Crystallisation, thermal analysis and acetal protection activity of new layered Zn(II) hybrid polymorphs

Article abstract of DOI:10.1039/c6ce00486e

Two new polymorphic mononuclear complexes, with analogous structural formula (C₆H₉N₂)₂[Zn(H₂O)₆](SO₄)₂·2H₂O, of zinc(ii) templated by 2-amino-6-methylpyridinium ligand have been discovered. These polymorphic hybrid crystals, which differ only in terms of their crystal structures, were prepared in one-step synthesis under ambient conditions and investigated for their thermal and catalytic properties. Single-crystal X-ray diffraction of the obtained materials revealed that polymorphs 1 and 2 crystallise in the triclinic system, space group P1. In an effort to further explore the form diversity of these compounds, the structural arrangements and intermolecular interactions such as hydrogen-bonding and π?π interactions are discussed from which supramolecular assembly was formed. Meanwhile, these new polymorphic forms can be described as the stacking of 3D layers where the interlayer distances are 13.23 and 12.59 ? for 1 and 2, respectively. The thermal behaviour of the precursors checked by TG-DTA analysis for both zinc sulfate polymorphs and variable temperature powder X-ray diffraction (VT-PXRD) show successive intermediate crystalline anhydrous phases upon dehydration in 1. The catalytic activity of both polymorphic structures has been tested in the acetalisation reaction of aldehydes as a benchmark process. Interestingly, both complexes showed excellent activity with almost total conversion in many examples and using MeOH as solvent and as the unique source of acetalisation.

Full text of DOI:10.1039/c6ce00486e

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Crystallisation, thermal analysis and acetal protection activity of
new layered Zn(II) hybrid polymorphs
Salem Saïd,a Houcine Naïlia*, Thierry Batailleb and Raquel P. Herrerac
Received 00th January 20xx,
Accepted 00th January 20xx
Two new polymorphic mononuclear complexes, with analogous structural formula (C6H9N2)2[Zn(H2O)6](SO4)2·2H2O, of
Zinc(II) templated by 2-amino-6-methylpyridinium ligand have been discovered. These polymorphic hybrid crystals, which
differ only in terms of their crystal structures, were prepared in one-step synthesis under ambient conditions and
investigated for their thermal and catalytic properties. Single-crystal X-ray diffraction of the obtained materials revealed
DOI: 10.1039/x0xx00000x
www.rsc.org/
that polymorphs 1 and 2 crystallise in the triclinic system, space group
.
In an effort to further explore the form
diversity of these compounds, the structural arrangements and the intermolecular interactions such as hydrogen-bonding
and π···π interactions are discussed from which supramolecular assembly was formed. Meanwhile, these new polymorphic
forms can be described as the stacking of 3D layers where the interlayer distances are 13.23 and 12.59 Å for 1 and 2,
respectively. The thermal behaviour of the precursors checked by TG-DTA analysis for both zinc sulfate polymorphs and
variable temperature powder X-Ray diffraction (VT-PXRD), show successive intermediate crystalline anhydrous phases
upon dehydration in 1. The catalytic activity of both polymorphic structures has been tested in the acetalisation reaction
of aldehydes as a benchmark process. Interestingly, both complexes showed excellent activity with almost total conversion
in many examples and using MeOH as solvent and as the unique source of acetalisation.
and inorganic chemistry. Hence, it is the property of a
substance to exist in different crystalline phases resulting from
different arrangements of the molecules in the solid state.6 In
Introduction
A very large number of solid substances can appear under
various phases if the environmental growth conditions enable
it. These phases can be polymorphic forms, microcrystalline
solids or amorphous materials. We will focus hereafter
completely on polymorphism, in which each polymorphic
variety has the same chemical composition, however presents
variations in the internal arrangement of atoms, ions or
molecules that constitute the crystal lattice. The phenomenon
of polymorphism – the ability of a compound to crystallise in
more than one crystal structure – has been the subject of
growing interest.1 Polymorphism2-5 is one of the most
fascinating phenomena, that confers great value in
pharmaceutical industry, on solid state chemistry and crystal
engineering. Indeed, it is a “difficult” phenomenon studied for
many decades mainly, and separately, in the fields of organic
spite of the huge efforts of many scientists, the knowledge
about polymorphic complexes is still embryonic. The
relationship between the growth of thermodynamically stable
(or metastable) crystalline phases and nucleation of the first
crystallites is still mysterious.7,8 A great number of metal
cations and organic entities that can be used to obtain
polymorphic compounds make it possible to have potential
applications in numerous fields, such as catalysis, biological
activities, electromagnetic and optical functions.9-18 In
a
supramolecular sense, polymorphism is the existence of more
than one type of network superstructure for the same
molecular building blocks. Hence, strong and rigid interactions
avoid changes in crystal structures and the formation of
different crystalline modifications of one chemical compound.
Although polymorphism is
a well-known crystallographic
phenomenon,19,20 no model exists to predict the polymorphic
behaviour of chemical compounds. Basically two reasons for
polymorphism are distinguishable: (1) the deviation of the
a.Laboratoire Physico-chimie de l’Etat Solide, Département de Chimie, Faculté des
Sciences de Sfax, B.P. 1171, 3000 Sfax, Université de Sfax, Tunisie. E-mail address:
houcine_naili@yahoo.com
b.Ecole Nationale Supérieure de Chimie de Rennes, 11 Allée de Beaulieu F-35708
RENNES cedex 7, France.
molecular arrangement at
a minimum of energy or
thermodynamic equilibrium and (2) a variety of possible
association modes or packing relations of the molecules, i.e.
geometric reasons. Recently, it has been shown that the
structural study toward the understanding of polymorphism in
framework structures can give rise to new insights especially in
the role of weak interactions. The non-covalent interactions
play an important role in organising structural units. They exert
important effects on the organisation and properties of many
c. Laboratorio de Organocatálisis Asimétrica, Departamento de Química Orgánica,
Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), CSIC-Universidad de
Zaragoza, C/Pedro Cerbuna 12, E-50009 Zaragoza, Spain.
† Footnotes relaꢀng to the ꢀtle and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [Crystallographic data for
polymorphs 1 and 2, are available from the Cambridge Crystallographic Data
Centre, with CCDC No. 1039275, for polymorph 1, and No. 1039276 for polymorph
2. Copies of these data can be obtained free of charge from the Cambridge
Crystallographic Data Centre, 12 Union Road, CambridgeCB2 1EZ, UK; fax: (+44)
1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 2016
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materials in areas such as biology,21,22 crystal engineering23,24 Synthesis
and material science.25,26 However, yet it is still a hard task to
Single crystals of the polymorphic phases were grown by solution
method with slow evaporative crystallisation technique at room
temperature (30 °C). Therefore, the concomitant polymorphism
occurs in the same crystallization vial, from the same solution under
the same conditions. Then the saturated solution was prepared and
it was constantly stirred for about 2 hours using a magnetic stirrer
and was filtered using 4 micro Whatmann filter papers. Then, the
filtered solution was kept in a borosil beaker covered with a porous
paper and kept in a dust-free atmosphere. After the successful
dissolving of the chemical species, two kinds of crystals were
harvested after a period of about two weeks. In addition, the
development of a process used for obtaining a given polymorphic
variety can be considered only if the data of thermodynamic and
kinetic conditions enable it. These concomitant polymorphs,
although rare, were prepared in one-step synthesis. Therefore, 1
mmol of Zinc(II) sulfate heptahydrate (ZnSO4·7H2O) and 2 mmol of
2-amino-6-methylpyridine (C6H8N2) were each dissolved in 10 mL of
distilled water. The Zn(II) salt solution was then added slowly to the
ligand solution. The pH of the resulting solution was adjusted
between 2 and 3 by dropwise addition of concentrated sulfuric acid
(H2SO4) until the solution becomes clear. Thereby, with the aim of
preventing any precipitation, the solution was filtered to remove
any solids. polymorph 1 appeared as colourless needles crystals and
polymorph 2 was found to have a colourless block morphology. The
crystals are stable for a long-time in normal conditions of
temperature and humidity.
predict and control the crystal structures of such compounds.
Then, intermolecular interactions are the essence of
supramolecular chemistry, and the field of crystal
supramolecularity seeks to understand intermolecular
interactions by analyses of crystal packing. One of the
compelling reasons for research in this area is to look for
newer structures and in some cases to investigate the close
structural relationships and possible transformations between
the structures within a family of compounds.27
A
number of papers have studied the structure–property
relationships of polymorphic forms. Then, the aim of this paper is
essentially twofold: (i) to take polymorphism from the more
traditional field of organic chemistry to the neighbouring area
of supramolecular chemistry, and (ii) to characterise and
discuss the chemical preparation, the catalytic properties, the
thermal behaviour and the crystallographic description of two
new hybrid polymorphic frameworks of Zn(II) sulfate
templated by 2-amino-6-methylpyridinium. The hope is to
stimulate research in this exciting field.
Experimental
Materials
In order to obtain crystals of high quality, purification of starting
materials was found to be an important step and hence, the re-
crystallised salts were used for the growth of crystals of the studied
materials. All chemicals and solvents were purchased from Sigma-
Aldrich and it was further purified by repeated re-crystallisation
processes for three times using de-ionised water as the solvent.
Single-Crystal X-ray Diffraction
Details of crystallographic data collection and refinement
parameters for polymorphic complexes 1 and 2 are given in Table 1.
Table 1. Crystallographic data and structure refinement parameters for polymorphs 1 and 2.
Structural parameter
formula
Polymorph 1
Polymorph 2
(C6H9N2)2[Zn(H2O)6](SO4)2·2H2O
(C6H9N2)2[Zn(H2O)6](SO4)2·2H2O
formula weight (g mol-1)
Temperature (K)
Crystal system
Space group
a(Å)
620
620
100 (2)
Triclinic
100 (2)
Triclinic
7.3614(18)
13.234(4)
7.3453(18)
12.594(4)
b (Å)
c (Å)
13.939(4)
20.074(6)
α (°)
106.77(4)
80.60(3)
β (°)
104.90(4)
83.38(3)
γ (°)
V (Å3)
98.84(4)
89.56(3)
1217.8(8)
1819.7(9)
Z
2
3
Diffractometer
Programs system
Absorption correction
ρcal (g cm-3)
Crystal size (mm3)
Crystal color/shape
Xcalibur, Ruby
SHELXL-2013 and SHELXS-97
Analytical
Xcalibur, Ruby
SHELXL-2013 and SHELXS-97
Analytical
1.691
1.697
0.32 × 0.23× 0.11
needle, colorless
1.262
0.35 × 0.20 × 0.11
block, colorless
1.266
μ (mm−1
)
θ range (deg)
hkl range
θmin = 2, θmax = 31
-9 ≤ h ≤ 10
-18 ≤ k ≤ 9
θmin = 2, θmax = 29
-8 ≤ h ≤ 9
-16 ≤ k ≤ 16
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-14 ≤ l ≤ 19
7447
-26 ≤ l ≤ 25
No. of reflection collected
9293
No. of independant reflection
5599
6332
F(000)
648
972
R1
0.0299
0.0757
1.067
321
0.0407
wR2
0.1078
GooF
1.038
No. param.
478
Transmission factors
Largest difference map hole
Tmin= 0.672; Tmax= 0.796
Tmin= 0.708; Tmax= 0.897
Δρmin = -0.580, Δρmax = 0.580
Δρmin = -0.480, Δρmax = 0.430
Both polymorphs were mounted onto glass fibre and cooled to low completely different, thus showing polymorphism. It is
temperatures. Data were performed on a, Xcalibur Ruby, noteworthy that the following driving forces, which influence
diffractometer with graphite monochromatised Mo kα radiation (λ the mode of packing, are to be considered as possible reasons
= 0.71073 Å), using an Oxford Cryosystems cooler. Data collection, for the polymorphism here: (a) hydrogen bonding and van-der
cell refinement, data reduction and analysis were carried out with Waals (vdW) forces (b) tendency to achieve a closer packed
CrysAlisPRO.28 Analytical absorption correction was applied to the structure, including π···π stacking, and (d) type of solvent used
data with the use of CrysAlisRED28. Structures of in the crystallisation process.32,33 The hybrid polymorphs
(C6H9N2)2[Zn(H2O)6](SO4)2·2H2O were solved with direct methods described within this work crystallise in the centrosymmetric
using SHELXS-97,29 and refined by a full-matrix least squares space groups
. Polymorph 1 consists of two zinc cations
technique with SHELXL-201330 with anisotropic thermal parameters octahedrally coordinated by six water molecules, two isolated
for all non H-atoms. All H-atoms were initially located in difference sulfate anions, two 2-amino-6-methylpyridinium cations and
Fourier maps, and in the final refinement cycles the C-bonded H two free H2O units (Figure 1).
atoms were placed in calculated positions, with C–H = 0.95 Å, and
refined with a riding model with Uiso(H) = 1.2Ueq(C). The O- and N-
bonded hydrogen atoms were located from difference maps and
treated with Uiso(H) = 1.5Uiso(O) and U(H) = 1.2Uiso(N). All figures
were made using DIAMOND program.31
Thermal analysis measurements
TGA-DTA measurements of complexes 1 and 2 were performed
using a 'SETSYS Evolution' (Pt crucibles, Al2O3 as a reference)
instrument, under air flow (100 ml/min), for any compound, with a
heating rate of 5 °C min-1 up to 600 °C. Variable temperature
powder X-ray diffraction (VT-PXRD) analysis was carried out using a
θ-θ Bruker AXS D8 Advance powder diffractometer, equipped with
a high-temperature Anton Paar HTK1200 oven camera and a
LynxEye detector. Powder patterns were collected sequentially
upon heating at 21.6 °C h-1 up to 450 °C, with the monochromatised
CuKα1 radiation (λ = 1.5406 Å). To ensure satisfactory counting
statistics, counting times of 20 min/pattern were selected for the
Fig. 1 Part of the crystal structure of polymorph
1 at 100 K
thermal decomposition of the precursors, so that any pattern could
be collected within a temperature range of 7.2 °C.
showing the asymmetric unit and atom-numbering scheme.
Displacement ellipsoids are drawn at the 50% probability level.
Hydrogen bonds are represented by dashed lines, [Zn1 and
Zn2 octahedra are completed by symmetry, symmetry code: i
= 1-x, 1-y, 1-z and ii = -x, 1-y, -z; respectively].
General catalytic procedure for the synthesis of acetals 4
Complex 1 (8 mg, 0.01292 mmol) and aldehydes 3a-l (0.323 mmol)
were dissolved in MeOH (0.25 ml) in a test tube. The resulting
mixture was stirred at 40 °C during 24 h. The reactions were
monitored by thin-layer chromatography. The yield of the reaction
is given by 1H NMR.
The zinc cations in
1 are located in special positions on
crystallographic inversion centres and each is coordinated by
six oxygen atoms from water molecules of which three are
crystallographically independent. While, the protonated
organic species, the sulfate tetrahedron and the
uncoordinated water molecules are located in general
Results and discussion
Description of Crystal Structures of Polymorphs
(C6H9N2)2[Zn(H2O)6](SO4)2·2H2O
positions. In contrast, in the structure of polymorph 2, each
The molecular geometry in the two polymorphic structures
1
zinc octahedron is surrounded by six water molecules to form
two hexaaquazinc(II) complexes, three sulfate tetrahedra,
and 2 is very similar. However, the packing arrangement is
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