Trimorphism of Zn(NCS)2(4-dimethylaminopyridine)2: Crystal Structures, Thermodynamic Relations, and Comparison with the Co(II) Polymorphs

Article abstract of DOI:10.1021/acs.cgd.8b01623

Reaction of Zn(NCS)₂ with 4-dimethylaminopyridine (DMAP) leads to the formation of three polymorphic modifications of Zn(NCS)₂(DMAP)₂ (I-Zn, II-Zn, and III-Zn), of which only the crystal structure of I-Zn is reported in literature. In all three modifications the Zn(II) cations are tetrahedrally coordinated by two terminal N-bonded thiocyanate anions and two DMAP ligands forming discrete complexes, which differ in the dihedral angle between the DMAP ring planes and the arrangement of the complexes in the crystals. All three modifications show melting, and the thermodynamic relations and the transition behavior were investigated by temperature-dependent X-ray powder diffraction, differential scanning calorimetry, thermomicroscopy, and solvent-mediated conversion experiments. These reveal that I-Zn and II-Zn as well as II-Zn and III-Zn are related by enantiotropism, whereas I-Zn and III-Zn are related by monotropism. The thermodynamic transition temperature between I-Zn and II-Zn is much lower than that between the isotypic Co(II) modifications I-Co and II-Co reported recently.

Full text of DOI:10.1021/acs.cgd.8b01623

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Article
Trimorphism of Zn(NCS)2(4-dimethylaminopyridine)2: Crystal Structures,
Thermodynamic Relations and Comparison with the Co(II) Polymorphs
Tristan Neumann, Inke Jess, Luzia S. Germann, Robert E. Dinnebier, and Christian Näther
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01623 • Publication Date (Web): 19 Dec 2018
Downloaded from http://pubs.acs.org on December 24, 2018
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Trimorphism of Zn(NCS) (4-dimethylaminopyridine) :
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Crystal Structures, Thermodynamic Relations and
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Comparison with the Co(II) Polymorphs
†
†
‡
‡
,†
Tristan Neumann, Inke Jess, Luzia S. Germann, Robert E. Dinnebier, and Christian Näther*
†Institut für Anorganische Chemie, Christian-Albrechts-Universität zu Kiel, Max-Eyth-Straße 2, 24118 Kiel,
Germany.
‡
Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany.
Abstract: Reaction of Zn(NCS) with 4-dimethylaminopyridine (DMAP) leads to the formation of three
2
polymorphic modifications of Zn(NCS) (DMAP) (I-Zn, II-Zn and III-Zn), of which only the crystal
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structure of I-Zn is reported in literature. In all three modifications the Zn(II) cations are tetrahedrally
coordinated by two terminal N-bonded thiocyanate anions and two DMAP ligands forming discrete
complexes, which differ in the dihedral angle between the DMAP ring planes and the arrangement of the
complexes in the crystals. All three modifications show melting and the thermodynamic relations and the
transition behavior were investigated by temperature dependent X-ray powder diffraction, differential
scanning calorimetry, thermomicroscopy and solvent mediated conversion experiments. These reveal that
I-Zn and II-Zn as well as II-Zn and III-Zn are related by enantiotropism, whereas I-Zn and III-Zn are
related by monotropism. The thermodynamic transition temperature between I-Zn and II-Zn is much lower
than that between the isotypic Co(II) modifications I-Co and II-Co reported recently.
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INTRODUCTION
The synthesis of new coordination compounds and polymers, inorganic-organic hybride compounds and
metal organic frameworks is still a fast growing field in chemistry, as they show potential for present or
future applications.1-10 For any application some information on the stability of a given compound is required,
which is especially the case for stable and metastable forms like, e.g. polymorphs or isomers. In this case,
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the chemical composition is constant and all differences in the physical properties, including thermodynamic
parameters, depend entirely on the differences in the crystal structures and can be compared directly.11-14 On
the other hand, even if several polymorphic or isomeric coordination compounds are reported, in most cases
their thermodynamic relations are usually not investigated.14-30 Sometimes conclusions on the stability were
drawn based on the comparison of the densities, the fact that one form transforms into another or by
computational methods.31-35 In other cases, crystals of two modifications were obtained and one of these
forms can be prepared pure, whereas no access to the other form was found and thus, it is concluded that the
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latter, frequently called “disappearing polymorph” is metastable, which is not necessarily the case. Finally,
these results are sometimes discussed in context of the crystal structure, where e.g., some short intermolecular
contacts are held to be responsible for the stability of a given form.37, 38 All these might be incomplete because
for definite conclusions the exact thermodynamic relations must be known which for coordination
compounds is difficult to investigate. In contrast to organic compounds, usually no melting or polymorphic
transition is observable and therefore, helpful thermodynamic rules like the heat of transition, the melting
enthalpy or melting entropy rule cannot be used.39-42 Therefore, there are only a few examples where the
thermodynamic relations of coordination compounds are investigated in more detail.25, 43-45 In some cases,
the transformation of one form into the other is observed by, e.g. X-ray diffraction. Nevertheless, without
measuring the direction of the heat-flow usually no conclusions can be drawn whether two forms are related
by enantiotropism or monotropism.38, 46-51 Finally, even if one form shows a transition into a different form
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in the solid state, this temperature is usually much higher than the true thermodynamic transition temperature
(TTrs). In summary, detailed investigations on the thermodynamic relations and the transition behavior of
polymorphs or isomers of coordination compounds are difficult to perform but of extraordinary importance
for any discussion on the stability of such compounds.
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In this context, we recently reported on two polymorphic modifications (I-Co and II-Co) of a simple
discrete complex with the composition Co(NCS) (DMAP) with 4-dimethylaminopyridine abbreviated as
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DMAP. The Co cations are coordinated by two N-bonding thiocyanate anions and two DMAP co-ligands.44
Form I-Co crystallizes in the monoclinic space group P2 /c and is exclusively obtained by water removal
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from the hydrate Co(NCS) (DMAP) (H O) ∙ 2 H O. Form II-Co also crystallizes in a monoclinic space
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group (P2 /m) and is synthesized by methanol removal from the solvate Co(NCS) (DMAP) (CH OH) .
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Solvent mediated conversion experiments prove, that form II-Co represents the thermodynamic stable form
at room-temperature. DSC measurements reveal that form I-Co has the higher melting point and the lower
heat of fusion and therefore, both modifications have to be related by enantiotropism with form I-Co being
stable at higher temperatures. From the melting points and the heats of fusion the thermodynamic transition
temperature was estimated to be about 153 °C, which is in good agreement with the transition temperature
determined by annealing experiments of 145 °C. For the room-temperature polymorph II-Co single crystals
were obtained, which was not possible for the high temperature (HT) form I-Co, which becomes
thermodynamic stable at about 145 °C. In this context, it is noted that in most cases the corresponding
Zn(NCS) compounds show a tetrahedral coordination and are frequently isotypic to their Co(II)
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counterparts. Therefore, to retrieve structural information on I-Co, we prepared Zn(NCS) (DMAP) and we
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found that it is isotypic to the HT form I-Co. The easy formation of these crystals was somehow surprising,
because the intermolecular interactions should in principle be dominated by the surrounding ligands and only
slightly influenced by the metal center that exhibits the same charge and a similar ionic radius. If there would
be a simple relationship between structure and stability, one would expect that the thermodynamic transition
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temperature for each pair of Co and Zn compounds would be similar. This means that in the case, where one
form is thermodynamically stable, e.g., at room-temperature, the isotypic form might behave similar, as
otherwise no reliable conclusions can be drawn. Consequently, we assumed that the crystals of the Zn
compound are also metastable at room-temperature and thus, were accidently obtained by kinetic control.
However, based on all these results we assumed that Zn(NCS) (DMAP) might crystallize in two different
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modifications too and thus we tried to prepare crystals of the form that should be isotypic to the form of
Co(NCS) (DMAP) that is thermodynamic stable at room-temperature (II-Co). Surprisingly, independent of
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the reaction conditions, at room-temperature always single crystals isotypic to the HT form (I-Co) were
obtained. According to this, we decided to investigate the solid-state properties of Zn(NCS) (DMAP) in
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more detail. In the course of this investigations we obtained three different polymorphic modifications of
which two (I-Zn and II-Zn) are isotypic to the corresponding Co(II) forms (I-Co and II-Co), whereas the
third form (III-Zn) was not detected with Co(II). All three forms show melting, which is rarely observed for
coordination compounds with the melting point of III-Zn being higher than that of II-Zn and lower than that
of I-Zn (T : II-Zn < III-Zn < I-Zn). Therefore, according to the generally accepted rules for the
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denomination of polymorphic forms, which suggests that the different forms should be sorted according to
their melting point, III-Zn should be denoted as II but for comparison with the corresponding Co(II)
modifications we deviate from this rule. Here we report on the results of these investigations.
EXPERIMENTAL SECTION
Reagents.Ba(NCS) ·3H O, was purchased from Alfa Aesar, KSCN, Zn(NO ) ∙6H O and Zn(SO )·H O were
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3 2
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bought from Merck. All chemicals were used without further purification. Zn(NCS) was prepared by the
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reaction of equimolar amounts of Zn(SO )·H O and Ba(NCS) ·3H O in water. The resulting white precipitate
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of BaSO was filtered off and the filtrate was concentrated to complete dryness resulting in white residue of
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Zn(NCS) . The purity was checked by X-ray powder diffraction (XRPD).
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Synthesis of I-Zn. The synthesis of single crystals is described elsewhere. Crystalline powders were
obtained by adding a methanolic solution (1.5 mL) of DMAP (122 mg, 1.0 mmol) to a stirred solution of
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Zn(NCS) (90 mg, 0.5 mmol) in methanol (2.5 mL).
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Synthesis of II-Zn. Crystalline powders were obtained by completely dissolving Zn(NCS) (0.25 mmol, 45
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mg) in water. Afterwards DMAP (0.5 mmol, 61 mg) was dissolved in water (2.5 mL) and added to the
solution of Zn(NCS) . The mixture was stirred for 4 d. It is noted that this procedure not always led to phase
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pure powders of II-Zn, in most cases I-Zn is contained as an impurity.
Synthesis of III-Zn. Crystalline powders were obtained by adding a methanolic solution (1.5 mL) of DMAP
(122 mg, 1.0 mmol) to a solution of Zn(NCS) (90 mg, 0.5 mmol) in methanol (2.5 mL) after stirring for 15
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s the precipitate of III-Zn was filtered off. Longer stirring resulted in mixtures of II-Zn and I-Zn.
IR and Raman spectroscopy. The IR data were measured using a Bruker Vertex70 FT-IR Spectrometer
-1
with VERTEX FM module with a resolution of 3 cm . The Raman data were measured using a Bruker RAM
II – 1064 FT-Raman module with R510-N/R Nd:YAG-Laser with the wavelength of 1064 nm and a D418-
-1
T/R Ge-detector with a resolution of 3 cm .
Thermoanalytical measurements. DTA-TG measurements were performed in a dynamic nitrogen
atmosphere in Al O crucibles using a STA PT 1600 thermobalance from Linseis. The DSC experiments
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were performed using a DSC 1 star system with STARe Excellence software from Mettler-Toledo AG under
dynamic nitrogen atmosphere and thermomicroscopy was performed using a hot-stage from Linkam and a
microscope from Olympus. All instruments were calibrated using standard reference materials.
X-Ray Powder Diffraction (XRPD). The XRPD measurements for phase identification were performed by
using a Stoe Transmission Powder Diffraction System (STADI-P) equipped with a Ge(111) Johann-type
monochromator from STOE & CIE with CuK radiation that was equipped with a linear position-sensitive
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MYTHEN 1K detector from Dectris Ltd. X-ray powder diffraction patterns for structure determination, low
temperature and high temperature measurements were performed on a Cu Kα Stoe STADI-P powder
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diffractometer, equipped with a Ge(111) Johann-type monochromator from STOE & CIE, an Oxford
cryostream 800 and an array of three Mythen 1K from Dectris Ltd. Details on the structure determinations
can be found in the Supplemental Material, selected crystal data and details on the structure refinements as
well as differences plots are given in Table S1 and Figures S1-S3 in the supplemental.
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CCDC-1875955 I-Zn, CCDC-1875956 II-Zn and CCDC-1875957 III-Zn contain the supplementary
crystallographic data for this paper. These data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.
RESULTS AND DISCUSSION
Preliminary synthetic investigations. Based on the synthetic results obtained for the corresponding Co
modifications, where both forms (I-Co and II-Co) were selectively prepared by solvent removal from
Co(NCS) (DMAP) (H O) ∙ 2 H O and Co(NCS) (DMAP) (CH OH) , we tried to prepare the corresponding
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solvato complexes with Zn(II) but none of these compounds formed. However, from ethanol I-Zn, isotypic
to I-Co, is obtained as a pure compound (Figure S4). Additional experiments with water as solvent sometimes
led to the formation of a further crystalline product that is isotypic to II-Co, confirmed by Rietveld refinement
using the crystallographic data of II-Co as starting model (Figure S2 and S5). In most cases, products were
synthesized that are contaminated with I-Zn, but a few batches of pure II-Zn were obtained for further
investigations (Figure S6). To find a reproducible procedure for the synthesis of II-Zn many experiments
were performed. Whenever the synthesis was performed in methanol and the product was immediately
filtered off, a further phase (III-Zn) has formed by kinetic control. In its powder pattern, no reflections of
I-Zn and II-Zn were observed, indicating that a pure sample was obtained. DTA-TG investigations reveal
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that no solvate was obtained and that a similar thermal behavior to that of I-Zn and II-Zn is observed (Figure
S7). Because no single crystals were available, its structure was solved ab initio from XRPD data. The
experimental pattern was compared with the calculated one, revealing that III-Zn was obtained as a pure
form (Figure S3 and S8). It is noted, that most batches of III-Zn very often contained I-Zn as an impurity
and we assumed that the former is metastable at room temperature and formed as kinetic product. Therefore,
several additional experiments were performed to obtain pure samples under kinetic control, but in most
cases I-Zn was obtained (Figure S9 and S10).
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To investigate if one can distinguish between the different modifications by spectroscopy, IR and Raman
measurements were performed. For all three forms values for the CN stretching vibration between 2056 and
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068 cm are observed in the IR and between 2064 and 2079 cm in the Raman spectra, which are typical
for terminal N-bonded thiocyanate anions (Figure S11-S13). In contrast to the other two polymorphs, for
-1
III-Zn only one peak is observed in the Raman spectrum at 2075 cm and this form also shows the lowest
value for the CN stretch in the IR spectrum, which is used to differentiate between the three polymorphs
(Figure S13). Distinct differences are also found in the IR and Raman spectra for I-Zn and II-Zn (Figure
S11 and S13).
Crystal structures of form I, II and III of Zn(NCS) (DMAP) . Form I-Zn was already reported in
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44
literature. It crystallizes in the monoclinic space group P2 /m with Z = 2 and the Zn atoms are located on a
1
crystallographic mirror plane (Table 1). This compound is isotypic to the corresponding Co(II) modification
4
4
I-Co. In its crystal structure the Zn(II) cations are coordinated by two terminal N-bonded thiocyanate anions
and two N atoms of the DMAP ligand, within a slightly distorted tetrahedral geometry (Figure 1: top). The
same coordination is found in the two other polymorphic forms II-Zn and III-Zn. These modifications
crystallize in different monoclinic space groups P2 /c with Z = 4 (II-Zn) and C2/c (III-Zn) with Z = 8 and
1
all atoms in general positions (Table 1).
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Table 1. Selected crystal data for I-Zn, II-Zn and III-Zn obtained by Rietveld refinements of X-ray powder
diffraction patterns measured at room-temperature.
Form
I-Zn
II-Zn
III-Zn
space group
a / Å
P2 /m
P2 /c
C2/c
1
1
5.50439(11) 13.9109(2) 7.44073(16)
15.2027(3) 9.69175(13) 18.0648(5)
12.0555(3) 14.7282(2) 31.4110(8)
97.3063(14) 90.2727(11) 91.0479
1000.63(4) 1985.64(5) 4221.41(18)
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b / Å
c / Å
β / °
volume / Å3
Z
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 . / cm ·g
1.414
1.425
1.340
calc
Differences between all three forms are found in the conformation of the DMAP ligands, because the dihedral
angles between the planes of the pyridine rings are always different, which is obvious if their crystal
structures are superimposed (Figure 1: bottom). In I-Zn, both pyridine rings are coplanar, whereas in II-Zn
they are nearly perpendicular to each other and in III-Zn the dihedral angle is the largest with 113.3 ° (Table
2). As expected for strong donor groups the dimethylamino groups are planar and are in the plane of the
6
-membered rings (Table 2).
Table 2. Interplanar angles between the planes of the 6-membered DMAP rings (A) and between these phenyl
rings and the dimethylamino groups (B), as well as angle sum around the dimethylamino N atom (C) in I-Zn,
II-Zn and III-Zn.
A / °
0
B / °
C / °
I-Zn
2, 2
360.0
II-Zn
III-Zn
113.3
80.8
0.1 (N12), 1.0 (N22)
0.4 (N12), 6.9 (N22)
360.0, 360.0
360.0, 359.5
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Figure 1. View of a discrete complex in the crystal structure of form I-Zn as a representative (top) and a plot
of the complexes observed in I-Zn (black), II-Zn (red) and III-Zn (blue; bottom), where the Zn(NCS) units
2
are superimposed.
In the crystal structures of all three modifications, the arrangement of the discrete complexes is completely
different. In I-Zn the building units are arranged into columns that elongate along the crystallographic a-
axis. Weak intermolecular C-H···S hydrogen bonding between the thiocyanate S atoms and the C-H
hydrogen atoms of the DMAP ligand are found, which finally link the discrete complexes into a 3D network
(Figure 2: top).
In the crystal structure of II-Zn neighboring complexes are arranged in a way, that each two DMAP ligands
form pairs, in which the dimethylamino groups of one complex are located above the 6-membered ring plane
of a neighboring complex (Figure 2: mid). Even in this structure the discrete complexes are linked by
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intermolecular C-H···S hydrogen bonding into a 3D network (Figure 2: mid). In the crystal structure of
III-Zn a more complex packing of the discrete complexes is observed. As it is the case for the other two
modifications the building units are linked by a number of C-H···S hydrogen bonds (Figure 2: bottom). It is
noted, that for the isotypic modifications I-Co and II-Co very similar intermolecular contacts were
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observed.44
Table 3. Hydrogen bonds [Å and °] in I-Zn, II-Zn and III-Zn.
I-Zn
D(H∙∙∙A) (Å) d(D∙∙∙A) (Å)
<(DHA) ( °)
138.3
C17-H17B...S1
C17-H17C...S1
C16-H16B...S2
II-Zn
2.97
2.90
2.93
3.756(3)
3.820(3)
3.841(3)
156.1
155.9
S1-H17A
S1-H17B
S2-H21
3.1
4.064
4.064
3.453
3.784
3.580
165.3
142.2
153.2
155.0
136.1
3.13
2.57
2.89
2.80
S2-H11
S2-H27B
III-Zn
S1-H22
3.02
3.19
3.33
3.20
2.94
2.91
3.826
3.923
4.291
4.175
3.808
3.740
141.8
135.1
165.1
170.1
151.3
145.9
S1-H24
S1-H26
S1-H27
S2-H12
S2-H14
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Figure 2. Crystal structure of I-Zn (top), II-Zn (mid) and III-Zn (bottom) with intermolecular C-H···S
interactions shown as dashed lines.
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Investigations on the transition behavior and the thermodynamic relations
Solvent mediated conversion experiments
To investigate which of the three forms represents the thermodynamically stable form at room-temperature,
solvent mediated conversion experiments were performed, in which mixtures of I-Zn and II-Zn were stirred
in either ethanol or n-butanol and the residues obtained at different times were identified by XRPD (Figure
S14 and S15). These experiments show that form II-Zn disappears after a few days and only I-Zn remains,
which indicates that I-Zn is the thermodynamically stable form at room-temperature. Similar experiments
for III-Zn show that this form also transforms into I-Zn, which means that II-Zn as well as III-Zn are
metastable at room-temperature (Figure 3). In a further experiment, a mixture of II-Zn and III-Zn was stirred
in ethanol, which reveals that the crystals of III-Zn disappear within a few minutes and that within 4 d the
remaining amount of II-Zn transforms into I-Zn (Figure 3).
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Figure 3. XRPD pattern of a suspension of II-Zn and III-Zn stirred in ethanol and of the residues obtained
after 10 min and 4 d together with the calculated powder pattern of I-Zn, II-Zn and III-Zn. Characteristic
reflections are indicated by red (III-Zn), blue (II-Zn) and black (I-Zn) perpendicular lines.
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Based on these experiments one can conclude that at room-temperature the order of stability is III-Zn <
II-Zn < I-Zn. This result is somehow surprising, because the corresponding Co(II) modification I-Co is
metastable at room-temperature and because both forms (I-Co and I-Zn) are isotypic, one would assume that
the intermolecular interactions and thus, the stability would be similar. Finally, similar experiments at lower
temperatures indicate, that I-Zn is also the thermodynamically stable polymorph at -34 °C (Figure S16 and
S17).
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Temperature dependent XRPD
To check if II-Zn or III-Zn show polymorphic phase transitions, high temperature XRPD experiments were
performed. II-Zn transforms into I-Zn at about 120 °C (Figure 4: top and Figure S18). A similar behavior is
observed for III-Zn, which also transforms into I-Zn but at significantly higher temperature (Figure 4:
bottom and Figure S19). It is noted, that the II-Zn sample contained a small amount of I-Zn as side phase
(ca. 2 w%), whereas III-Zn seemed to be phase pure, see Figure 4. In both cases, the low temperature (LT)
and high temperature (HT) phase coexist over a larger temperature range, which is typical for polymorphic
solid to solid polymorphic transformations that proceed via nucleation and growth of a new phase, which
also means that in this case a transformation via melting and crystallization can be excluded (Figure S20 and
S21). There is no indication that one of the forms II-Zn or III-Zn occurs as intermediate in the heating of
the other polymorph, before their transformation into I-Zn is observed. On further heating the reflections of
I-Zn disappear, which indicates melting of I-Zn. After cooling to room-temperature, the reflections of I-Zn
are observed again, which proves that melting and crystallization had occurred (Figure 4). Because both
polymorphs (II-Zn and III-Zn) transform into I-Zn, the latter should also be thermodynamically stable at
high temperatures. No reversible polymorphic transition from I-Zn into II-Zn nor III-Zn was observed by
low temperature XRPD measurements down to -180 °C (Figure S22). From these experiments no conclusions
can be drawn whether these forms are related by enantiotropism or monotropism for which the direction of
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the heat-flow needs to be measured and therefore, measurements using differential scanning calorimetry were
performed.
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Figure 4. Temperature dependent XRPD measurements of II-Zn (top) and III-Zn (bottom).
DSC and thermomicroscopic investigations
To investigate the transition behavior of all forms DSC measurements were performed at a heating rate of
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0 °C/min. Upon heating I-Zn an endothermic peak is observed at 173 °C, which should correspond to the
melting point of this form, because melting of the corresponding Co(NCS) (DMAP) polymorph I-Co is
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2
observed at exactly the same temperature (Figure 5).44 A similar measurement on II-Zn shows two
endothermic events at 148 and 172 °C and one exothermic peak at 154 °C (Figure 5). The overall behavior
is similar to that of the isotypic modification II-Co, which transforms into I-Co via melting and
crystallization and if one assumes that the first endothermic event corresponds to the melting of II-Zn, its
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melting temperature is about 20 °C lower than that of II-Co (T = 168 °C). After melting of II-Zn,
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recrystallization is observed at 154 °C, which might lead to the formation of I-Zn. It is noted, that there is an
additional but very weak signal at 169 °C, which in previous investigations was not observed in the DSC
measurements of II-Co.44
DSC measurements for III-Zn show only one peak at 167 °C, which is nearly at the same temperature as the
second endothermic event in the measurement of II-Zn. Thus, it can be assumed that in this case II-Zn
transforms via melting and recrystallization into a mixture of I-Zn and III-Zn or into pure III-Zn that upon
further heating transforms into I-Zn via melting and recrystallization, which cannot be resolved. The overall
thermal behavior is much more complicated than that of the two Co(II) dimorphs (I-Co and II-Co).
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Figure 5. Heating and cooling DSC curves for I-Zn, II-Zn and III-Zn at 10 °C/min. Given are the peak
temperatures (T ) in °C.
p
However, upon cooling after the last endothermic signal, no exothermic peak is observed and XRPD
investigations prove, that cooling the melt of I-Zn leads to an amorphous residue, whereas the same
procedure for III-Zn leads to the formation of a poorly crystalline sample, in which a few and very weak
reflections of III-Zn and an unknown crystalline phase are observed (Figure S23). The IR and Raman spectra
-1
of the amorphous residue show values of the CN stretch at 2056 and 2075 cm , close to that for III-Zn and
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typical for terminal N-bonded thiocyanate anions (Figure S24). It is also noted, that on reheating the
amorphous samples a very broad exothermic signal is observed that corresponds to incomplete
recrystallization leading to the formation of III-Zn together with an unknown residue (Figure S25 and S26).
To additionally prove that the first endothermic event of II-Zn and III-Zn corresponds to melting of these
forms, additionally DSC measurements were performed, in which a small amount of I-Zn was added, which
in this case should act as nuclei for the crystallization of this form (Figure 6). In this case the integral of the
melting peak of II-Zn is much lower and that of I-Zn much higher, obviously because crystallization of I-Zn
immediately starts during melting of II-Zn (Figure 6). A similar observation was made for III-Zn, which
only shows one peak, which should correspond to melting. If a small amount of I-Zn is added, the integral
of the melting peak of III-Zn is lower and a second peak is observed that corresponds to melting of I-Zn
(Figure 6). These experiments strongly indicate that in this case the transformations proceeds via melting and
recrystallization of a new form.
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Figure 6. DSC curves of II-Zn and III-Zn and a mixture of II-Zn or III-Zn with a small amount of I-Zn at
10 °C/min. Given are the peak temperatures (T ) in °C.
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This is further supported by thermomicroscopic measurements on all forms, which additionally prove that
the melting temperatures of III-Zn and I-Zn are very similar (Figure 7). In thermomicroscopic measurements
of II-Zn only melting of I-Zn, which formed upon heating, can clearly be identified. There are some optical
changes in the powders at temperatures, where II-Zn should melt, but obviously melting and crystallization
cannot successfully be resolved in this case (Figure S27).
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Figure 7. Microscopic images of I-Zn (left) and III-Zn (right) at different temperatures measured by
thermomicroscopy.
Unfortunately, in the DSC measurements presented above only transformations via the melt are observed and
thus, the heat of transition rule cannot be used to determine the thermodynamic relations. Therefore, we
investigated whether the transformations of II-Zn and III-Zn into I-Zn can be suppressed by faster heating
to finally determine the melting enthalpies (Figure 8). Therefore, heating rate depend measurements were
performed (Figure 8). In this context it is noted that at 1 °C/min the endo-exo-endo sequence for II-Zn and
III-Zn is much better resolved, which proves again that these transitions proceed via the melt.
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Figure 8. DSC curves of II-Zn (top) and III-Zn (bottom) with different heating rates.
If the heating rate is increased, only one single endothermic peak is observed for both forms, which in the
case of II-Zn needs a heating rate of at least 40 °C/min. This indicates that the transition of II-Zn respectively
III-Zn into I-Zn is suppressed (Figure S8 and Figure S28). If the melting enthalpies were determined always
strongly varying values were obtained for different batches of II-Zn, which despite the experimental error
might be traced back to the fact that crystallization into I-Zn was not be completely suppressed. Therefore it
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was not possible to extract reliable melting enthalpies from these measurements. However, for II-Zn
integration over the first endothermic and the exothermic signal always leads to positive values, indicating
an overall endothermic process and therefore, an enantiotropic relationship can be expected.
However, as shown in the temperature dependent XRPD measurements, for some batches a solid to solid
polymorphic transition of II-Zn or III-Zn was observed, which is obvious because the corresponding
transition temperatures are much lower than the melting temperatures of both forms. The fact, that in the
temperature dependent XRPD the transition proceeded via the solid, whereas in the DSC measurements via
melting and crystallization of a new phase, is not unusual. On one hand, such transitions depend on the history
of a sample and the kinetics of a transition. It is typical for polymorphic transitions, that each nuclei has its
own pre-coded transition temperature, which also depends on defects. This means that in the solid, deviating
transition temperatures are usually observed and that in some cases, e.g. by fast heating of well crystalline
samples the polymorphic transition can be suppressed, which allows to determine the melting point of a
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metastable form. Compared to DSC measurements, the temperature dependent XRPD measurements need
more time to be measured, which is similar to annealing a sample below the melting temperature, which can
lead to the transformation into the stable form. Finally, it is also possible that traces of I-Zn were present that
induced the transformation into this phase. In this context it is mentioned that for one batch of II-Zn a less
intense and very broad endothermic signal is observed at 130 °C, notably below the melting point of this
form, which is typical for polymorphic solid-to-solid transitions (Figure 9: top). If the residues formed at the
peak of this signal and after this signal are investigated by XRPD, it is obvious that a mixture of I-Zn and
II-Zn, respectively nearly pure I-Zn has formed (Figure S29). Because this transition is endothermic,
according to the heat of transition rule both forms have to be related by enantiotropism, as it is the case for
the corresponding Co(II) modifications. From the heat of transition H (II-Zn I-Zn) (3.3-3.5 kJ/mol) and
trs
the melting enthalpy of I-Zn H (I-Zn) (30.6-31.1 kJ/mol) formed by the polymorphic transition, the
fus
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melting enthalpy of II-Zn H (II-Zn) can be estimated to be between 34.1 and 34.4 kJ/mol (Table S2 and
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S3). It is noted that H (I-Zn) determined directly from DSC measurements of I-Zn (32.5-34.0 kJ/mol) is
fus
always higher than the former value, which indicates that the endothermic transition does not lead to pure
samples of I-Zn (Table S4).
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Figure 9. DSC curves of additionally batches of II-Zn and III-Zn. The inset shows an enlargement of the
preliminary endothermic, respectively exothermic transition.
Finally, it is mentioned that DSC measurements of III-Zn lead to values for the melting enthalpy

H (III-Zn) between 28.4 and 29.9 kJ/mol, which are always lower than those for I-Zn and II-Zn (Table
fus
S5 and S6). Moreover, for one batch of III-Zn a very weak exothermic signal is observed before melting,
which indicates a polymorphic transition into I-Zn (Figure 9). If the residue obtained after the exothermic
event is investigated by XRPD it is obvious that some amount of III-Zn has been transformed into I-Zn
(Figure 10: A).
In a second DSC run a sample of III-Zn was seeded with a small amount of I-Zn and in this case the
exothermic polymorphic transition is nearly complete (Figure 10: B). Finally, if this mixture is annealed for
2
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(Figure 10: C). Because the polymorphic transition of III-Zn into I-Zn is exothermic, both forms should be
related by monotropism.
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Figure 10. XRPD pattern of the residues obtained after the exothermic event in a DSC measurement of
III-Zn (A) and a mixture of III-Zn and I-Zn (B) and of a mixture of these forms that was annealed at 150 °C
for 2 hours (C) together with the calculated powder pattern of I-Zn and III-Zn.
Thermodynamic relations, energy temperature diagram and comparison with the cobalt polymorphs
From all previous results, which are summarized in Figure 11, the semi-empirical energy temperature
diagram can be established, showing the thermodynamic relations between all three polymorphic
modifications.
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Figure 11. Overview of the experimental results on the thermal and transition behavior of I-Zn, II-Zn and
III-Zn, from which the thermodynamic relations can be determined (see below). Please note that batch 1 and
2 refer to arbitrary batches that behave differently depending on the history of the sample, the actual heating
rate or if, e.g. some very small amount of I-Zn are present that cannot be proven by XRPD.
II-Zn exhibits the highest density (Table 1) and according to the density rule should exhibit the lowest Gibbs
energy at absolute zero and therefore, should represent the thermodynamic stable form at low temperatures
followed by I-Zn and III-Zn (Figure 12). Upon heating II-Zn transforms endothermically into I-Zn, which
proves that both forms behave enantiotropically with II-Zn being stable at low temperatures, in agreement
with the density of the forms. I-Zn exhibits the highest melting temperature and thus is the thermodynamic
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stable form at high temperatures up to the melting point. The enantiotropic relation between I-Zn and II-Zn
is also in agreement with the melting enthalpy rule, because the higher melting polymorph I-Zn exhibits the
lower melting enthalpy, even if the difference seems to be extremely small and deviating values were
obtained (Table 4).
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Table 4. Melting enthalpies and melting temperatures for I-Zn, II-Zn and III-Zn.
-1
T / °C
fus
ΔH / kJmol
fus
I-Zn
II-Zn
III-Zn
173
148
167
30.6 - 34.0
34.1 - 34.4
28.4 - 29.9
Figure 12. Qualitative energy temperature diagram for I-Zn, II-Zn and III-Zn.
Concerning the thermodynamic relation between I-Zn and the new form III-Zn the exothermic transition of
III-Zn into I-Zn indicates that both forms are related by monotropism with I-Zn as the stable polymorph.
However, in the case of enantiotropism an exothermic transition is also expected if the transformation occurs
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below the thermodynamic transition temperature but because III-Zn definitely exhibits a lower melting point
and a lower melting enthalpy than I-Zn the monotropic relation is obvious (Figure 12). Finally, even if no
transition between III-Zn and II-Zn was observed, we found that the melting point of III-Zn is significantly
higher than that of II-Zn and that the higher melting polymorph III-Zn definitely exhibits the lower melting
enthalpy and therefore, these two forms should be related by enantiotropism (Figure 12).
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The enantiotropic relation between II-Zn and I-Zn is similar to that of the cobalt forms II-Co and I-Co but
surprisingly, unexpected differences in the thermodynamic transition temperatures between that of II-Zn and
I-Zn and that of II-Co and I-Co were found, which means that I-Co is metastable at room-temperature,
whereas I-Zn is thermodynamically stable. This is surprising, because both isotypic modifications I-Co and
I-Zn as well as II-Co and II-Zn consist of very similar cations of identical charge and an identical or at least
very similar surrounding, which should lead to comparable intermolecular interactions. Therefore, at first
glance, for both forms a similar thermodynamic stability would be expected. We also carefully checked the
results of the solvent mediated conversion experiments, because unstable solvates might form, that on
isolation of the solids decompose into a metastable form. Therefore, these experiments were repeated in a
closed capillary, from which experiments no hints for an additional phase were found. Even in this case it is
obvious that from a mixture of I-Zn and II-Zn the crystals of II-Zn start to disappear at first.
Moreover, according to our findings unexpected differences in the thermodynamic transition temperatures
between that of II-Zn and I-Zn and that of II-Co and I-Co were found, which means that I-Co is metastable
up to about 145°C, whereas I-Zn is already stable just below room-temperature. This large difference in the
thermodynamic transition temperatures appears to be unusual large. However, the thermodynamic transition
temperature of the Co(II) polymorphs was determined by thermal annealing of powders and one can assume
that in fact it is lower than determined. But it is unlikely that mixtures of I-Co and II-Co transform into I-Co
at 145°C but not at 135°C if the thermodynamic transition temperature would be much lower and the
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experimental value of 145°C is in good agreement with that estimated from the melting temperatures and
melting enthalpies of 153°C.
We also estimated the thermodynamic transition temperature between II-Zn and I-Zn from the melting
points and melting enthalpies according to the following equation:
0
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푇 푇f(II)(∆퐻f(II) ― ∆퐻 )
f(I)
f(I)
푇 =
trs
푇 ∆퐻
f(II)
― 푇f(II)∆퐻f(I)
f(I)
This lead to transition temperatures between -55 and 32 °C, depending on the values that were used for the
melting enthalpies for I-Zn and II-Zn. However, in any case they are much smaller than those estimated for
the corresponding Co(II) polymorphs. In this context, it is noted that the calculated transition temperature
decreases if the difference between the melting enthalpies decreases and from our DSC measurements it is
indicated, that this difference between I-Zn and II-Zn is very small and within the experimental error of the
measurements.
That in any case relatively large differences between the Co(II) and the Zn(II) polymorphs must occur is
already obvious, because the melting temperatures of I-Co and I-Zn are identical, whereas that of II-Zn is
much lower than that of II-Co. Moreover, the difference in melting enthalpy between the Co(II) polymorphs
is significantly larger than that between the Zn(II) forms, which also shifts the thermodynamic transition
temperature for the latter to lower temperatures.
CONCLUSION
This work is based on previous investigations on discrete complexes with the composition
Co(NCS) (DMAP) that exist in two polymorphic modifications with II-Co being thermodynamically stable
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below about 145 °C and I-Co above this temperature up to the melting point. For structure determination
of I-Co which is metastable at room-temperature the corresponding Zn(II) compound I-Zn was crystallized,
which was always obtained in these experiments. Consequently, it was assumed that I-Zn is metastable at
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room-temperature and was accidently obtained by kinetic control. Later it was found that the situation is
completely different, because I-Zn is already thermodynamically stable at room-temperature. This is
surprising, because each pair of the Co and the Zn polymorphs is isotypic, which means that similar
intermolecular interactions are expected. In this context it is noted that the difference in the thermodynamic
transition temperatures appears to be very large, but originates from our experimental investigations.
However, independent on the exact value of this difference one can imagine that both cations, Co(II) and
Zn(II) fit perfectly into the structure type of form I, leading to comparable melting points, which might not
be the case for form II. This will not only influence the enthalpies of the modifications but also the
contribution of the entropy to the free energy. In any case the difference in the stability of the low temperature
forms II seems to be an important observation, because it indicates that definite conclusions on the stability
of compounds only on the basis of crystal structures and intermolecular interactions should be drawn very
carefully, which possibly accounts for more cases as just polymorphic modifications.
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ACKNOWLEDGEMENTS
This project was supported by the Deutsche Forschungsgemeinschaft (Project No. Na 720/5-2) and the
State of Schleswig-Holstein. We thank Prof. Dr. Wolfgang Bensch for access to his experimental facilities.
ASSOCIATED CONTENT
The Supporting Information is available free of charge on the ACS Publications website at DOI:
AUTHOR INFORMATION
Corresponding Author
*
Email: cnaether@ac.uni-kiel.de.
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Notes
The authors declare no competing financial interest.
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