Ligand preferences in ytterbium ions complexation with carboxylate-based metal-organic frameworks

Article abstract of DOI:10.1080/00958972.2017.1375098

Two coordination polymers of ytterbium were synthesized by employing 4,4′,4″-s-triazine-2,4,6-triyl-tribenzoic acid (H₃TATB), 4,4′,4-benzene-1,3,5-triyl-tribenzoic acid (H₃BTB), and 3,5-pyridinedicarboxylic acid (3,5-PDC) ligands and were characterized by single-crystal X-ray diffraction analysis. Reaction of ytterbium(III) chloride in the presence of H₃BTB and 3,5-PDC ligands gives preferred complexation with the 3,5-PDC ligand, producing [Yb₂(3,5-PDC)(ClO₄)₃][NH(Me)₃] (1). However, under exactly the same reaction conditions, reaction of ytterbium(III) chloride in the presence of 3,5-PDC and H₃TATB resulted in complexation with H₃TATB to form [(CH₃)₂NH₂][Yb₄(TATB)₄(HCO₂)(H₂O)₂]·3H₂O (2). The crystal structure results showed a layered structure for 1 and a metal-organic framework structure for 2. This indicates that the complexation preference of the ytterbium ion is H₃TATB?≥?3,5-PDC?≥?H₃BTB. Conversely, the uncomplexed ligand in the metal-organic framework (2) is an auxiliary agent during the synthesis, which shows polytopic linker controls crystal properties, to form suitable crystals for single-crystal structure determination. The prepared coordination compounds were used as heterogeneous catalysts in an oxidation amidation reaction with different aldehydes and benzylamine hydrochloride.

Full text of DOI:10.1080/00958972.2017.1375098

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Ligand preferences in ytterbium ions
complexation with carboxylate-based metal-
organic frameworks
Alireza Hashemzadeh, Mostafa M. Amini, Hamid Reza Khavasi & Seik Weng
Ng
To cite this article: Alireza Hashemzadeh, Mostafa M. Amini, Hamid Reza Khavasi & Seik Weng
Ng (2017): Ligand preferences in ytterbium ions complexation with carboxylate-based metal-
organic frameworks, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2017.1375098
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Date: 10 September 2017, At: 04:37

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://doi.org/10.1080/00958972.2017.1375098
Ligand preferences in ytterbium ions complexation with carboxylate-based
metal-organic frameworks
ALIREZA HASHEMZADEH†, MOSTAFA M. AMINI*†, HAMID REZA KHAVASI† and SEIK WENG NG‡
†Department of Chemistry, Shahid Beheshti University, G.C., Tehran 198396311,
Iran
‡School of Pharmacy, University of Nottingham, Malaysia Campus, 43500 Semenyih,
Selangor Darul Ehsan, Malaysia
Two coordination polymers of ytterbium were synthesized by employing 4,4′,4″-s-triazine-2,4,6-
triyl-tribenzoic acid (H3TATB), 4,4′,4-benzene-1,3,5-triyl-tribenzoic acid (H3BTB), and 3,5-
pyridinedicarboxylic acid (3,5-PDC) ligands and were characterized by single-crystal X-ray
diffraction analysis. Reaction of ytterbium(III) chloride in the presence of H3BTB and 3,5-PDC
ligands gives preferred complexation with the 3,5-PDC ligand, producing [Yb2(3,5-
PDC)(ClO4)3][NH(Me)3] (1). However, under exactly the same reaction conditions, reaction of
ytterbium(III) chloride in the presence of 3,5-PDC and H3TATB resulted in complexation with
H3TATB to form [(CH3)2NH2][Yb4(TATB)4(HCO2)(H2O)2]·3H2O (2). The crystal structure
results showed a layered structure for 1 and a metal-organic framework structure for 2. This
indicates that the complexation preference of the ytterbium ion is H3TATB ≥ 3,5-PDC ≥ H3BTB.
Conversely, the uncomplexed ligand in the metal-organic framework (2) is an auxiliary agent
during the synthesis, which shows polytopic linker controls crystal properties, to form suitable
crystals for single-crystal structure determination. The prepared coordination compounds were
used as heterogeneous catalysts in an oxidation amidation reaction with different aldehydes and
benzylamine hydrochloride.
Keywords: Inorganic polymer; MOF; Ytterbium; Single-crystal; Amidation
*Corresponding author. Email: m-pouramini@sbu.ac.ir
1

1. Introduction
Metal-organic frameworks (MOFs) are crystalline materials constructed from organic struts and
metal-containing nodes as secondary building units (SBUs) [1, 2]. These compounds are of
considerable interest owing to their remarkable framework topologies and design flexibility,
which has led to their widespread use in gas storage [3-8], gas separation [9-13], catalysis
[14-16], sensor [17-19], and drug delivery applications [20-23].
Many techniques have been used to synthesize MOFs and grow high-quality single
crystals [24], including the diffusion method, hydrothermal and solvothermal process,
microwave-assisted synthesis, sonochemical and mechanochemical techniques, and the
modulating ligands approach. In the modulator ligand approach, modulators can control crystal
properties, such as the shape, size, stability, and growth rate of the crystal. Typically,
monocarboxylate molecules, such as acetic acid, benzoic or formic acid, and other
monocarboxylic acids, are used as modulators [25, 26]. In research on the various methods of
preparing MOFs, the effect of polytopic linkers as auxiliary modifying ligands has not been
investigated; further work on this issue will help in understanding the formation of MOFs in the
presence of at least two types of polytopic linkers.
In this work, we report the synthesis of two new ytterbium complexes with
polycarboxylates. The crystal structures and catalytic performances were also investigated. We
used three types of carboxylate-based ligands (scheme 1) under certain conditions. To study the
role of the auxiliary modifying ligand in the formation of the two synthesized ytterbium
coordination polymers, we applied polycarboxylate framework-forming linkers, which competed
with the other ligand in the framework formation. Reaction of YbCl3 with 4,4′,4″-s-triazine-
2,4,6-triyl-tribenzoic acid (H3TATB) in the presence of 3,5-pyridinedicarboxylic acid (3,5-PDC)
resulted in the formation of a three-dimensional MOF with rod-shaped SBUs. Conversely, when
4,4′,4-benzene-1,3,5-triyl-tribenzoic acid (H3BTB) was used instead of H3TATB under exactly
the same reaction conditions, an unexpected layered structure coordination polymer with
3,5-PDC, perchlorate, trimethylammonium ion, and Yb(II) and Yb(I) ions was formed. The
single-crystal structure of the latter compound, along with the infrared spectroscopy and
elemental analysis, demonstrate that perchlorate was formed in situ through an oxidation and
reduction reaction. There is a preference between the two carboxylic-based ligands toward
2

complexing with Yb(III), in which one of the ligands acts as a modifier. Furthermore, in
extension of our work, we used the prepared coordination polymers as heterogeneous catalysts in
an oxidation amidation reaction from different aldehydes and benzylamine hydrochloride.
Scheme 1. Schematic representation of ligands.
2. Experimental
2.1. Materials and methods
All chemicals were purchased from Aldrich and Merck and used without purification. The
syntheses of 1,3,5-tris(4-acetylphenyl)benzene, 4,4',4''-benzene-1,3,5-triyl-tribenzoic acid
(H3BTB) and 4,4′,4″-s-triazine-2,4,6-triyl-tribenzoic acid (H3TATB) were adapted from
published methods [27, 28]. FT-IR spectra were recorded on a Bomem MB-Series FT-IR
spectrometer. Thermal analysis (TGA-DTA) was carried out using a Bahr STA-503 instrument
at a heating rate of 10 °C min-1 in air. The N2 adsorption-desorption analysis was performed at
77 K using a BELSORP-mini instrument (BEL Japan, Inc.). CHN analyses were performed on a
Thermo Finnigan Flash EA112 elemental analyzer. X-ray powder diffraction (XRD) pattern was
recorded on a STOE diffractometer with Cu-Kα radiation ( = 1.5418 Å). Single-crystal structure
data were collected on a STOE IPDS diffractometer equipped with graphite monochromated
Mo-Kα radiation at 298 K. The structures were solved by direct methods and refinement was
achieved by full matrix least squares of F2 using the SHELXL97 crystallographic software
package. Crystallographic data along with the refinement parameters for 1 and 2 are listed in
3

table S1.
2.2. Synthesis of [Yb2(C7H3NO4)(ClO4)3][NH(Me)3]2 (1)
For the synthesis of 1, 3,5-PDC (0.01 g, 0.06 mmol) and H3BTB (0.01 g, 0.23 mmol) were
dissolved in a solution containing DMF (4.0 mL, 52 mmol), H2O (1.0 mL, 55 mmol) and 5 drops
HNO3. To this solution, YbCl3·6H2O (0.03 g, 0.08 mmol) was added, and the mixture was stirred
at room temperature until complete dissolution. The final solution was transferred to a small
glass container and sealed. The sample was heated to 110 °C with a rate of 0.5 °C min-1 and kept
at this temperature for 72 h, and then cooled to room temperature over 48 h. Colorless needle-
like crystals were obtained in 29.4% yield (based on Yb). FTIR (KBr, cm-1): 2974, 2938, 2739,
2677, 2495, 1610, 1547, 1473, 1433, 1397, 1144, 1089, 1034, 845, 805, 772, 719, 627. Anal.
Calcd. (Found) for C13H23Cl3N3O16Yb2: C 16.79 (17.28); H 2.49 (2.67); N 4.52 (5.16).
2.3. Synthesis of [(CH3)2NH2][Yb4(C24H12N3O6)4(HCO2)(H2O)2]·3H2O (2)
Compound 2 was synthesized exactly like 1 except using H3TATB instead of H3BTB. In detail,
3,5-PDC (0.01 g, 0.06 mmol) and H3TATB (0.01 g, 0.23 mmol) were dissolved in a solution
containing DMF (4.0 mL, 52 mmol), H2O (1.0 mL, 0.055 mol) and 5 drops HNO3. Then
YbCl3·6H2O (0.03 g, 0.08 mmol) was added into the primary solution and dissolved while
stirring at room temperature. The final solution was transferred to a glass container and sealed.
The sample was heated to 110 °C with a rate of 0.5 °C min-1 and kept at this temperature for 72 h
and then cooled to room temperature over 48 h. White cubic crystals were obtained. FTIR (KBr,
cm-1): 3048, 2923, 2852, 1677, 1548, 1503, 1420, 1400, 1193, 1100, 883, 1089, 819, 777, 700,
565, 500, 399. Anal. Calcd. (Found) for C99H67N13O31Yb4: C 45.27 (44.32), H 2.57 (2.76), N
6.93 (7.32).
3. Results and discussion
3.1. Synthesis
In an attempt to prepare ytterbium(III) MOF from ytterbium(III) chloride, 4,4',4''-benzene-1,3,5-
triyl-tribenzoic acid, and 3,5-pyridinedicarboxylic acid by solvothermal method in DMF, an
unprecedented coordination polymer with a layer structure was formed,
[Yb2(3,5-PDC)(ClO4)3][NHMe3] (1). When 4,4',4''-benzene-1,3,5-triyl-tribenzoic acid was
4

replaced by 4,4′,4″-s-triazine-2,4,6-triyl-tribenzoic acid and the reaction was carried out under
exactly the same conditions, the formation of a new MOF was observed,
[(CH3)2NH2][Yb4(C24H12N3O6)4(HCO2)(H2O)2]·3H2O (2). Evidence and characterization data
showed that during the formation of 1, a perchlorate ion was produced and integrated into the
structure. The FT-IR spectrum clearly showed the formation of the perchlorate ion in the in situ
reaction. The perchlorate ion must have originated from an oxidation-reduction process during
the hydrothermal reaction. Moreover, single-crystal X-ray diffraction showed that a
trimethylammonium cation formed and combined into the structure of 1, which is probably due
to the reduction of DMF [29]. Since YbCl3·6H2O was the only chlorine source to be oxidized,
Yb(III) and DMF were reduced in a parallel process. A redox electron exchange resulted in
chloride and DMF transformation. The formation of perchlorate in an in situ catalytic reaction
during the solvothermal processes is unusual. For example, a catalytic reaction in the
solvothermal synthesis of coordination polymers and a chemical transformation of perchlorate to
chloride have previously been reported [30], but the reverse of this, i.e., a transformation of
chloride to perchlorate, has not been observed.
Addition of modulating ligands during MOF synthesis can control crystal quality
[31, 32]. The modulating agents are usually monocarboxylic acids, such as trifluoroacetic acid,
acetic acid, and benzoic or formic acids, which slow the nucleation of MOF crystals and lead to
the growth of larger and better-shaped crystals. Herein, H3BTB acted as a modifying agent and
competed with 3,5-PDC in the formation of 1, whereas in the formation of 2, 3,5-PDC acted as a
modifying agent, and polytopic H3TATB integrated into the MOF and suitable crystals formed.
Other lanthanide complexes with similar structures have been synthesized in the absence of a
“modifying agent” [33]. We also utilized the same procedure, but the resulting patterns are
different from the reported structures. Therefore, it can be postulated that the presence of both
ligands is necessary to successfully generate the reported MOFs. The tendency of coordination of
H3BTB to Yb in 1 indicated that H3BTB is an auxiliary agent over 3,5-PDC, and in 2, it is
evident that the coordination power of the ligands is in order of H3TATB ≥ 3,5-PDC. Herein, in
both cases, the polytopic linkers, H3BTB and 3,5-PDC, act as modifier agents, and the modifiers
form complexes during the synthesis of the coordination polymers. Concomitantly, modifiers
slow down the synthetic process of more thermodynamically favorable compounds and finally
facilitate the formation of high-quality crystals. It seems that the coordination motif of TATB is
5

almost the same as that of BTB, but they show different preferences under the same reaction
conditions and compositions. The difference in the structural preferences of TATB-3 and BTB-3
ligands can be attributed to the middle ring of the BTB-3 ligand, which is spatially more hindered
than the TATB-3 ligand because of the hydrogens on the middle benzene ring that directly
influence properties and behavior of the ligand. This could be a reason for the different
coordination preference of the ligands under the same conditions.
3.2. Single-crystal structures
3.2.1. Molecular structure of [Yb2(C7H3NO4)(ClO4)3][NH(Me)3]2 (1). Single-crystal X-ray
analysis revealed that 1 crystallized in the monoclinic system in the space group of C2/m.
Selected bond lengths and angles are listed in table S2. The asymmetric unit contains half Yb(I)
and half Yb(II) ions, half 3,5-PDC ligand, one and half µ3-bridging perchlorates, and one
trimethylammonium cation. Carboxylate oxygens of 3,5-PDC are coordinated bidentate through
bridging of O1 and O2 to two crystallographically independent pairs of Yb ions. These two
crystallographically independent ytterbium ions, Yb1 and Yb2, exhibit two different coordination
geometries in the complex. As illustrated in figure 1, the Yb1 has a seven-coordinate pentagonal
bipyramidal geometry, with two oxygens from two different 3,5-PDC ligands occupying the
axial sites with an O1—Yb1—O1ii angle of 167(3), and five further oxygens, O3, O6, O9, O9i
and O11, from perchlorate ions were positioned in the equatorial sites of the pentagonal plane.
The O1—Yb1 and O1ii—Yb1 bond lengths, 2.187(6) Å, are in the range of those observed in
other carboxylate-bridged ytterbium coordination polymers, such as [Yb2(bdc)3(e-urea)4] [34],
Yb-PVDC-2 [35] and [Yb2(TP)3(H2O)2]2 [36]. The other Yb1—O bonds are formed by
coordination of oxygens from three crystallographically independent perchlorates. These
oxygens that formed the pentagonal plane can be differentiated into two types. In the first type,
two crystallographically independent perchlorates adopt a µ3-bridging mode to link three
ytterbium ions while in the second type, the third perchlorate forms a µ-O9 bridge between Yb1
and Yb1i while also forming a chelate with Yb1 through O9i and O11. The bond lengths of O3—
Yb1 and O6—Yb1 are shorter than those of O9—Yb1, O9i—Yb1 and O11—Yb1. The angles of
O1—Yb1—Oeq (eq: O3, O6, O9, O9i and O11) and O1ii—Yb1—Oeq are ca. 84 to 95. The sum
of the angles in the pentagonal plane, O3—Yb1—O6, O3—Yb1—O9, O6—Yb1—O11, O9—
Yb1—O9i, and O11—Yb1—O9i, is very close to 360°. The extrusions above 72° are to
6

compensate for the smaller value of O9—Yb1—O9i and the chelate angles. In contrast to the
coordination geometry of Yb1, the coordination geometry of Yb2 is best described as distorted
octahedral with four oxygens of four different µ3-bridged perchlorate anions, O4, O4iii, O7 and
O7iii, and two carboxylate oxygens, O2 and O2iii, from two 3,5-PDC ligands. The span of the
O2—Yb2 and O2iii—Yb2 bond lengths is in the range of that reported in previous studies for
CUmof-9 [37] and [NH3(CH2)2NH3]0.5[Yb(OBA)2(H2O)] [38]. Apparently, for each octahedral
center, the µ3-bridged perchlorates are a monodentate ligand. The perchlorates that bonded to
Yb2 trans with bond lengths of 2.218(5) and 2.215(5) Å are almost identical. Although the bond
angles O2—Yb2—O7iii, O4iii—Yb2—O4 and O7—Yb2—O7iii are nearly in accord with
octahedral geometry, the other bond angles around Yb2 with values of 85.1(2) and 94.9(2) are
somewhat different. Moreover, the bond lengths show that the octahedral coordination sphere
around Yb2 is distorted from an ideal octahedral environment. Finally, 1 possesses a 2-D
polymeric layer structure and is connected to a 3-D network by supramolecular interactions
(figure 2). 2D layers are separated by 3.21 Å, in which trimethylammonium cations stabilize the
molecular structure by inter-layer NH…O hydrogen bonding and weak CH…O interactions,
forming a 3-D supramolecular network in the ab plane.
3.2.2. Molecular structure of [(CH3)2NH2][Yb4(C24H12N3O6)4(HCO2)(H2O)2]·3H2O (2).
Complex 2 crystallized in the orthorhombic system with the Fddd space group. Selected bond
lengths and angles are listed in table S3. The asymmetric unit contains one crystallographically
distinct Yb(III), one TATB3-, half a water molecule, one-quarter of a formate, one-quarter of a
dimethylammonium, and 0.75 of the uncoordinated water molecule in the chemical formula. The
chemical formula of 2 can be expressed by
[(CH3)2NH2)]0.25[Yb(C24H12N3O6)(HCO2)0.25(H2O)0.5]·0.75(H2O). The chemical formula moiety
is multiplied by four in order to make the formula moiety a whole number, and this may give an
impression that there is more than one type of ytterbium ion.
In 2, the ytterbium is connected to a chelating-bridging -CO2 group as well as to oxygens
of five other bridging -CO2 groups. Disorder resulted from occupation of a coordination site by a
water molecule for half of the time and a unidentate formate for half of the time. This disorder
site along with seven ligating oxygens of the TATB3- ligand brought the coordination number of
ytterbium to eight. However, multiplication of the formula moiety by four can change the
7

interpretation. In the new formula, [(CH3)2NH2][Yb4(C24H12N3O6)4(HCO2)(H2O)2]·3H2O, three
species, Yb1−formate−Yb1, Yb1(water) and Yb1i(water), overlapped. Similarly, the eighth
coordination site was occupied by a bridging formate ion for half of the time (figure 3a). Partial
occupancy feature can also explain the in situ catalysis of DMF to formate. In this configuration,
the O1i-O3ii-O5iv and O2-O4iii-O6v planes form the two triangular bases of the prism with
dihedral angle 9.03°. The distances between Yb1 and the centroid of these two bases are 1.476
and 1.457 Å, respectively. This geometry is not regular among the three other possible choices
for eight-coordinate complexes. A half of O1W and half of O7 caps the lateral face of O2-O3ii-
O5iv-O6v, and O1 caps the lateral face of O1i-O3ii-O2-O4iii. The criteria for assignment of
geometry are based on a comparison of the observed polyhedra with the most favorable one
following a set defined by PoraiKoshits and Aslanov, as δ1 = 8.06°, δ2 = 11.58°, δ3 = 37.81,
49.77°, δ4 = 26.45°, 1 = 16.60, 9.75°, 2 = 30.64° and  = 89.97 and 87.84°. The theoretical
values for a bicapped trigonal prism are δ1 = 0°, δ2 = 21.7°, δ3 = δ4 = 48.2°, 1 = 16.1° and  =
83.7° [39, 40]. The coordination geometry can be best described as a distorted bicapped trigonal
prism or square-face bicapped; however, it should be noted that the Yb1 coordination geometry
can be described as an intermediate between two ideal forms, i.e., square antiprism (δ1 = 0°, δ2 =
0°, δ3 = δ4 = 52.5°, 1 = 24.5°,  = 77.4°) and bicapped trigonal prism. The Yb–O bond lengths
vary from 2.219(6) to 2.878(8) Å, and a mean value for Yb-O distances, as statistically analyzed
in version 1.18 of ConQuest, is 2.326 Å with a maximum of 3.187 and minimum of 1.708 Å.
This is in the normal range of values observed for other compounds. The O–Yb–O bond angles
are 49.0(2)154.3(3)°. Each carboxylate connects two identical YbIII ions and the YbO8
polyhedron constructed a 1D inorganic chain with Yb…Yb distances of 4.086 and 5.343 Å and
extended in two directions that are almost mutually perpendicular. This compound represents
another example of MOFs constructed with rod-shaped SBUs. Three SBUs are connected by
rigid TATB3- to form a 3D coordination framework (figure 3b). The formate ions also connect
two SBUs in different orientations, but merely consolidate the network. The dihedral angles
between the central triazine and the three benzene rings are 3.00°, 4.77° and 18.89° and the
dihedral angles among the three benzene rings are 7.59°, 19.84° and 20.33°. The dihedral data
indicate that the TATB3- is rather unsymmetrical and not as flat as reported [41, 44]. Herein, the
solvent-accessible volume (SAV) was calculated by PLATON analysis as 1491.6 Å3, which is
8.2% of the unit-cell volume. There are eight voids with approximately 170 Å3 in the unit cell.
8

The low porosity can be associated with the higher coordination number (eight) of the Yb1 ions.
S-triazine rings of each TATB3- were rotated and oriented to stack on top of one another to form
a parallel-displaced - stacking, which can help improve thermal stability.
3.3. X-ray powder diffractions
The powder X-ray diffraction (PXRD) patterns of compounds were recorded and compared to
those generated from corresponding single-crystal data (figure 4). The good correlation between
the simulated patterns and the ones obtained from PXRD indicates the high phase purity of the
solids. The slight differences between some of the reflection intensities of the simulated and
experimental patterns can be attributed to the variation in the orientation of the powder samples
during collection of the experimental PXRD data.
3.4. Infrared spectroscopy
The symmetric and asymmetric stretching vibrations of carboxylate in the FT-IR spectrum of 1
(figure 5a) were observed at 1470 and 1610 cm-1, respectively. The  (as-s) value is below
200 cm-1, which indicates that carboxylate coordinated bidentate to ytterbium [45]. Meanwhile,
the stretching vibrations of perchlorate were observed at 1096 and 629 cm-1, confirming the
formation of perchlorate during synthesis [46].
In contrast to the FT-IR spectrum of 1, the FT-IR spectrum of 2 is attributed to formation
-
of HCO2 , and various coordination modes of the carboxylate are present. The FT-IR spectrum of
2 (figure 5b) showed three intense bands at 1622, 1544 and 1401 cm-1 in addition to three
shoulders at 1561, 1508 and 1359 cm-1 in the carbonyl region. These imply two coordination
modes for the carboxylate ligand, as confirmed by single-crystal X-ray analysis. Furthermore,
the FT-IR spectrum showed bands at 2921 and 2847 cm-1 which correspond to the C-H
stretching of formate ions [47]. The formation of formate is associated with the hydrolysis or
decarboxylation of DMF, which is very common in high-temperature solvothermal synthesis
[48]. For further clarification of the formate ion decomposition and its conversion to CO2 during
heat treatment, as noticed in TGA-DTA analyses (in situ), the FT-IR spectrum of the compound
after heat treatment at 350 C was recorded. As figure 5c shows, the C-H stretch of the formate
ion after heat treatment collapsed, which indicates decomposition of the formate ion. Notably,
identical X-ray powder diffraction patterns of 2 before and after heat treatment prove that the
9

structure remains intact after heat treatment.
3.5. Thermal analysis
To investigate the thermal stability of the prepared compounds for possible use as catalysts and
precursors for fabrication of ytterbium oxide, thermal analyses (TGA-DTA) were carried out in
air from 30-750 °C. As the TGA curve of 1 in figure 6a shows, there is a slight weight loss from
150-200 C. The weight loss is accompanied with an endothermic peak in the same temperature
range, which is attributed to loss of solvent. Furthermore, the TGA curve shows about 53 wt.%
loss of 1 from 350-500 C. This weight loss, along with an exothermic peak in the DTA curve in
the same temperature, implies collapse of the structure due to combustion of the 3,5-PDC and
reflects the stability of the complex up to 300 °C.
The thermal behaviors of 1 and 2 are similar to some extent. However, the latter does not
exhibit significant weight loss to 400 C, which illustrates the higher stability of 2 with respect
to 1. This higher stability of 2 could be attributed to its 3-D network. As figure 6b shows, there
are two distinct weight losses for 2 with a total weight loss of approximately 64%. The first
weight loss of 11% up to 440 C, along with an endothermic peak in the DTA curve centered at
380 C, can be attributed to the decomposition of dimethyl ammonium formate and its
transformation to CO2 and the release of trapped and coordinated water without collapse of the
structure up to 440 C. The second weight loss at 460-670 C is associated with the combustion
of the TATB-3 as indicated by the exothermic peak in its DTA curve.
3.6. Surface area analysis
The N2 adsorption-desorption behavior of 2 after activation was examined at 77 K. Analysis of
isotherm (figure S1) led to the BET surface area of 5.6 m2/g (Langmuir 50.8 m2/g), a pore
volume of 0.0097 cm3/g and a pore size of 1.2 nm. The low surface reveals that nitrogen
molecules with a kinetic diameter of 3.64 Å scarcely adsorb on the activated compound owing to
its narrow windows [49, 50]. As shown by the isolated yellow spheres in figure 7, there are no
open channels to connect pores and this reflects in the low surface area of 2. Such a low N2
adsorption isotherm was previously reported for isostructures of a series of multifunctional
lanthanide-organic frameworks [33].
10

3.7. Catalytic activity studies of prepared compounds
The amide functionality is one of the important factors in the preparation of drugs and
biopolymers. The amides were traditionally prepared by activation of a carboxylic acid by toxic
and corrosive reagents such as thionyl or oxalyl chlorides in high yields and conversions, but this
process is not environmentally friendly. Therefore, to avoid these adverse effects, other
methodologies, such as oxidative amidation of alcohols and aldehyde, were introduced.
Preparation of benzamide derivatives through the oxidative amidation reaction from aldehydes
and amine hydrochlorides is quite slow in the absence of catalysts. Recently, to advance the
related methodologies, catalytic amidation of aldehydes with amines by homoleptic lanthanide
amido complexes has been investigated [51-54]. The promising catalytic activities of lanthanide
complexes also encouraged us to explore the catalytic behavior of the new Yb-coordination
polymers in the oxidative amidation reaction of aldehydes with benzylamine hydrochloride.
A simple comparison between two newly synthesized coordination polymers showed that
2 has the potential to act as a Lewis acid site; therefore, it was a better candidate for
heterogeneous metal-based catalysis in reactions such as an oxidative amidation. In the outset of
our work, we utilized a model reaction of benzaldehyde and benzylamine hydrochloride to form
N-benzylbenzamide in order to evaluate the catalyst performance of the prepared complexes at
room temperature with TBHP, as an oxidant. As can be seen from the entries in table 1, the
presence of a catalyst, TBHP, and CaCO3 is essential to obtain acceptable yields. As entries 2
and 3 show, 2 provided a much higher yield of amide in comparison with 1. The higher activity
can be attributed to its pore and framework structure. To study the role of the solvent in the yield,
the reaction was carried out using various solvents, and the yields were obtained in the order of
acetonitrile > dioxane > water > tert-butanol > tetrahydrofuran > toluene. The role of the solvent
in the yield is not well understood at this time; however, the higher stability of 2 with the
framework structure in acetonitrile can be speculated. Finally, the efficacy of 2 as the catalyst in
the oxidative amidation reaction of benzaldehyde with benzylamine hydrochloride is compared
with several previously reported catalysts for the same reaction. As table 2 shows, the efficiency
is superior or at least equal, but unfortunately, the base and oxidants are essential for the catalyst.
The catalyst recycling test was also performed for the model reaction, and it was found that after
four consecutive runs, the isolated yield drops slightly, only 2-3%. With these optimized
conditions, this reaction was further explored and generalized with various aldehydes and
11

benzylamine hydrochloride, and fairly good yields were obtained (table 3). It seems that the
conversions of benzamide derivatives were slightly changed by the electron-donating or
electron-withdrawing groups.
The plausible reaction mechanism for the oxidation amidation of aldehydes, as shown in
scheme 2, begins with deprotonation of the amine hydrochloride salt to produce the
corresponding amine. Compound 2 after activation creates open metal coordination sites.
Therefore, it can act as a Lewis acid site for the activation of both substrates. The amine is
reacted with ytterbium Lewis-acid sites to form a ytterbium amido complex, which consequently
reacts with the aldehyde to afford the reactive species B. Further reaction of B with another
amine yields C, which may be oxidized by the oxidant to generate the desired product [54, 60].
Cl-
+H3N
H2N
CaCO3
+
Activated MOF
Yb(MOF)
(MOF)Yb
N
N
O
N
Yb(MOF)
(A)
(MOF)Yb
H
TBHP
O
HN
H
Yb(MOF)
H
O
HN
H
O
(C)
Yb(MOF)
HN
(B)
H2N
Scheme 2. The plausible reaction mechanism for oxidation amidation of aldehydes.
12

4. Conclusion
We have prepared two coordination polymers under hydrothermal conditions. Compound 1
features a two-dimensional layered anionic structure with trimethethylammonium cations
between the layers, and 2 exhibits a metal-organic framework with tritopic H3TATB ligands.
Single-crystal diffraction studies showed that ligand preferences through complexation with
ytterbium ions are as follows: H3TATB ≥ 3,5-PDC ≥ H3BTB. In the preparation of both
compounds, one polytopic ligand acts as an auxiliary modifying agent in the reaction. The results
reveal that the carboxylic ligands show a significant role in the formation of high-quality
crystals, and it seems that polycarboxylate molecules can also be used as a modifier. Finally, 2
with the framework structure acted as an efficient catalyst in the formation of different
benzamides from benzaldehyde derivatives.
Supplementary materials
Electronic Supplementary Information (ESI) available: synthesis of 1,3,5-triphenylbenzene,
synthesis of 1,3,5-tri(4',4',4''-acetylphenyl)benzene, synthesis of 4,4',4''-benzene-1,3,5-triyl-
tribenzoic acid (H3BTB), synthesis of 2,4,6-tri-p-tolyl-1,3,5-triazine, synthesis of 4,4′,4″-s-
triazine-2,4,6-triyl-tribenzoic acid (H3TATB), crystal data for 1 and 2 (table S1), selected bond
lengths and angles for 1 (table S2) and selected bond lengths and angles for 2 (table S3).
Crystallographic data of 1 and 2 have been deposited with the Cambridge Crystallographic Data
Centre, CCDC No. 151026 (1) and 1517025 (2). These data can be obtained free of charge from
the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgement
The authors thank the Vice-President’s Office for Research Affairs of Shahid Beheshti
University for supporting this work.
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16

Figure 1. The coordination environment of ytterbium ions in 1 with the ellipsoids drawn at the
30% probability level.
17

Figure 2. The dotted lines indicate inter-layer NH…O hydrogen bonding and weak CH…O
interactions with the ellipsoids drawn at the 30% probability level.
18

(a)
(b)
Figure 3. The coordination environment of ytterbium ions in 2 with the ellipsoids drawn at the
30% probability level (a); SBUs are connected by rigid TATB3- ligands to form a 3D
framework (b).
19

Figure 4. Simulated and experimental powder X-ray diffraction patterns of 1 and 2.
20

Figure 5. FT-IR spectra of 1 (a) and 2 before (b) and after activation (c).
21

Figure 6. TGA-DTA analyses of 1 (a) and 2 (b).
22

Figure 7. The yellow spheres illustrate the isolated volume fitted inside the pores without
contacting van der Waals surface. Yb green, O red, N blue, C gray and H white.
23

Table 1. Catalytic activity of 2 in oxidative amidation of benzaldehyde
with benzylamine hydrochloride saltsa.
Entry
Solvent
Time (h)
Yield (%)
1
CH3CN
CH3CN
CH3CN
H2O
16
16
16
16
16
16
16
16
12
8
<5
87
21c
47
39
16
73
11
79
65
48
2
3
4
5
t-BuOH
THF
6
7
Dioxane
Toluene
CH3CN
CH3CN
CH3CN
8
9
10
11
4
(a) Reaction conditions: catalyst (0.03 mmol), benzaldehyde (2 mmol),
benzylamine hydrochloride salt (2.4 mmol), base (2.2 mmol), TBHP
(70 wt.% in water, 2.2 mmol), solvent (2 mL), at room temperature, base:
CaCO3; (b) isolated yield; (c) compound 1.
24

Table 2. Comparison of the catalytic efficiency of 2 in oxidative amidation of benzaldehyde with several reported
catalysts.
Entry
1
Catalyst
Solvent
CH3CN
T (°C) Time (h)
Yield (%)
78
Ref.
[55]
Fe(NO3)3.9H2O (0.05 mmol), CaCO3
(1.1 mmol) and TBHP (1.1 mmol)
60
60
25
60
60
40
25
25
16
6
2
MagCNTs@SiO2–linker–CuI (20 mg),
Na2CO3 (2 mmol.), TBHP (1.5 mmol)
CH3CN
85
86
72
78
71
75
87
[56]
[57]
[58]
[59]
[60]
[61]
3
CuSO4·5H2O (5.0 mol%), CaCO3 (1.1 mmol), CH3CN
TBHP (1.1 mmol)
24
16
6
4
FeSO4·7H2O (5.0 mol%), CaCO3 (1.1 mmol), CH3CN
TBHP (1.1 mmol)
5
Na2CO3 (2 mmol.), TBHP (1.5 mmol), Cu/C
(3.68 mol%)
CH3CN
CH3CN
CH3CN
6
CaCO3 (1.1 mmol), TBHP (1.1 mmol), CuI
(1.0 mol%), AgIO3 (1.0 mol%)
6
7
CuBr (20 mol%), CaCO3 (2.5 mmol),
PhI(OAc)2 (50 mol%), TBHP (1.0 mmol)
24
16
10
Compound 2 (1.5 mol%), CaCO3 (1.1 mmol), CH3CN
This
TBHP (1.1 mmol)
work
25

Table 3. Compound 2-catalyzed synthesis of amides from aldehydes and benzylamine
hydrochloride salts.
Entry
R1
R2
Yield (%)
87
Melting point
Ref.
[62]
1
2
H
H
H
101-103/102-104
138-140/137-139
4-Me
69
[63]
3
4
5
4-OMe
4-Cl
H
H
H
74
71
67
127-129/128-131
162-164/161-163
165-167/166-168
[64]
[65]
[65]
4-Br
6
4-NO2
H
65
134-137/134-137
[64]
26

27