Synthesis of Hofmann-type Zn(H2O)2Ni(CN)4.nG (G = water and 1,4-dioxane)clathrates and the determination of their structural properties by variousspectroscopic methods

Full text

Turk J Chem
(2019) 43: 1608 – 1621
© TÜBİTAK
doi:10.3906/kim-1906-26
Turkish Journal of Chemistry
http://journals.tubitak.gov.tr/chem/
Researc h Article
Synthesis of Hofmann-type Zn(H2O) 2Ni(CN) 4.nG (G = water and 1,4-dioxane)
clathrates and the determination of their structural properties by various
spectroscopic methods
Zeki KARTAL1,∗,, Onur ŞAHİN2,, Abdülkerim YAVUZ3,
1Department of Physics, Faculty of Arts and Sciences, Kütahya Dumlupınar University, Kütahya, Turkey
2Scientic and Technological Research Application and Research Center, Sinop University, Sinop, Turkey
3Institute of Science and Technology, Kütahya Dumlupınar University, Kütahya, Turkey
Received: 18.06.2019 • Accepted/Published Online: 17.10.2019 • Final Version: 09.12.2019
Abstract: Two new 2-dimensional cyanide-bridged coordination polymers [Zn(H2O)2Ni(CN)46(H2O) and Zn(H2O)2
Ni(CN)43(C4H8O2)], which were similar to Hofmann-type clathrates, were synthesized based on [Ni(CN)4]2−and
Zn2+ as building blocks. These substances were synthesized as compounds in crystalline form. Thes tructures of the
crystalline compounds were characterized via their spectral analyses. General information about the structures of the
newly obtained Hofmann-type clathrates was obtained from their vibration spectra by considering signicant changes
in the vibration peaks of the cyanide group, water ligand molecule, and guest molecules (water and 1,4-dioxane). The
thermal behavior of the Hofmann-type clathrates was investigated in the range of 25–500 °C. In addition, experimental
data on the magnetic properties of the Hofmann-type clathrates were obtained using the Gouy method under normal
conditions. Information on the properties of the structures of the Hofmann clathrates was obtained by applying the
single crystal diraction technique. The asymmetric unit of the rst Hofmann-type clathrate contained 1 Zn(II) ion, 1
Ni(II) ion, 1 cyanide ligand, 1 water ligand molecule, and 2 guest water molecules. The asymmetric unit of the second
Hofmann-type clathrate contained 1 Zn(II) ion, 1 Ni(II) ion, 2 cyanide ligands, 1 water ligand molecule, and 2 half guest
1,4-dioxane molecules.
Key words: Hofmann-type clathrates, vibrational spectra, single crystal X-ray diraction analysis, host structure, guest
molecules.
1. Introduction
There are many important chemical groups in organic chemistry. They play a major role in many important
events in our universe and in the formation of the environment we live in. The cyanide group is one of these
important chemical groups and is represented by the formula C≡N. The cyanide group forms a wide variety of
compounds with metal atoms and other molecules. The Ni(CN)4group is an important group in terms of both
chemical and physical properties formed by binding of the Ni atom to the carbon atom of the cyanide structure.
Some of these compounds are 1-dimensional (1D), some are 2D, and some are 3D. Based on the potassium-Ni
cyanide compound, which is one of the cyanide compounds, many new compounds have been synthesized and
are still being synthesized. Hofmann-type compounds are among the most important compounds synthesized
based on the potassium-Ni cyanide compound.
∗Correspondence: zeki.kartal@dpu.edu.tr
This work is licensed under a Creative Commons Attribution 4.0 International License.
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Since the discovery of an interesting compound by Hofmann in 1897, a number of host structures
called Hofmann-type compounds have been synthesized and these compounds are given by the general formula
M(II)LM'(II)(CN) 4. In this formula, M and M'represent 1 transition metal atom, while L represents either 2
ligand molecules capable of a single chemical bond or a single ligand molecule capable of 2 chemical bonds [1,2].
If some suitable molecules, as guests, enter the cavities of the Hofmann-type compounds, these new structures
are called Hofmann-type clathrates and are given by the formula M(II)LM'(II)(CN) 4•nG. In this formula, G
shows the guest molecule in the Hofmann-type clathrates, whereas n shows the number of the guest molecules
[3,4].
Dierent ligand molecules containing nonmetal atoms, such as N atoms, sulfur (S) atoms, and O atoms,
are used to obtain Hofmann-type complexes and clathrates. These nonmetal atoms in the ligand molecules
are electron-giving atoms in the formation of Hofmann-type complexes and clathrates. When the transition
metal atoms M'(II), such as nickel (Ni), palladium, and platinum (Pt), are used to obtain Hofmann compounds,
the compound formed is called a Hofmann-type complex. If the transition metal atoms zinc , cadmium, and
mercury are used to obtain the Hofmann compounds, the resulting compounds are called Hofmann-Td-type
complexes and clathrates [1–4].
Broad and descriptive information about Hofmann-type compounds and clathrates can be found in various
studies [1–7].
The various structural properties of Hofmann-type complexes and clathrates can be explained by various
spectroscopic methods, such as vibrational (infrared (IR) and Raman), nuclear magnetic resonance, electron
spin resonance, thermal analysis, single-crystal X-ray diraction (SC-XRD) techniques, and magnetic moment
spectroscopy. The formation of Hofmann-type complexes and clathrates and/or interactions of the guest
molecule with the host structure can be easily understood by means of vibrational spectroscopy [5–7].
Hofmann-type complexes are compounds with 2D polymeric layers consisting of M'(CN)−2
4ions bonding
to one another with M(L)+2 cations. These multilayer transition metal complexes are chemically and biolog-
ically very important macromolecules consisting of metal-metal or metal-ligand-metal bridges with 1, 2, or 3
dimensions [8,9].
The aim of this study was to obtain new Hofmann-type clathrates in crystal structures using the water
molecule as a ligand molecule and the water molecule and the 1,4-dioxane molecule as guest molecules. This
was done because the water molecule is a substance that is both very abundant and very cheap compared to
other ligand molecules.
Water is known as a universal solvent due to its ability to dissolve many substances in our environment,
as well as being a good ligand molecule [10]. The 1,4-dioxane material is similar to water in terms of its chemical
functions, and it also acts as a ligand molecule in some chemical reactions, while in others it acts as a guest
molecule and is also a good solvent liquid. Because 1,4-dioxane is present in greater amounts than other isomers
(for example, 1,2-dioxane and 1,3-dioxane), it is usually only referred to as dioxane (D) [11].
The D molecule is structurally centrosymmetric, meaning that it is in the form of a typical chair
conformation shown by cyclohexane and its derivatives. However, the D molecule is structurally highly exible
and can easily change to boat conformation due to the changes in its environmental conditions [12].
Solutions of Zn(CH3COO)2and K2Ni(CN)4•H2O in distilled water and D (C4H8O2)compound were
used to obtain single crystal structures of Hofmann-type clathrates given by the formula Zn(H2O)2Ni(CN)4•nG.
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2. Experimental
2.1. Materials
All chemicals used to obtain the crystals of the Hofmann-type clathrates, namely potassium tetracyanonickelate
hydrate {K2[Ni(CN) 4]•H 2O, Fluka, 96%}, [1,4-dioxane, (C4H8O2), Fluka, 98%], Zn(II) acetate anhydrous
[Zn(CH3COO)2, Alfa Aesar, 99.9%], and ammonia solution (NH3, Merck, 25%), were used without any further
modications.
2.2. Syntheses of Hofmann-type clathrates Zn(H2O) 2Ni(CN) 4•nG(G =H2O and C 4H8O2)
For synthesis of the compounds obtained in this study, the synthesis method of Kartal et al. [7] was used. The
colorless, transparent crystal for the Hofmann-type clathrate Zn(H2O)2Ni(CN)4•n(H2O) (hereafter referred
to as 1) formed over a period of 6 weeks.
The other crystal was obtained in a similar manner as crystal 1. In contrast to crystal 1, while the other
crystal was obtained in the third step, about 5 mmol of guest D molecules was added to the mixture medium.
The colorless, transparent crystal for the Hofmann-type clathrate Zn(H2O)2Ni(CN)4•n(C4H8O2)(hereafter
referred to as 2) formed over a period of 7 weeks.
2.3. Instrumentation
The FT-IR spectra of crystals 1and 2were obtained with a Bruker Optics Vertex 70 FT-IR spectrometer
(Billerica, MA, United States) at a resolution of 2 cm−1and room temperature, at a wavenumber range of
3500–400 cm−1. The FT-Raman spectra of crystals 1and 2were obtained with a Bruker Senterra dispersive
Raman microscope (Bruker Optics), on the 532-nm line of a 3B diode laser and at room temperature, at a
wavenumber range of 3500–150 cm−1. The data of the crystal structures of crystals 1and 2were collected
with a D8-QUEST diraction meter (Bruker Optics) equipped with a graphite-monochromatic Mo-Kα(λ=
0.71073 Å) radiation. The thermal gravimetric analysis (TGA) and dierential thermal analysis (DTG) curves
of crystals 1and 2were recorded in a static air atmosphere at a heating rate of 10 °C/min at temperatures of
25–500 °C, using Pt crucibles with a SII EXSTAR 6000 TG/DTA 6300 thermal analyzer (LabMakelaar Benelux
B.V., Zevenhuizen, the Netherlands). Magnetic properties of crystals 1and 2were recorded with a Sherwood
Scientic Magway MSB MK1 model magnetic balance (Sherwood Scientic Ltd., Cambridge, UK).
The amounts of Ni and Zn in the structures of crystals 1and 2were determined using a PerkinElmer
optima 4300 DV ICP OES (PerkinElmer Inc., Waltham, MA, USA), and the amounts of C, H, and N were
determined using a CHNS-932 element analyzer (LECO Corp., St. Joseph, MI, USA). According to the results
of the elemental analysis, the number of guest molecules in the clathrates was n =6 for crystal 1and n
=3 for crystal 2. The results of all of the analyses for crystal 1were as follows: [found/(calculated)%] C,
12.62/(12.91)%; H, 3.98/(4.33)%; N, 15.65/(15.05)%; Ni, 15.21/(15.77)%, and Zn, 17.24/(17.56)%. The results
of all of the analyses for crystal 2were as follows: [found/(calculated)%] C, 36.09/(36.36)%; H, 4.93/(5.34)%;
N, 10.93/(10.60)%; Ni, 11.29/(11.11)%, and Zn, 12.64/(12.37)%.
2.4. SC-XRD analyses of crystals 1 and 2
Suitable crystals of 1and 2were selected for data collection, which was performed on a D8-QUEST diractome-
ter equipped with a graphite-monochromatic Mo-Kαradiation at 296 K. The structure was solved by direct
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methods using SHELXS-2013 [13] and rened by the full-matrix least-squares method on F2using SHELXL-
2013 [14]. All nonhydrogen atoms were rened with anisotropic parameters. The H atoms of the C atoms were
located from dierent maps and then treated as riding atoms with C-H distances of 0.96 Å. The other H atoms
were located on a dierence map rened freely. The following procedures were implemented in the analysis:
data collection: Bruker APEX2 [15], program used for molecular graphics: MERCURY program [16], and soft-
ware used to prepare material for publication: WinGX [17]. The experimental conditions for the solution of the
structures of crystals 1and 2and the data of their crystal structures are shown in Table 1.
Table 1. Crystal data and structure renement parameters for crystals 1and 2.
Crystal data 1 2
Empirical formula C4H16N4NiO8Zn C12 H20N4NiO6Zn
Formula weight 372.29 440.40
Crystal system Tetragonal Monoclinic
Space group P-4 P2/c
a(Å) 7.6491 (11) 7.1859 (9)
b(Å) 7.6491 (11) 7.4416 (9)
c(Å) 8.1137 (14) 16.0571 (18)
β(º) 90.00 94.421 (4)
V(Å3)474.72 (16) 856.09 (18)
Z 1 2
Dc(g cm−3)1.302 1.708
µ(mm−1)2.28 2.54
θrange (°) 3.7–28.3 3.0–26.1
Measured res. 2003 34154
Independent res. 768 2113
Rint 0.045 0.050
S 1.00 1.19
R1/wR2 0.088/0.236 0.042/0.090
Tmax/Tmin 1.29/–0.89 0.61/–0.91
3. Results and discussion
3.1. Crystallographic analysis of crystals 1 and 2
SC-XRD studies of crystals 1and 2showed that the structures of these crystals had a 2D polymeric coordination
compound structure. The asymmetric unit of heterometallic crystal 1consisted of 1 Zn(II) ion, 1 Ni(II) ion,
1 cyanide ligand, 1 coordinated water molecule, and 2 noncoordinated water molecules, as shown Figure 1.
The asymmetric unit of heterometallic crystal 2consisted of 1 Zn(II) ion, 1 Ni(II) ion, 2 cyanide ligands, 1
coordinated water molecule, and 2 half noncoordinated D molecules, as shown Figure 2. In crystals 1and 2,
each Zn(II) ion was located on the inversion center and coordinated by 4 N atoms (Zn1-N1 =2.375(17) Å in
crystal 1and Zn1-N1 =2.131(3) Å in crystal 2) from cyanide ligands, and 2 O atoms (Zn1-O1 =2.33(2) Å
in crystal 1and Zn1-O1 =2.117(3) Å in crystal 2) from water molecules, thus showing a distorted octahedral
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coordination geometry. Each Ni(II) ion was coordinated by 4 carbon atoms (Ni1-C1 =1.85(2) Å in crystal 1
and Ni1-C1 =1.881(3) Å and Ni1-C2 =1.877(3) Å in crystal 2) from cyanide ligands, thus showing a square
planar coordination geometry. The Zn(II) and Ni(II) ions were bridged by cyanide ligands to generate grid-like
2D sheets, with a grid dimension of 5.409 Å in crystal 1and 5.163 ×5.182 Å in crystal 2, as shown Figures
3 and 4 (dened by Zn•••Ni distance). These values are clearly seen in Tables 2 and 3. The most striking
structural feature of crystals 1and 2was that they possessed a 3D network, as shown in Figures 5 and 6,
which contained 1D channels along the aaxis (7.649 ×8.114 Å 2in crystal 1and 7.186 ×8.033 Å 2in crystal 2)
(dened by Zn•••Zn distance), which were lled with guest water molecules in crystal 1and D molecules in
crystal 2. The unit cells of crystals 1and 2are shown in Figures 7a and 7b, respectively. In order to provide a
more comfortable view, the hydrogen atoms have been deleted in Figure 7b.
Figure 1. Molecular structure of crystal 1showing the
atom numbering scheme.
Figure 2. Molecular structure of crystal 2showing the
atom numbering scheme [(i) –x+1, –y +2, –z +1].
Figure 3. An innite 2D layer in crystal 1.Figure 4. An innite 2D layer in crystal 2.
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Table 2. Selected bond distances for crystals 1and 2(Å, °).
Crystal 1
Zn1-N1 2.375(17) Zn1-O1 2.33(2)
Ni1-C1 1.85(2)
Crystal 2
Ni1-C2 1.877(3) Ni1-C1 1.881(3)
Zn1-O1 2.117(3) Zn1-N1 2.131(3)
Zn1-N2vi 2.165(3)
O1-Zn1-O1v177.96(16) O1v-Zn1-N1 89.12(11)
O1-Zn1-N1 92.35(11) N1-Zn1-N1v88.33(15)
O1-Zn1-N2vii 88.27(12) O1-Zn1-N2vi 90.27(11)
N1-Zn1-N2vi 177.39(12) N1-Zn1-N2vii 91.71(10)
Symmetry codes: (v) –x+1, y, −z+1/2; (vi) x+1, y+1, z; and (vii) –x, y+1,−z+1/2 for crystal 2.
Table 3. Hydrogen-bond parameters for crystal 2(Å, °).
D-H•••A D-H H•••A D•••A D-H•••A
C4—H4B•••O1viii 0.97 2.44 3.328 (5) 151
O1—H1A•••O3 0.82 (6) 1.95 (6) 2.765 (4) 172
O1—H1B•••O2 0.82 (6) 1.91 (6) 2.728 (4) 173
Symmetry code: (viii) –x+1, −y+1, −z+1 for crystal 2.
Figure 5. The 3D open framework lled by H2O
molecules in crystal 1.
Figure 6. The 3D open framework lled by the guest D
molecules in crystal 2.
3.2. Spectral characterization of crystals 1 and 2
The vibrational (IR and Raman) spectra of crystals 1and 2are given in Figures 8a and 8b, respectively. The
presence of characteristic vibration peaks of the water ligand molecule and guest water molecule with guest D
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molecules in the vibrational spectra of crystals 1and 2indicated that these involved molecules played a major
role in the formation of crystals 1and 2.
Figure 7. Unit cells of crystal 1(a) and 2(b). Figure 8. Vibrational spectra of crystal 1(a) and 2(b).
The data of the vibration spectra of crystals 1and 2were interpreted separately for the vibrations of the
H2O ligand molecule, [Ni(CN)4]2−ion, and guest H2O with D molecules. According to this interpretation, the
eects of the formation of crystal structures on the spectral data of all of the molecules were clearly revealed.
3.2.1. Vibrations of the H2O ligand and guest molecules
The water molecule is not a linear molecule because it has an angle of about 104.52°between the hydrogen
bonds forming the water molecule. Furthermore, since the O atom that forms the water molecule has higher
electronegativity than the hydrogen atoms in the structure of the water, the O atom carries some negative
charge, while the hydrogen atoms have some positive charges. As a result, the water molecule is a polar
molecule with an electrical dipole moment [18–20].
The water molecule has 3 vibration modes, namely asymmetric stretching, symmetric stretching, and
the bending vibrations of the O-H bond. These vibration modes are theoretically calculated in vibrational
spectroscopy at wavenumbers of 3625, 3520, and 1641 cm−1, respectively [18,20]. These vibration modes were
obtained experimentally in IR and Raman spectroscopies, respectively, as 2 vibration peaks at 3431 and 3448
cm−1with wavenumbers of 1641 and 1648 cm−1in our laboratory studies. Due to the overlapping of the
asymmetric and symmetrical stretching vibrations of the water molecule, a very broad peak with a maximum
value at wavenumbers of 3431 and 3448 cm−1was generated in the IR and Raman spectroscopies, respectively.
Similar situations have been observed in the work of some previous researchers [18,19].
Many scientic studies can be found in the literature about the various properties of the water molecules
and dierent studies in which water molecules were used as ligand molecules [21–25].
The broad peaks seen at 3327 and 3336 cm−1for crystal 1and 3304 and 3312 cm−1for crystal 2,
respectively, in the IR and Raman spectra of the crystals correspond to the asymmetric and symmetric stretching
vibration peaks overlapping with the water ligand molecule. These peaks were shifted to low wavenumbers of
104 and 112 cm−1for crystal 1and 127 and 136 cm−1for crystal 2, according to the liquid water molecule
in the IR and Raman spectra, respectively. The reason for these shifts was the binding of the water ligand
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molecule from the O atom to the Zn transition metal atom. Another shift to a high wavenumber also occurred
in the H-O-H bending vibrations of the water ligand molecule. The reason for these shifts was that the water
ligand molecule bound to the Zn transition metal atom. The values of shifting to high frequencies in the IR and
Raman spectra of the crystals were 18 and 21 cm−1for crystal 1and 21 and 27 cm−1for crystal 2, respectively.
Shifts to these high and low wavenumbers were also observed in studies conducted by other researchers with
dierent ligand molecules [5–7].
There were also some peaks that could not be identied in the vibration spectra of crystals 1and 2. It
was thought that these peaks were composed of their combinations or overtones with the other basic peaks of the
water molecule at smaller wavenumbers. Similar situations have also been observed in other studies [19,24,25]. In
addition, because weak interactions were formed between all of the guest water molecules in crystal 1, the free
OH stretching vibration peak, which should be observed at about 3600 cm−1, was not observed in the IR and
Raman spectra of crystal 1.
3.2.2. [Ni(CN)4]2−group vibrations
The Ni[(CN)4]2−ion has 21 basic vibration modes and a D4hsymmetry group. The IR active vibrational
modes of Ni[(CN)4]2−ions are ν(C≡N), ν(Ni–CN), π(Ni–CN), and δ(Ni–CN). It was clear that the resulting
Hofmann-type clathrates had a square planar structure because the 4 vibration bands of the Ni[(CN)4]2−ions
that dened this structure were found in the IR spectra of crystals 1and 2. The vibration frequencies of the
Ni(CN)4groups in the structures of crystals 1and 2were interpreted by making use of the studies conducted
by McCullough [26].
The characteristic vibration bands belonging to the Ni(CN)4group in K2[Ni(CN)4]•H2O compound
with 1 and 2 crystals were obtained. These vibration bands and the frequency shift values that occur due to the
formation of Hofmann-type compounds are given in Table 4. The frequency shifts in some vibration modes of a
molecule may be to a low frequency region or high frequency region due to changes in environmental conditions
or symmetry conditions during the formation of the compounds. In Table 4, some of these frequency shifts
are shown to have a positive sign for those that shifted to the high frequency region. In addition, some other
frequency shifts shifted to the low frequency region and they are shown with negative signs.
Table 4. Vibrational wavenumbers (cm−1)Ni(CN)4group in crystals 1and 2. Bands in the Raman spectra appear
within parentheses.
Assignment aK2[Ni(CN)4]•H2O 1 ∆2∆
ν(C≡N), A1g(2160) vs (2173) vs +13 (2179) vs +19
ν(C≡N), B1g(2137) m n. o. - n. o. -
ν(C≡N), Eu2122 vs 2156 vs +34 2157 +35
ν(Ni–CN), Eu544 w 540 w –4 544 0
δ(Ni–C–N), B2g(488) w (485) w –3 (480) w -8
π(Ni–C–N), A2u442 w 446 w +4 456 w +14
δ(Ni–C–N), Eu420 s 424 s +4 429 s +9
π(Ni–C–N), Eg(303) s (308) w +5 (312) w +9
ν:Stretching, δ: in plane b ending, π: out-of-plane bending, vs: very strong, s: strong, m: medium, w: weak,
vw: very weak, n. o.: not observed. aTaken from [26].
Furthermore, frequencies of the C≡N group vibrations in the compounds obtained depended on the
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electronegativity of the transition metal atom in that compound, its environment, and the number of bonds it
formed with the oxidation state. The frequency values of the C≡N vibrations in a chemical compound depend
on the type of metal, electronegativity, chemical environment, oxidation state, and number of bonds it makes
[5–7,24,25].
These shifts in the stretching vibration of the cyan group showed that the N atom of the cyan group was
bound to the Zn transition metal atom. These shifts in the stretching vibration of the cyan group were due to
the coupling eect of C≡N and metal-N bond stretching vibrations [27,28].
3.2.3. Guest D molecule vibrations
The D molecule has a centrosymmetric chair conformation, which is typical, such as relatives of cyclohexane
[29,30]. The D molecule is conformationally exible, and the boat conformation is easily adopted, as required
for chelating to metal cations. However, as shown in Figures 4 and 6, the guest D molecule in crystal 2has a
chair conformation.
The frequencies of the vibrational bands arising from the enclathrated D observed in the spectrum of
crystal 2are given in Table 5. The wavenumbers of the D in the liquid and gas phases found by Shimanouchi
are also given in Table 5 for comparison [30]. The vibrational studies showed that the D molecule had a chair
conformation with a center of symmetry of C2hin the gas and liquid phases [29,30]. In our clathrate (in crystal
2), several vibrational bands of the guest D molecule [for example: ν17 (au),ν33 (bu),ν18 (au), and ν35 (bu)]
were split into doublets. When the vibrations of the free liquid phase D were compared with the vibrations of
the guest D molecule in crystal 2, they showed a large variation relative to each other. This was probably due to
the interactions between the vibrations of the guest D molecule and the vibrations of the H2O ligand molecule,
and the hydrogen bond formed between the guest D molecule and the ligand water molecule (see Table 3).
These results indicated that the interactions between the host structure Zn(H2O) 2Ni(CN) 4and the guest (D)
molecule, which formed crystal 2, were strong (the crystal eld eects). In Figures 2 and 6, the presence of
hydrogen bonds between the ligand water molecule and the guest molecule D is clearly apparent in the packed
structure of crystal 2.
Similar observations have also been seen for Hofmann-type D clathrates [31–33] and Hofmann-Td-type
D clathrates [34].
3.3. Thermal behavior of crystals 1 and 2
The TGA and DTG graphics of crystals 1and 2are shown in Figures 9a and 9b. When heated, crystals 1
and 2gradually lost their guest molecules. In the structure of crystal 1, water molecules were present as both
ligand and guest molecules. In the rst step of the thermal analysis, the 6 guest water molecules, which had not
bonded to the crystalline structure, were separated from the crystal structure due to the increased temperature
of the environment. This rst step of the thermal analysis took place for crystal 1in a low temperature range
of 74–92 °C, with a maximum peak of temperature of about 84 °C [found/(calc.)% =28.85/(29.03)%].
In the second step of the thermal analysis, the 2 ligand water molecules, which had bonded to the crys-
talline structure, were separated from the crystal structure due to the increased temperature of the environment.
This second step of the thermal analysis took place for crystal 1in a temperature range of 98–116 °C, with a
maximum peak of temperature of about 108 °C [found/(calc.)% =8.87/(9.68)%].
In the third stage of the thermal analysis, the Ni(CN)4bridges forming the structure of crystal 1were
broken down and the Zn and Ni oxides remained in a temperature range of 398–469 °C, with a maximum peak
of temperature of about 448 °C [found/(calc.)% =40.87/(41.92)%].
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Table 5. Vibrational wavenumber (cm−1)of the D in crystal 2.
Assignment aLiquid D 2
IR Raman IR Raman
ν1(ag),ν20(bg)ia 2966 vs ia 2989 m
ν11(au),ν28 (bu)2961 vs ia 2949 m ia
ν2(ag),ν21(bg)ia 2854 ia 2861 w
ν12(au),ν29 (bu)2854 vs ia 2864 m ia
ν22(bg)ia 1459 ia n.o.
ν30(bu)1453 s ia 1452 m ia
ν3(ag)ia 1443 ia 1443 w
ν4(ag)ia 1396 ia 1400 vw
ν14(au)1366 s ia 1375 w ia
ν23(bg)ia 1334 ia n.o.
ν5(ag)ia 1303 ia 1307 w
ν32(bu)1289 s ia 1293 m ia
ν15(au)1255 s ia 1256 m ia
ν24(bg)ia 1216 ia 1216 w
ν25(bg)ia 1126 ia 1127 w
ν16(au)1122 vs ia 1121 s ia
ν6(ag)ia 1109 ia n.o.
ν17(au)1084 s ia 1084 m
1079 m
ia
ν33(bu)1048 s ia 1049 m
1043 m
ia
ν7(ag)ia 1014 ia 1013 w
ν18(au)874 s ia 875 s
866 s
ia
ν26(bg)ia 853 ia n.o.
ν8(ag)ia 834 ia 831 w
ν35(bu)614 s ia 611 m
622 m
ia
ν27(bg)ia 486 ia Obs.
N9(ag)ia 432 ia Obs.
N10(ag)ia 424 ia Obs.
S, Strong; vs, very strong; m, medium; w, weak; vw, very weak; ia, inactive; n.o., not observed;
Obs. =obscured. aTaken from [30].
Similar thermal degradation steps were observed for crystal 2. In the rst stage of the thermal analysis
of crystal 2, the 3 guest D molecules, each of which formed a hydrogen bond with the ligand water molecule,
were separated from the crystal structure due to an increase in the temperature of the environment. This rst
step of the thermal analysis for crystal 2was in the range of 86–98 °C, with a maximum temperature peak of
about 94 °C [found/(calc.)% =48.87/(50.00)%].
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Figure 9. Thermal graphics of crystals 1(a) and 2(b).
In the second step of the thermal analysis, the 2 ligand water molecules, which had bonded to the crys-
talline structure, were separated from the crystal structure due to the increased temperature of the environment.
This second step of the thermal analysis took place for crystal 2at a temperature range of 100–119 °C, with a
maximum peak of temperature of about 110 °C [found/(calc.)% =6.23/(6.82)%].
In the nal stage of the thermal analysis, the Ni(CN)4bridges forming the structure of crystal 2were
broken down and the Zn and Ni oxides remained in a temperature range of 402–474 °C, with a maximum peak
of temperature of about 441 °C [found/(calc.)% =28.36/(29.53)%].
These thermal decomposition results showed that crystals 1and 2were 2 new examples of Hofmann-type
clathrates. Similar decomposition stages were observed for other Hofmann-type clathrates [5,33,35–39].
3.4. Magnetic moments of crystals 1 and 2
Since crystals 1and 2both had a square planar coordination structure, and their structures had a square
planar Ni2+ (d8)ion and an octahedral Zn2+ (d10 )ion, the total number of unpaired electrons for both of
these crystals was zero. For crystals 1and 2, the theoretically calculated values of the magnetic moments were
0.000 BM, and the experimental magnetic moment values at room temperature under normal conditions were
measured as 0.002 and 0.005 BM, which were very close to the theoretical value, respectively.
4. Conclusions
In this study, 2 new Hofmann-type Zn(H2O)2Ni(CN)4•6(H2O) and Zn(H2O)2Ni(CN)4•3(C4H8O2)clathrates
were successfully synthesized in crystal form. Since the reaction medium contained a sucient number of
water molecules as both the ligand and the guest molecule, a Hofmann-type clathrate with the formula
Zn(H2O)2Ni(CN)4•2(H2O) was formed in the form of a powder in an aqueous medium. In these Hofmann-
type clathrates, the H2O ligand molecule acted only as a monodentate ligand molecule by binding to the Zn
transition metal atoms from the O atom. At the same time, the guest H2O molecules in crystal 1formed O-O
and O-hydrogen bonds among themselves. These bonds played a very important role in the stability of the
crystalline structure.
While the molecules and the groups that made up the structure of both crystal 1and crystal 2were
the same, the volumes of the crystals were determined by the type and number of molecules entering them.
For example, the volume of crystal 2was greater than the volume of crystal 1, because the D molecule was
considerably larger than the water molecule.
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KARTAL et al./Turk J Chem
Furthermore, due to weak interaction forces between the ligand water molecules constituting the host
structure, it was thought that the volume of the host structure could easily change depending on the structures
and volumes of the guest molecules entering the volume of the host structure. Therefore, because of the easily
exible volumes of the host structures formed by the ligand water molecule, it can be considered that they were
more suitable for the storage of certain gases than host structures formed by larger ligand molecules.
In addition, in crystals 1and 2, the Ni(II) ions were surrounded by 4 carbon atoms of 4 cyanide groups
in a square planar arrangement, while the Zn(II) ions were surrounded by 4 N atoms of 4 cyanide groups and
2 O atoms of 2 water molecules in octahedral arrangement.
All of the properties and spectral data of the crystalline compounds obtained in this study showed that
they were in the structure of polymeric clathrates with 2 transition metal atoms of Hofmann-type clathrates. In
some future studies, the crystalline structures of Hofmann-type compounds or clathrates could be also obtained
with other transition metal atoms or dierent guest molecules using the same water ligand molecule. Thus, the
eects of dierent transition metal atoms and dierent guest molecules on the structural properties of the new
Hofmann-type clathrates can be investigated.
Supplementary material
Crystallographic data for the structural analysis have been deposited at the Cambridge Crystallographic Data
Centre, CCDC No. 1851096 for 1and 1854594 for 2. Copies of this information may be obtained free of
charge from the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336033; e-mail:
deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
Acknowledgments
The authors wish to particularly thank Kütahya Dumlupınar University for their technical support (Department
of Physics and Chemistry) and nancial support under project number 2017/25. The authors acknowledge the
Scientic and Technological Research Application and Research Center, Sinop University, Turkey, for the use
of their Bruker D8 QUEST diractometer.
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