Synthesis, Crystal Structures, and Properties of Ni(NCS)2-4-methoxypyridine Coordination Compounds

Article abstract of DOI:10.1002/zaac.201800332

Two polymorphs of Ni(NCS)₂(4-methoxypyridine)₄ (1-I, 1-II) as well as compounds with the compositions Ni(NCS)₂(4-methoxypyridine)₄(MeCN)_1.33 (1-MeCN), Ni(NCS)₂(4-methoxypyridine)₂(MeCN)₂ (2) and [Ni(NCS)₂(4-methoxypyridine)₂]_n (3) are reported. For 1-II no single crystals were obtained, but this compound is isotypic to the Fe analog 1-II-Fe. The crystal structures of 1 and 2 show discrete octahedral Ni^II complexes, whereas in 3 the cations are connected by pairs of thiocyanate anions into chains. Solvent mediated conversion experiments reveal that 1-I is thermodynamically stable at room temperature and differential scanning calorimetry shows no indication for a polymorphic transition. The thermal behavior of 1-I, 1-II, and 2 was investigated by simultaneous thermogravimetry and differential scanning calorimetry, which shows that they decompose in two discrete steps. In the first step compound 3 is formed as an intermediate, which decomposes on further heating into Ni(NCS)₂. Time dependent XRPD investigations prove, that 2 transforms already at room temperature into 3. Upon heating 1-MeCN transforms into 1-I in the first TG step, which decomposes on further heating into 3. Investigation of the magnetic behavior of 3 shows dominating ferromagnetic exchange interactions along the chains with J = 5.9(1) K. Susceptibility measurements indicate antiferromagnetic ordering, which was confirmed by specific heat measurements, revealing that magnetic ordering occurs at 1.91(5) K.

Full text of DOI:10.1002/zaac.201800332

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.201800332
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Synthesis, Crystal Structures, and Properties of Ni(NCS) -4-
2
methoxypyridine Coordination Compounds
Aleksej Jochim,[a] Magdalena Ceglarska,[b] Michał Rams,[b] and Christian Näther*[a]
Dedicated to Prof. Dr. Wolfgang Bensch on the Occasion of his 65th Birthday
Abstract. Two polymorphs of Ni(NCS)
as well as compounds with the compositions Ni(NCS)
idine) (MeCN)1.33 (1-MeCN), Ni(NCS) (4-methoxypyridine)
2) and [Ni(NCS) (4-methoxypyridine) (3) are reported. For 1-II no investigations prove, that 2 transforms already at room temperature
2
(4-methoxypyridine)
(4-methoxypyr- In the first step compound 3 is formed as an intermediate, which de-
(MeCN) composes on further heating into Ni(NCS) . Time dependent XRPD
4
(1-I, 1-II)
calorimetry, which shows that they decompose in two discrete steps.
2
4
2
2
2
2
(
2
2 n
]
single crystals were obtained, but this compound is isotypic to the Fe
into 3. Upon heating 1-MeCN transforms into 1-I in the first TG step,
which decomposes on further heating into 3. Investigation of the
magnetic behavior of 3 shows dominating ferromagnetic exchange in-
teractions along the chains with J = 5.9(1) K. Susceptibility measure-
ments indicate antiferromagnetic ordering, which was confirmed by
specific heat measurements, revealing that magnetic ordering occurs
at 1.91(5) K.
analog 1-II-Fe. The crystal structures of 1 and 2 show discrete octahe-
II
dral Ni complexes, whereas in 3 the cations are connected by pairs
of thiocyanate anions into chains. Solvent mediated conversion experi-
ments reveal that 1-I is thermodynamically stable at room temperature
and differential scanning calorimetry shows no indication for a poly-
morphic transition. The thermal behavior of 1-I, 1-II, and 2 was inves-
tigated by simultaneous thermogravimetry and differential scanning
Introduction
of many different coordination modes of these anionic li-
gands.[
31,37–48]
Unfortunately for less chalcophilic metal cat-
The synthesis of new magnetic coordination compounds is
still an important topic in modern coordination chemistry and
an extremely large number of compounds showing a variety
of magnetic properties has been reported.
one-dimensional coordination polymers, for which the mag-
ions, the coordination to the N atom is preferred and therefore,
in most of them the anions are only terminally N-coordinated.
Consequently the compounds with μ-1,3-bridging anions are
usually more difficult to prepare and therefore, some time ago,
we have reported an alternative approach that is based on ther-
[1–10]
In this regard
netic properties largely depend on the strength of the exchange
along the chains are of special interest.[11–17] This interaction
mal decomposition of suitable precursor compounds, in which
the anionic ligands are only terminally coordinated.[
46,49]
Upon
is predominantly determined by the bridging ligand that medi-
ates the magnetic exchange and therefore, replacement of one
ligand by a different one is one possibility to influence the
magnetic properties of such compounds in a desired way.
heating, the neutral co-ligands are usually removed stepwise,
which enforces the formation of the desired compounds with
bridging anionic ligands and in several cases metastable poly-
In this context numerous coordination polymers based on, morphic modifications or different isomers can be obtained,
e.g. transition metal cyanides or azides have been which cannot be synthesized or that are at least difficult to
reported.[
17–29]
Compared to these, less compounds based on prepare from solution.[45,49–52] Therefore, we are generally in-
thio- or selenocyanate anions are known, which was one of the terested in the thermal properties of this class of compounds.
reasons why we became interested in this class of compounds
If the precursors consist of discrete complexes with pyridine
While the magnetic exchange in com- derivatives as co-ligands, usually 1D compounds are obtained,
several years ago.[
30–36]
pounds containing these anionic ligands is usually weaker than in which the metal cations are octahedrally coordinated by two
that in azides they show a larger structural diversity because N-donor co-ligands as well as four μ-1,3-bridging anionic li-
gands and are linked by pairs of anionic ligands into
*
Prof. Dr. C. Näther
[
51,53–55]
chains
although examples of 2D networks are also
The 1D compounds with Co are of special
E-Mail: cnaether@ac.uni-kiel.de
known.[
51,56–58]
II
[
a] Institute of Inorganic Chemistry
Christian-Albrechts-University of Kiel
Max-Eyth-Straße 2
interest, because the magnetic exchange along the chain is fer-
romagnetic and several of these coordination polymers show
single chain magnetism.
netic properties largely depend on the strength of the magnetic
exchange and therefore, we became generally interested in the
relation between chain structure and chemical composition as
well as the strength of the magnetic exchange. In this context
24118 Kiel, Germany
[
b] Institute of Physics
Jagiellonian University
Łojasiewicza 11
[54,55,59]
For such compounds the mag-
30348 Kraków, Poland
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201800332 or from the au-
thor.
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II
it is noted that the corresponding compounds with Ni show a
similar structural behavior and are in several cases isotypic to
II
its Co counterparts. Moreover, in these compounds the ex-
change along the chain is also ferromagnetic and in contrast to
II
the Co compounds models are available, which can be used
to extract the magnetic exchange constant J for all kinds of
chain structures. This is crucial, because dependent on the co-
ligand these compounds can show a different chain arrange-
ment and a different metal coordination.
As mentioned above, most compounds consist of linear
chains, in which the thiocyanate N and S atoms as well as the
co-ligand N atoms are in trans position. In the course of our
systematic work we also prepared an isomer, in which the thio-
cyanate N and the thiocyanate S atoms are cis, while the co-
ligand is still trans, which also leads to the formation of linear
chains.[ Dependent on the co-ligand additional isomers were Figure 1. Experimental XRPD pattern of 1-II (top) and 1-I (bottom).
60]
obtained, which also show a cis-cis-trans coordination or an
alternating all-trans and cis-cis-trans coordination but in this
During the attempts to synthesize single crystals of 1-II in
acetonitrile fifth compound with the composition
Ni(NCS) (4-methoxypyridine) ·(MeCN) (1-MeCN) was
accidently obtained. Unfortunately, for 1-II no single crystals
could be obtained and thus, another way was used for structure
determination.
case the thiocyanate N atoms are cis, leading to the formation
of corrugated chains.[
61]
a
For these compounds the magnetic exchange constant J was
extracted from magnetic measurements, which indicate that the
magnetic exchange in corrugated chains is lower than in linear
chains. Unfortunately, no simple rules can be extracted regard-
ing the influence of geometrical parameters like distances and
angles or the nature of the co-ligand on J, which indicates
that even small variations have a large impact on the magnetic
exchange. This might be traced back to the relatively low data
basis, because for these isomers only a limited number of ex-
amples are available. All this is the reason why we are still
interested in the syntheses, crystal structures, and magnetic
2
4
1.33
In previous investigations using other transition metal thio-
II
II
II
cyanates we found that the substitution of Ni with Co or Fe
usually leads to the formation of isotypic compounds. In some
other cases, the change of the transition metal cations leads to
the formation of different structures. Therefore, we tried to
prepare a compound with the composition Fe(NCS) (4-meth-
2
oxypyridine) , which might be isotypic to 1-II. Even if this
4
II
compound cannot be prepared as a pure phase, XRPD investi-
gations indicate that this crystalline phase (1-II-Fe) is isotypic
to 1-II. Single crystals could be easily obtained and were char-
acterized by single-crystal X-ray diffraction. Finally, a Pawley
fit of a powder of 1-II was made using the crystallographic
parameters of 1-II-Fe as the initial values, which shows that
both compounds are isotypic (Figure S1, Supporting Infor-
mation). For all Ni compounds XRPD patterns were calculated
and compared with the experimental patterns, which prove that
always pure crystalline phases were obtained (Figures S2–S6,
Supporting Information). For the calculation of the powder
pattern of 1-II, the lattice parameters determined by the Paw-
ley fit were used.
properties of such Ni chain compounds and in the course of
this project we selected 4-methoxypyridine as co-ligand, which
exhibits a strong donor substituent in 4-position. In this context
it would be of interest, which chain isomer is obtained and
which value for the magnetic exchange constant can be found.
We also tried to prepare precursor complexes, which on ther-
mal decomposition might transform into different isomers, as
recently observed for other compounds.[
45,51]
Herein we report
on our investigations.
Results and Discussion
Synthesis and Characterization
Thermodynamic Stability of Form 1-I and 1-II
The reactions of Ni(NCS) with 4-methoxypyridine in dif-
To investigate, which of the two polymorphic modifications
2
ferent molar ratios and solvents lead to the formation of three represents the thermodynamically stable form at room tem-
different crystalline compounds with the composition perature, a physical mixture of 1-I and 1-II in equimolar quan-
Ni(NCS) (4-methoxypyridine) (1-I), Ni(NCS) (4-methoxy- tities was prepared, dissolved in water, ethanol, and methanol
2
4
2
pyridine) (MeCN) (2), and [Ni(NCS) (4-methoxypyridine) ]
with an excess of solid forming a suspension, which was
3). Time dependant studies revealed that another compound stirred for 1 h. As the precipitates were investigated afterwards
could be obtained under kinetic control. This crystalline phase by XRPD all crystals of 1-II disappeared and only reflections
1-II) has the same composition as 1-I, but its powder diffrac- indicating the presence of 1-I were observed, which proves
tion pattern is completely different, indicating the formation of that the latter form represents the thermodynamically stable
2
2
2
2 n
(
(
a polymorphic modification (Figure 1).
modification at room temperature (Figure 2).
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Crystal Structures
The room-temperature stable form 1-I crystallizes in the or-
thorhombic space group Pccn with the asymmetric unit con-
taining one thiocyanate anion and two 4-methoxypyridine mol-
II
ecules in general positions as well as one Ni cation being
situated on a twofold rotational axis. The Ni cation is octahe-
drally coordinated by two thiocyanate anions and four 4-meth-
oxypyridine ligands forming discrete complexes (Figure 3).
Figure 2. XRPD patterns of an equimolar mixture of compounds 1-I
and 1-II, the calculated patterns for compounds 1-I and 1-II as well
as the XRPD pattern of the solid obtained after stirring the mixture of
1-I and 1-II for 1 h in methanol as a representative for the experiments
obtained from water, ethanol, and methanol.
Figure 3. View of the coordination sphere of the NiII cation in com-
pound 1-I as a representative for the discrete complexes found in com-
pounds 1-I, 1-II-Fe, and 1-MeCN (Ni: orange; S: yellow; N: blue; C:
Additionally measurements using differential scanning calo-
rimetry (DSC) show decomposition at about 214 °C, with very
similar decomposition temperatures for 1-I and 1-II (Figure black; O: red; H: grey). ORTEP plots of 1-I, 1-II-Fe, and 1-MeCN
S7, Supporting Information). Because this temperature de- can be found in Figures S10–S12 (Supporting Information).
pends on the kinetics of these reactions, no conclusion on the
The Ni–NPy distances amount to 2.111(2) Å and 2.118(2) Å,
stability of the forms at higher temperatures can be drawn.
whereas the corresponding values for the thiocyanate anions
There is also no sign for a polymorphic transformation of one
are slightly shorter with Ni–NNCS distances amounting to
form into the other below the decomposition temperatures.
2
.072(2) Å (Table S1, Supporting Information). The N–Ni–N
However, in the case where both forms are related by mono-
tropism an exothermic phase transition is expected, which is
usually difficult to detect in DSC measurements. Therefore,
these measurements were repeated and stopped before decom-
position takes place. If the XRPD patterns of these residues
are compared with those calculated for 1-I and 1-II no changes
are detected (Figures S8 and S9, Supporting Information).
Finally, the unit cell volume of 1-I and 1-II was determined
by Pawley fits, from which the density was calculated, which
shows that the density of 1-II is higher than that of 1-I
angles deviate slightly from the ideal values indicating a slight
distortion of the coordination octahedron. This can be quanti-
fied using the octahedral angle variance σθ‹oct›2 and the mean
[
65]
quadratic elongation λoct introduced by Robinson et al.
The
resulting values of σθ‹oct›2 = 1.3 and λoct = 1.001 confirm the
presence of a minor distortion of the coordination octahedron.
Compound 1-II-Fe is isotypic to 1-II and therefore, its
structure is discussed as a representative for the form that is
thermodynamically metastable at room temperature. 1-II-Fe
crystallizes in the orthorhombic space group Pbca. The asym-
metric unit consists of two thiocyanate anions, four 4-meth-
(Table 1). If the density rule is valid, this would mean that 1-II
becomes thermodynamically stable at low temperatures, which
indicates that both forms are related by enantiotropism.[62–64]
II
oxypyridine molecules, and one Fe cation, which are all lo-
cated in general positions. The Fe cation is octahedrally coor-
II
dinated by the two anionic as well as the four neutral co-li-
gands forming discrete complexes (Figure 3). The Fe–NPy and
Fe–NNCS distances are slightly larger than those found in 1-I
ranging from 2.198(2) to 2.241(2) Å and 2.115(2) to
Table 1. Selected crystal data for 1-I and 1-II determined by Pawley
fits using powder patterns measured at room temperature.
II
1-I
1-II
2.125(2) Å, which originates from the larger radius of the Fe
cation (Table S2, Supporting Information). The values for the
octahedral angle variance (σθ‹oct› = 2.9) and the mean qua-
Crystal system
Space group
a /Å
b /Å
c /Å
orthorhombic
Pccn
19.0128(4)
9.2982(2)
17.1355(3)
90
90
90
3029.3(1)
orthorhombic
Pbca
10.2874(3)
17.0150(4)
34.2684(7)
90
90
90
5998.3(2)
1.354
2
dratic elongation (λoct = 1.002) also indicate a slight distortion
of the coordination octahedron. It shall be noted that the coor-
dination octahedron of 1-II-Fe is stronger distorted than that
in 1-I. To emphasize the differences between the discrete com-
plexes in both modifications 1-I and 1-II-Fe an overlay plot is
presented in Figure 4, which shows that the orientation and
conformation of the ligands is different.
α /°
β /°
γ /° 3
V /Å
Density calcd. /g·cm–3 1.340
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Figure 4. Overlay plot of the discrete complexes found in compound
-I (transparent) and 1-II-Fe (opaque) with labeling of the ligands (Ni/
1
Fe: orange; S: yellow; N: blue; C: black; O: red; H: grey).
Two of the four neutral ligands (1 and 4) show a similar
orientation of the 4-methoxypyridine ligands in both modifica-
tions, whereas this is different for the co-ligands 2 and 3. Large
differences are also found in the conformation of the methoxy
group in ligand 2, which, in contrast to the methyl groups of
the other co-ligands, is rotated by 167.7(4)° along the C –O
Py
axis in 1-II-Fe.
Figure 5. Crystal structure of 1-I (top) and 1-II-Fe (bottom) with view
along the b axis and C–H···S short contacts shown as dashed lines
Ni: orange; S: yellow; N: blue; C: black; O: red; H: grey).
Differences are also found in the packing of the discrete
complexes in both modifications. In both forms short contacts
between H and S atoms of adjacent complexes indicate the
(
presence of weak C–H···S interactions, by which the discrete Table 2. Hydrogen bonding /Å,° for 1-I, 1-II-Fe, 1-MeCN, 2, and 3.
complexes in 1-I are connected into layers parallel to the ab
D–H···A
-I
C(22)–H(22)..S(1)A
-II-Fe
d(H···A)
d(D···A)
Ͻ(DH···A)
plane, whereas in 1-II-Fe they form a three-dimensional net-
work (Figure 5 and Table 2). In 1-II-Fe some of the six-mem-
bered rings of 4-methoxypyridine molecules of neighboring
1
2.92
3.788(2)
152.9
1
complexes are nearly coplanar with a dihedral angle between C(16)-H(16B)···S(1)B
2.90
2.80
3.839(3)
3.720(3)
161.5
156.7
C(36)-H(36B)···S(1)C
the pyridine planes of 4.23(8)°, which is indicative for π–π
1-MeCN
interactions (Figure 5). However, the intermolecular distance
between these molecules amounts to 6.671(1) Å and thus, is
too long for such interaction.
C(22)–H(22)···S(1)
2
C(16)–H(16B)···S(1)D
2.99
3.863(4)
157.4
2.83
3.02
2.56
3.788(4)
3.945(5)
3.535(5)
164.7
157.7
171.9
Compound 1-MeCN crystallizes in the cubic space group C(22)–H(22A)···S(1)E
¯
C(22)–H(22C)···N(1)F
3
C(16)–H(16B)···O(11)G
C(26)–H(26B)···O(21)H
P43n with the asymmetric unit containing one 4-methoxypyr-
idine ligand in a general position, one thiocyanate anion on a
2.63
2.64
2.83
3.522(6)
3.501(6)
3.715(5)
151.5
146.1
151.0
II
twofold rotational axis, one Ni cation situated on the intersec-
tion point of three twofold rotational axes as well as of one C(26)–H(26C)···S(1)I
acetonitrile molecule that is located on a threefold rotational
axis. The metal cation is octahedrally coordinated by two thio- C: x + 1/2, –y + 3/2, –z + 1, D: –x + 5/4, y + 5/4, z + 1/4, E:
Symmetry codes: A: –x + 1, y + 1/2, –z + 3/2, B: x–1/2, y, –z + 3/2,
cyanate anions and four 4-methoxypyridine molecules forming –x + 3/2, –y + 1, z + 1/2, F: x, y + 1/2, z + 1/2, G: –x + 1, –y, –z + 2,
H: –x, –y + 2, –z–1, I: x, y, z–1.
discrete complexes analogous to those in 1-I and 1-II-Fe (Fig-
ure S12, Supporting Information). The Ni–NPy and Ni–NNCS
distances amount to 2.120(2) and 2.060(5) Å and are compar- quadratic elongation values amounting to σθ‹oct›2 = 0.2 and
able to those in 1-I (Table S3, Supporting Information). Be- λoct = 1.000.
cause of the high symmetry the coordination octahedron is
In the crystal structure the discrete complexes are linked by
nearly ideal with the octahedral angle variance and the mean weak C–H···S interactions and are arranged in a way that cavi-
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ties are formed in which acetonitrile solvate molecules are lo-
cated (Figure S13, Supporting Information, and Table 2).
The second compound containing acetonitrile also forms
discrete complexes. Compound 2 crystallizes in the ortho-
rhombic non-centrosymmetric space group Fdd2. The asym-
metric unit consists of one thiocyanate anion, one 4-methoxy-
pyridine molecule and one acetonitrile molecule in general po-
II
sitions as well as one Ni cation that is situated on a twofold
rotational axis. The metal cations are octahedrally coordinated
by two thiocyanate anions, two 4-methoxypyridine co-ligands,
and two acetonitrile molecules (Figure 6 and Figure S14, Sup-
porting Information) with a Ni–NPy distance of 2.105(2) Å and
a Ni–NNCS distance of 2.047(3) Å. The distance to the aceto-
nitrile molecules of 2.108(3) Å is comparable to that to the 4-
methoxypyridine co-ligand (Table S4, Supporting Infor-
mation). The coordination polyhedron is slightly distorted with
octahedral angle variance and mean quadratic elongation val-
2
ues of σθ‹oct› = 3.1 and λoct = 1.001.
Figure 7. View of a chain in compound 3 (top) and crystal structure
with view along the chains (bottom) with C–H···O and C–H···S short
Figure 6. View of the coordination sphere of the NiII cation in com-
pound 2 (Ni: orange; S: yellow; N: blue; C: black; O: red; H: grey). An contacts shown as dashed lines (Ni: orange; S: yellow; N: blue; C:
ORTEP plot of 2 can be found in Figure S14 (Supporting Information). black; O: red; H: grey). An ORTEP plot of 3 can be found in Figure
S16 (Supporting Information).
In the crystal structure of 2 the discrete complexes are ar-
ranged into stacks along the crystallographic b axis and are
connected by weak C–H···S and C–H···N interactions (Figure
are nearly perpendicular with dihedral angles between the pyr-
idine ring planes of 84.2(2)° and the Ni–NPy vectors of neigh-
boring cations being tilted by 7.1(2)°. The chains are arranged
S15, Supporting Information, and Table 2).
Compound 3 crystallizes in the triclinic space group P1 with
¯
in a way that the Ni(NCS) Ni planes of neighboring chains are
2
the asymmetric unit consisting of two 4-methoxypyridine mol-
ecules and two thiocyanate anions in general positions and two
parallel, which corresponds to one of the typical arrangement
[66]
in such 1D compounds.
The chains are linked by relatively
II
crystallographically independent Ni cations that are located
weak intermolecular C–H···S and C–H···O hydrogen bonding
into a three-dimensional network (Figure 7 bottom, Table 2).
on centers of inversion (Figure S16, Supporting Information).
The metal cations are octahedrally coordinated by four thiocy-
anate anions, of which two bind via the nitrogen atom and two
via the sulfur atom as well as two 4-methoxypyridine mol- IR Spectroscopy
ecules in axial positions. The Ni cations are linked into chains
To determine the position of the asymmetric CN stretching
by pairs of μ-1,3-bridging anionic ligands (Figure 7 top).
The Ni–NPy distances range from 2.090(4) to 2.121(3) Å
and the Ni–NNCS distances from 2.011(4) to 2.045(4) Å, and
are comparable to the values known from literature (Table S5,
Supporting Information). The Ni–S distances range from
band of the thiocyanate anions IR spectroscopic measurements
were performed (Figures S17–21, Supporting Information,
Table 3). For compounds containing N-terminally coordinated
Table 3. Wavenumbers of the asymmetric CN stretching vibration of
the thiocyanate anions in compounds 1-I, 1-II, 1-MeCN, 2, and 3 in
2.525(1) to 2.531(1) Å and are also consistent with the corre-
sponding values reported in the literature. The Ni coordination
–1
cm .
is slightly distorted with values for the octahedral angle vari-
1
-I
1-II
1-MeCN
2
3
2
ance and the mean quadratic elongation of σ
λoct = 1.021 for Ni1 and σθ‹oct› = 5.9 and λoct = 1.021 for Ni2.
= 6.9 and
θ‹oct›
2
Position
2071
2070
2082
2083
2113,
2
126
Along the chains neighboring 4-methoxypyridine co-ligands
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thiocyanate anions the CN stretching vibration is usually found
–
1
below 2100 cm , whereas for those containing μ-1,3 bridging
thiocyanate anions this vibration is observed above
–
1 [67]
2
2
100 cm .
The IR spectrum of compound 1-I shows a band at
–
1
071 cm , which is in good agreement with what is expected
for N-terminally coordinated thiocyanate anions. For the poly-
morphic modification 1-II the corresponding band is found at
–
1
2
070 cm , which is expected because of the structural similar-
ity of both discrete complexes. In fact, two different values are
expected because this form contains two crystallographically
independent anions but obviously, they cannot be resolved. In
the IR spectrum of compound 1-MeCN the CN stretching vi-
–
1
bration is shifted to 2082 cm , indicating that the incorporated
acetonitrile solvate molecules might have an influence on this
–
1
–1
vibration. The two signals at 2285 cm and 2313 cm can be
assigned to the CN stretch of the acetonitrile solvent mol-
ecules. For compound 2 the value of the CN stretching vi-
–
1
bration (2083 cm ) is very similar to that in 1-MeCN but still
in the range expected for N-terminally bonded thiocyanate Figure 8. TG curves of 1-MeCN, 1-I, 1-II, and 2 at 4 K·min–1 in a
nitrogen atmosphere.
anions. In contrast, for compound 3 that contains μ-1,3-bridg-
ing thiocyanate anions the CN stretching vibration is shifted
–
1
–1
above 2100 cm . Two different values at 2113 cm and
2
Compounds 1-I, 1-II, and 2 decompose in two steps upon
heating and for each of these steps an endothermic event is
observed in the DSC curve (Figures S23–S25, Supporting In-
formation). For 1-I and 1-II the mass loss in each step is very
similar for both compounds and in good agreement with that
expected for the removal of two 4-methoxypyridine molecules
in each step (Δmcalcd = 35.7%). For the acetonitrile compound
–
1
126 cm are observed that originate from the two crystallo-
graphically independent thiocyanate anions. In contrast to 1-II
these two values can be resolved, which might be traced back
to the fact, that each of the thiocyanate N atoms is connected
to a crystallographically different metal cation.
2
the mass loss of the first mass step corresponds to that calcu-
Thermoanalytical Investigations
lated for the removal of the MeCN molecules (Δmcalcd
=
As shown in this work most of the compounds obtained 17.3%), whereas in the last step the 4-methoxypyridine ligands
from solution are discrete complexes containing only ter- are removed.
minally bonded thiocyanate anions. In this context the question
To identify the intermediates that formed after the first TG
arose if these compounds can be transformed by thermal de- step of compound 1-MeCN a TG measurement was stopped
gradation into the chain compound 3 or into a different poly- after the first step and the residue was investigated using
morphic or isomeric modification as shown in recent work.[45] XRPD, which shows that polymorph 1-I formed (Figure 9).
It is also of interest, which of the two polymorphic modifica-
tions 1-I or 1-II is obtained by removal of the acetonitrile solv-
ate molecules in compound 1-MeCN. Therefore, compounds
1-MeCN, 1-I, 1-II, and 2 were investigated by simultaneous
thermogravimetry and differential scanning calorimetry (TG-
DSC). For clarity only the thermogravimetric curves are shown
in Figure 8 but complete TG, DSC, and DTG curves can be
found in Figures S22–S25 (Supporting Information).
Upon heating 1-MeCN decomposes in three different steps
with each step accompanied by endothermic events in the DSC
curve (Figure 8 and Figure S22, Supporting Information). The
first mass loss is slightly higher than that calculated for the
removal of 1.33 acetonitrile molecules per formula unit
(
Δmcalcd = 8.2%) indicating the presence of additional solvent
molecules in this compound. This is expected as single crystal
structure analysis showed that 1-MeCN contains a small
amount of disordered solvent molecules. However, upon fur-
ther heating two mass steps are observed, which are in reason-
able agreement with the removal of two 4-methoxypyridine
ligands per formula unit (Δmcalcd = 32.8%) each.
Figure 9. XRPD pattern of the residue obtained after the first step of
the thermal degradation of compound 1-MeCN (A), and the calculated
patterns for 1-I (B) and 1-II (C). The reflection marked with an aster-
isk does not correspond to the XRPD pattern of 1-I.
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There is an additional reflection, which indicates that a further
and unknown crystalline compound was also obtained.
Similar experiments for 1-I, 1-II, and 2 prove that they
transform into the chain compound 3 after removal of half of
the 4-methoxypyridine ligands for compounds 1-I and 1-II and
after complete removal of the acetonitrile molecules in com-
pound 2 (Figure 10). There is no indication for the formation
of an additional crystalline phase.
Figure 12. Temperature dependence of the magnetic susceptibility χ
measured at 1 kOe for compound 3 presented as the χT product.
Magnetic Properties
The magnetic properties were studied only for compound 3,
because for the remaining presented structures only single-ion
magnetic properties are expected, due to lack of exchange
paths between the magnetic Ni ions. For 3, the temperature
dependence of the magnetic susceptibility χ is presented in
Figure 12. Starting from room temperature, the χT product in-
creases as the temperature decreases, which means that the
Figure 10. XRPD pattern of the residues obtained after the first step
of the thermal degradation of compound 1-I (A), 1-II (B), and 2 (C)
as well as the calculated pattern for 3 (D).
exchange interaction J along the Ni(NCS)
chains is ferromag-
2
netic. To quantify J, the Heisenberg chain Hamiltonian with
Subsequent investigations using time-resolved XRPD isotropic spins (S = 1) was used:
showed that compound 2 loses the acetonitrile molecules al-
ready at room-temperature and transforms into compound 3
within a few hours (Figure 11). However, under acetonitrile
atmosphere it is stable over a long period. To investigate the
reversibility of the acetonitrile removal, compound 3 was kept
in a MeCN atmosphere for 14 d, but no changes were observed
The values J and g were determined by fitting the experi-
mental data above 30 K using the approximation formula valid
[45]
for ferromagnetic exchange J Ͼ 0:
(Figure S26, Supporting Information).
The values obtained are J = 5.92(10) K and g = 2.21(1).[45]
This J value is at the lower limit of those reported for linear
Ni(NCS)2 chains but it is still higher than values recently
found for corrugated chains and therefore is in agreement with
previous investigations.[
45,61]
Below 30 K the course of χT(T) experimental points differs
from the extrapolated curve, which is probably due to an an-
II
isotropy of the single Ni ion and interchain interactions,
which are not accounted for in the above Hamiltonian. The
low-temperature dependence of the susceptibility measured at
100 Oe is shown in Figure 13 top. There is no difference be-
tween zero-field cooled and field cooled susceptibility (Figure
S27, Supporting Information) and the out-of-phase ac suscep-
tibility is practically zero (Figure S28, Supporting Infor-
mation). All these low field susceptibility data show a χ(T)
maximum around 2.05 K, which suggests an onset of magnetic
Figure 11. Time dependent XRPD pattern of 2 measured each 2 h at
room temperature.
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Journal of Inorganic and General Chemistry
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ordering due to antiferromagnetic interchain interactions. To mentally available temperature range, which is limited to
check if the ordering transition really occurs, specific heat 1.8 K.
measurements were performed (Figure S29, Supporting Infor-
mation).
Conclusions
The reaction of Ni(NCS) and 4-methoxypyridine leads to
2
the formation of several compounds with 4-methoxypyridine
as co-ligand, with most of them consisting of discrete com-
II
plexes, in which the Ni cations are octahedrally coordinated
by four co-ligands and two terminally N-bonded anions. This
also includes two polymorphic modifications, for which the
thermodynamic stability at room-temperature was investigated.
The structure of the thermodynamically metastable form was
clarified by crystallization of the corresponding Fe compound,
which shows that crystallization of compounds with cations of
similar coordination behavior is an alternative tool to extract
structural information in those cases, in which no single crys-
tals are available. Both discrete complexes with the composi-
tion Ni(NCS) (4-methoxypyridine) lose half of their 4-meth-
2
4
oxypyridine ligands upon heating transforming into a coordi-
nation polymer with the composition [Ni(NCS) (4-methoxy-
2
pyridine) ] , in which the cations are linked by pairs of anionic
2
n
ligands into linear chains. This transformation also occurs for
Ni(NCS) (4-methoxypyridine) (MeCN) , but in this case the
2
2
2
acetonitrile molecules are emitted. For [Ni(NCS) (4-methoxy-
2
pyridine) ] the exchange constant was determined by mag-
2
n
netic measurements leading to a value of J = 5.92(10) K,
which is at the lower limit compared to the corresponding val-
ues found for similar compounds with linear chains, but still
higher than what is usually found for corrugated chains. Heat
Figure 13. Temperature dependence of the susceptibility χ measured capacity measurement proved that antiferromagnetic ordering
at 100 Oe and the magnetic contribution to the specific heat over tem- occurs at 1.91(5) K.
perature Cmagn/T (top) as well as Cmagn together with the d(χT)/dT
derivative (bottom) for compound 3.
Experimental Section
Above 10 K the dominant contribution to the specific heat
4 2 2 2
originates from the crystal lattice. This lattice contribution to Synthesis: NiSO ·6H O was obtained from Merck, Ba(NCS) ·3H O
was obtained from Alfa Aesar and 4-methoxypyridine from TCI. All
chemicals were used without further purification. If not noted other-
wise all crystalline powders were prepared by stirring the reactants in
solution at room temperature. The residues were filtered off and dried
in air.
the specific heat was estimated by fitting the Debye phonon
model in the range from 16 to 20 K and extrapolating to lower
temperatures. The magnetic contribution to the specific heat
Cmagn that was obtained by subtracting the lattice contribution
is also shown in Figure 13 top. The small peak of Cmagn that
is visible at T = 1.91(5) K corresponds to the magnetic order- Ni(NCS) : Ni(NCS) was prepared by the reaction of equimolar
c
2
2
ing. There is also a broad peak at around 6 K, which originates amounts of NiSO
4
·6H
2
O and Ba(NCS)
was filtered off and the filtrate was concen-
trated to complete dryness resulting in a greenish-brown residue of
Ni(NCS)
2 2
·3H O in water. The resulting
white precipitate of BaSO
4
from the exchange interaction along the Ni(NCS) chains and
2
II
zero-field splitting of Ni . The determined magnetic entropy
–
1
–1
2
.
change from 0.4 to 16 K equals ΔS = 7.4 J·mol ·K , which
is slightly lower than the theoretical value R·ln(2S + 1) = 9.13
Synthesis of 1-I: To a solution of Ni(NCS)
2
(87.4 mg, 0.50 mmol) in
–
1
–1
J·mol ·K .
4.0 mL water 4-methoxypyridine (203.0 μL, 2.00 mmol) was added.
The positions of Cmagn and χ maxima are not exactly at the After stirring the mixture for 4 d, the blue residue was filtered of and
same temperature (see Figure 13 top). The reason may be that washed with water. Single crystals were obtained by layering a solu-
for an antiferromagnetic ordering, the critical temperature Tc tion of Ni(NCS) (26.2 mg, 0.15 mmol) in 2.0 mL water with 2.0 mL
2
of ethyl ether. 4-Methoxypyridine (60.8 mL, 0.60 mmol) was carefully
added to the mixture and after 1 d deep blue crystals were obtained.
should be determined from magnetic data as the position of
the d(χT)/dT maximum, which in some cases is shifted from
–
1
the position of the χ(T) maximum.[
68,69]
C
26
H
28
N
6
NiO
4 2
S (611.3681 g·mol ): calcd. C 51.08, H 4.62, N 13.75,
To check this, d(χT)/
S 10.49%; found: C 51.04, H 4.66, N 13.70, S 10.58%.
dT was plotted together with Cmagn for comparison as a func-
tion of temperature (Figure 13 bottom). However using these Synthesis of 1-II: To a solution of Ni(NCS) (87.4 mg, 0.50 mmol) in
2
data the d(χT)/dT maximum is not observed due to the experi- 1.0 mL water 4-methoxypyridine (76.2 μL, 0.75 mmol) was added.
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Journal of Inorganic and General Chemistry
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The resulting blue precipitate was filtered of immediately and dried in
Synthesis of 3: A microcrystalline powder on larger scale was synthe-
sized by stirring a mixture of Ni(NCS) (174.8 mg, 1.00 mmol) and 4-
methoxypyridine (101.5 μL, 1.00 mmol) in 3.0 mL of ethanol for 3 d.
Single crystals were obtained by layering a solution of Ni(NCS)
52.4 mg, 0.30 mmol) in 2.0 mL water with 2.0 mL ethyl ether and
–1
air. C26
H
28
N
6
NiO
4
S
2
(611.3681 g·mol ): calcd. C 51.08, H 4.62, N
2
13.75, S 10.49%; found: C 51.04, H 4.78, N 13.59, S 10.58%.
2
Synthesis of 1-II-Fe: Single crystals were prepared in a similar way to
those obtained for compound 1-I. A mixture of FeSO ·7H O (27.8 mg,
.10 mmol) and KSCN (19.5 mg, 0.20 mmol) in 1.0 mL water was
layered with 2.0 mL ethyl ether. Subsequently 4-methoxypyridine
60.8 μL, 0.60 mmol) was carefully added to the mixture. After one
(
4
2
carefully adding 4-methoxypyridine (15.2 μL, 0.15 mmol). After sev-
0
eral days green crystals suitable for X-ray structure determination
–1
14 4 2 2
formed. C14H N NiO S (393.1128 g·mol ): calcd. C 42.78, H 3.59
(
N 14.25, S 16.31%; found: C 42.34, H 3.65, N 14.14, S 16.88%.
day light yellow crystals were obtained, which were suitable for X-ray
structure determination.
Single-crystal Structure Analysis: Data collection was performed
with an imaging plate diffraction system (IPDS-2) from STOE & CIE
2
Synthesis of 1-MeCN: Ni(NCS) (43.7 mg, 0.25 mmol) was stirred
using Mo-K
SHELXS-97
α
radiation. Structure solution was performed with
with 4-methoxypyridine (101.6 μL, 1.00 mmol) in 2.5 mL MeCN in a
closed culture tube at 90 °C until complete dissolution of the reactants
was observed. The solution was let to cool to room temperature, upon
which blue crystals suitable for single crystal diffraction formed. For
powders of this compound the crystals obtained were grinded and the
resulting powder stored in an acetonitrile atmosphere.
[
70]
[71]
or SHELXT
and structure refinements were per-
formed against F using SHELXL-2014.[72] A numerical absorption
2
correction was applied using programs X-RED and X-SHAPE of the
[
73]
program package X-Area.
All non-hydrogen atoms were refined
with anisotropic displacement parameters. The C–H hydrogen atoms
were positioned with idealized geometry (methyl H atoms were al-
lowed to rotate but not to tip) and were refined isotropically with
–1
C
28.67
H
32
N7.33NiO
4
S
2
(666.1048 g·mol ): calcd. C 51.69, H 4.84, N
15.42, S 9.63%; found: C 51.97, H 4.79, N 15.80, S 9.34%.
Uiso(H) = 1.2 Ueq(C) (1.5 for methyl H atoms) using a riding model.
Synthesis of 2: A mixture of Ni(NCS)
2
(43.7 mg, 0.25 mmol) and 4- The absolute structure of 1-MeCN and 2 was determined and is in
methoxypyridine (25.4 μL, 0.25 mmol) was stirred in 3.0 mL aceto-
nitrile until complete dissolution occurred. The resulting solution was
concentrated at ambient conditions until a blue precipitate was ob-
agreement with the selected setting [Flack-x parameter: –0.030(8) for
1-MeCN and –0.002(9) for 2]. After the structure refinement of 1-
MeCN a very small amount of residual electron density was found in
served, which was filtered of and stored under acetonitrile atmosphere. the cavities indicating the presence of disordered solvent. Therefore
Single crystals were grown from the filtrate by slow evaporation of the data were corrected for disordered solvent using the SQUEEZE
the solvent. It shall be noted that compound 2 was only obtained as a routine in PLATON. For this structure PLATON detected pseudo-sym-
¯
phase pure powder after a complete dissolution of the reactants and
subsequent removal of the solvent. If the product was isolated before
metry leading to the centrosymmetric space group Pn3n. Refinement
in this space group yielded significantly worse R values and symmetry
a
C
clear solution had formed and compound 3 was obtained. related disorder of the methoxy groups. Because this was not the case
–1
¯
18
H
20
N
6
NiO
2
S
2
(475.2178 g·mol ): calcd. C 45.49, H 4.24, N 17.68,
in the non-centrosymmetric space group P43n, where the absolute
S 13.50%; found: C 42.26, H 3.93, N 16.49, S 13.54%. The large
structure can also be determined, this space group was chosen. The
differences in the elemental analysis can be traced back to the fact that crystal of compound 2 was merohedrally twinned and therefore, a twin
this compound was already decomposed at room temperature.
refinement was performed leading to a BASF parameter of 0.42 (2).
Table 4. Selected crystal data and details of the structure refinements.
1
-MeCN
1-I
1-II-Fe
28FeN
608.51
orthorhombic
Pbca
10.2901(2)
16.7120(3)
34.5051(10)
90
90
90
5933.8(2)
170(2)
8
2
3
formula
MW /g·mol–1
Crystal system
Space group
a /Å
b /Å
c /Å
a /°
β /°
C
28.67
H
32
N7.33NiO
4
S
2
C
26
H
28
N
6
NiO
4
S
2
C
26
H
6
O
4
S
2
C
18
H
20
N
6
NiO
2
S
2
C
14 14 4 2 2
H N NiO S
666.11
cubic
P43n
17.2262(5)
17.2262(5)
17.2262(5)
90
90
90
5111.7(4)
200(2)
6
611.37
orthorhombic
Pccn
18.8975(5)
9.2301(2)
16.9150(5)
90
475.23
orthorhombic
Fdd2
38.9287(3)
8.0570(7)
14.3752(5)
90
90
90
4508.8(6)
170(2)
8
393.12
triclinic
P1
8.9076(7)
10.4170(9)
10.8294(9)
65.877(6)
67.720(6)
83.224
847.83(13)
170(2)
2
¯
¯
90
90
γ /°
V /Å3
2950.41(13)
170(2)
4
T /K
Z
D
μ /mm
–3
calcd /g·cm
1.298
1.376
1.362
1.400
1.540
–
1
0.734
0.840
0.690
1.071
1.403
θ
max /°
26.991
17673
1846
27.001
19356
3215
25.093
24654
5269
27.003
8978
2476
25.198
6127
3008
Measured refl.
Unique refl.
Refl. F
o
Ͼ 4σ(F
o
)
1589
2734
4202
2328
2362
Parameter
101
179
356
136
214
R
R
wR
int
0.0283
0.0411
0.1154
–0.030(8)
1.093
0.0355
0.0358
0.0968
–
1.070
0.283/ –0.342
0.1007
0.0433
0.01153
–
1.101
0.635/ –0.580
0.0289
0.0299
0.0812
–0.002(9)
1.072
0.0950
0.0591
0.1751
–
1.057
0.599/ –0.967
1
[F
0 0
Ͼ 4σF )]
[all data]
2
Flack x
GOF
Δρmax/min /e·Å–3
0.180/ –0.344
0.303/ –0.288
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Journal of Inorganic and General Chemistry
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Selected crystal data and details of the structure refinements are sum-
marized in Table 4.
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Received: July 25, 2018
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A. Jochim, M. Ceglarska, M. Rams, C. Näther* ........... 1–12
Synthesis, Crystal Structures, and Properties of Ni(NCS) -4-
2
methoxypyridine Coordination Compounds
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