New microporous lanthanide organic frameworks. synthesis, structure, luminescence, sorption, and catalytic acylation of 2-Naphthol

Article abstract of DOI:10.3390/molecules25133055

New metal-organic frameworks (MOF) with lanthanum(III), cerium(III), neodymium(III), europium(III), gadolinium(III), dysprosium(III), and holmium(III)] and the ligand precursor 1,3,5-tris(4-carboxyphenyl)-2,4,6-trimethylbenzene (H3L) were synthesized unde

Full text of DOI:10.3390/molecules25133055

Article
New Microporous Lanthanide Organic Frameworks.
Synthesis, Structure, Luminescence, Sorption, and
Catalytic Acylation of 2-Naphthol
Dana Bejan 1,*, Lucian Gabriel Bahrin 1, Sergiu Shova 1, Narcisa Laura Marangoci 1,
Ülkü Kökҫam-Demir 2, Vasile Lozan 1,3,* and Christoph Janiak 2,*
1
“Petru Poni” Institute of Macromolecular Chemistry, Romanian Academy, 700487 Iasi, Romania;
bahrin.lucian@icmpp.ro (L.G.B.); shova@icmpp.ro (S.S.); nmarangoci@icmpp.ro (N.L.M.);
2
Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine Universität Düsseldorf,
Universitätsstr. 1, D 40225 Düsseldorf, Germany; Uelkue.Koekcam@uni-duesseldorf.de (Ü.K.-D.)
3
Institute of Chemistry of MECR, Academiei str. 3, MD2028 Chisinau, Moldova;
* Correspondence: bejan.dana@icmpp.ro (D.B.); lozan.vasile@icmpp.ro (V.L.);
janiak@uni-duesseldorf.de (C.J.)
Academic Editors: Irina Savina and T. Jean Daou
Received: 18 June 2020; Accepted: 2 July 2020; Published: 3 July 2020
Abstract: New metal-organic frameworks (MOF) with lanthanum(III), cerium(III), neodymium(III),
europium(III), gadolinium(III), dysprosium(III), and holmium(III)] and the ligand precursor 1,3,5-
tris(4-carboxyphenyl)-2,4,6-trimethylbenzene (H3L) were synthesized under solvothermal
conditions. Single crystal x-ray analysis confirmed the formation of three-dimensional frameworks
of [LnL(H2O)2]n ·xDMF·yH2O for Ln = La, Ce, and Nd. From the nitrogen sorption experiments, the
compounds showed permanent porosity with Brunauer-Emmett-Teller (BET) surface areas of about
400 m2/g, and thermal stability up to 500 °C. Further investigations showed that these Ln-MOFs
exhibit catalytic activity, paving the way for potential applications within the field of catalysis.
Keywords: metal-organic frameworks; lanthanides; luminescence; sorption; catalysis
1. Introduction
Metal-organic frameworks (MOFs) form a class of porous materials that have been intensively
studied in the last decade [1–6]. MOFs provide diverse and easily modifiable architectures and
topologies and play important roles in synthetic chemistry [7], catalysis [2,8–10], molecular
separation [11], magnetism [12,13], optics [14,15] possess luminescence [16,17], and are used as
potential drug delivery carriers [18–20]. Mostly, MOFs are of interest [21–23] as microporous
materials in gas/solvent sorption applications due to their high porosity and tunable pore structure.
Amongst the intensely studied carboxylate ligands is 1,3,5-benzenetricarboxylate, the trianion of
trimesic acid (TMA) [24,25]. Extended derivatives of TMA, namely 1,3,5-benzenetribenzoic acid
(BTBA) and mesitylenetribenzoic acid (MTBA) are also exploited as organic spacers in MOFs (see
Figure 1).
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OH
O
OH
O
O
OH
O
Me
OH
OH
O
O
OH
Me
Me
HO
O
TMA
HO
O
HO
BTBA
O
MTBA, H3L
Figure 1. Tritopic linkers used in the construction of metal-organic frameworks (MOFs).
MOFs designed with these ligands exhibit properties and functions that are attractive to
researchers due to their rigidity and trifunctional identity [26–28]. Even though the BTBA and MTBA
systems are at first glance quite similar, their geometries differ markedly. The three methyl groups in
MTBA induce steric hindrance and lead to a structure in which the three carboxylate groups are
almost 90° out of plane with the central benzene moiety. Attempts to crystallize BTBA and establish
its crystal structure have been unsuccessful thus far [29], while hydrogen bond-mediated self-
assembly of MTBA was found to result in honeycomb networks with enlarged pores of ca. 24–26 Å
in diameter, which can accommodate K+-coordinated 18-crown-6 and dibenzo-18-crown-6, as shown
by a series of x-ray diffraction structures reported recently [30]. BTBA and MTBA were used in the
development of new design strategies for MOF materials with the desired porosity, pore size, and
functionality [31]. Mixed-ligand Archimedian polyhedron-based MOFs assembled from equilateral
triangles (BTBA or MTBA) and squares (porphyrin) were shown to be efficient photocatalysts for CO2
reduction upon visible light irradiation [31]. Mesoporous MOFs consisting of M6 nodes (M = Zr, Hf,
Ce, Th) and MTBA as a tritopic linker have been prepared and used as supporting materials for a
vanadium catalyst in the aerobic oxidation of 4-methylbenzyl alcohol [32].
Coordination geometry flexibility and high coordination numbers make lanthanide-based MOFs
a promising class of porous materials with useful properties such as luminescence, thermal stability,
magnetism, and catalytic behavior [26,33–36]. MTBA possesses intrinsic fluorescence. Even though a
number of complexes with MTBA as a linker have been documented in the literature [28,30,31,37–
41], lanthanide(III)-based MOFs with this tritopic ligand have not been reported as yet.
Herein, we report on the synthesis and characterization of a series of lanthanide-based MOFs of
general formula [LnL(H2O)2]n where Ln = La (1), Ce (2), Nd (3), Eu (4), Gd (5), Dy (6), and Ho (7), and
1,3,5-tris(4-carboxyphenyl)-2,4,6-trimethylbenzene (H3L) is the linker. The aim of this work was the
synthesis of new MOFs with the tribasic tricarboxylate H3L ligand (MTBA) and the investigation of
their physicochemical properties, crystal structures, thermal, and luminescence properties.
Additionally, for these new Ln-MOFs, their potential catalytic activity was established for O-
acylation reactions.
2. Results and Discussion
The linker precursor 1,3,5-tris(4-carboxyphenyl)-2,4,6-trimethylbenzene (H3L), also called
mesitylenetribenzoic acid (MTBA), previously reported by Bajpai et al. [30], was prepared in
moderate yield (50%) as described in the Supplementary Materials. The structure and purity of H3L
were confirmed by IR and NMR spectroscopy. The lanthanide complexes 1–7 were prepared by the
solvothermal reaction of H3L with Ln(NO3)3·6H2O [Ln = La; Ce; Nd; Eu; Gd; Dy; Ho]. The optical
microscopy images (Figure S1) of all lanthanide complexes are given in the SM. To optimize the
reaction conditions, different solvents and mixtures were used (DMF, DMSO, H2O, and ethanol). In
addition, different reaction temperatures and times were used in order to obtain crystalline products
suitable for x-ray diffraction. The optimal conditions were found to be an EtOH/H2O/DMF solvent
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mixture, a reaction temperature of 80 °C, and a reaction time of one to five days (see Table S1, in the
SM).
The chemical robustness of the Ln-MOF 3 was exemplarily checked by immersing in a 2 mol/L
solution of NaOH and HNO3, respectively, for one day. Compound 3 (10 mg) was suspended in 2
mL of acidic/basic solution for 24 h, filtered off, washed, dried in air, and investigated by PXRD
analysis. As can be seen in Figure S14 (SM), compound 3 decomposed in 2 mol/L of aqueous acid or
base. The Ln-MOFs 1–7 were, however, stable in air and solvent (ethanol, DMF, H2O-mother liquor)
for more than one week. Powder x-ray diffraction patterns recorded after two weeks showed that
these compounds are unaffected by the solvents.
2.1. Structural Characterization
The results of single-crystal x-ray diffraction studies are shown in Figure 2; Figure 3. It should
be noted that the three complexes described here exhibited similar structural features, being
constructed in the same way. Moreover, compounds 1 and 2 were isostructural, therefore only the
structure of compound [LaL(H2O)2]n (1) is described below in detail (Figure 2). Complex 1 crystallizes
in the orthorhombic space group Pnna. As shown in Figure 2a, the asymmetric unit comprises a La
atom in a special position on a two-fold axis, half of the deprotonated L3− ligand, and one coordinated
water molecule. The L3− tricarboxylate linker acts as an eight-dentate ligand, bridging and chelating
toward five La atoms. Two of the carboxylate groups are coordinated in μ2-к3O,O’:O tridentate-
bridging mode, while the third group exhibits a к2O,O’ bidentate-chelating function. The La atom is
coordinated by ten oxygen atoms in a distorted square antiprism geometry (Figure 2a polyhedron).
The La–O bond distances vary in the range 2.503(7)3.017 Å and 2.461(4)3.177(4) Å for [LaL(H2O)2]n
(1) and [CeL(H2O)2]n (2), respectively. In the crystal, the asymmetric units are assembled to form a
three-dimensional coordination network (Figure 2b–d). It can be described as formed by infinite
linear arrays of La atoms bridged by μ2-к3O,O’:O tridentate-bridging/chelating carboxylate groups
along the a axis. These linear chains are further interconnected via tripodal spacers L3−. Within the
linear chain, the two neighboring La atoms are separated at 4.690(4) Å across the tricarboxylate
spacers at 16.356(4) Å. The network structure forms large rectangular channels of 17.5 × 12.7 Å (taking
into account the van-der-Waals surfaces) directed along the b axis (Figure 2c), indicating the potential
porosity of this compound. The solvent-accessible volume of the channels is equal to 2060 Å3/unit
cell, which constitutes 48% of the unit cell volume.
Compared with compounds 1 and 2, the x-ray diffraction study for 3 yielded very similar unit
cell dimensions. Nevertheless, compound 3 crystallized in the monoclinic space group P21/n, having
a β angle of 93.098(18)°. The crystal structure is formed by the [NdL(H2O)2]n coordination polymer
and solvate molecules of H2O and DMF in a 1:2:1.33 stoichiometric ratio. A view of the asymmetric
unit is shown in Figure 3a. The coordination environment of Nd comprises nine oxygen atoms, seven
of which come from five carboxylate groups and two from coordinated water molecules. The
coordination polyhedron was different compared to compounds 1 and 2: this can be characterized as
a tricapped trigonal prism (Figure 3a polyhedron). The Nd–O bond distances vary in the range of
2.464(6)2.947(6) Å. As shown in Figure 3, the three-dimensional network in 3 closely resembles that
found for 1 and 2 (cf. Figure 2b,c). In the structure of 3, the positions of the H2O and DMF solvent
molecules could be easily determined from Fourier maps and refined. A view of the crystal structure
along the b axis showing the channels accommodating the solvent molecules is depicted in Figure 3d.
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Figure 2. Crystal structure of compound [LaL(H2O)2]n (1). Extended view of the asymmetric part and
coordination environment around La in 1 along with the atom labelling scheme and thermal ellipsoids
at the 40% probability level. Symmetry generated fragments are drawn with 50% transparency (a).
Crystal structure packing viewed along the a axis (b). View of the crystal structure showing formation
of the channels along the b axis (c). Space filling representation for (d). Symmetry codes: I = 1  x, 1 
y, 1  z; II = 1.5  x, 0.5 + y, 0.5  z; III = x, 0.5  y, 0.5  z; IV = 2  x, 1  y, 1  z; V = 1.5  x,  y, z; 0.5 +
x, y, 1  z.
Figure 3. Crystal structure of [NdL(H2O)2]1.33DMF2H2O (3). Extended view of the asymmetric part
and coordination environment around the Nd atom in 3 along with the atom labeling scheme and
thermal ellipsoids at the 40% probability level. Symmetry generated fragments are drawn with 50%
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transparency (a). Crystal structure packing viewed along the a axis (b). View of the crystal structure
showing the formation of the channels along the b axis (c); with space filling representation of the co-
crystallized DMF and H2O solvent molecules (d). Symmetry codes: I = 0.5 + x, 1.5  y, 0.5 + z; II = 0.5 
x, 0.5 + y, 1.5  z; III 0.5 + x, 0.5  y, 0.5 + z; IV = 0.5  x, −0.5 + y, 1.5  z; V = 1  x, 1  y, 2  z; VI x, 1 
y, 2  z.
2.2. Infrared Spectroscopy Attenuated Total Reflectance (ATR)
For all synthesized lanthanide metal-organic frameworks, the ATR spectra were quite similar
and are given in the Supplementary Materials (Figures S2–S9). The IR spectra showed strong
characteristic adsorptions in the 1610–1520 cm−1 and 1420–1370 cm−1 interval, assigned to the
symmetric/asymmetric COO− vibrations of the ligand carboxylate groups. The broad bands between
3600 and 3200 cm−1 can be attributed to the hydrogen bonded υOH groups from adsorbed water.
2.3. Powder X-Ray Diffraction
Powder X-ray diffraction (PXRD) reveals information about the crystallinity and crystalline
phase purity of the material. The PXRD patterns were recorded after the compounds had been
filtered, air-dried, and exposed to air for at least a few days (Figures S10–S20). The patterns recorded
for all compounds were in good agreement with each other, as can be seen in Figure S20. The
simulation versus experimental patterns for 1–3 are represented in Figures S11–13.
2.4. Thermogravimetric Analysis (TG/DTG)
Thermal stability and the pathways of thermal decomposition of all complexes were investigated
by means of the TG-DTG (nitrogen) methods (see Figures S21–S28). The newly synthesized Ln-MOFs
were found to have high thermal stability compared with the ligand. For compounds 1 and 2, the first
weight loss of 3–4% corresponded to the loss of water molecules. Above around 480 °C, the
decomposition process of the framework and organic ligand takes place. The TG trace of the as-
synthesized 3 showed a mass loss of ~11% up to 220 °C, which can be attributed to the loss of water
molecules and part of the DMF molecules (theor. ~21% for water and DMF). When compound 3
was analyzed after activation and gas sorption measurements, no mass loss up to 480 °C was
observed, which is evidence of successful solvent removal. During heating of 5, the desolvation
process took place in three steps. The first weight loss (~5%) found in the temperature range of 40–
220 °C is associated with the loss of solvent molecules (calc. 4.5%). Further heating caused the
decomposition of the organic ligand. The thermal decomposition curves of compounds 4, 6, and 7
showed a similar behavior and high thermal stability at about 500 °C, as presented in the SM. It is
worth pointing out that the MOFs presented here have a high thermal stability that is similar to other
lanthanide metal organic frameworks [42].
2.5. Sorption Properties
In order to assess the porous nature of compounds 1–7, the nitrogen adsorption–desorption
isotherms at −196 °C were recorded. Prior to measurement, the samples were outgassed for 4 h under
a high vacuum (2.7 × 10−2 mbar) at 80 °C. The compounds exhibited permanent porosity with BET
surface areas from 110 to 470 cm2/g, and total pore volumes between 0.1 to 0.2 cm3/g (Table 1). The
obtained nitrogen isotherms are shown in Figures S30–S36 in the Supplementary Materials.
Table 1. BET surface area and total pore volume for Ln-MOFs compound.
Total Pore Volume
Ln-MOFs—Activated Compounds
BET Surface Area [m2/g]
[cm3/g]
[LaL(H2O)2]n—act (1-act)
[CeL(H2O)2]n—act (2-act)
[NdL(H2O)2]n—act (3-act)
405/395*
467/460*
426/404*
0.19
0.20
0.20
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[EuL(H2O)2]n—act (4-act)
114
348
0.08
0.18
0.16
0.16
0.15
[GdL(H2O)2]n—act (5-act)
[DyL(H2O)2]n—act (6-act) [DyL(H2O)2]n—act (6-act)
[HoL(H2O)2]n—act (7-act)
202 (Figure S35a)
298 (Figure S35b)
286
* The same compound, second measuring, without exposing to air. The experimental error for a BET
surface area is about 5–10%.
The structurally authenticated compounds 1–3 exhibited adsorption isotherm branches as a
composite of Types I and II with a H4 hysteresis loop. The pronounced uptake at low p/p0 is
associated with the filling of micropores. H4 loops are often found with aggregated crystals or micro-
mesoporous materials [43,44]. Of note, structurally similar compounds 1–3 (see Figure 4) also
exhibited rather similar surface areas between 400–460 m2/g and a total pore volume of 0.19–0.20
cm3/g. In a repeated adsorption/desorption cycle, the surface area remains unchanged within the
experimental error [44].
160
140
120
100
80
adsorption [LaL(H2O)2]n - act (1-act)
desorption [LaL(H2O)2]n- act (1-act)
adsorption [CeL(H2O)2]n- act (2-act)
desorption [CeL(H2O)2]n- act (2-act)
60
40
20
0
.
.
adsorption [NdL(H2O)2]n1.33DMF 2H2O - act (3-act)
.
.
desorption [NdL(H2O)2]n1.33DMF 2H2O - act (3-act)
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure, p/p0
Figure 4. N2 sorption isotherms of the activated compounds of 1–3 at 77 K. Closed and open symbols
refer to adsorption and desorption, respectively.
Compounds 4–7 still feature adsorption isotherm branches that can be viewed as a composite of
Types I and II. However, there is essentially no desorption until p/p0 <~0.1–0.15 for 4–6 (for 6 after the
second adsorption run). For 7, the desorption starts around p/p0 <~0.3. For 4–7, about 80% of the
adsorbed N2 is still retained at a very low p/p0 of ~0.01. There is a final steep desorption branch at
p/p0 of ~0.01 for these remaining 80%. Such a very steep desorption branch is also a characteristic
feature of H2(a) hysteresis loops and can be attributed either to pore-blocking/percolation in a narrow
range of pore necks or to cavitation-induced evaporation [44].
A similar sorption behavior with broad isotherms and steep desorption steps at low pressures
was frequently observed for H2 and CH4 adsorption in IFP MOFs [45–48] and in supramolecular
hydrogen-bonded imidazolate frameworks (HIFs) [49,50] with gate effects (IFP = imidazolate
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framework Potsdam), in part also for CO2 adsorption in IFPs [47,51]. Such a broad desorption
behavior is rarely observed in microporous MOFs and only observed for MOFs with flexible
substituents. The structurally authenticated IFPs possess very small, less than 1 Å, pore aperture
windows to the cavities and flexible ethoxy or methoxy groups form the pore aperture windows
[47,48]. For H2 sorption at 77 K IFP-7 [48], IFP-8 [45,46] and IFP-9 [48] exhibits an open-loop hysteresis
due to almost irreversible adsorption at higher relative pressure. Although H2 is a small molecule
with a kinetic diameter of only 2.89 Å at 77 K, the sorption isotherms can deviate from ideal
equilibrium experiments as pronounced kinetic effects occur because of the small channel size and
the gate effect. In IFP-7, 98% of the adsorbed H2 is trapped in the framework when the pressure is
reduced from 840 mmHg to 100 mmHg, and 70% of the adsorbed H2 remains when the pressure is
further reduced to 7 mmHg [48]. In IFP-8, 98% of the adsorbed H2 is trapped in the framework when
the pressure is reduced from 760 mmHg (1 bar) to 114 mmHg (0.15 bar) [45].
2.6. Luminescence
It is known that lanthanides often exhibit luminescence properties [1,16,52]. In order to study
the luminescence properties of the synthesized frameworks, the fluorescence spectra of Ln-MOFs (1,
2, 3, and 4) as well as ligand H3L were recorded in solid state (powder) at room temperature. It was
found that H3L displays an emission maximum at 437 nm upon excitation at λ = 365 nm. Similar
emissions in the UV region were observed for frameworks 1, 2, and 3 (Figure 5), which can be
attributed to the organic ligand. In the case of the Eu based framework 4, besides the weak blue
emission of the ligand, typical red emissions attributed to the Eu3+ ion were observed. The
7
7
characteristic sharp emissions at λ = 561 nm (very weak, 5D0→ F0), λ = 589 nm (medium, 5D0→ F1), λ
5
7
5
7
= 611 nm (very strong, D0→ F2), λ = 616 nm (very weak, D0→ F3), and λ = 697 nm (very weak,
7
5D0→ F4) were in good correlation with the data reported in the literature [53–55]. The red emissions
of framework 4 were confirmed through fluorescence microscopy by irradiating at λ = 365 nm, as can
be seen in the SM, Figure S37.
2x106
Ligand
.
.
compound [EuL(H2O)2]n1.33DMF 2H2O
1x106
1x106
1x106
8x105
6x105
4x105
.
.
compound [NdL(H2O)2]n1.33DMF 2H2O
compound [LaL(H2O)2]n
compound [CeL(H2O)2]n
λex=365
2x105
0
350
400
450
500
550
600
650
700
Wavelength (nm)
Figure 5. Emission spectra of ligand and lanthanide MOF 1–4 obtained under excitation at 365 nm.
2.7. Catalytic Activity
Two Ln-MOFs (1 and 3) were investigated in O-acylation reactions to explore their Lewis-acidic
properties. Despite the large number of reported O-, N-, S- acylation processes [56,57], in the last
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decade, the involvement of MOFs in heterogeneous catalysis has greatly increased [58,59]. The
advantages of MOF materials are solid-state catalytic activity, together with the robustness of their
structure, allowing facile separation after the catalytic process [56,59,60].
In the present work, the conversion of 2-naphthol to 2-naphthyl acetate by the reaction with
(CH3COO)2O in the presence of lanthanide-MOFs as catalysts (Scheme 1) was investigated. The
reactions were carried out with 1.5 equivalents of acetic anhydride at room temperature in the
presence of 1 mol% of catalysts and chloroform as the solvent for 24 h.
OH
+ (CH3CO)2O
OC(O)CH3
+
Ln-MOF 1 or 3
CH3COOH
RT, 24 h, CHCl3
Scheme 1. Acylation of 2–naphthol with acetic anhydride in the presence of 1 or 3 as the catalyst.
In the control experiment, in the absence of the catalyst, the yield of the acylated product was
lower than 10%. Before each catalytic experiment, the catalyst was activated by washing several times
with chloroform and then drying in an oven at 80 °C for 20 h. The catalytic activity of the lanthanide-
based MOFs was tested in multiple run experiments (see Table 2). When the reactions were catalyzed
by 1 (1 mol%), the yield was 88%. When the catalyst used was 3, the yields were 98% (run 1), 96%
(run 2), and 92% (run 3) (see Table 2, Entry 1–4). The spectral data of the isolated yellowish powders
of 2-naphthylacetate were in full agreement with the authentic samples (Mp, NMR, see Figure S29,
SM) [61]. The results were expressed as the average of at least two independent measurements
performed for each sample.
Table 2. Acylation of O–substrates with Ac2O in the presence of 1 and 3 as catalysts.
Run
Entry Substrate Conversion * Time Catalyst
Number
1
2
3
4
2-naphthol
2-naphthol
2-naphthol
2-naphthol
88
98
96
92
65
65
80
72
24
24
24
24
5
1
3
3
3
3
-
1
1
2
3
1
2
1
2
5
2-naphthol
2-naphthol
24
24
24
3
3
6 **
* Every experiment was performed at least in duplicate; ** The catalyst is dried in air.
Critically important for MOF catalysts is the framework stability during catalysis. For compound
3, this was tested over three runs and the results showed a stable catalytic activity. In addition, after
the first and the third run, the catalyst was analyzed for crystallinity by PXRD. The results indicate
that the compound was only stable over one run, but had significantly deteriorated after the third
run. (Figure S15, SM). Moreover, two catalytic reactions were carried out with the activated and non-
activated catalyst. It is clear that without activating the catalyst, the yield is much lower (see Table 2,
entry 6).
In order to prove the heterogeneity of the catalytic process, a room temperature filtration test
was performed. The catalyst was removed from the reaction mixture after 5 h using a glass frit funnel.
The results show that after removing the catalyst, the reaction did not proceed further, indicating that
no catalytically active materials remained in the filtrate (Table 2, entry 5).
3. Material and Methods
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All chemicals were purchased from commercial suppliers and used as received. The Nuclear
Magnetic Resonance (NMR) spectra were recorded on a BRUKER AVANCE III 400 (Bruker BioSpin
1
Rheinstetten, Germany) instrument operating at 400.1 and 100.6 MHz for H and 13C nuclei,
respectively. Carbon, Hydrogen, Nitrogen elemental analysis was performed on a vario Micro cube
CHNS analyzer from Elementar Analysensysteme GmbH, Germany. Infrared (IR) spectra were
recorded on a BRUKER VERTEX 70 (Bruker Optik GmbH, Ettlingen, Germany) FTIR spectrometer
with the measurements performed in ATR (attenuated total reflectance) Golden Gate mode in the
600–4000 cm−1 range at room temperature with a resolution of 4 cm−1 and accumulation of 64 scans.
Powder x-ray diffraction (PXRD) patterns were collected on a Bruker AXS D2 Phaser powder
diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) equipped with a flat silicon, low
background sample holder using Cu-Kα radiation ( = 1.5418 Å) with a scan speed of 0.2 s/step and
a step size of 0.02° (2) at 290 K in the range of 2 = 5–40°. Simulated PXRD patterns were calculated
with the CCDC Mercury 3.1 program (Cambridge, UK) using the single-crystal data of the
compounds. A STA 449F1 JUPITER (Netzsch, GmbH, Selb, Germany) thermal analyzer from Netzsch
was employed for the thermogravimetric (TG) measurements at a heating rate of 5 °C min−1 between
30 and 700 °C. The data were processed with the NETZSCH PROTEUS 4.2 software (Netzsch, GmbH,
Selb, Germany). Nitrogen sorption experiments (up to 1 bar) for the BET surface area and porosity
determination were measured with a Quantachrome NOVA 4200e (Quantachrome GmbH & Co. KG,
Odelzhausen, Germany) at 77 K. About 20 to 40 mg of freshly synthesized samples were weighed
before and after the degassing procedure to confirm the evacuation of the solvent. The fluorescence
microscopy imaging was performed on a Leica DMI 3000 B (Leica Microsystems, Germany)
microscope equipped with “A” and “GFP” filter cubes, with UV (365 nm) and green excitation (470
nm), respectively. The solid-state fluorescent study was conducted at room temperature on a Horiba
Fluoromax-4 fluorescence spectroscopy instrument (HORIBA Jobin Yvon GmbH, Bensheim,
Germany).
3.1. Synthesis of the Ligand 1,3,5-tris(4-carboxyphenyl)-2,4,6-trimethylbenzene (H3L)
The linker H3L was prepared according to a procedure described in the literature [30] and its
structure and purity were confirmed by NMR and IR spectroscopy.
1H NMR (DMSO-d6, 400.1 MHz, ppm): δ = 12.9 (s, 3H); 8.04 (d, 3JH,H = 8.04 Hz, 6H); 7.35 (d, 3JH,H
= 8.04 Hz, 6H); 1.62 (s, 9H). 13C NMR (DMSO-d6, 100.6 MHz, ppm): δ = 167.6; 146.3; 139.2; 132.3; 130.1;
129.9; 129.8; 19.6. IR (ATR): ν (cm−1) = 640.33 (m); 661.54 (w); 682.76 (w); 715.50 (m); 759.91 (s); 796.55
(m); 864.06 (s); 960.49 (w); 1018.35 (m): 1043.42 (w); 1060.78 (w); 1097.43 (m); 1143.72 (vw); 1178.43
(w); 1253.65 (w); 1278.73 (w); 1305.73 (m); 1411.81 (vs); 1519.81 (s); 1583.46 (vs); 1664.47 (s); 1820.69
(vw); 1944.13 (vw); 2279.72 (vw); 2561.31 (vw); 2750.32 (vw); 2923.90 (w); 3355.93 (w); 3512.16 (w);
3807.25 (vw).
3.2. Typical Synthesis of Ln-MOFs Where Ln = La (1), Ce (2), Nd (3), Eu (4), Gd (5), Dy (6), Ho (7)
All complexes were prepared in the same manner using the following procedure. To a solution
of H3L (0.01 g, 0.02 mmol) in dimethylformamide (DMF) (except La(NO3)3·6H2O and Ce(NO3)3·6H2O
where ethanol was used) in a 20 mL culture tube was added Ln(NO3)3·6H2O (~0.036 g, 0.08 mmol)
dissolved in ethanol/water at room temperature (see Table S1, Supplementary Materials (SM) for the
volumes and solvents used). The tube with a clear solution was kept under static conditions for 1–5
days at 80 °C (reaction times are detailed in Table S1, SM). After cooling, the crystalline product was
collected by centrifugation and washed with DMF and ethanol. Finally, the crystals were dried at
room temperature. Specific conditions for each Ln-MOF are given in section S1, SM.
[LaL(H2O)2]n (1): Yield 62%; Elemental analysis calcd. (%) for C30H25LaO8 (M = 652.41 g/mol): C
55.26, H 3.86; found: C 55.26, H 3.79. IR (ATR):  (cm−1) = 638.41 (m); 675.06 (w); 717.49 (m); 758.01
(vs); 858.29 (s); 958.58 (m); 1062.74 (w); 1105.17 (w); 1141.82 (m); 1176.53 (m); 1236.32 (m); 1284.54 (m);
1307.69 (m); 1375.29 (vs); 1529.85 (s); 1585.43 (s); 1602.78 (s); 2929.76 (w); 2987.62 (w); 3620.25 (w).
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[CeL(H2O)2]n (2): Yield 88%; Elemental analysis calcd. (%) for C30H25CeO8 (M = 653.63 g/mol): C
55.13; H 3.86 found C 55.83, H 3.92. IR (ATR):  (cm−1) = 638.42 (w); 715.57 (m); 758.01 (s); 796.58 (m);
862.16 (m); 958.60 (w); 1016.46 (m); 1037.68 (w); 1101.33 (w); 1139.90 (w); 1176.55 (m); 1276.85 (m);
1305.78 (m); 1406.07 (vs); 1523.73 (s); 1583.52 (s); 1608.59 (s); 2922.09 (w); 3429.35 (w); 3616.44 (w).
[NdL(H2O)2]1.33DMF2H2O (3): Yield: 84%; Elemental analysis calcd. (%) for
C34H38.3O11.33N1.33Nd (M = 791.21 g/mol): C 51.59, H 4.87; N 2.35 found: C 52.30, H 4.57, N 2.56. IR
(ATR):  (cm−1) = 638.41 (m); 675.06 (w); 715.76 (m); 758.01(s); 796.57 (m); 862.15 (s); 958.58 (m); 1016.45
(m); 1101.31 (m); 1139.89 (w); 1176.53 (m); 1267.53 (m); 1305.76 (m); 1382.91 (vs); 1402.2 (vs); 1523.71
(s); 1581.57 (s); 1604.71 (s); 2923.37 (w); 3627.96 (w).
[EuL(H2O)2]1.33DMF2H2O (4): Yield: 66%; Elemental analysis calcd. (%) for C34H38.3O11.33N1.33Eu
(M = 797.09 g/mol): C 51.08, H 4.83, N 2.33 (calc idem Nd), found C 51.45, H 5.01; N 2.50. IR (ATR): 
(cm−1) = 640.34 (m); 715.56 (s); 758 (vs); 790.79 (w); 864.08 (s); 958.58 (m); 1022.53 (m); 1203.24 (m);
1141.82 (m); 1176.53 (m); 1240.18 (m); 1280.68 (m); 1307.69 (m); 1377.12 (vs); 1402.02 (vs); 1519.85 (s);
1583.5 (s); 1583.5 (s); 1606.71 (s); 2927.83 (w); 2981.83 (w); 3649.18 (w).
[GdL(H2O)2]2DMF2H2O (5): Yield: 82%; Elemental analysis calcd. (%) for C36H43N2O12Gd (M =
852.98 g/mol): C 50.69, H 5.08, N 3.28, found C 50.87, H 4.75; N 3.85. IR (ATR):  (cm−1) = 640.33 (m);
675.04 (w); 715.55 (s); 757.98 (vs); 794.62 (m); 864.06 (s); 958.56 (m); 1016.42 (m); 1103.21 (m); 1143.72
(m); 1176.50 (m); 1245.94 (m); 1278.73 (m); 1303.80 (m); 1384.80 (vs); 1406.02 (vs); 1514.03 (s); 1527.53
(s); 1606.67 (s); 1934.748(w); 2923.63 (w); 3039.63 (w); 3406.08 (w); 3616.31 (w); 3652.95 (w).
[DyL(H2O)2]n (6): Yield: 77%; Elemental analysis calcd. (%) for C30H25O8Dy (M = 677.08 g/mol): C
53.30, H 3.73; found: C 53.81, H 4.08. IR (ATR):  (cm−1) = 639.33 (m); 715.55 (m); 757.98 (s); 794.65 (m);
865.98 (s); 958.56 (w); 1016.42 (m); 1101.21 (m); 1141.72 (m); 1176.50 (m); 1278.72 (m); 1377.09 (vs);
1583.46 (s); 1936.48 (w); 2854.44 (w); 2921.9 (w); 3419.65 (w); 3508.37 (m).
[HoL(H2O)2]1.33DMF2H2O (7): Yield 45%; Elemental analysis calcd. (%) for C34H38.3O11.33N1.33Ho
(M = 810.06 g/mol): C 50.26, H 4.76, N 2.22, found: C 50.73, H 5.06., N 2.90. IR (ATR):  (cm−1) = 640.33
(m); 715.55 (s); 757.98 (vs); 794.65 (m); 865.98 (s); 958.56 (w); 1016.42 (m); 1016.21 (m) 1103.21 (m);
1143.72 (m); 1176.50 (m); 1278.72 (m); 1305.73 (m);1376.02 (vs) 1527.53 (s); 1583.46 (vs); 1814.9 (vw);
1936.48 (w); 2281.65 (vw); 2559.38 (vw); 2856.40 (w); 2923.9 (w); 3425.37 (w); 3514.09 (w).
3.3. Synthesis of 2-Naphthyl Acetate
A mixture of the substrate, 2–naphthol (0.07 g, 0.48 mmol), acetic anhydride (0.07 g, 0.72 mmol,
1.5 equiv.), and Ln-MOFs as the catalyst (1 mol%—related to substrate) was stirred at room
temperature with CHCl3 (2 mL) as the solvent for one day. The catalyst was collected by filtration
and the organic phase was washed successively with 2% aqueous NaOH (5 mL) and saturated NaCl
solution (5 mL). After drying with MgSO4, filtration and evaporation of the solvent at reduced
pressure, the solid product was isolated and analyzed (yellowish powder; Mp: 72 °C—lit. 71 °C [61]).
Yield 98% (3 as catalyst). 1H NMR (CDCl3-d3, 400.1 MHz, ppm): δ = 7.89–7.86 (m, 2CH); 7.84–7.82 (m,
3
3
CH; JH,H = 7.9 Hz); 7.61 (d, CH; JH,H = 2.3 Hz); 7.51 (m, 2CH); 7.27 (dd, CH; 3JH,H = 9.03 Hz); 2.38 (s,
CH3). 13C NMR (CDCl3-d, 100.6 MHz, ppm); δ = 169.7; 148.3; 133.7; 131.4; 129.3; 127.7; 127.6; 126.5;
125.7; 121.1; 118.5; 21.2.
3.4. Crystal Structure Determination
X-ray diffraction measurements were carried out with an Oxford-Diffraction Xcalibur E CCD
diffractometer equipped with graphite-monochromated MoKα radiation. Single crystals were
positioned at 40 mm from the detector and 163, 157, and 257 frames were measured each for 70, 10,
and 75 s over a 1° scan width for 1, 2, and 3, respectively. The unit cell determination and data
integration were carried out using the CrysAlis package of Oxford Diffraction [62]. The structures
were solved by direct methods using Olex2 [63] and refined by full-matrix least-squares on F² with
SHELXL-2015 [64] using an anisotropic model for non-hydrogen atoms. The positional parameters
for H atoms attached to C atoms were introduced in idealized positions (dCH = 0.96 Å) using the riding
model with their isotropic displacement parameters fixed at 120% of their riding atom. Positional
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parameters of the H attached to O atoms were obtained from different Fourier syntheses and verified
by the geometric parameters of the corresponding hydrogen bonds. Crystal structures 1 and 2
contained large areas of disordered solvent molecules that could not be localized. In these cases, the
contribution of disordered solvent molecules to structure factors was extracted using the “Solvent
Mask” tool available in Olex2. Crystals of 3 were found to be a non-merohedral twin with two
components related by 180° rotation about the [100] direct lattice direction. The twin component ratio
was refined at 0.52:0.48. The molecular plots were obtained using the Olex2 program. The
crystallographic data and refinement details are quoted in Table 3, while bond lengths are
summarized in Tables S2–S4 (Supplementary Materials). CCDC-1904154 (1), CCDC-1904155 (2), and
CCDC-1904156 (3) contain the supplementary crystallographic data for this contribution. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or
deposit@ccdc.ca.ac.uk).
Table 3. Crystal data and details of the data collection for 1–3.
Compound
3984 (1)
C30H25LaO8
652.41
Pnna
3971 (2)
C30H25CeO8
653.62
Pnna
3972 (3)
C34H38.33N1.33NdO11.33
791.21
empirical formula
Fw
space group
a [Å]
P21/n
9.3345(5)
16.6700(9)
27.2304(19)
90
9.5029(6)
16.6504(10)
27.1399(15)
90
9.5200(13)
16.473(3)
27.389(6)
90
b [Å]
c [Å]
a [°]
B [°]
90
90
93.098(18)
90
g [°]
90
90
V [Å3]
4237.2(4)
4
4294.3(4)
4
4288.9(13)
4
Z
−
3
1.023
1.011
1.225
rcalcd [g cm ]
Crystal size [mm]
T [K]
0.20 × 0.20 × 0.10 0.40 × 0.20 × 0.20
0.70 × 0.60 × 0.18
180
180
1.040
293
1.092
−
1
1.260
 [mm ]
2È range
5.11 to 50.054
9929
4.542 to 50.052
9514
2.886 to 50.054
11840
Reflections collected
Independent reflections
Data/restraints/parameters
3744 [Rint = 0.0515] 3780 [Rint = 0.0952] 11840 [Rint = 0.1178]
3744/54/181
0.0852
3780/12/180
0.0538
11840/48/439
0.0651
[a]
R1
[b]
wR2
0.2280
0.0983
0.1431
GOF [c]
1.048
1.002
1.040
Largest diff. peak/hole/e Å−
1.16/−1.45
3
1.23/−1.58
1.40/−1.00
2
2
2
2
2
[a] R1 = ||Fo|  |Fc||/|Fo|. [b] wR2 = {[w(Fo  Fc )2]/[w(Fo )2]}1/2
.
[c] GOF = {[w(Fo  Fc )2]/(n  p)}1/2
,
where n is the number of reflections and p is the total number of parameters refined.
4. Conclusions
A series of new lanthanide metal-organic frameworks with the general formula [LnL(H2O)2]n
(where Ln = La, Ce, Nd, Eu, Gd, Dy and Ho, and L = deprotonated 1,3,5-tris(4-carboxyphenyl)-2,4,6-
trimethylbenzene) were synthesized. The crystal structures of the La(III), Ce(III), and Nd(III)
complexes were successfully studied by the single-crystal diffraction method. It was demonstrated
that all of these compounds showed high thermal stability (up to ~480 °C), while the gas sorption
studies indicated good permanent porosity with SBET up to ~400 m2/s. The catalytic activity for a
model reaction, namely O-acetylation was also evaluated and it was found that the synthesized MOFs
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successfully catalyzed the acetylation of 2-napthol. The novel [LnL(H2O)2]n MOFs can serve as
promising materials for gas storage as well as for heterogeneous catalysis.
Supplementary Materials: The following are available online, Synthesis of the ligand 1,3,5-tris(4-
carboxyphenyl)-2,4,6-trimethylbenzene (H3L) and of Ln-MOFs compounds (1–7). Table S1: Overview of the
experimental details for the synthesis of 1–7. Figure S1: Optical microscopy image of 1–7, with Leica ICC50 W,
4x/0.10. Figure S2: IR spectrum of ligand H3L. Figure S3: IR spectrum of compound [LaL(H2O)2]n (1). Figure S4:
IR spectrum of compound [CeL(H2O)2]n (2). Figure S5: IR spectrum of compound [NdL(H2O)2]n·1.33DMF·2H2O
(3). Figure S6: IR spectrum of compound [EuL(H2O)2]n·1.33DMF·2H2O (4). Figure S7: IR spectrum of compound
[GdL(H2O)2]n·2DMF·2H2O (5). Figure S8: IR spectrum of compound [DyL(H2O)2]n (6). Figure S9: IR spectrum of
compound [HoL(H2O)2]n·1.33DMF·2H2O (7). Figure S10: PXRD patterns of the two isostructural compounds
[LaL(H2O)2]n (1) and [CeL(H2O)2]n (2) recorded at different times (6 min and 30 min). Figure S11: PXRD patterns
of [LaL(H2O)2]n (1) recorded at different times (6 min and 30 min). Figure S12: PXRD patterns of [LaL(H2O)2]n
(2). Figure S13: PXRD patterns of [NdL(H2O)2]n·1.33DMF·2H2O (3). Figure S14: PXRD patterns of compound
[NdL(H2O)2]n·1.33DMF·2H2O (3)-red, compound 3 after basic condition-black; compound 3 after acidic
condition-green. Figure S15: PXRD patterns of activated compound [NdL(H2O)2]n·1.33DMF·2H2O (3)-black and
compound (3) after catalytic reaction (first cycle) -red and (3) -blue, after third cycle in catalytic processes. Figure
S16: PXRD patterns of [EuL(H2O)2]n·1.33DMF·2H2O (4). Figure S17: PXRD patterns of [GdL(H2O)2]n·2DMF·2H2O
(5). Figure S18: PXRD patterns of [DyL(H2O)2]n (6). Figure S19: PXRD patterns of [HoL(H2O)2]n·1.33DMF·2H2O
(7). Figure S20: PXRD patterns of all compounds (1–7). Figure S21: Thermogravimetric analysis of ligand H3L.
Figure S22: Thermogravimetric analysis of compound [LaL(H2O)2]n (1). Figure S23: Thermogravimetric analysis
of
compound
[CeL(H2O)2]n
(2).
(3).
(4).
Figure
Figure
Figure
S24:
S25:
S26:
Thermogravimetric
Thermogravimetric
Thermogravimetric
analysis
analysis
analysis
of
of
of
compound
compound
compound
[NdL(H2O)2]n·1.33DMF·2H2O
[EuL(H2O)2]n·1.33DMF·2H2O
[GdL(H2O)2]n·2DMF·2H2O (5). Figure S27: Thermogravimetric analysis of compound [DyL(H2O)2]n·(6). Figure
S28: Thermogravimetric analysis of compound [HoL(H2O)2]n·1.33DMF·2H2O (7). Figure S29: Thermogravimetric
analysis of 2-naphyl acetate. Figure S30: Nitrogen sorption isotherms of [LaL(H2O)2]n (1) recorded at 77 K. Filled
symbols are for adsorption, empty symbols are for desorption (SBET = 405 m2/g). Figure S31: Nitrogen sorption
isotherms of [CeL(H2O)2]n (2) recorded at 77 K. Filled symbols are for adsorption, empty symbols are for
desorption (SBET = 467 m2/g). Figure S32: Nitrogen sorption isotherms of [NdL(H2O)2]n·1.33DMF·2H2O (3)
recorded at 77 K. Filled symbols are for adsorption, empty symbols are for desorption (SBET = 426 m2/g). Figure
S33: Nitrogen sorption isotherms of [EuL(H2O)2]n·1.33DMF·2H2O (4). Filled symbols are for adsorption, empty
symbols are for desorption (SBET
=
114 m2/g). Figure S34: Nitrogen sorption isotherms of
[GdL(H2O)2]n·2DMF·2H2O (5) recorded at 77 K. Filled symbols are for adsorption, empty symbols are for
desorption (SBET = 348 m2/g). Figure S35: (a) Nitrogen sorption isotherms of [DyL(H2O)2]n (6). Filled symbols
are for adsorption, empty symbols are for desorption (SBET = 202 m2/g), (b) Nitrogen sorption isotherms of
[DyL(H2O)2]n (6) (different synthesis, the same conditions). Filled symbols are for adsorption, empty symbols
are for desorption (SBET = 298 m2/g). Figure S36: Nitrogen sorption isotherms of [HoL(H2O)2]n·1.33DMF·2H2O
(7). Filled symbols are for adsorption, empty symbols are for desorption (SBET = 286 m2/g). Figure S37:
Fluorescence imaging of representative samples (Ligand; C1–7) excited with (a) green light, or (b) UV Objective
20x. Table S2: Bond distances (Å) and angles (°) for shI_3971_BeDa. Table S3: Bond distances (Å) and angles (°)
for shI_3972_BeDa. Table S4: Bond distances (Å) and angles (°) for shI_3984_BeDa.
Author Contributions: D.B. performed the experiments, interpreted the data, and evaluated the results,
writing—review and editing the manuscript; L.G.B. provided the ligand, experimental data acquisition, and
analysis; S.S. provided the x-ray data expertise, writing and review; N.L.M. conducted the TG data analysis,
project administration, and funding acquisition; Ü.K.-D. conducted the PXRD analysis; V.L. undertook the
review process; J.C. conducted the scientific validation and manuscript revision. All authors discussed the
results, commented on the manuscript, and agreed to the published version of the manuscript.
Funding: The financial support of European Social Fund for Regional Development, Competitiveness
Operational Program Axis 1—Project “Novel Porous Coordination Polymers with Organic Ligands of Variable
Length for Gas Storage”, POCPOLIG (ID P_37_707, Contract 67/08.09.2016, cod MySMIS: 104810) is gratefully
acknowledged.
Conflicts of Interest: The authors declare no conflicts of interest.
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