Thermally stable and robust gadolinium-based metal-organic framework: Synthesis, structure and heterogeneous catalytic O-arylation reaction

Article abstract of DOI:10.1016/j.poly.2020.114934

Hydrothermal treatment of gadolinium nitrate and 2,6-naphthalenedicarboxylic acid (H₂NDC) afforded a new metal-organic framework compound, {[Gd₄(NDC)₆(H₂O)₆]·2H₂O}_n(1). Compound 1 has been characterized by single-crystal X-ray crystallography, elemental analysis, FT-IR spectroscopy, therrmogravimetric analysis (TGA) and powder X-ray diffraction analysis. It is crystallized in the monoclinic system with the P2₁/n space group. Four crystallographically distinct Gd (III) centres are interconnected with each other through bridged carboxylato oxygen atoms and water molecules to form tetranuclear secondary building units, which are further connected through the carboxylato ligand and the network propagates along the crystallographic ac plane to form a 2D structure. Subsequent reinforcement from the remaining carboxylato oxygen atoms gives rise to a robust 3D framework structure. Thermogravimetric analysis demonstrates that compound 1 is fairly stable after dehydration under a nitrogen atmosphere. Notably, compound 1 is capable of catalyzing the O-arylation reaction efficiently between substituted phenols and bromoarene under heterogeneous conditions at 80 °C to afford unsymmetrical diarylethers.

Full text of DOI:10.1016/j.poly.2020.114934

Polyhedron 194 (2021) 114934
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Thermally stable and robust gadolinium-based metal-organic
framework: Synthesis, structure and heterogeneous catalytic
O-arylation reaction
Pameli Ghosh a, Tanmoy Maity a,b, Saptarshi Biswas c, Rakesh Debnath a, Subratanath Koner a,
⇑
a Department of Chemistry, Jadavpur University, Kolkata 700 032, India
b Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, Karnataka 560 012, India
c Katwa College, Katwa, West Bengal 713 130, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 July 2020
Accepted 16 November 2020
Available online 25 November 2020
Hydrothermal treatment of gadolinium nitrate and 2,6-naphthalenedicarboxylic acid (H2NDC) afforded a
new metal-organic framework compound, {[Gd4(NDC)6(H2O)6]Á2H2O}n(1). Compound 1 has been charac-
terized by single-crystal X-ray crystallography, elemental analysis, FT-IR spectroscopy, therrmogravimet-
ric analysis (TGA) and powder X-ray diffraction analysis. It is crystallized in the monoclinic system with
the P21/n space group. Four crystallographically distinct Gd (III) centres are interconnected with each
other through bridged carboxylato oxygen atoms and water molecules to form tetranuclear secondary
building units, which are further connected through the carboxylato ligand and the network propagates
along the crystallographic ac plane to form a 2D structure. Subsequent reinforcement from the remaining
carboxylato oxygen atoms gives rise to a robust 3D framework structure. Thermogravimetric analysis
demonstrates that compound 1 is fairly stable after dehydration under a nitrogen atmosphere.
Notably, compound 1 is capable of catalyzing the O-arylation reaction efficiently between substituted
phenols and bromoarene under heterogeneous conditions at 80 °C to afford unsymmetrical diarylethers.
Ó 2020 Elsevier Ltd. All rights reserved.
Keywords:
Gadolinium
2,6-Naphthalenedicarboxylic acid (H2NDC)
Metal-organic framework
Hydrothermal synthesis
Heterogeneous catalysis
O-Arylation
1. Introduction
homogeneous processes by efficient heterogeneous catalytic sys-
tems. Heterogeneous catalysts have many practical advantages in
Metal-organic frameworks (MOFs) and coordination polymers
(CPs) have received ever-growing attention for their fascinating
architectures and variable dimensionality, and have enticed
researchers to use MOFs in a variety targeted applications, like
gas storage and separation [1], luminescence and sensing [2], catal-
ysis [3], magnetism [4] and proton conduction [5], to mention a
few. As crystal engineering and supramolecular chemistry have
evolved, the design of metal-organic frameworks has attained a
higher level of sophistication [6]. However, to have control over
the structure of a material still remains a challenge.
Amongst the various applications of MOFs, one of the most dis-
tinct research areas is their use in heterogeneous catalysis. Never-
theless, there are some important catalytic reactions where MOF
based catalysts have still not been employed extensively. Hence,
there are increasing demands for MOFs being employed as cata-
lysts in heterogeneous catalysis [7]. For environmental and eco-
contrast to analogous homogeneous catalysts because of their
reusability, easier separation and close relevance to green and
clean process [8]. As a result, in last two decades, MOFs have
attracted immense attention in heterogeneous catalysis research.
The metal centers of the network, especially coordinatively unsat-
urated metal centers present in the catalyst or such centers gener-
ated in situ under the reaction conditions, act as active sites for the
catalysis [9]. Basic requirements for employing MOFs in heteroge-
neous catalysis include their chemical and thermal stability under
the reaction conditions. Low thermal stability of an MOF limits its
practical application in catalytic reactions. Further, leaching of
metal ions during the reaction is undesirable in liquid phase
heterogeneous catalysis. Therefore, the thermal and chemical sta-
bility of MOFs are of paramount importance as regards to hetero-
geneous catalysis. Metal-organic framework based catalysts have
been successfully employed in organic transformations, such as
Suzuki cross-coupling [10], the Sonogashira reaction [11] and
epoxide ring opening reactions [12] etc. We have also reported
the use of porous MOFs as heterogeneous catalysts in a variety of
catalyzed organic reactions, such as CAC, CAN and CAO coupling
nomic reasons, there is
a
huge demand for replacing
⇑
Corresponding author.
E-mail address: subratan.koner@jadavpuruniversity.in (S. Koner).
https://doi.org/10.1016/j.poly.2020.114934
0277-5387/Ó 2020 Elsevier Ltd. All rights reserved.

P. Ghosh, T. Maity, S. Biswas et al.
Polyhedron 194 (2021) 114934
reactions, aldol condensation, hydrogenation, epoxidation [13] etc.
Many lanthanide based metal-organic framework compounds have
been synthesized mainly to study their magnetic and photo-lumi-
nescence properties, however, their catalytic efficacy has not been
so well explored to date [14].
A few years ago, our group reported the hydrothermal synthesis
and structural study of a series of lanthanide containing MOFs
[13,15]. Maintaining the pH of the hydrothermal medium by con-
trolled hydrolysis allowed us to isolate a fascinating Gd26 cluster
grade) were procured from Merck (India). Solvents were distilled
and dried before use.
2.2. Physical measurements
IR spectra were recorded in KBr pellets (4000–400 cmÀ1) on a
Perkin-Elmer SPECTRUM II FTIR spectrometer. Elemental analysis
was performed using a Vario-Micro V2.0.11 elemental (CHNSO)
analyzer. Thermo-gravimetric (TGA) analysis was done on a Per-
kin-Elmer Pyris Diamond TG unit. The heating rate was pro-
grammed at 10 °C/min with a protecting stream of N2 flow,
20 mL/min. Powder X-ray diffraction (PXRD) pattern of the sam-
ples were recorded with a Bruker D8 Conquest X-ray diffractome-
based 3D framework compound in which a spherical ‘‘CO3@Gd26
”
shell acts as the building block of the network. The MOF compound
efficiently catalyzes the heterogeneous epoxidation of olefinic sub-
strates including a,b-unsaturated ketones [15]. Further exploration
of the gadolinium(III) carboxylate system availing hydrothermal
synthetic route has afforded us a new structurally robust and ther-
mally stable three dimensional MOF compound.
Carboxylate ligands with diverse coordination abilities are very
useful for preparing metal-organic frameworks (Fig. 1). With much
promise for versatility, the H2NDC ligand contains two carboxylic
acid groups that can afford four oxygen atoms for binding metal
ions. 2,6-Naphthalenedicarboxylic acid is a fascinating molecule,
with regard to its rigidity as well as its different modes of coordi-
nation ability [16]. MOFs like MIL-140B [16a] MIL-69 [16b] and
ter using Cu-Ka radiation (k = 1.54056 Å) at room temperature.
Diffraction lines were measured in the 2h = 5–50° range with a
scanning speed of 0.1°/sec. Mass spectra were measured on a
Waters XEVO–G2QTOF#YCA351 high resolution mass spectrome-
ter. 1H NMR spectra were measured on a 300 MHz Bruker Advance
DPX-300 NMR spectrometer.
2.3. Synthesis of {[Gd4(NDC)6(H2O)6]Á2H2O}n (1)
The salt of gadolinium nitrate hexahydrate (0.25 mmol, 0.112 g)
was dissolved in 2 mL of water and a 3 mL solution of 2,6-naph-
thalenedicarboxylic acid (0.25 mmol, 0.027 g) in water was pre-
pared separately. The two solutions were mixed thoroughly, then
stirred for a few minutes. The resulting mixture was placed in a
15 mL Teflon-lined Parr acid digestion bomb and kept at 170 °C
for two days. After two days, the solution was allowed to cool
down slowly at the rate of 10 °C/h to room temperature. Hexago-
nal-shaped yellow coloured crystals were collected from the diges-
tion bomb. The crystals were washed with distilled water and
dried in air (yield 60% based on metal). The phase purity was ver-
ified through elemental analysis and X-ray powder diffraction
analysis (Fig. S1; see Supporting Information). Anal. Calc. for
DUT-4 [16g] have been synthesized by using H2NDC as
a
secondary building unit. Recently, a europium-NDC MOF system
has been isolated that displayed an interesting thermo-sensitive
fluorescent property [16c].
Here we report the hydrothermal synthesis of the Gd-carboxy-
late (NDC2À) based MOF {[Gd4(NDC)6(H2O)6]Á2H2O}n (1), which
exhibits desirable thermal stability after dehydration, without
breaking its network structure. Notably, compound 1 efficiently
catalyses an O-arylation reaction between substituted phenols
and bromoarenes. To the best of our knowledge, this is the first
example of a lanthanide-based MOF catalyzing the O-arylation
reaction under heterogeneous conditions.
C
72H48O32Gd4 (FW = 2054.17), C, 42.06, H, 2.33; Found C, 42.5, H,
2.3%. Selected IR peaks (KBr pellet,
, cmÀ1) (Fig. S2; see Supporting
Information): 1600, 1527 [mas (COÀ2 )], 1406 [ s(CO2À)], 1334 [ms
(CAO)], 3300 s.br [ (OAH)].
m
2. Experimental
m
m
2.1. Materials
2.4. X-ray crystallography
Gadolinium(III) nitrate hexahydrate and 2,6-naphthalenedicar-
boxylic acid (H2NDC) were purchased from Sigma-Aldrich and
used as received without further purification. Cesium carbonate,
substituted phenols, other chemicals and solvents (analytical
A single crystal suitable for X-ray diffraction was selected from
the bulk material under a microscope and carefully glued onto a
thin glass fiber. The X-ray diffraction data of 1 was collected on a
Microfocus Bruker D8 Conquest X-ray single crystal diffractometer
using graphite-monochromated Mo-Ka radiation (k = 0.71073 Å).
Cell refinement and determination of the integrated intensities
were performed with the SAINT [17] software package, using a nar-
row frame integration alogorithm. An empirical absorption correc-
tion (SADABS) [17] was applied. The structure was solved by direct
methods and refined by full-matrix least squares on F2 using the
SHELXL-2014 package [18], SHELXL 2018[19] and Olex2 programs
[20]. Non-hydrogen atoms were refined with anisotropic thermal
parameters. All other hydrogen atoms were placed in their geo-
metrically idealized positions, except for the hydrogen atoms of
the coordinated water molecules (O18 and O25) in complex 1,
which could not be located in the Fourier map. Successful conver-
gence was indicated by the maximum shift/error of 0.001 for the
last cycle of the least squares refinement. All calculation and
molecular graphics were performed with WinGX [21], Diamond
[22], Mercury [23] and PLATON [24]. A summary of the crystallo-
graphic data and the relevant structural refinement parameters
for 1 are given in Table 1. Selected bond distances and angles are
summarized in Tables S1 and S2 (see Supporting Information).
The thermal ellipsoid crystal structure of 1 is shown in Fig. S3
Fig. 1. Diversity in coordination modes of the H2NDC ligand.
2

P. Ghosh, T. Maity, S. Biswas et al.
Polyhedron 194 (2021) 114934
Table 1
present in the crystal structure. Two different types of geometries,
tricapped-trigonal prism and bicapped-trigonal prismatic polyhe-
dron, are exhibited in complex 1. Among four different Gd metal
centers, the Gd1 centre (Fig. S4a) is coordinated by five carboxylato
oxygen atoms (O14, O23, O26, O12 and O20) of five NDC2À ligands
each in a monodentate mode (Fig. 1a), two oxygen atoms (O16 and
O17) in a chelate- bidentate mode (Fig. 1b), another two oxygen
atoms from coordinated water molecules (O24 and O25), while
the ninth position is satisfied by an oxygen (O17) atom from a car-
boxylato group through semi-coordination (with a long GdAO
bond = 2.911 Å) [25]. Similarly, all the coordination positions of
the Gd4 centre are occupied by nine oxygen atoms (Fig. S4b), of
which, five are occupied by oxygen atoms (O5, O13, O17, O15
Crystal data and refinement parameters of 1.
Formula
C72H52O32Gd4
2058.17
Monoclinic
P21/n
17.0880(13)
15.2662(12)
25.0888(19)
90
106.260(2)
90
6283.1(8)
4
2.171
4.271
1.8–27.1
Formula Weight
Crystal System
Space Group
a (Å)
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (g cmÀ3
)
(mmÀ1
)
and O2) from five NDC2À ligands in
a monodentate mode
l
h Range (°)
(Fig. 1a), another two oxygen atoms (O20 and O21) from one
NDC2À ligand in a bidentate chelate mode (Fig. 1b) and the remain-
ing two coordination positions are occupied by oxygen atoms of
Intervals of reflection indices
Reflections measured
Rint
Unique data
R indices (all data)
À21ꢀhꢀ21, À19ꢀkꢀ19, À32ꢀlꢀ32
89,844
0.056
13,844
R1 = 0.056, wR2 = 0.0872
water molecules. However, the Gd4
À
O18 (2.907 Å) and
Gd4 À O20 (2.887 Å) bond lengths are rather longer than normal
Gd À O coordination bonds [25]. Both the Gd1 and Gd4 centers dis-
play a distorted tricapped-trigonal prism geometry (Figs. S6a, e and
b,f; see Supporting Information). The Gd1 center is situated
between the Gd4 and Gd3 centres,(Fig. 2), where Gd1 and Gd4
are connected in a syn-syn and syn-anti bridging fashion through
the NDC2À ligand (Fig. 2c and d) and Gd1 is connected to the
Gd3 center by bridging carboxylate (O22, O23, O26 and O27) and
water oxygen (O25) atoms.
Both the Gd2 and Gd3 centers are eight-coordinated and feature
[GdO8] chromophores (Fig.S5a, b; see Supporting Information). The
coordination positions are satisfied by six oxygen atoms (O1, O3,
O29, O10, O8 and O6 for Gd2; O9, O11, O27, O22, O28 and O4 for
Gd3) from six NDC2À ligands in a monodentate fashion (Fig. 1a),
one from a coordinated water molecule (O7 for Gd2 and O30 for
Gd3) and another oxygen atom from a bridging water molecule
(O18 and O25 for Gd2 and Gd3, respectively), which form distorted
bicapped-trigonal prismatic polyhedrons (Fig. S6c, h and d, g); see
Supporting Information).
The gadolinium ions are inter-connected with each other
through bridging carboxylato groups in syn-syn and syn-anti fash-
ions and chelation in the bridging mode (Fig.2), and with an alter-
nate bridging water molecule to form a secondary building unit.
The tetra-nuclear units are connected through the oxygen atoms
of the NDC2À ligand to form a 1D chain along the crystallographic
a-axis (Fig. 3a). The 1D chains are further connected by NDC2À
ligands to construct 2D layers along the crystallographic ac-plane
(Fig. 3b). Finally, 2D layers are again pillared by 2,6-naphthalenedi-
carboxylic acid ligands to give rise to a 3D network structure that
propagates along the crystallographic a-axis (Fig. 4) (Fig. S7; see
Supporting Information).
Final R indices[I > 2
Data with I > 2(I)
R1 (I > 2(I)
r
(I)]
0.0385
10,704
0.0385
0.0872
3968
wR2(I > 2(I)
F(000)
Goodness-of-Fit on F2
1.072
P
P
P
P
R1 = ||Fo| À |Fc||/ |Fo|, wR2 = { [w(F2o À Fc2)2]/ w(Fo2).
(see Supporting Information). CCDC-2013484 contains the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from Cambridge Crystallographic Data Cen-
tre via www.ccdc.cam.ac.uk/data_request/cif.
2.5. General procedure for the catalytic reactions
0.151 g (1 mmol) of p-nitrobromobenzene and 0.103 g (1 mmol)
of p-cresol were added with 0.487 g (1.5 mmol) of solid cesium car-
bonate in 3 mL dehydrated dimethylformamide (DMF) solvent to a
15 mL round-bottom flask. 0.005 g of compound 1 was added to
the reaction medium and the resulting mixture was stirred under
continuous heating at 80 °C for seven hours. The conversion of
the reaction was monitored by the thin layered chromatography
(TLC) method. After completion of the reaction mixture, it was
cooled to room temperature and purged on ice cold water. The
aqueous layer was extracted three times with ethylacetate
(6 mL). The combined organic layer thus collected was washed
with a brine solution, dried over anhydrous sodium sulfate and
concentrated in vacuum. The collected residue was further purified
by column chromatography on silica-gel (60–120 mesh), eluting
with an n-hexane/ethyl acetate mixture to get the desired product.
The product was analyzed by 1H NMR spectroscopy and elemental
analysis, and compared with the literature data.
3.2. Thermal study
3. Results and discussion
Thermogravimetric analysis was carried out to study the ther-
mal stability of the framework in the solid state. Thermal analysis
of the compound {[Gd4(NDC)6(H2O)6]Á2H2O}n (1) was performed
using a powdered sample from room temperature to 900 °C
under an N2 atmosphere (Fig. 5). Compound 1 showed mass
losses in two clear stages. In the first step, upon heating, a sharp
mass loss of 6.6% from 100 to 230 °C corroborates with the the-
oretical value of 6.99% owing to loss of two lattice and six coor-
dinated water molecules. The dehydrated framework then
showed no mass loss up to 530 °C. On further heating, 1 decom-
poses steadily to form the corresponding metal oxide at ~870 °C.
In the second step, a total mass loss of about 65% matches well
with the theoretical value of 64.7% due to decomposition of the
organic parts of the MOF further to form gadolinium oxide. To
3.1. X-ray structure
The single-crystal X-ray structure analysis reveals that com-
pound 1 is crystallized in the monoclinic P21/n space group with
Z = 4. The molecular structure of 1 consists of tetra-nuclear
homo-metallic gadolinium moieties. The tetra-nuclear unit, with
the numbering scheme, of {[Gd4(NDC)6(H2O)6]Á2H2O}n (1) is shown
in Fig. 2 (the thermal ellipsoid structure with 50% probability is
given in Fig. S3). The structure contains four Gd-metal centres of
which Gd1, Gd4 (nine-coordinated) and Gd2, Gd3 (eight-coordi-
nated) have the same coordination mode. Along with this, six coor-
dinated water molecules and two lattice water molecules are
3

P. Ghosh, T. Maity, S. Biswas et al.
Polyhedron 194 (2021) 114934
Fig. 2. Tetra-nuclear unit of compound 1 with the atom numbering scheme.
Fig. 3. Two-dimensional connectivity of compound 1.
verify the thermal stability of the dehydrated species, MOF sam-
ples were kept at the desired temperature for 3–4 h and subse-
quently the collected solid samples were subjected to PXRD
analysis (Fig. S8; see Supporting Information). It was observed
that the dehydrated framework compound remained stable at
least up to around 350 °C, thereafter it started to decompose
and finally gadolinium oxide was formed.
comparison of the experimental and simulated (from single crystal
data) PXRD patterns (Fig. S1, see Supporting Information) showed
that all the major diffraction lines of the experimental PXRD of 1
matched well with those of the simulated ones, indicating its rea-
sonable crystalline phase purity. The PXRD patterns of the com-
pound were retained upon prolonged storage (for at least
6 months). No significant changes in the PXRD patterns were
observed upon exposure to various solvents, such as water, metha-
nol, ethanol, DMF and DMA. To study this, 1 was kept in these sol-
vents as a suspension and PXRD were then recorded (Fig. S9; see
Supporting Information). All these facts suggest the robustness of
framework compound 1.
3.3. PXRD study
To confirm the phase purity of the bulk material of 1, powder
X-ray diffraction (PXRD) measurements were carried out.
A
4

P. Ghosh, T. Maity, S. Biswas et al.
Polyhedron 194 (2021) 114934
Fig. 4. a) 3D network structure of compound 1; b) 3D topological view of compound 1 (the carboxylato bridge is represented by the magenta rod and the H2O bridge is
represented by the cyan rod). Organic moieties have been omitted for clarity.
particular, demonstrate excellent catalytic efficacy [13,32]. This
gives us impetus to undertake the O-arylation reaction using 1 as
a catalyst.
To start with, optimization of the O-arylation reactions was
undertaken using p–cresol and p–nitrobromobenzene as the sub-
strates under various reaction conditions, catalyzed by compound
1 (Table 2). We have explored the effect of solvents, since solvent
plays an important role in O-arylation reactions. Among the vari-
ous solvents used (entries 1–7), the highest yield of the desired
product was obtained with DMF (entry 14). Generally, O–arylation
reactions show better reactivity with Cs2CO3 (base) than with
K2CO3 or Na2CO3 due to the higher solubility and fine powder form
of Cs2CO3. In the present case, Cs2CO3 also was the best choice as
the base among the other tried options (entries 8–12, 14). The iso-
lated yield increases sharply with the increase of the reaction tem-
perature (entries 14, 15–17), however, it does not significantly
improve beyond 80 °C. The catalyst concentration of a catalytic
reaction is another very important factor to investigate. Accord-
ingly, the O-arylation reaction was carried out in the presence of
0.01, 0.02, 0.025 and 0.05 mol% of the catalyst (entries18–21).
The reaction with 0.025 mol% of catalyst showed the maximum
Fig. 5. Thermogravimetric plot of 1.
conversion in 7 h. On increasing the catalyst concentration above
0.05 mol%, no noticeable increase in yield was observed. No reac-
tion occurred in the absence of catalyst in 7 h (entry 13). In addi-
tion, mixture of gadolinium nitrate salt and 2,6-
3.4. Catalytic activity study
a
naphthalenedicarboxylic acid (as a mixture in a stoichiometric
ratio of 4:7), and neat gadolinium nitrate salt have been tested as
catalysts, but only moderate to low conversion was observed (en-
tries 22 and 23). Thus, the optimum conditions for the catalytic
reaction are as follows: Cs2CO3 (base), DMF (solvent), 0.025 mol%
catalyst and reaction temperature 80 °C.
Under the optimized reaction conditions, the scope and applica-
bility of the coupling reaction using various bromoarenes with
substituted phenols to unsymmetrical diaryl ethers were exam-
ined (Table 3). At first, the investigation was carried out using var-
ious substitutions (electron donating or withdrawing) on the
phenol moiety (Table 3, entries 1, 2, 5, 10 and 11). Electron donat-
ing methyl group containing phenols yield higher quantities of
products in comparison to simple phenol. The yield of the diaryl
ether products decreased in the order p- > o- > m-cresol. It may
be realized that the methyl substituent at the ortho/para position
affords a positive inductive effect and a hyper-conjugation effect,
which increases the nucleophilicity of the phenolate oxygen atoms,
whereas substitution at the meta position affords only a positive
Diaryl ethers are important structural motifs of many biologi-
cally active compounds (K13, perrottetin, teicoplanin and van-
comycin) as well as synthetic polymers [26]. Traditionally, diaryl
ethers are synthesized employing copper(I) catalyzed Ullmann
type cross-coupling reactions from aryl iodide or bromide and phe-
nols. However, drastic reaction conditions, such as very high tem-
perature (125–200 °C), requirement of stoichiometric amounts of
the catalyst, purification problems and low to moderate yields,
limit its utility [27]. Therefore, searches for new methods for the
synthesis of diaryl ethers under milder conditions are of interest
[26,28,29]. Buchwald, Hartwig and later Beller have developed sev-
eral palladium catalyzed processes for CAO cross–coupling reac-
tions under mild conditions [30]. Despite being highly efficient,
palladium catalyzed methods have limited prospects in application
for large scale industrial production, not only due to high cost and
toxicity of palladium but also for the requirement of suitable
ligands in the process [31].In the course of our continuing investi-
gation in different organic reactions using a variety of solid mate-
rials as heterogeneous catalysts, we have found that MOFs, in
5

P. Ghosh, T. Maity, S. Biswas et al.
Polyhedron 194 (2021) 114934
Table 2
Optimization of reaction conditiona.
Entry
Catalyst
Base
Solvent
IsolatedYieldb (%)
1
2
3
4
5
6
7
8
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
–
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Compound 1
Gd(NO3)3ÁxH2O + NDC
Gd(NO3)3ÁxH2O
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
Na2CO3
CH3COONa
t-BuOK
DABCO
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Ethanol
DMSO
Toluene
Acetonitrile
Methanol
Ethyleneglycol
Dioxane
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
39
79
18
36
64
58
42
87
84
trace
38
29
–
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
94
27c
69d
95e
49f
88g
94h
94i
41
DMF
DMF
DMF
29
aReaction conditions, p-cresol (1 mmol), p-nitrobromobenzene (1 mmol), catalyst (0.025 mol%), base (1.5 mmol), solvent (3 mL) and temperature 80 °C. bisolated yield.c
reaction temperature 40 °C.d temperature of the reaction 60 °C.ereaction temperature 100 °C.f0.01 Mol% compound 1.g0.02 Mol% compound 1. h0.025 Mol% compound 1.
i
0.05Mol% compound 1.
inductive effect. Between para and ortho cresol, more steric crowd-
ing at the ortho position might lead to a lower yield. Interestingly,
in the case of p-methoxyphenol, the yield is lower compared to p-
cresol (entries 10 and 16). Though the methoxy group has a very
strong positive resonance effect, it also has the ability to interact
with metal centers [33]. Maybe due to this reason the methoxy
group loses its power to increase the nucleophilicity at the pheno-
late oxygen atom. Moreover, the coupling reactions of electron–de-
ficient phenols with aryl halides have been challenging as the
corresponding phenolates are weak nucleophiles [34]. Catalyst 1
also activates its catalytic efficiency towards weak nucleophiles
e.g. p-hydroxyacetophenone (entry 11). Apart from bromoni-
trobenzene, bromobenzonitrile and bromobenzaldehyde were also
used as electrophiles and the corresponding products were
obtained in good yields. A representative plot of the progress of
the catalytic reaction is shown in Fig. S10 (see Supporting
Information).
solution. This experiment clearly demonstrates that there was no
leaching of gadolinium(III) ions from the solid catalyst during the
reactions (Fig. S11; see Supporting Information)
In order to check the stability of the catalyst, we characterized
the recovered material. After completing the catalytic reactions,
the solid catalyst was recovered by centrifugation, washed thor-
oughly with dichloromethane and dried under vacuum at 100 °C.
The recovered catalyst was then subjected to X-ray powder diffrac-
tion analysis and IR spectral measurement. A comparison of the
PXRD patterns (Fig. S12; see Supporting Information) and the IR
spectra (Fig. S13; see Supporting Information) with the pristine
compound and recovered catalyst convincingly demonstrated that
the structural integrity of the compound is unaltered after the
reaction.
For the recycling study, the O-arylation reaction was performed
using p-cresol with p-nitrobromobenzene. After the first cycle of
the reaction, the catalyst was recovered as mentioned earlier.
The performance of the recycled catalyst was then studied for up
to five successive runs (Fig. S14; see Supporting Information).
The catalytic efficacy of the recovered catalyst remained almost
same in each run.
3.4.1. Separation, catalyst reuse and heterogeneity test
As gadolinium(III) ions in water are toxic and it is a costly metal,
the reusability and heterogeneity are important criteria for use of 1
as a catalyst. To ascertain that the catalysis is indeed heteroge-
neous, we performed a hot filtration test. The solid catalyst was fil-
tered hot after 2 h of reaction and the residual activity of the
supernatant solution after separation of the catalyst was studied.
The supernatant solution was kept under the reaction conditions
for another 7 h and the composition of the solution was analyzed
from time to time. No progress of reaction was observed during
this period, which excludes the presence of active species in the
4. Conclusion
In essence, we have successfully synthesized a thermally stable
and robust tetra-nuclear Gd-carboxylate based three dimensional
metal-organic framework compound, {[Gd4(NDC)6(H2O)6]Á2H2O}n
via a hydrothermal route. The structure of the complex was estab-
lished by X-ray diffraction analysis. The dehydrated compound is
6

P. Ghosh, T. Maity, S. Biswas et al.
Polyhedron 194 (2021) 114934
Table 3
O-arylation reaction between substituted bromobenzene with phenolsa
Entry
1
Aryl alcohol
Product
Isolated Yieldb (wt%)
84
TOFc (hÀ1
12.5
)
2
90
13.4
3
4
5
90
85
90
13.4
12.5
13.4
6
7
31
91
4.61
13.5
(continued on next page)
7

P. Ghosh, T. Maity, S. Biswas et al.
Polyhedron 194 (2021) 114934
Table 3 (continued)
Entry
8
Aryl alcohol
Product
Isolated Yieldb (wt%)
90
TOFc (hÀ1
13.4
)
9
83
84
12.3
12.5
10
11d
39
5.8
12
83
12.35
13
14
79
93
11.8
13.8
15
87
12.9
8

P. Ghosh, T. Maity, S. Biswas et al.
Polyhedron 194 (2021) 114934
Table 3 (continued)
Entry
16
Aryl alcohol
Product
Isolated Yieldb (wt%)
81
TOFc (hÀ1
12.1
)
aReaction conditions: Nitroarene (1 mmol), aryl alcohol (1 mmol), Cs2CO3 (1.5 mmol), catalyst (0.025 mol%), DMF (3 mL) at 80 °C for 7 h. bIsolated yield. cTOF = Mol.
diarylether converted/mol. Gd used/ h. d10 h.
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CRediT authorship contribution statement
Pameli Ghosh: Visualization, Investigation, Data curation, Writ-
ing - original draft, Software. Tanmoy Maity: Writing - review &
editing. Saptarshi Biswas: Software. Rakesh Debnath: Software.
Subratanath Koner: Conceptualization, Methodology, Supervision,
Project administration, Funding acquisition.
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Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
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Acknowledgements
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We acknowledge the Council of Scientific and Industrial
Research (CSIR), New Delhi for funding a project (to SK) (No. 01
(2944)/18/EMR-II). PG expresses her gratitude to the University
Grants Commission, New Delhi, India for providing a senior
research fellowship. We also thank the Department of Science
and Technology, Government of India for funding the Department
of Chemistry, Jadavpur University to procure single crystal X-ray
facility (Microfocus) and 300 MHz NMR spectrometer. Financial
assistance received under the UGC-CAS program is also
acknowledged.
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