Synthesis and characterization of a novel phosphonate metal organic framework starting from copper salts

Article abstract of DOI:10.1080/10426507.2013.843004

Increased interest in the area of metal phosphonate inorganic-organic frameworks is exemplified with a high range of applications and a rich synthetic and structural chemistry of these compounds. The synthesis and potential applications of a novel metal p

Full text of DOI:10.1080/10426507.2013.843004

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Phosphorus, Sulfur, and Silicon and the
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Synthesis and Characterization of
a Novel Phosphonate Metal Organic
Framework Starting from Copper Salts
Aurelia Visaa, Bianca Maranescua, Alexandra Bucurb, Smaranda
Iliescua & Konstantinos D. Demadisc
a Institute of Chemistry Timişoara of the Romanian Academy,
Timişoara, Romania
b Analysis and Characterization Department, National Institute for
Research and Development in Electrochemistry and Condensed
Matter, Timisoara, Romania
c Crystal Engineering, Growth and Design Laboratory, Department of
Chemistry, University of Crete, Heraklion, Crete, Greece
Accepted author version posted online: 23 Sep 2013.Published
online: 14 May 2014.
To cite this article: Aurelia Visa, Bianca Maranescu, Alexandra Bucur, Smaranda Iliescu & Konstantinos
D. Demadis (2014) Synthesis and Characterization of a Novel Phosphonate Metal Organic Framework
Starting from Copper Salts, Phosphorus, Sulfur, and Silicon and the Related Elements, 189:5, 630-639,
DOI: 10.1080/10426507.2013.843004
To link to this article: http://dx.doi.org/10.1080/10426507.2013.843004
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Phosphorus, Sulfur, and Silicon, 189:630–639, 2014
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2013.843004
SYNTHESIS AND CHARACTERIZATION OF A NOVEL
PHOSPHONATE METAL ORGANIC FRAMEWORK
STARTING FROM COPPER SALTS
Aurelia Visa,1 Bianca Maranescu,1 Alexandra Bucur,2
Smaranda Iliescu,1 and Konstantinos D. Demadis3
1Institute of Chemistry Timis¸oara of the Romanian Academy, Timis¸oara, Romania
2Analysis and Characterization Department, National Institute for Research and
Development in Electrochemistry and Condensed Matter, Timisoara, Romania
3Crystal Engineering, Growth and Design Laboratory, Department of Chemistry,
University of Crete, Heraklion, Crete, Greece
GRAPHICAL ABSTRACT
Abstract Increased interest in the area of metal phosphonate inorganic–organic frame-
works is exemplified with a high range of applications and a rich synthetic and structural
chemistry of these compounds. The synthesis and potential applications of a novel metal phos-
phonate, namely Cu(II) phenylvinylphosphonate (PVP) is described in this paper. Syntheses
were performed starting from a 1:1 molar ratio of a Cu(NO3)2·6H2O or CuSO4·5H2O and
1-phenylvinylphosphonic acid under hydrothermal conditions at pH values ranging between
2.8 and 3.1. The influence of different counterion for the copper salt used as the Cu(II) source
on the structure and crystallinity of the final product was studied. The obtained copper(II)
phenylvinylphosphonate compounds were characterized by X-ray powder diffraction, FT-IR
Received 18 June 2013; accepted 16 August 2013.
This work was supported by a grant of the Romanian National Authority for Scientific Research,
CNCS—UEFISCDI, project number PN-II-RU-TE-2011-3-0092. The authors would like to thank Dr. eng. Ilia
Gheorghe and Prof. Dr. Mircea Mracec for useful discussions.
Address correspondence to B. Maranescu and K. D. Demadis, Institute of Chemistry Timis¸oara of the
Romanian Academy, 300223, Timis¸oara, Romania, and Crystal Engineering, Growth and Design Laboratory,
Department of Chemistry, University of Crete, Voutes Campus, Heraklion, Crete GR-71003 Greece. E-mail:
biancamaranescu@yahoo.com; bmaranescu@acad-icht.tm.edu.ro; demadis@chemistry.uoc.gr (K.D.D.)
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gpss.
630

NOVEL PHOSPHONATE METAL ORGANIC FRAMEWORK
631
spectroscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and
energy-dispersive X-ray spectrometry (EDAX). A possible crystal structure for the copper (II)
phenylvinylphosphonate products is proposed using semiempirical approaches.
Keywords Copper; metal phosphonates; metal organic frameworks; hydrothermal synthesis;
thermal stability
INTRODUCTION
Modern technological advances need advanced, multifunctional materials with mul-
tiple and diverse properties. Since the first reports in the 1978 by G. Alberti of the layered
metal-organophosphonates,1 the research on phosphonate-based metal organic frameworks
has been intensively investigated because of the great variety of structures available, with
the goal of efficient preparation of functional materials useful in the field of optoelectronics,
gas storage, mechanics, biology, and molecular recognition.2–8
The majority of metal organic frameworks utilize organic carboxylic acids and deriva-
tives. Phosphonate-based metal organic frameworks are a fast growing area among the
various types of MOF materials. The phosphonic acid group exhibits certain similarities
to the carboxylic acid group, such as acidity and ability to coordinate to virtually every
metal cation and the propensity to form bridges between two or more metal ions. How-
ever, there are several profound differences as well. In Scheme 1, a simple comparison is
shown between the carboxylic and phosphonic acid groups. The carboxylic acid shows one
deprotonation step, whereas the phosphonic acid, two. The tetrahedral structure of the P
atom increases the potential of the phosphonate ligand to form metal phosphonate com-
pounds with novel structures. The selection of the organic group R can greatly influence
the properties of the resulting materials. During the last 20 years, it has become evident
that phosphonates metal organic frameworks are fundamentally different from carboxylic
derivatives, and lead to high structural diversity.9
Scheme 1 Carboxylate-based (upper) and phosphonate-based (lower) building units.
The self-assembly of phosphonate metal organic frameworks depends on numerous
structural factors, such as: nature (ionic radius and charge) of incorporated metal, proto-
nation degree, geometry of the phosphonic acid, presence of other coordinating moieties

632
A. VISA ET AL.
on the phosphonate backbone, and synthetic parameters related to the reaction conditions:
temperature, time, reactant molar ratio, pH, and pressure.10–12
A large number of copper complexes containing auxiliary ligands based on nitrogen
play an important role in the structural topology of metal organic frameworks. In order to
act as a linker, the ligand needs to have at least two coordination sites which allow coor-
dination to the building blocks. Frequently used N-containing ligands are: 4,4ꢁ-bipyridine
(4,4ꢁ-bipy), 2,2ꢁ-bipyridine (2,2ꢁ-bipy),13,14 4,4ꢁ-azobisdipyridine (4,4ꢁ-azpy),14,15 1,10-
phenanthroline (phen), 2,2ꢁ:6,2ꢁꢁ-terpyridine (terpy),10,16–18 2,2ꢁ:4,4ꢁꢁ:2ꢁꢁ,2ꢁꢁꢁ-quarterpyridyl,
6ꢁ,6ꢁꢁ-di-2-pyridine (bisterpy),18,19 and 5-(4-pyridyl)tetrazole (azolate or 4-Hpt)21 with
various phosphonic acids: 1-aminoethylidene-diphosphonate, 1-hydroxyethylidene-
diphosphonate, and xylyl-bis-phosphonate linkers.
Only a few copper phosphonates were investigated starting from copper salts
with phosphonic acids without N-containing ligands. Three new copper phosphonates
have been synthesized by Yao et al. from the reaction of copper (II) nitrate with
1-hydroxycyclohexanephosphonic acid. This type of compound has a dinuclear structure
in which two {CuO5} square pyramids are bridged by two {CPO3} tetrahedra through
corner sharing. The dimers are connected through intermolecular hydrogen bonds, form-
ing supramolecular layers.22 Structural chemistry of conjugated π-bonded systems is well
known for their ability to absorb UV light. Zinc and copper vinylphosphonates were char-
acterized by X-ray crystallography.23,24
This paper investigates a novel metal phosphonate inorganic–organic framework
starting from Cu(NO3)2·6H2O or CuSO4·5H2O and 1-phenylvinylphosphonic acid (PVP)
under hydrothermal conditions, with the formation of products named Cu1 (from nitrate
salt), and CuPVP (from sulfate salt). These are compared to previously reported copper
vinylphosphonates (CuVP) by our research group.25 The two synthetic approaches yield
the same monophasic product as evidenced by powder X-ray diffraction patterns. The
presence of the organic functional groups on copper phosphonate is determined by FT-IR
spectroscopy. Scanning electron microscopy (SEM), thermogravimetric analysis (TGA),
and energy dispersive X-ray (EDAX) techniques were also performed. The structure was
further elucidated by quantum chemical calculations.
RESULTS AND DISCUSSION
The sample is characterized by a high degree of crystallinity, judging from the high
intensity of the peak in the 2θ region 8◦–9◦. The presence of intense, narrow peaks in the
pattern is an indication of a monodirectional crystalline growth (Figure 1).
The Cu(O3PCH CH2)·H2O material exhibits a high degree of crystallinity, which is
similar in both compounds Cu1 and CuPVP. The crystallinity of the CuVP sample is higher
than that of Cu1 and CuPVP.
The study of the surface of the crystal gives valuable information about crystal
morphology. The morphology of Cu(O3PC(C6H5) CH2)·H2O (Figure 2) was studied by
SEM, which reveals (at magnification × 1600) that the material is composed of needle-like
crystals. The images also reveal a plate-like crystalline structure, in good correlation with
the previous assumption of a monodimensional/monodirectional growth.
The composition of the aforementioned crystals was studied by EDAX. The spectra
contain small aluminum peaks, due to the Al coating. The EDAX image (Figure 3) provides
information of the elements on phosphonate metal organic framework: copper, phosphorus,
oxygen, and carbon are clearly identified.

NOVEL PHOSPHONATE METAL ORGANIC FRAMEWORK
633
Figure 1 X-ray diffraction patterns: (a) for Cu(O3PC(C6H5)=CH2)·H2O (Cu1 and CuPVP) and
Cu(O3PCH=CH2)·H2O (CuVP), and (b) the superposition of Cu1 and CuPVP.
Figure 2 SEM image of Cu(O3PC(C6H5)=CH2)·H2O.

634
A. VISA ET AL.
Figure 3 EDAX spectra of phosphonate metal organic framework (CuPVP).
The FT-IR spectra of copper phenylvinylphosphonates (both Cu1 and CuPVP)
recorded as a KBr pellet are shown in Figure 4. The broad absorption at 3300 cm−1 is due to
the O–H stretching vibration of the copper-coordinated water molecule. The peak position
and width suggest that it is interacting strongly through hydrogen bonding. The weak band
centered at 1600 cm−1 corresponds to the C=C double bonds of the phenylvinylphos-
phonate ligand. The absence of bands in the FT-IR spectra centered between 2600 and
2300 cm−1 (normally assigned to PO-H vibrations)21 point to complete deprotonation of
the phosphonic acid. The bands near 1300 cm−1 are due to the P–C stretching vibrations
of the phosphonates. The spectrum also has two intense bands at 1100–900 cm−1 which
correspond to –PO3 stretching modes and a weaker band at 800–600 cm−1 belonging to
Figure 4 FT-IR spectra of Cu1, CuPVP, CuPVP after heating at 260◦C, and CuVP.

NOVEL PHOSPHONATE METAL ORGANIC FRAMEWORK
635
Figure 5 X-ray powder diffraction plot for copper phenylvinylphosphonates.
the disubstituted vinyl moiety. These bands are comparable with those found in hydrated
copper vinylphosphonate.26
The stability of Cu(O3PC(C6H5)=CH2)·H2O as a function of temperature (up to
300◦C) was studied by powder X-ray thermodiffractometry, using a high-temperature cham-
ber mounted onto the X’Pert Pro diffractometer. The patterns were recorded every 50◦C,
with the final one collected again at room temperature (Figure 5). The structure appears
to be stable up to 150◦C. However, increasing the temperature from 150 to 200◦C causes
a structural change as evidenced by the splitting of the peak at ∼8◦. Further temperature
increase to 250◦C results in complete amorphization of the sample. The sample does not
recover its crystallinity upon cooling and rehydration for 3 h at room temperature (Figure 5).
Thermogravimetric analysis was also performed on the crystals of
Cu(O3PC(C6H5)=CH2·H2O (Figure 6). The TGA results confirm those from ther-
modiffractometry. The curves indicate that the compound is stable up to 180◦C, after
which it undergoes dehydration in a well-defined step corresponding to the loss of the
metal-coordinated water molecule between 180 and 230◦C (experimental weight loss
6.68%, calculated 6.83%). The TGA results are in good agreement with those from the
high-temperature XRD, where the structure change appears between 150 and 200◦C.
After completion of the dehydration process, the residue was investigated with FT-IR
spectrum and no trace of stretching bands associated to water molecules were observed.
At this temperature, the compound is less crystalline and the bands are broader with
several vibrations in the phosphonates region at 1100–900 cm−1 (Figure 4). The complex
undergoes decomposition in a two-step process, with a final weight loss of 32.31% up to
680◦C.
Using the existing X-ray structure of copper vinylphosphonates, a structure for Cu-
PVP (Figure 7) was proposed. Gas phase equilibrium geometry of conformer was obtained
by semiempirical PM3-RHF calculations using the Polak–Ribiere conjugate gradient algo-
rithm for geometry optimization. Stop criteria were: SCF convergence of 10−5 and RMS

636
A. VISA ET AL.
Figure 6 Thermal behavior of Cu(O3PC(C6H5) CH2 ·H2O (Cu1 and CuPVP).
1
gradient of 10−2 kcal/A·mol. For the structures with spin number, we used half electron
2
approximation.27–31 Calculations have been performed with HyperChem 7.52 package.32
Starting from the X-ray structure of copper vinylphosphonates, copper phenyl
vinylphosphonates geometry is best described by a distorted tetragonal pyramid: the five
ligands are oriented as follows: an oxygen atom from a vinyl phosphonic group is oriented
in axial position and four equatorial positions are occupied by three atoms of oxygen derived
from the phosphonate group and the fourth by a water molecule. The studied compound has
a dinuclear structure in which two {CuO5} square pyramids are bridged by two {CPO3}
Figure 7 Proposed copper phenylvinylphosphonate structure.

NOVEL PHOSPHONATE METAL ORGANIC FRAMEWORK
637
tetrahedra through corner sharing, forming an octahedron with 2Cu, 2P, and 4O atoms. The
bond lengths are presented in Figure 7. Comparing the two structures CuVP and CuPVP, a
major difference occurs in the network layers, namely the distance between the two layers
is higher in the case of CuPVP as the phenyl radical is bulky and placed between layers.25
Tests for electrical conductivity studies of this compound are currently being inves-
tigated and will be reported in due course.
CONCLUSIONS/PERSPECTIVES
In summary, a novel copper phosphonate inorganic–organic framework was obtained.
Starting from different copper salts with the same phosphonic acid (i.e., phenylvinylphos-
phonic acid) under similar experimental conditions, the same final product is obtained.
The product was characterized by powder X-ray diffraction, FT-IR, and thermogravimetric
analysis. The structure was proposed by semiempirical approaches. The present work fur-
ther extends the rich chemistry of metal ions with unsaturated phosphonate ligands25 and
adds to the already rapidly expanding chemistry of metal phosphonate materials.33
EXPERIMENTAL
Materials and Methods
All chemicals were obtained for commercial sources and used without further pu-
rification. Bis distilled water was used for all syntheses. The reagents were stirred briefly,
and the pH was measured before heating. The pHs were measured using a pH HI 2221
Calibration Check pH/ORP Meter by Hanna Instruments.
The crystallinity of the samples was examined using X-ray diffraction (XRD) analysis
with Cu Kα radiation (PANalytical X’Pert Pro MPD diffractometer equipped with PixCel
detector and spinning sample holder, working parameters 45 kV and 30 mA). The same
equipment with an additional high-temperature Anton Paar HTK 2000 chamber (Pt heating
strip) was used in order to study the dependence of structure changes with temperature.
The microstructure of the synthesized samples was analyzed using the scanning
electron microscope model Inspect S Phillips and their composition was determined using
energy-dispersive X-ray diffraction (EDAX). Elemental composition of the compounds
was determined by EDAX, phosphorus elemental analysis was determined by modified
Scho¨niger method,24,25 while C and H analyses were performed using an automatic CE-440
Elemental Analyzer from Exeter Analytical Inc.
FT-IR spectra were recorded on a Jasco-FT/IR-4200 instrument in the range
400–4000 cm−1 on compressed KBr pellets. The thermal stability of the new materials
was monitored between 30 and 700◦C under a flow of air or N2 with a heating rate of
10◦C/min, using a Perkin Elmer Diamond thermogravimetric analyzer.
Synthesis of Cu-PVP (PVP = Phenylvinylphosphonate)
A 100 mL one neck round-bottom flask was charged with the copper salt of choice
(sulfate or nitrate, 50.0 mmol), phenylvinylphosphonic (50.0 mmol), urea (50.0 mmol),
and bidistilled water (50 mL). The pH was adjusted to 2.8–3.2 with an aqueous solution of
NaOH (0.10 M). The reactant solution was heated in an oil-bath at 65–80◦C for 75–80 h. The
resulting blue crystals were collected by filtration, washed with distilled water, and dried

638
A. VISA ET AL.
in air (yield: 60–70%). Elemental analysis confirmed that there is one H2O molecule per
formula unit. Calcd (for the formula Cu(O3PC(C6H5)=CH2)·H2O): C, 36.44%; P, 11.75%
H, 3.44%, Found: C, 36.52%; P 11.9%; H, 3.38%, IR (KBr pellet, cm−1): 3223 (s), 1569
(w), 1490 (m), 1385 (w), 1055 (s), 1009 (s), 940 (m), 861(m), 692(m), 565 (m).
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