and 30 mA
with monochromated Cu Kα radiation (λ = 1.5405 Å) with a scan
speed of 5°/min and a step size of 0.02° in 2θ. Thermogravimetric
(TGA) and differential thermal analysis data were recorded on a
Thermal Analysis Instrument (SDT 2960, TA Instruments, New
Castle, DE, USA) from room temperature to 800 °C with a heating
rate of 10 °C min−1 under a nitrogen atmosphere. Solid-state UV−
visible absorption measurement was performed using a Hitachi U-
4100 spectrophotometer. Infrared spectra were collected using a
Nicolet 6700 FT-IR spectrometer with a diamond ATR objective. The
photoluminescence (PL) excitation and emission spectra were
recorded with a Hitachi F-7000 luminescence spectrometer equipped
with a xenon lamp of 450 W as an excitation light source.
Synthesis of 4,4′,4″-(Phenylsilanetriyl)tribenzoic acid. The
H3L ligand was synthesized according to the method described in the
literature by a two-step procedure. First, a mixture of 4-bromotoluene
(5 g, 29 mmol) and butyllithium (12 mL, 2.5 mol/L) in diethyl ether
was stirred at 0 °C under N2 for 4 h. Then, phenyltrichlorosilane (9.6
mmol) was added, and the mixture was stirred at room temperature
overnight. The reaction mixture was quenched by water (100 mL) at 0
°C. The organic layer was washed with water three times. A white solid
was obtained by evaporation, washed with water, and dried under
vacuum. Yield: 75%. Then, a mixture of diphenyldi-p-tolylsilane (4.00
g, 11.0 mmol), 90 mL of pyridine, and 30 mL of water was transferred
to a 1000 mL round-bottomed flask, heated, and refluxed; at the same
time KMnO4 (13.9 g, 87.8 mmol) was added in portions until the dark
purple solution became light yellow, and the mixture was then cooled
to room temperature. MnO2 was removed by suction filtration. The
filtrate was concentrated to about 15 mL, and concentrated HCl was
added until the pH was 1. White solids were precipitated and collected
by suction filtration, then dried at 70 °C.
Synthesis of Compound 1. A mixture of H3L(30 mg, 0.064
mmol), 0.1 M UO2(NO3)2 aqueous solution (1.0 mL, 0.100 mmol)
and 1,10-phenanthroline (phen) (20 mg, 0.1 mmol) was loaded into a
20 mL Teflon-lined stainless steel autoclave. The autoclave was sealed
and heated at 160 °C for 2 days, and then cooled to room
temperature. The solution pH was 3.0 before the reaction and 2.5 at
the end. The target product, yellow single crystals, was isolated after
filtration and washed thoroughly with distilled water. Yield: ca. 35%
(based on uranium).
Synthesis of Compound 2. A mixture of H3L (30 mg, 0.064
mmol), 0.1 M UO2(NO3)2 aqueous solution (1.0 mL, 0.100 mmol),
and tetraethylammonium hydroxide (30 μL) was loaded into a 20 mL
Teflon-lined stainless steel autoclave. The autoclave was sealed and
heated at 160 °C for 2 days and then cooled to room temperature. The
solution pH was 3.2 before the reaction and 2.7 at the end; yellow
needle-like crystals of the title compound were isolated after filtration
and washed thoroughly with distilled water. Yield: ca. 45% (based on
uranium).
tetrahedral center and features a tripodal geometry with a
single metal-coordination-free phenyl ring, has caught our
attention. H3L shows a variety of coordination modes and
conformations to bind uranyl cations because of its feature of
three carboxylate groups and intrinsically three-dimensional
orientation. Moreover, the nonbinding phenyl ring has large
steric hindrance and will alter the coordination mode of the
carboxylate group. Therefore, attempts to construct UOFs
based on H3L are expected to form diverse topological
structures.18
Entanglement is a common natural phenomenon and has
been observed in many areas of biology and chemistry.19
According to the reviews by Ciani et al.,20 entangled systems
are divided into several types, including interpenetrating
networks, polyknotting networks, polycatenated networks,
and polythreading networks. Entangled coordination polymers
have been extensively studied for transition/lanthanide metal
organic species,21 but uranium-containing compounds remain
relatively rare. Currently, only a few cases of interpenetrating or
polycatenating UOFs have been reported.16,22−28 For example,
a 2D → 2D parallel interpenetrating UOF with a honeycomb
(6,3) net has been synthesized using 1,4-benzenedicarbox-
ylate.22 Wang and co-workers reported a rare case of a 2D →
3D polycatenating uranyl network based on 3,5-di(4-
carboxyphenyl)benzoate.24 Another polycatenating uranyl
framework was prepared by Thuery using 4,4′-biphenyldicar-
̇
boxylate.23 However, polythreading networks, as another
important branch of entangled systems, are relatively sparse
in relation to uranyl organic systems. Polythreading systems can
be regarded as infinite periodic arrays of molecular rotaxanes or
pseudo-rotaxane analogues. The basic principle for the design
of those polythreading networks requires the presence of closed
loops, while a rigid motif serves as the rod to penetrate through
the loops. These two basic moieties may possess the same unit
or have different structures. The resultant crystal structure may
exhibit the same or an increased dimensionality, depending on
the nature of the motif.20,29 So far, 1D and 2D uranyl
polyrotaxane networks have been reported by Shi and co-
workers by combining uranyl-bearing units and pseudorotaxane
precusors.30,31 Current research interest will focus on the
development of 3D self-assembly interlocked UOFs, which may
exhibit intriguing topological structures.
In this paper, we prepared two 3D UOFs based on H3L, i.e.,
(HPhen)2[(UO2)L2]·4.5H2O (1) and [(UO2)3(H2O)4L2]·
6H2O (2). Interestingly, these two compounds exhibit mutual
entanglement with different entangled modes. Compound 1
features 2-fold parallel interpenetration with uninodal (10,3)-b
topology. Compound 2 comprises three interlocked sets of
X-ray Crystal Structure Determination. Suitable single crystals
were selected for single-crystal X-ray diffraction analyses. Crystallo-
graphic data were collected at 298 K on a Bruker Apex II CCD
diffractometer with graphite-monochromated Mo Kα radiation (λ =
B
DOI: 10.1021/acs.inorgchem.6b00582
Inorg. Chem. XXXX, XXX, XXX−XXX