Facile synthesis of novel zinc-based infinite coordination polymer nanoparticles

Full text of DOI:10.1016/j.inoche.2017.02.013

Accepted Manuscript
Facile synthesis of novel zinc-based infinite coordination polymer
nanoparticles
M. Mohammadikish, A. Ahmadvand-Akradi
PII:
S1387-7003(17)30016-3
doi: 10.1016/j.inoche.2017.02.013
INOCHE 6564
DOI:
Reference:
To appear in:
Inorganic Chemistry Communications
Received date:
Revised date:
Accepted date:
6 January 2017
3 February 2017
7 February 2017
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of novel zinc-based infinite coordination polymer nanoparticles. The address for the
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appropriate. Inoche(2017), doi: 10.1016/j.inoche.2017.02.013
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ACCEPTED MANUSCRIPT
Facile synthesis of novel zinc-based infinite coordination polymer nanoparticles
*
M. Mohammadikish and A. Ahmadvand-Akradi
Faculty of Chemistry, Kharazmi University, Tehran, Iran
Tel./fax: +98 26 34551023.
E-mail address: mohammadikish@khu.ac.ir
Uniform zinc-based infinite coordination polymer (ICP) nanoparticles were prepared via a facile
precipitation method. The analysis data confirmed the proposed structure containing one
bicarboxylic salen type ligand and two zinc cations in each monomeric unit. One of the metals was
placed in N O salen environment and the other used as node to connect metal-organic linkers to
2
2
each other.

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1
-3
One new class of materials is metal-organic coordination polymers , which are based on metal
ions coordinated by polydentate organic ligands and explored for potential use in a variety of
important applications. In a subset of these materials, the use of metalloligand complexes as
ligand provides an additional level of tailorability.
4
, 5
Infinite coordination polymers (ICPs)
which consist of repeating ligands interconnected by
6
, 7
metallic nodes is rapidly growing area due to their potential use as gas storage
, catalysis
8
9
10
11
12, 13
14
,
photonics , separation , optoelectronics , small molecule detection
, drug delivery and
1
5
antisense gene regulation . This type of polymers is attractive because they can be made easily
from available and highly tailorable metal and ligand precursors.
Similar to organic polymers, the bulk feature of a coordination polymer is dependent not only to
the chemical nature of the building blocks, but also the chain monodispersity, degree of
polymerization, and intermolecular forces.
The infinite coordination polymer particles offer a high degree of tailorability through the choice
of transition metal connectors and predesigned functional organic linkers which have useful
1
6-18
16
chemical and physical properties
. According to previous results , these structures exhibit fast
ion exchange properties and it has been shown that one particle type can be rapidly interconverted
into several others through simple exchange reactions.
Coordination polymers can be made either as fully crystalline structures, often named to as metal–
organic frameworks (MOFs), or amorphous infinite coordination polymers (ICPs). In addition,
they can be prepared in macroscopic forms or as dispersible nano and microparticles, which have
properties distinct from their macroscopic counterparts. For the first time, in two individual
4
,
groups, development of synthetic methods for these new types of materials was investigated
5
.Unlike their bulk counterparts, these structures have well-defined shapes and sizes; can be
dispersed in many solvents, and exhibit properties such as rapid ion exchange, which are often a
function of their nano- and microscale architectures. Herein, we report a new carboxylic acid-
2
+
functionalized salen type ligand (CFSL) that can be used to generate ICP with Zn cation as
coordination metal and ligand interconnecting node.
The carboxylic acid-functionalized salen-based ligand (CFSL) was synthesized according as
1
9
follow and used for the preparation of ICP particles . This ligand (C) was synthesized in two
steps. An initial nucleophilic reaction of 5-(chloromethyl)-2-hydroxybenzaldehyde (A) with
sodium isonicotinate led to the formation of 4-carboxy-1-(3-formyl-4-hydroxy-benzyl)-
pyridinium chloride (B). Subsequent Schiff base condensation of the functionalized aldehyde (B)
2
0
with ethylenediamine led to the formation of target compound (C) (Scheme 1) .
2

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Zn based-ICP nanoparticles, Zn-CFSL-Zn (D) was prepared by slow addition of methanolic
solution of Zn(OAc) .2H O to ligand solution (C) with 2:1 stiochiometric ratio (Scheme 1). The
2
2
nanosized particles were collected from the reaction mixture by centrifugation and washed with
methanol via successive centrifugation and redispersion cycles.
Spherical nanoparticles composed of polymeric metal-ligand networks can be made by the
coordination assembly of metal ions and carboxylate-functionalized metallosalen Schiff base
building blocks (Scheme 1). Chloromethylsalicylaldehyde (A) was synthesized from
salicylaldehyde in the presence of paraformaldehyde in hydrochloric acid. The nucleophilic attack
on compound (A) by nitrogen of isonicotinic acid resulted in the preparation of carboxylate-
functionalized salicylaldehyde (
ethylenediamine produced carboxylate-functionalized salen Schiff base building blocks (
addition of metal solution to methanolic solution of salen type ligand ( ) (with 2:1 ratio) results
in the rapid formation of spherical inorganic polymer particles ( ). These particles are formed via
B
) . Schiff base condensation of compound
B
with
C
). Slow
C
D
coordination of the carboxylate groups on the ligand precursor to one equivalent of metal ions
supplied by the acetate salt. Another equivalent of metal ions was involved in N O coordination
2
2
sphere of ligand.
The chemical composition of Zn-CFSL-Zn was determined by inductively coupled plasma-
optical emission spectroscopy (ICP-OES) and elemental analysis which was consistent with the
formation of 1:2 structures derived from the salen type ligand and zinc ion precursors. As shown
in Scheme 1, each monomeric unit contains one acid-functionalized salen type ligand (CFSL) and
two zinc cations. One of the metals is coordinated by N O salen environment and the other metal
2
2
used as node to connect metal-organic linkers to each other.
O
O
HC
HC
HCl, (CH2O)n
days, r.t.
THF, reflux, 48h
HCl 6M, pH = 2
HO
HO
CH2Cl +
N
CO2Na
5
A
O
HC
CH2Cl2, 72h, r.t.
HO
CH2
N
CO2H
+
2 2 2
0.5 eq. NH CH CH NH
2
B
N
N
2
eq. Zn(OAc)2.2H2O
CH3OH, 1h
HO2C
N
CH2
OH
HO
CH2
N
CO2H
C;
(CFSL)
N
N
O
Zn
O2C
N
CH2
O
CH2
N
CO2 Zn
n
D;
Zn-CFSL-Zn
3

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Scheme 1. Synthesis of carboxylic acid-functionalized salen type ligand (CFSL) and Zn-CFSL-Zn
particles.
Fig. 1. FT-IR spectra of a) carboxylic acid functionalized salen ligand (CFSL) and b) its Zn-CFSL-Zn
coordination polymer.
All characteristic bands of desired ICP can be seen in the FT-IR spectrum (Fig. 1). The presence
−
1
of water molecules is confirmed by a broad absorption band at 3410 cm in the FT-IR spectrum
of the polymer, indicating intermolecular H-bonding. The imine band in the Zn-CFSL-Zn was
appeared in the same frequency compared to the free ligand. Infrared spectrum of the Zn-CFSL-
Zn nanoparticles confirms the coordination of the carboxylate groups to Zn cation, as evidenced
−
1
by a shift of the carboxylate stretching frequency from 1675 cm in salen type ligand (CFSL) to
−
1
−1
1
537 cm and 1477 cm for the Zn-CFSL-Zn. These values compare very well with the
−
1
−1
2
stretching frequencies of Zn(OAc) at 1558 cm and 1450 cm , consistent with η -coordination
2
2
+
21
of the carboxylate groups to the Zn center through their anionic O atoms .
Fig.2 shows SEM images of the as-prepared precipitate, which clearly indicates that the
precipitate consists of a large quantity of monodispersed nanoparticles. The average diameter of
the Zn-CFSL-Zn is 90 nm.
4

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Fig. 2. The SEM images of Zn-CFSL-Zn nanoparticles.
Scheme 2. Proposed mechanism for the Zn-CFSL-Zn nanoparticles formation.
Tiny seeds of the polymer are formed immediately after mixing the metal and ligand solutions.
These seeds are agglomerated and start to form bigger spherical particles with rough surfaces. The
formation mechanism can be involved oligomerization and nucleation, aggregation of seeds,
fusion and growth steps (Scheme 2). At the beginning step of reaction, with high concentrations
of ligand and metal ion, these steps are very fast. By proceeding of the reaction, with decreasing
the concentration of precursor, the fusion and growth processes become dominant and spherical
ICP particles are produced with smooth surfaces. The particle growth step involves both
aggregation of the nanometer-sized seeds along with the subsequent adsorption of additional
nanoparticles onto the larger particle core. With progressing the reaction, the Ostwald ripening
process dominated, and then the nanoparticles with smooth surfaces were generated. Note that the
surfaces of the particles contain many active sites for further polymerization and nucleation via
coordination
Thermogravimetric analysis (TGA) of Zn-CFSL-Zn shows that the ICP particles are stable up to
2
50ºC after an initial weight loss due to solvent liberation in the 100–250ºC temperature range
(
Fig. S1). The next two steps from 250ºC to 600ºC are corresponding to removal of the organic
ligand CFSL, (observed: 71.35%). Further heating of the sample to 600ºC does not show any
weight loss. The final residual weight is 21.16% (calculated: 21.03%) corresponding to ZnO.
5

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Thermogravimetric analysis clearly shows the presence of two water molecules and confirms the
molecular weight of the monomeric unit of the polymer.
The prepared ICP consists of metalloligand (Zn-CFSL) building blocks and metal
interconnecting nodes. This ICP contains two different types of cations, those that are coordinated
by the tetradentate pockets of the ligands and those that link the Schiff base ligands through the
2
+
carboxylate groups to form the polymers. Mirkin and coworkers reported that when Zn is used
as the metal ion, it can be readily replaced with a variety of metal ions from the tetradentate Schiff
1
6
base pockets but did not from the carboxylate sites .
2
+
We prepared a mixture of Zn-CFSL-Zn nanoparticles and Cu methanolic solution under
2+
ambient conditions. The mixture contained Cu ions in a slight excess to force the replacement of
2+
all the Zn ions coordinated by the tetradentate Schiff base moieties within the particles. The
reaction mixture was stirred overnight, collected and washed with methanol. In contrast with the
1
6, 22
previous works
, the ion exchange did not occur in this polymer. The inductively coupled
plasma-optical emission spectroscopy (ICP-OES) experiment showed the presence of 4.04%
copper in the product that presumably was related to trapped copper ions in the cavities of the
ICP. The similar experiment was done with Zn-CFSL-Zn and MoO (acac) as base polymer and
2
2
ion exchange agent, respectively. Again, the amount of the molybdenum content was very low,
.06%, and showed that the ion exchange did not occur.
4
ICP like MOF counterpart could be good precursor for metal oxide preparation. To clarify this
capability, Zn infinite coordination polymer was individually placed in a furnace and calcined at
5
00 ºC for 6 h in air. It was found that zinc oxide with the same morphology as ICP precursor
could be produced.
As can be seen from the SEM image (Fig. S2a) the ICP maintained its original shape after
calcination and transforming into zinc oxide. Because of high temperature of the reaction, the
agglomeration of nanoparticles and production of microparticles can be observed clearly.
The wurtzite structure of ZnO (card no. 80-0075), indicated sharp and strong di
ff raction peaks
confirming the well-developed crystallization of the metal oxides products (Fig. 2b).
In conclusion, zinc-based coordination polymer nanoparticles with 90 nm diameters were
synthesized by precipitation method. In contrast with previous work, the prepared ICP did not
show any ion exchange reaction in the presence of two different metal cations. The well-
crystallized ZnO obtained from calcination of Zn-CFSL-Zn at 500 ºC.
6

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Graphical abstract
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Highlights




Synthesis of zinc oxide from ICP nanoparticles.
Synthesis of new infinite coordination polymer.
Represent a reasonable mechanism for growth of crystals.
Investigation of ion exchange properties of ICP.
1
0