Selective Synthesis and Thermodynamic Relations of Polymorphic Co(NCS)2-4-Dimethylaminopyridine Coordination Compounds

Article abstract of DOI:10.1002/ejic.201800741

Reaction of Co(NCS)₂ with 4-dimethylaminopyridine (DMAP) leads to the formation of four new compounds. The crystal structures of Co(NCS)₂(DMAP)₂(H₂O)₂·2H₂O (1), Co(NCS)₂(DMAP)₂(MeOH)₂ (2) and Co(NCS)₂(DMAP)₂(MeCN)₂ (3) consist of discrete simple solvato complexes in which the Co cations are octahedrally coordinated by two terminally N-bonded thiocyanate anions, two DMAP ligands as well as two solvato ligands. Co(NCS)₂(DMAP)₂ (4/II) also forms discrete complexes, but the Co cations are tetrahedral coordinated by two anionic ligands and two DMAP ligands. Upon heating, the hydrate 1 transforms into 4/II, whereas the methanol solvate 2 transforms into a new polymorphic modification of Co(NCS)₂(DMAP)₂ (4/I). For structure determination the corresponding Zn(NCS)₂ compound was prepared that is isotypic to 4/I. Solvent mediated conversion experiments prove that 4/II represents the thermodynamic stable form at room-temperature and density functional theory (DFT) calculations indicate that this form is also stable at 0 K. Upon heating both modifications show melting with the higher melting polymorph 4/I having the lower melting enthalpy. Temperature dependent X-ray powder diffraction shows that 4/II transforms into 4/I upon heating. All experimental results indicate, that both modifications are related by enantiotropism.

Full text of DOI:10.1002/ejic.201800741

DOI: 10.1002/ejic.201800741
Full Paper
Polymorphism
Selective Synthesis and Thermodynamic Relations of
Polymorphic Co(NCS)2-4-Dimethylaminopyridine Coordination
Compounds
Tristan Neumann,[a] Inke Jess,[a] Florian Pielnhofer,[b][‡] and Christian Näther*[a]
Abstract: Reaction of Co(NCS)2 with 4-dimethylaminopyridine Co(NCS)2(DMAP)2 (4/I). For structure determination the corre-
(DMAP) leads to the formation of four new compounds. sponding Zn(NCS)2 compound was prepared that is isotypic to
The crystal structures of Co(NCS)2(DMAP)2(H2O)2·2H2O (1), 4/I. Solvent mediated conversion experiments prove that 4/II
Co(NCS)2(DMAP)2(MeOH)2 (2) and Co(NCS)2(DMAP)2(MeCN)2 (3) represents the thermodynamic stable form at room-tempera-
consist of discrete simple solvato complexes in which the Co ture and density functional theory (DFT) calculations indicate
cations are octahedrally coordinated by two terminally N- that this form is also stable at 0 K. Upon heating both modifica-
bonded thiocyanate anions, two DMAP ligands as well as two tions show melting with the higher melting polymorph 4/I hav-
solvato ligands. Co(NCS)2(DMAP)2 (4/II) also forms discrete com- ing the lower melting enthalpy. Temperature dependent X-ray
plexes, but the Co cations are tetrahedral coordinated by two powder diffraction shows that 4/II transforms into 4/I upon
anionic ligands and two DMAP ligands. Upon heating, the heating. All experimental results indicate, that both modifica-
hydrate 1 transforms into 4/II, whereas the methanol solvate 2 tions are related by enantiotropism.
transforms into
a
new polymorphic modification of
Introduction
be investigated how a specific form can be prepared phase
pure, which of these forms is thermodynamic stable and how
these forms can be transformed into each other.
Investigations on the synthesis, structures and properties of
compounds with defined structures and desired physical prop-
erties are still an important goal in solid state chemistry.[1–8]
For this purpose coordination compounds can be very useful
because based on simple chemical knowledge the connectivity
of the cations or networks can be predicted to some extent.[9]
Unfortunately, the energy difference between possible poly-
morphs or isomers is frequently small, which can lead to the
formation of different products under kinetic or thermodynamic
control.[10] In this context, a large number of polymorphic and
isomeric coordination compounds were reported in the last
years.[1,11–29] From a viewpoint of material synthesis this might
be of disadvantage but there is also some benefit from these
phenomena. For polymorphs or isomers the chemical composi-
tion is constant and all changes in the properties of the differ-
ent forms can be directly correlated with their structural differ-
ences, allowing the investigation of structure–property relation-
ships. However, if polymorphs or isomers are detected it should
One class of compounds for which polymorphs or isomers
frequently occur is represented by coordination compounds
based on 3d transition metal thiocyanates with additional coor-
dinating N-donor co-ligands. This anionic ligand can coordinate
to metal cations in many different ways, which leads to a large
structural diversity of such compounds and this might be one
of the reasons why many different polymorphs or isomers were
reported for this class of compounds.[30–38] The reason why we
became interested in this class of compounds originates from
the idea for the preparation of compounds that might show
different magnetic properties including cooperative magnetic
phenomena, because this ligand can mediate reasonable mag-
netic exchange.[39–54] In this context 1D compounds based on
CoII are of special interest, because several of them show a slow
relaxation of the magnetization, which can partly be traced
back to single chain magnetism.[55–60] However, for the synthe-
sis of such compounds paramagnetic metal cations must be
linked by these anionic ligands into, e.g., 1D or 2D networks
and even if several of such compounds are reported this is
sometimes a difficult task. For the less chalcophilic metal cat-
ions like, e.g. MnII, FeII, CoII or NiII, the compounds with only
terminally N-bonded anionic ligands are usually more stable
and easy to synthesize, whereas the desired compounds with a
bridging coordination are frequently difficult to synthesize in
pure form if they are prepared by crystallization from solution.
This is one of the main reasons why we have developed an
alternative procedure for the synthesis of the bridging com-
[a] Institute of Inorganic Chemistry, Christian-Albrechts-University of Kiel
Max-Eyth-Strasse 2, 24118 Kiel, Germany
E-mail: cnaether@ac.uni-kiel.de
http://www.ac.uni-kiel.de/de/naether
[b] Max-Planck-Institut for Solid State Research,
Heisenbergstraße 1, 70569 Stuttgart, Germany
[‡] Current adress: Institute of Inorganic Chemistry,
University of Regensburg,
Universitätsstraße 31, 93040 Regensburg, Germany
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201800741.
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pounds, which is based on thermal decomposition of precur- compound 2 is contaminated with a further crystalline phase
sors. The precursors mostly consist of discrete complexes with (Figure S5–7). The amount of this new form increases with in-
terminal anionic ligands that upon heating loose the organic creasing storage time, which indicates that 2 is unstable and
co-ligands or solvate molecules in separate steps forming the loses methanol already at room-temperature (Figure S6). Thus,
desired bridging compounds as intermediates in pure form and it must be assumed that solvent removal from the methanol
quantitative yield.[61] One additional advantage of this ap- solvate will lead to a different crystalline form (4/I) with the
proach is the fact that metastable compounds can be obtained same composition as that of 4/II. All batches of 4/II prepared
that are difficult to prepare or that are not accessible from from water contain predominantly the hydrate 1, which is more
solution and this might be also one reason why so many poly- stable and thus, this compound cannot be obtained pure under
morphs or isomers were detected by us following this these conditions (Figure S8).
route.[29,62]
In the course of our investigations we became interested in
Thermoanalytical Investigations
4-dimethylaminopyridine (DMAP) as co-ligand, because of its
strong donor substituent. Therefore, we investigated if 1D com-
pounds with Co(NCS)2 chains might be available, which would
be of extraordinary importance for the entire project. It is
noted, that no crystal structures with Co(NCS)2 and DMAP are
reported in the CCDC database but some discrete complexes
with other transition metal cations are known.[63–68] However,
for compounds with other anions in which Co is coordinated
by DMAP, more octahedral than tetrahedral compounds are
known.[69–72] It is also noted that a 1 D compound is reported,
in which Co cations are linked by pairs of azido ligands into
chains, an arrangement that exactly corresponds to that what
we expected for thiocyanate coordination polymers and which
indicates that also with Co(NCS)2 the desired chain compound
might be available.[73] Here we report on the results of our in-
vestigations.
To investigate which crystalline form is obtained on solvent re-
moval, the thermal properties of 1–3 were investigated by si-
multaneous thermogravimetry and differential thermoanalysis
(TG-DTA). Upon heating compound 1–3 in
a thermo-
balance, two mass losses are observed in the TG curves that are
accompanied with endothermic events in the DTA curve (Fig-
ure 1 and Figures S9–S11). For the methanol solvate 2 there is
an indication that the first mass step consists of two not well
resolved steps. For all three compounds the experimental mass
loss in the first step is in reasonable agreement with that ex-
pected for the removal of the water, methanol or acetonitrile
molecules, indicating that intermediates with the composition
Co(NCS)2(DMAP)2 are formed. The experimental mass loss in the
second TG step is larger than that expected for the removal of
each two DMAP ligands and thus, ligand removal and decom-
position of Co(NCS)2, that must form as intermediate, occur si-
multaneously. It is noted that for all three compounds after
Results and Discussion
Synthesis and Thermoanalytical Investigations
Reaction of Co(NCS)2 with DMAP in different molar ratios in
water, methanol or acetonitrile lead to the formation of four
new compounds with the compositions Co(NCS)2(DMAP)2-
(H2O)2·2H2O (1), Co(NCS)2(DMAP)2(MeOH)2 (2), Co(NCS)2-
(DMAP)2(MeCN)2 (3) and Co(NCS)2(DMAP)2 (4/II). It is noted that
the composition of 4/II corresponds exactly to that of the de-
sired chain compound. The asymmetric CN stretching vibration
of the thiocyanate anions is observed at 2090 cm–1 for 1, 2
and 3, which is in the range expected for terminally N-bonded
thiocyanate anions with an octahedral coordination indicating
that simple discrete solvato complexes have formed with addi-
tional water solvate molecules in 1 (Figure S1–S3 in the SI). For
compound 4/II, the CN stretch is found at 2054 cm–1, which
is much lower than expected for bridging anionic ligands and
therefore, it can be assumed that a tetrahedral complex with
terminal anionic ligands instead of a chain has formed (Figure
S4).
For all compounds single crystals were prepared and charac-
terized by single-crystal X-ray diffraction, which proves that
compounds 1–3 consist of octahedral and 4/II of tetrahedral
discrete complexes (see below). Comparison of the experimen-
tal X-ray powder diffraction pattern (XRPD) with those, calcu-
lated from the single crystal structures shows that compounds
1 and 3 were obtained as pure phases, whereas the methanol
Figure 1. TG curves of compounds 1–3 measured at 8 °C/min. DTG, TG and
DTA curves curves can be found Figures S9–S11.
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solvent removal an additional endothermic peak is observed at observed in similar compounds reported in literature. Presuma-
about 177 °C, where the sample mass does not change, which bly, the Co–N bond to the DMAP ligands is stronger because of
indicates that the intermediates show an additional transition the strong donor-substituent in para position.
(Figure S9–S11).
If the residues obtained after the first mass loss are investi-
gated by XRPD it is proven that compound 1 has been trans-
formed into the discrete complex 4/II, whereas compound 2
decomposes into a new crystalline form with the composition
Co(NCS)2(DMAP)2 (4/I) that is different from 4/II (Figure S12–
S13). The intermediates obtained by acetonitrile removal from
3 at 8 °C/min and 1 °C/min are different and correspond to
crystalline phases that are unknown (Figure S14). The CN
stretching vibration of the thiocyanate anion in 4/I measured by
IR spectroscopy occurs at 2060 cm–1, indicating that the anionic
ligands are still terminally N-bonded (Figure S15). Therefore, it
can be assumed that a further polymorphic modification of a
tetrahedral discrete complex has formed. In this context it is
noted that tetrahedral discrete complexes with the composition
M(NCS)2(L)2 (M = divalent metal cation; L = pyridine derivative)
are also formed with Zn(NCS)2. In previous investigations we
have shown, e.g. that the corresponding compounds with
Co(NCS)2 and Zn(NCS)2 and 2-methylpyridine form isotypic
pairs with each pair crystallizing in two different polymorphic
modifications.[12] Therefore, we reacted Zn(NCS)2 with DMAP in
ethanol which was overlayed with heptane, which easily lead to
crystals of a discrete tetrahedral complex with the composition
Zn(NCS)2(DMAP)2 (5), for which the CN stretching vibration is
observed at 2073 cm–1 and 2097 cm–1 (Figure S16) If the calcu-
lated XRPD pattern of 5 is compared with the experimental
pattern of the residue obtained after methanol removal (4/I), it
is obvious that both compounds are isotypic (Figure S17). This
is proven by a Pawley-Fit of 4/I, which also shows that this
Figure 2. View of a discrete complex of 1 (top) and crystal structure with view
compound is obtained as a pure phase (Figure S18 and S19).
along the crystallographic c axis (bottom). Intermolecular O–H···O and
O–H···S hydrogen bonding is shown as dashed lines and an ORTEP plot can
be found in Figure S21 in the Supplemental.
Finally, we checked if compounds 4 can be retransformed into
the solvates 1–3. Therefore, a sample of 4/II was stored for 12 h
in a water, a methanol and an acetonitrile atmosphere. XRPD
investigations of the residue formed in acetonitrile reveal that
4/II has been transformed into 3, whereas the residues stored
in a water and methanol atmosphere still correspond to 4/II
(Figure S20).
The discrete complexes are linked via intermolecular O–H···O
hydrogen bonding between the coordinated and the solvate
water molecules into chains, that elongate along the crystallo-
graphic a axis (Figure 2: bottom and Table S2). Each of the two
H atoms of the coordinated water molecules are connected to
a water hydrate molecule forming four-membered (H2O)4 rings.
The H atoms of the two hydrate water molecules that do not
participate in this interaction are hydrogen bonded to two thio-
cyanate S atoms by intermolecular O–H···S hydrogen bonding
forming four-membered (H2O)2S2 rings (Figure 2: bottom and
Table S2). There are additional short S···H contacts indicating
for weak C–H···S interactions (Table S2).
Crystal Structures
Crystal Structure of Co(NCS)2(DMAP)2(H2O)2·2 H2O (1)
Compound 1 crystallizes in the triclinic space group P1¯ with
one formula unit in the unit cell. The asymmetric unit consists
of one CoII cation located on a center of inversion, one thio-
cyanate anion and one DMAP ligand as well as two water mol-
ecules in general positions (Figure 2: top and Figure S21). The
Crystal Structure of Co(NCS)2(DMAP)2(MeOH)2 (2)
CoII cations are coordinated by two N-bonded thiocyanato an- Compound 2 also crystallizes triclinic in space group P1. The Co
¯
ions, two DMAP ligands and two water molecules in a slightly cations are located on centres of inversion and are coordinated
distorted octahedral geometry to form discrete complexes (Fig- by two N-bonded anionic ligands, two DMAP ligands as well as
ure 2: top). The Co–N bond lengths to the thiocyanate N atom two methanol molecules (Figure 3: top and Figure S22). As in
of 2.1140 (14) Å and to the DMAP ligands of 2.1134 (14) Å are compound 1, the Co–N bond lengths to the anionic and the
surprisingly very similar (Table S1). This points to a stronger neutral ligands are very similar (Table S3). The angles around
interaction between the CoII cation and the pyridine N atom, the Co cations show that the octahedra is slightly distorted
because usually a shorter bond length to the anionic ligands is (Table S3).
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Crystal Structures of M(NCS)2(DMAP)2 with M = Co (4/I) and
M = Zn (5)
As mentioned above, compound 4/I is isotypic to the corre-
sponding Zn compound 5. Compound 4/II crystallizes mono-
clinic in space group P21/c with all atoms located in general
position, whereas compound 5 crystallizes in space group
P21/m with the cations and the thiocyanate anions located on
a twofold rotation axis (Figures S24 and S25). In contrast to
most other Co(NCS)2 coordination compounds with same metal
thiocyanate to co-ligand ratio, the metal cations are tetrahedral
coordinated by two terminally N-bonded thiocyanate ligands
and two DMAP co-ligands into discrete complexes (Figure 5 and
Table S7 and S9). The formation of a tetrahedral instead of an
octahedral coordination is surprising. It is noted that a tetrahe-
dral coordination of the Co cation is frequently found if the
pyridine substituent is directly neighboured to the coordinating
N atom in 2-position[12] and this might be traced back to steric
reasons, but this cannot be valid for compound 4. Therefore,
electronic reasons might be responsible that the tetrahedral co-
ordination is preferred over the octahedral environment. In this
context it is mentioned that the Co–N bond lengths to the
DMAP ligands in compound 1–4 are much shorter than that to
other pyridine co-ligands, which points to a stronger interac-
tion. Therefore, it is assumed that the strong electron donor
DMAP transfers more electron density to the Co cation as other
pyridine derivatives and that the metal cation in an octahedral
coordination can accept only a limited amount and as a conse-
quence the tetrahedral coordination might become energeti-
cally favoured.
Figure 3. View of a discrete complex of 2 (top) and crystal structure with view
along the crystallographic c axis (bottom). Intermolecular O–H···S hydrogen
bonding is shown as dashed lines and an ORTEP plot can be found in Figure
S22 in the Supplemental.
In the crystal structure the discrete complexes are linked into
chains along the a axis by pairs of intermolecular O–H···S
hydrogen bonding between the thiocyanate S atom and the
hydroxyl H atom (Figure 3: bottom and Table S4). There are also
weak C–H···S interactions between the anionic and the neutral
N-donor ligands (Table S4).
Crystal Structure of Co(NCS)2(DMAP)2(MeCN)2 (3)
Compound 3 also forms discrete complexes with octahedral co-
ordinated Co cations (Figure 4 and Figure S23). As compounds
1 and 2 it also crystallizes in space group P1¯ with the Co cations
located on centres of inversion. In contrast to the former com-
plexes, the differences in the bond lengths between the anionic
and the neutral ligands are more pronounced (Table S5). The
discrete complexes are linked via weak C–H···S interactions into
a hydrogen-bonded network (Table S6).
Figure 5. Crystal structure of 4/II (top) and the corresponding Zn compound
5 that is isotypic to 4/I (bottom) with view of one discrete complex. An ORTEP
plot can be found in Figure S24 and S25.
Figure 4. Crystal structure of 3 with view of one discrete complex. An ORTEP
plot can be found in Figure S23.
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However, concerning the structural differences between 4/I
The question why the solvent removal from compound 1
and 4/II or 5 it is noted that the arrangement of the DMAP leads to the formation of 4/II, whereas 4/I is exclusively ob-
ligands is already different as it is obvious when both discrete tained from the methanol solvate 2 is difficult to answer. First
complexes are superimposed (Figure S26). In form 4/I, the of all, the formation of 4/II is not surprising, because it can be
planes of the six-membered rings are nearly perpendicular to assumed that this form represents the thermodynamic stable
each other, which is not the case in 5 or 4/II.
modification starting from room-temperature (see below).
Therefore, form 4/I should be formed by kinetic control and
this might be easier if there would be some structural relation
between the solvate 2 and the form obtained by solvent re-
moval (4/I), even if this is no necessary condition. This does not
mean that the reaction is topotactic, because it is obvious that
large structural changes are needed to transform 2 into 4/I.
However, as mentioned above, in 4/I the building blocks are
arranged in layers and the DMAP ligands of neighbouring com-
plexes are shifted by half of the a axis (Figure 6: bottom). A
similar arrangement is also found in the methanol solvate 2,
which indicates that the arrangement of the complexes in 4/I
is somehow predefined in compound 2 (Figure S29).
Differences in both forms are also found in the arrangement
of the complexes in the crystal. In form 4/II the complexes are
interwoven leading to a relatively dense packing (Figure 6: top).
Moreover, each two DMAP ligands forms pair, that are located
around centres of inversion (Figure S27).
Investigations on the Transition Behaviour and the
Thermodynamic Relation between 4/I and 4/II
To prove which of the two polymorphic modifications 4/I and
4/II is thermodynamically stable at room-temperature, a mix-
ture of both forms was stirred in ethanol with an excess of solid.
If the solid residues obtained after 7 d are investigated by XRPD,
it is obvious that all crystals of form 4/I disappeared and that
only crystals of 4/II are present (Figure 7). This indicates, that
4/II represents the thermodynamically stable form at room-
temperature, where 4/I is metastable.
Figure 7. Experimental XRPD pattern of a mixture of 4/I and 4/II (B) and after
stirring this mixture for 7 d in ethanol (A) together with the pattern calculated
for 4/II (C) and 4/I (D).
Figure 6. Crystal structure of 4/II (top) and 5 (bottom) with view along the
crystallographic a axis.
To compare the densities of both forms, their unit cell param-
eters were determined by Pawley fits, using XRPD data meas-
ured at room temperature, which proved that the density of form
4/II is significantly higher than that of form 4/I (Table S11).
According to the density rule this indicates that 4/II should also
be thermodynamically stable at low temperatures.[74]
In 5 or 4/I the complexes are arranged in columns, that elon-
gate in the direction of the crystallographic a axis (Figure 6:
bottom). These columns are packed in a less dense arrange-
ment. Moreover, the DMAP ligand as well as the Co(NCS)2 build-
ing blocks are arranged in layers, that are parallel to the a/c
plane. In each second Co(NCS)2 layers the anionic ligands point
into different directions, from which the centrosymmetric ar-
rangement becomes obvious (Figure S28).
The stability of both polymorphs was further compared by
their total electronic energies (at 0 K) as obtained by quantum
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chemical calculations. 4/II is energetically favored to 4/I by
Thermomicroscopic measurements on both forms are in full
11 kJ mol–1, and therefore predicted to be the thermodynami- agreement with the interpretation of the DSC measurements,
cally stable modification which is in good agreement with ex- because clearly melting is observed with a melting point for
perimental observations.
4/I that is higher than that of 4/II (Figure 9). All this strongly
indicates that form 4/I becomes thermodynamically stable at
higher temperatures and that therefore, both modifications
should behave enantiotropically with the free energy tempera-
ture curves crossing somewhere between room temperature
and the melting point of 4/II.
Measurements on 4/I using differential scanning calorimetry
(DSC) show one endothermic event upon heating, that is ab-
sent in the cooling curve (Figure 8: top). On reheating a very
broad exothermic event is observed, that is followed by an en-
dothermic peak at the same temperature as observed in the
first heating run. The DSC curve for 4/II is very similar but the
endothermic transition in the first heating run is at significantly
lower temperature than in the second heating (Figure 8).
Figure 9. Microscopic images of 4/II (left) and 4/I (right) at different tempera-
tures.
Additional DSC measurements on form 4/II at 1 °C/min show
an endo-exo-endo sequence, which might be traced back to the
fact that 4/II melts in the beginning and that on further heating
4/I crystallizes from the melt because its melting point is higher
(Figure S34). This is in agreement with XRPD measurements of
residues obtained after the first endothermic and exothermic
event (Figure S35). After the first endothermic peak an amor-
phous samples is obtained, because this event correspond to
the melting of 4/II (Figure S35). If the measurement is stopped
after the exothermic event 4/I is obtained, which can be traced
back to the crystallization of this form from the melt (Figure
S35). Interestingly, the same endo-exo-endo sequence is ob-
served for a different batch already at a heating rate of
10 °C/min and in this case it can be assumed that a small
amount of 4/I is present with a content too low to detect it by
XRPD. In this case these crystals act as nuclei for the crystalliza-
tion of this form as previously observed (Figure S34).[75] For this
batch (batch 2) the heat of fusion 4/II is much lower than that
for batch 1, because the intensity ratio between both endother-
mic peaks, will depend on how much of 4/I crystallizes from
the melt upon heating.
Figure 8. DSC curves for 4/I (top) and 4/II (bottom) at 10 °C/min.
If the residues obtained after the heating and cooling cycle
are investigated by XRPD, it is proven that amorphous samples
have formed (Figure S30). This indicates that the first endother-
mic event corresponds to the melting of both forms with the
melting point of 4/I significantly higher than that of 4/II. XRPD
investigations of the residue obtained after the first exothermic
event in the second heating run shows that 4/I has formed
(Figure S31). This is in agreement with the DSC results on 4/II
because melting of this residue occurs at the same temperature
as observed for 4/I (Figure 8: top). It is also noted that XRPD
investigations of samples of both forms heated just before their
Finally, form 4/II was investigated by temperature dependent
XRPD, which shows that at a temperature of about 158 °C new
reflections occur, which definitely belong to that of 4/I (Fig-
ure 10). Upon further heating all reflections of 4/II disappear
slowly and the transformation into 4/I is completed at about
178 °C (Figure 10).
Interestingly, from these measurements it is obvious that
first endothermic event show no changes and thus, gave no both forms coexist over a large temperature range and this is
hint for any further transformation (Figure S32 and S33).
typical for a solid-to-solid polymorphic transition, that proceed
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However, the sample used for the DSC measurements was
prepared from ethanol and even if the identity of 4/II was
proven by XRPD we decided to prepare this form by a different
route. Therefore, the water was removed by heating from the
hydrate 1 and the sample was stored at 110 °C until the meas-
urement. DSC measurements on this batch show two endother-
mic peaks, that correspond to the melting of both forms, where
the latter is formed by consecutive melting and crystallization
of 4/II (Figure 11). To suppress the transformation of 4/II into
4/I via the melt measurements with faster heating rates were
performed (Figure 11).
Figure 10. Temperature dependent XRPD pattern of 4/II. For the full tempera-
ture range see Figure S36.
via nucleation and growth of a new phase and for which each
nucleus has his own predetermined transition temperature.
Therefore, in contrast to the DSC measurements presented
above, where the transition proceeds via melting and crystalli-
zation of a new form (4/I), in this case it must proceed via
the solid state, because otherwise the powder pattern should
change in a much smaller temperature range.
All these findings indicate that both forms are related by
enantiotropism, with 4/I thermodynamically stable at high and
4/II at low temperatures. To additional prove this, the heat of
fusion and heat of entropy rule might be useful, which state
that in those cases where the higher melting polymorph has
the higher melting enthalpy or entropy, both forms are related
by monotropism. In the case where its melting enthalpy or en-
tropy is lower they are enantiotropically related.[76] Surprisingly,
the melting enthalpy and entropy of the high temperature
polymorph 4/I is always higher than that of 4/II and this indi-
cates a monotropic relation, which is in complete contradiction
with our previous results. There might be some exceptions from
this rule but only if the melting temperatures and heat-capacity
are very different, which seems to be not the case here. In this
case it is possible that the outcome of the solvent mediated
conversion experiment performed in ethanol might be wrong,
because, e.g. an unstable solvate will form that decomposes on
isolation into a metastable form. Therefore, the solvent medi-
ated conversion experiment was repeated at room-temperature
in butanol, which again leads clearly to the formation of 4/II
which proves that it must be the thermodynamically stable
form at this temperature (Figure S37).
Figure 11. DSC curves of 4/I and 4/II at different heating rates.
These experiments reveal that the heat of fusion (ΔHf) for
4/II is about 34.8 kJ mol–1, whereas that for 4/I is about
25.8 kJ mol–1, which indicates that the higher melting poly-
morph has a lower heat of fusion and therefore that both poly-
morphs are related by enantiotropism (Table S12). Neglecting
the differences in the heat capacity, the energy difference be-
tween both forms is about 9 kJ/mol, which is much for poly-
morphic modifications. From these values the entropy of fusion
(Sf) was calculated according to
which also proves the enantiotropic relation (Table S12).
Based on the melting temperatures (Tm in K) and melting en-
thalpies (ΔHf) the thermodynamic transition temperature (Tt)
can be estimated using the following equation:[77]
We additionally determined the solubility of both forms in Using the values given above as well as the onset temperatures
ethanol by atomic absorbtion spectroscopy (AAS). Therefore, a (Table S12), the transition temperature is calculated to be about
saturated solution with excess of 4/I was stirred in ethanol at 153 °C, even if there surely is an error of several degrees. This
room-temperature before transformation into 4/II was observed value is lower than that observed in the temperature depend-
and the Co content in the solution was determined by AAS. The ent XRPD measurements, which is expected because of the ki-
same procedure was used for 4/II using longer reaction times. netics of this reaction. Therefore, a sample of 4/II was annealed
This proves that the solubility of 4/II (160 mg/L) is lower than at different temperatures. At 135 °C no changes are observed,
that of 4/I (170 mg/L), which is in agreement with previous but at 145 °C the transformation into 4/I is completed within 92
investigations, because the latter is metastable at room-temper- hours (Figure S38). This transition temperature is in reasonable
ature.
agreement with that calculated. From all these results the free
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energy temperature diagram can be drawn, from which the enthalpies the thermodynamic transition temperature was esti-
thermodynamic relation becomes obvious (Figure 12).
mated, which is in good agreement with that determined ex-
perimentally by annealing samples of 4/II at different tempera-
tures. Finally, we have demonstrated that crystallization of the
corresponding Zn(NCS)2 compounds can be an useful alterna-
tive to structurally characterize Co(NCS)2 intermediates with a
tetrahedral coordination.
Experimental Section
Synthesis: Ba(NCS)2·3H2O, Co(NCS)2 and 4-dimethylaminopyridine
(DMAP) were purchased from Alfa Aesar, Zn(SO4)·H2O was bought
from Merck. All chemicals were used without further purification.
Zn(NCS)2 was prepared by the reaction of equimolar amounts of
Zn(SO4)·H2O with Ba(NCS)2·3H2O in water. The resulting white pre-
cipitate of BaSO4 was filtered off and the filtrate was concentrated
to complete dryness resulting in a white residue of Zn(NCS)2. All
compounds were obtained in almost quantitative yields.
Synthesis of 1: Single crystals suitable for single-crystal X-ray struc-
ture determination were obtained by the reaction of Co(NCS)2
(56.0 mg, 0.32 mmol) with DMAP (18.7 mg, 0.15 mmol) in 1.0 mL of
demin. water at room temperature after several days. A crystalline
powder was obtained by the reaction of Co(NCS)2 (87.5 mg,
0.5 mmol) with DMAP (122 mg, 1.0 mmol) in 5.0 mL of demin.
water.
Figure 12. Qualitative energy-temperature diagram showing the thermo-
dynamic relation between 4/I and 4/II (G = free energy; H = enthalpy; Tt =
transition temperature; Tm = melting point; ΔHf = enthalpy of fusion).
Synthesis of 2: Single crystals suitable for single-crystal X-ray struc-
ture determination were obtained by the reaction of Co(NCS)2
(109.5 mg, 0.63 mmol) with DMAP (20.2 mg, 0.17 mmol) in 1.0 mL of
methanol at room temperature after several days. A microcrystalline
powder was obtained by the reaction of Co(NCS)2 (87.5 mg,
0.5 mmol) with DMAP (122.0 mg, 1.0 mmol) in 5.0 mL of MeOH.
Conclusions
In the present contribution two polymorphic modifications of
the compound Co(NCS)2(DMAP)2 were obtained selectively by
solvent removal from two different precursors. The form ob-
tained from the hydrate (4/II) is thermodynamically stable at
room-temperature, whereas that obtained from the methanol
complex is metastable at this temperature. For the latter there
is some rough relation between the structure of the reactant
and that of the product, but it is difficult to prove if this is
responsible for the formation of this form. Interestingly, both
modifications show melting, which is rarely observed for coordi-
nation compounds, but which is of advantage to investigate
the thermodynamic relations. Moreover, at fast heating rates
melting of each form is observed exclusively, whereas at lower
heating rates 4/II transforms into 4/I via melting and crystalliza-
tion, which shows that the kinetics play an important role for
this transition. At very low heating rates, like in the temperature
dependent XRPD measurements, there is a strong indication
that the reaction proceeds in the solid state as frequently ob-
served for polymorphic transitions. Most experiments strongly
indicate that both forms are related by enantiotropism with
4/II thermodynamic stable at low and 4/I at high temperatures,
but this was in contradiction to our quantitative DSC measure-
ments on samples of 4/II prepared from ethanol. The reason for
this observation is not clear and there were no indications for
the formation of a solvate or that 4/II is hygroscopic. However,
if 4/II is prepared by water removal from the hydrate and care-
fully dried, the melting enthalpy of the higher melting poly-
morph is definitely lower than that of 4/II, which indicates that
the enantiotropic relation is correct, in agreement with all previ-
Synthesis of 3: Single crystals suitable for single-crystal X-ray struc-
ture determination were obtained after a few hours by solving of
Co(NCS)2 (4.4 mg, 0.03 mmol) and DMAP (25.1 mg, 0.10 mmol) in
1.5 mL of boiling acetonitrile. The heat source was removed, and
the still warm solution was overlayed with 1.5 mL n-heptane. A
crystalline powder was obtained by the reaction of Co(NCS)2
(175.1 mg, 1:0 mmol) with DMAP (61.0 mg, 0.5 mmol) in 3.5 mL of
MeCN.
Synthesis of 4/II: Single crystals suitable for single-crystal X-ray
structure determination were obtained by the reaction of Co(NCS)2
(26.3 mg, 0.15 mmol) with DMAP (36.7 mg, 0.30 mmol) in 1.0 mL
of demin. water at room temperature after several days.
Synthesis of 5: Single crystals suitable for single-crystal X-ray struc-
ture determination were obtained by overlaying a solution of
Zn(NCS)2 (18.2 mg, 0.1 mmol in 1.5 mL of EtOH) with 2.0 mL n-
heptane to which a solution of DMAP (6.1 mg, 0.05 mmol in 0.5 mL
of EtOH) was carefully added.
XRPD Investigations: The XRPD measurements for phase analysis
and temperature dependent XRPD measurements were performed
using a Stoe Transmission Powder Diffraction System (STADI P) with
Cu-Kα1 radiation (λ = 1.540598 Å) equipped with a MYTHEN 1K
detector (Dectris Ltd.) and a Ge(111) Johann monochromator. For
the temperature dependent XRPD measurements a STOE capillary
furnace was used.
Quantum Chemical Calculations: Quantum chemical calculations
in the framework of density functional theory (DFT) with CRYS-
TAL17[78,79] were performed to compare total energies of both in-
ous results. Based on the melting temperatures and melting vestigated polymorphs. For that purpose full structural optimiza-
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tions using the hybrid-functional HSE06[80] were carried out. Opti-
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basis4, basis5, basis6].[81–84] The Kohn–Sham matrix was diagonal-
ized on a k-mesh of 2 × 2 × 2 for 4/II and 4 × 4 × 4 for 4/I. The
convergence criterion was set to 10–7 a.u. The optimized lattice pa-
rameters are given in Table S13.
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This project was supported by the Deutsche Forschungsge-
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Received: June 12, 2018
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Polymorphism
Two polymorphic Co(NCS)2 coordina-
tion compounds were synthesized by
solvent removal from different precur-
sors, structurally characterized and
investigated for their stability, thermo-
dynamic relations and transition be-
haviour. Both forms are related by en-
antiotropism and the thermodynamic
transition temperature was estimated
from their melting enthalpies and con-
firmed by annealing experiments.
T. Neumann, I. Jess, F. Pielnhofer,
C. Näther* .......................................... 1–11
Selective Synthesis and Thermody-
namic Relations of Polymorphic
Co(NCS)2-4-Dimethylaminopyridine
Coordination Compounds
DOI: 10.1002/ejic.201800741
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