Cu(II)/Cu(II)-Mg(II) containing pyridine-2,5-dicarboxylate frameworks: Synthesis, structural diversity, inter-conversion and heterogeneous catalytic epoxidation

Article abstract of DOI:10.1016/j.poly.2018.02.023

Ligand concentration dependent structural diversity and inter-conversion of hydrothermally synthesized metal carboxylate compounds, {[Mg(H₂O)₆][Cu(pdc)₂]·2H₂O}_n (1), {[CuMg(pdc)₂(H₂O)₄]·2H₂O}_n (2), {2(Him)·[Cu(pdc)₂]}_n (3), and {[Cu(pdc)(im)₂]·2H₂O}_n (4) (H₂Pdc = pyridine-2,5-dicarboxylic acid and im = imidazole), have been thoroughly investigated. Formation of compounds 1–4 depends on the concentration of imidazole. Structure of all four compounds has been verified by single crystal X-ray diffraction and other physicochemical studies. Compound 3 catalyzes olefin epoxidation reaction in heterogeneous condition.

Full text of DOI:10.1016/j.poly.2018.02.023

Accepted Manuscript
Cu(II)/Cu(II)-Mg(II) Containing Pyridine-2,5-dicarboxylate Frameworks: Syn-
thesis, Structural Diversity, Inter-conversion and Heterogeneous Catalytic Ep-
oxidation
Debraj Saha, Saikat Gayen, Subratanath Koner
PII:
S0277-5387(18)30107-4
https://doi.org/10.1016/j.poly.2018.02.023
POLY 13058
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
10 January 2018
23 February 2018
Please cite this article as: D. Saha, S. Gayen, S. Koner, Cu(II)/Cu(II)-Mg(II) Containing Pyridine-2,5-dicarboxylate
Frameworks: Synthesis, Structural Diversity, Inter-conversion and Heterogeneous Catalytic Epoxidation,
Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.02.023
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Cu(II)/Cu(II)-Mg(II) Containing Pyridine-2,5-dicarboxylate
Frameworks: Synthesis, Structural Diversity, Inter-conversion
and Heterogeneous Catalytic Epoxidation
Debraj Saha*,†, Saikat Gayen‡, Subratanath Koner*,‡
†Department of Chemistry, Darjeeling Government College, Darjeeling 734101, India
‡Department of Chemistry, Jadavpur University, Kolkata 700032, India
Abstract
Ligand concentration dependent structural diversity and inter-conversion of
hydrothermally
synthesized
metal
carboxylate
compounds,
(2),
{[Mg(H2O)6][Cu(pdc)2]∙2H2O}n
(1),
{[CuMg(pdc)2(H2O)4]∙2H2O}n
{2(Him)∙[Cu(pdc)2]}n (3), and {[Cu(pdc)(im)2]∙2H2O}n (4) (H2Pdc = pyridine-2,5-
dicarboxylic acid and im = imidazole), have been thoroughly investigated. Formation of
compounds 1-4 depends on the concentration of imidazole. Structure of all four
compounds has been verified by single crystal X-ray diffraction and other
physicochemical studies. Compound 3 catalyzes olefin epoxidation reaction in
heterogeneous condition.
1. Introduction
Metal organic framework (MOF) is nothing but a symmetric arrangement of
metal ions which are connected through organic linkers and extended into 1, 2 and 3-
dimensional networks via coordination bonding and other weak bonding interactions [1].
MOF has wide range of application potential in various fields, including gas adsorption,

gas storage, gas separation or gas purification, membranes and coating, sensing,
luminescence, drug delivery, artificial nucleases for the hydrolytic cleavage of DNA and
RNA, magnetic, catalysis etc. [2-11]. Designing of metal organic framework has now
reached at higher level of sophistication, however, challenges still remain to control over
the structure of the material that deals with supramolecular chemistry and crystal
engineering [12-13]. There are various synthesis methods to design metal organic
frameworks (MOFs) such as diffusion based crystallization and hydro-, solvo- and
ionothermal synthesis etc. Hydrothermal method is one of the most popular methods
utilized in the preparation of MOFs [14]. However, the control of products in
hydrothermal reactions pose great challenge to chemists, since there are many factors
influence the crystallization of MOFs and the structural motifs that acquire by the
prepared compound. This includes the solvent and reaction temperature, pH of the media,
molar ratio of raw materials, reaction time, and so on [15-19]. Therefore, investigations to
get deeper insight into the interplay between the structure of compounds and various
stimuli are still important.
A variety of reactions have been studied using MOF as catalyst [20]. C=C bond
functionalization reaction is one of the important reactions in organic chemistry that can
lead to a diverse array of derivatives. Epoxidation is one of these types of reactions, and
epoxides are active intermediates for industrial applications as feedstock for value added
products [21]. Olefin epoxidation catalyzed by transition metal complexes has been a
subject of great interest in the past few decades [22]. In this context several copper
catalyzed epoxidation reaction have been reported [23]. Usually epoxidation is carried
out in the presence of an oxidant such as hydrogen peroxide, TBHP or peracids.
2

Hydroperoxides in combination with suitable catalysts constitute an adequate system
from the viewpoint of green chemistry.
In this study, we report our research over the structural inter-conversion amongst
four metal carboxylate framework compounds, {[Mg(H2O)6][Cu(pdc)2]∙2H2O}n (H2Pdc
=
pyridine-2,5-dicarboxylic
acid)
(1),
{[CuMg(pdc)2(H2O)4]∙2H2O}n
(2),
{2(Him)∙[Cu(pdc)2]}n (im = imidazole) (3), and {[Cu(pdc)(im)2]∙2H2O}n (4). Synthesis,
X-ray crystal structure and catalytic efficacy of the new compound 3 in olefin
epoxidation reaction are also described here. Synthesis, characterization and catalytic
efficacy of others compounds were reported elsewhere [24].
2. Experimental section
2.1. Materials
Pyridine-2,5-dicarboxylic acid, magnesium nitrate hexahydrate, copper nitrate
trihydrate, imidazole and alkenes were purchased either from Sigma–Aldrich or Merck
(India) and were used as received. Ethanol was purchased from Merck (India) and was
distilled and dried before use.
2.2. Physical measurements
Elemental analysis was performed on a Vario-Micro V2.0.11 elemental (CHNSO)
analyzer. Fourier-transformed infrared spectra of KBr pellets were measured using a
Perkin Elmer RX I FT-IR spectrometer. PXRD was recorded on a Bruker D8 Advance X-
ray powder diffractometer (PXRD) equipped with Cu-Kα radiation (λ = 1.54 Å). In situ
temperature-resolved PXRD patterns were performed using a similar device equipped
with a high-temperature chamber (Anton Paar HTK 16N) and TCU 2000N temperature
3

control unit using Cu-Kα radiation. The sample was loaded on a Pt plate and heated at a
rate of 5 °C min−1 under nitrogen. TG-DT analysis was made using a PerkinElmer
(SINGAPORE) Pyris Diamond TG/DTA unit. The heating rate was programmed at 5°C
min-1 with a protecting stream of N2 flowing at a rate of 150 mL min-1. The metal content
of the sample was estimated on a Varian Techtron AA-ABQ atomic absorption
spectrometer. The products of the catalytic reactions were identified and quantified by a
Varian CP-3800 Gas Chromatograph using a CP-Sil 8 CB capillary column.
2.3. Synthesis of the compounds
All compounds were prepared from same reaction mixture. Pyridine-2,5-
dicarboxylic acid (0.167 g, 1 mmol) and imidazole first added in 10 ml of milli-Q water
and the mixture was stirred for 0.5 h, to this mixture magnesium nitrate hexahydrate
(0.128 g, 0.5 mmol) and copper nitrate trihydrate (0.120 g, 0.5 mmol) were added and the
final mixture was stirred for 0.25 h. Compounds were obtained in crystalline form in a 20
ml capacity Teflon-lined acid digestion bomb, at 175 °C for 3 days followed by slow
cooling at a rate of 5 °C h−1 to room temperature. Compounds 1, 2, 3 and 4 were obtained
respectively from 1, 2, 3 and 4 mmol of imidazole. For characterization of the bulk
compound, PXRD study (Fig. S1), elemental analysis and IR spectroscopic study were
undertaken. Blue block crystals of compound 3 suitable for X-ray analysis were obtained
in about 62% yield based on Copper. Anal. Calc. for {2(Him)∙[Cu(pdc)2]}n 3: C, 45.16%;
H, 3.03%; N, 15.80%. Found: C, 45.22%; H, 3.05%; N, 15.73%. Selected IR peaks (KBr
-
-
disk, ν, cm-1): 1616, 1555 [υas (CO2 )], 1396 [υs (CO2 )], 1336, 1274 [υs (C-O)], 3112-
3160 [υs (N-H)], and 3457 s.br [υ (O-H)].
4

PXRD patterns (Figs. S2, S3 and S4), elemental analysis data and IR
spectroscopic data of other compounds were given in supporting information.
2.4. X-ray crystallography
X-ray diffraction data for 3 was collected at 293K on a Bruker SMART APEX
CCD X-ray diffractometer using graphite-monochromated MoKα radiation (λ =
0.71073Å). Determination of integrated intensities and cell refinement were performed
with the SAINT software package using a narrow-frame integration algorithm [25]. An
empirical absorption correction (SADABS) was applied [26]. The structure was solved by
direct methods and refined using full-matrix least-squares technique against F2 with
anisotropic displacement parameters for non-hydrogen atoms with the programs
SHELXS97 [27] and SHELXL2014/7 [28]. The C-bound Hydrogen atoms were freely
refined. In the final difference Fourier maps there were no remarkable peaks except the
ghost peaks surrounding the metal centers. A summary of crystal data and relevant
refinement parameters for compound 3 is given in Table S1. X-ray diffraction data for
compound 1, 2, and 4 were also collected in the same manner as of compound 3 as
described above to determine the cell parameters (Table S1) and matched with the
corresponding data of previously reported {[Mg(H2O)6][Cu(pdc)2]∙2H2O}n (1),
{[CuMg(pdc)2(H2O)4]∙2H2O}n (2), and {[Cu(pdc)(im)2]∙2H2O}n (4) [24].
2.5. Catalytic reaction
The catalytic reactions were carried out in a glass batch reactor according to the
following procedure. Substrate (10 mmol), ethanol (10 ml) and catalysts (2 mg) were first
mixed. The mixture was then equilibrated to desired temperature in an oil bath. After
addition of hydrogen peroxide (14.4 mmol, 1.5 equiv.) the reaction mixture was stirred
5

continuously. Reactions were performed in open air. The products of the reactions were
collected at different time intervals and were identified and quantified by gas
chromatography.
3. Results and discussion
3.1. Imidazole dependent structural diversity and inter-conversion
Compound 1, 2, 3 and 4 were synthesized from an identical reaction mixture with
different imidazole concentration (Fig. 1). All compounds except 4 crystallized in
triclinic space group P-1 with Z = 1. Increase of the imidazole concentration led to the
formation of compounds 1 to 4. Basic structure of 1, 2 and 3 are same. They are 1-D
coordination polymer contains copper(II) ions connected by pdc dianions forming
{[Cu(pdc)2]2-}n ribbon like chains. Coordination geometry of copper(II) in 1, 2 and 3 is
distorted octahedron where two nitrogen atoms and two oxygen atoms from two chelating
pdc dianions formed the basal plane of the octahedron while axial positions are occupied
by two oxygen atoms from two different pdc dianions. When 1 mmol of imidazole is
used, compound 1 is formed where negative charge of {[Cu(pdc)2]2-}n is satisfied by the
[Mg(H2O)6]2+ ions and ultimately forming a hydrogen bonded 2D network through
hydrogen of coordinated water molecule and free carboxylato oxygen atom. Upon
increasing the amount of imidazole to 2 mmol, hydrothermal treatment of the reaction
mixture afforded compound 2 where {[Cu(pdc)2]2-}n chains are linked with Mg(II) ions
through carboxylate oxygen atoms and a 2D network was formed. In compound 2, free
carboxylate oxygen atoms present in compound 1 involve in bonding with Mg(II) ions
and replace two coordinated water molecules of Mg(II) ions present in 1. Upon adjusting
6

the amount of imidazole to 3 mmol, magnesium ions were replaced by the imidazolium
ions and compound 3 was thus formed. Structure of compound 3 is close resembles with
compound 1 but only difference is that the place of hexa-aqua Mg(II) ions are occupied
by uncoordinated imidazolium ions. When amount of the imidazole was adjusted to 4
mmol, imidazole coordinates to the Cu(II) ion and compound 4 was formed. In
compound 4, each Cu(II) ion is coordinated by two nitrogen atoms from two imidazole
molecules, one carboxylato oxygen atom and one nitrogen atom from a pdc dianions and
forms a chelate, and one carboxylato oxygen atom from another pdc2- ligand. Here, pdc2-
ligand connects two copper(II) ions in a similar fashion as in other compounds forming
1D chain of distorted square pyramidal Cu(II) coordination geometry. Another difference
is that instead of four connecting pdc2- ligands with one Cu(II) center, here involve only
two.
Structural inter-conversions are shown in Fig. 1. Upon increasing imidazole
concentration 1 was converted to 2, 3 and 4 through hydrothermal treatment. Similarly 3
was converted to 4. But our attempts in converting 2 to 3 and 2 to 4 were not successful.
It is pertinent to mention that in presence of excess magnesium nitrate, compound 3
converted to 1 upon hydrothermal treatment.
3.2. X-ray structure of {2(Him)∙[Cu(pdc)2]}n (3)
Compound 3 crystallizes in the triclinic space group P-1 with Z = 1 and possesses
a polymeric ribbon like 1D chain structure consist of copper(II) ion and carboxylate
linker. The chain propagates parallel to the crystallographic a axis (Fig. 2). Chain is
anionic in nature and the overall charge is balanced by uncoordinated imidazolium
cations. Crystallographically unique [Cu(pdc)2]2- units construct the ribbon, where each
7

Cu2+ adopts tetragonally distorted octahedron geometry (Fig. 3). An ORTEP diagram
with atom-numbering scheme is shown in the Fig. S5. Selected bond distances and the
bond angles are collated in Table S2. Four donor atoms (N1, O3 and cN1, cO3; c = –x, –y,
–z) from two chelating pdc2- ligands occupy the equatorial positions where N1 and O3 of
pdc2- ligand form chelate with metal center and axial positions are occupied by two
d
carboxylato oxygen atoms (aO1 and O1; a = –1+x, y, z; d = 1–x, –y, –z) from two
different pdc2- ligands. The pdc2- ligand shows μ3 connectivity mode. In one site it form a
chelate with copper center through one heterocyclic nitrogen atom (N1) and one
carboxylato oxygen (O3) atom adjacent to ring nitrogen and in another site it links with
another copper center through oxygen (O1) of different carboxylate group. Among the
four carboxylato oxygen atoms of pdc2- ligand, two (O2 and O4) remain uncoordinated.
Those 1-D chains are connected through imidazolium ions by intermolecular H-bonding
between hydrogen bonded with the ring nitrogen of imidazolium ion and free carboxylato
oxygen atom of pdc2- ligand resulting a 2D supramoleculer structure (Table S3 and Fig.
2).
3.3. Thermal analysis of compound 3
TG measurement confirms that the compound 3 was thermally stable up to around
325 °C (Fig. S6). Compound 3 shows a single stage mass loss. A sharp mass loss at
around 325 °C indicates the decomposition of the compound 3. The corresponding DTA
curve of compound 3 shows an endothermic peak centered at 353 °C. The final solid
product of thermal decomposition was identified as CuO (Fig. S7) with total mass loss of
88.57 % (calcd. 88.05 %). Being a 1D chain structure, the high thermal stability of this
MOF can be explained in terms of the salt like nature, rigidity of the linkers, strong bonds
8

between the metal ions and the organic linkers, absence of solvent molecules, etc.
Thermal stability of compound 3 also verified by variable temperature PXRD study.
Major experimental PXRD peaks of compound 3 are well matched with the simulated
one up to ca. 300 °C (Fig. 4).
3.4. Catalytic activities of compound 3
Catalytic epoxidation of alkenes using compound 3 was carried out at 60 °C in
heterogeneous conditions. We examined the epoxidation of 1-hexene and cyclooctene in
six different solvents to find the optimal solvent. The best conversion and selectivity were
found in ethanol. Although acetonitrile acts as very good solvent, but selectivity is less in
case of styrene and cyclohexene epoxidation. The conversion increases in the order
CH2Cl2<CHCl3<MeOH<CH3COCH3<EtOH<CH3CN (Fig. S8). But the selectivity
increases in the order CH2Cl2<CHCl3<CH3COCH3<CH3CN<MeOH<EtOH for styrene
and cyclohexene (Fig. S9). To monitor the temperature effect, we tuned the reaction
temperature from 30–78 °C for the epoxidation of cyclooctene and 1-hexene, a good
yield with good product selectivity was obtained at a moderate temperature of 60 °C (Fig.
S10) in ethanol. At higher temperatures the conversion decreases, as H2O2 starts to
degrade at higher temperatures. The epoxidation of alkenes were studied here goes
smoothly, showing an excellent conversion (80% to 100%) to form epoxides as the major
product. Results of the catalytic epoxidation of different substrates are given in Table 1.
Oxidation of cyclohexene proceeded smoothly, showing 100% conversion after 8 h of
reaction to form epoxycyclohexane with ca. 72% selectivity and some amount of alkenes
were converted to 2-cyclohexene-1-ol and 2-cyclohexene-1-one. Cyclooctene was
effectively converted to epoxycyclooctane with ca. 100% selectivity and conversion. In
9

case of linear alkenes, 1-hexene shows almost complete conversion and producing
epoxide as the sole product, but in case of 1-octene the conversion was not complete; the
conversion only limited to 80% after 8 h of reaction. It suggests that the catalytic activity
decreases along with chain length of alkene. This may due to the larger hexyl group of 1-
octene connected to double bond sterically hinders it in approaching to the active site
with respect to 1-hexene in which its double bond carries a smaller butyl group.
Consequently, the conversion was lowered in catalytic reaction. To study conjugated
olefins, styrene, 4-methylstyrene and trans-stilbene were used. Styrene and 4-
methylstyrene were converted with 100% efficiency, and the product selectivity remained
around 78 and 70%, respectively, with benzaldehyde as the usual corresponding bi-
product. The epoxidation of trans-stilbene with H2O2 gives stilbene oxide in 82%
conversion but 100% selectivity. A different substrate, limonene, was also subjected to
oxidation reaction; 100% conversion with a product selectivity of 86% and along with
some unidentified products. A graphical representation of relative efficacy of 3 for the
epoxidation of various alkenes in ethanol with time is given in Fig. S11. Compound 1
catalyses the same reaction but in homogeneous condition [24a]. Therefore, it can be said
that counter ion in compound 3 and also the nature of supramolecular interactions play
crucial role to maintain heterogeneity of the reaction. To ascertain the heterogeneous
nature of catalyst, a hot filtration test was performed. The liquid phase of the reaction
mixture was collected by filtration after ca. 36% of the reaction was completed for
epoxycyclooctane. The supernatant solution was then kept in the reaction conditions for
another 24 h. Almost no progress of the reaction was observed during this period, which
excludes the presence of active species in the solution. Besides, Atomic absorption
10

spectrometric analysis (sensitivity up to 0.001 ppm) of the supernatant solution of
reaction mixture thus collected by filtration confirms the absence of copper ions in the
liquid phase. For the recycling study, catalyst was recovered by centrifugation, washed
thoroughly with ethanol, then activated at 140 °C under vacuum for 3 h, and reused five
times using cyclooctene. All repeated reactions gave almost identical conversion (Fig. 5).
The IR spectra and PXRD patterns of a fresh and a used sample of the catalyst did not
show any significant differences which convincingly demonstrated that its structural
integrity is well maintained after several cycles of reactions (Figs. S12, S13).
4. Conclusion
In essence, we have investigated structural inter-conversion of four metal
carboxylate
compounds,
{[Mg(H2O)6][Cu(pdc)2]∙2H2O}n
{2(Him)∙[Cu(pdc)2]}n (3),
(1),
{[CuMg(pdc)2(H2O)4]∙2H2O}n
(2),
and
{[Cu(pdc)(im)2]∙2H2O}n (4) (H2Pdc = pyridine-2,5-dicarboxylic acid and im =
imidazole). All are synthesized through hydrothermal route. All of them except
compound 4 crystallized in same space group and there is a close similarity of some of
the basic structural features among them. Compound 3 catalyzes olefin epoxidation
reaction in heterogeneous condition where H2O2 acts as oxidizing agent.
Acknowledgements
D. Saha acknowledges University Grants Commission, New Delhi, India for
financial assistance (Minor Research Project No. F. PSW-188/15-16 (ERO)). S. Gayen is
thankful to the University Grants Commission (UGC), New Delhi, (Award letter No.
11

22/12/2013(ii)EU-V) through National eligibility test (NET) Fellowship. Funding
received from UGC in a major project (to SK) is also gratefully acknowledged.
Supplementary data
CCDC 1586363 contains the supplementary crystallographic data for compound 3. These
data can be obtained free of charge via http:// www.ccdc.cam.ac.uk/conts/retrieving.html,
or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary
data associated with this article can be found, in the online version, at
http://xxxxxxxxxxxxxxxxxxx.
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18

Fig. 1. Imidazole dependent structural variation and transformation
Fig. 2. 1D Chain of compound 3. Dotted lines are showing hydrogen bonded 2D net.
19

Fig. 3. Metal-ligand coordination of compound 3
20

Fig. 4. Variable temperature PXRD pattern of compound 3
21

Fig. 5. Catalytic activity of 3 in different catalytic cycles for cyclooctene oxidation.
Reaction conditions are the same as given in the footnote of Table 1.
Table 1. Oxidation of selected olefins by H2O2 with catalyst 3[a]
% yield of product
Substrate
Conversion (wt %)
100
TOF[b] (h-1)
Epoxide
Others
72
28[c]
332
100
100
-
332
92
80
92
80
-
-
305
265
100
100
78
70
22[d]
30[e]
332
332
22

82
100
86[f]
-
271
332
100
14[g]
[a] Reaction condition: Substrate (10 mmol), ethanol (10 ml) and catalysts (2 mg);
hydrogen peroxide (1.5 equiv.); temperature: 60 °C; reaction time: 8 h; The products of
the reactions were collected at different time intervals and were identified and quantified
by Varian CP-3800 gas chromatograph equipped with an FID detector and a CP-Sil 8 CB
capillary column. [b] Turn over frequency (TOF) = moles converted per moles of active
site per hour. [c] 2-cyclohexen-1-ol and 2-cyclohexene-1-one. [d] Benzaldehyde.
Benzoicacid. [e] 4-Methylbenzaldehyde. [f] Limonene oxide. [g] Unidentified product.
23

Cu(II)/Cu(II)-Mg(II) Containing Pyridine-2,5-dicarboxylate
Frameworks: Synthesis, Structural Diversity, Inter-conversion
and Heterogeneous Catalytic Epoxidation
Debraj Saha*,†, Saikat Gayen‡, Subratanath Koner*,‡
†Department of Chemistry, Darjeeling Government College, Darjeeling 734101, India
‡Department of Chemistry, Jadavpur University, Kolkata 700032, India
Graphical abstract for pictogram
24

Graphical Abstract
Structural
conversion
of
four
metal
carboxylate
compounds,
(2),
{[Mg(H2O)6][Cu(pdc)2]∙2H2O}n
(1),
{[CuMg(pdc)2(H2O)4]∙2H2O}n
{2(Him)∙[Cu(pdc)2]}n (3), and {[Cu(pdc)(im)2]∙2H2O}n (4) (H2Pdc = pyridine-2,5-
dicarboxylic acid and im = imidazole) have been investigate through hydrothermal
reaction. The new compound, 3, has been characterized by single crystal X-ray
diffraction and other physicochemical studies. High thermal stability of compound 3 was
established by variable temperature PXRD study. Compound 3 catalyzes olefin
epoxidation reaction in heterogeneous condition where H2O2 acts as oxidizing agent.
Catalytic activities were tested in different temperature and solvent medium.
25