Structures, Thermodynamic Relations, and Magnetism of Stable and Metastable Ni(NCS)2 Coordination Polymers

Article abstract of DOI:10.1021/acs.inorgchem.8b00092

Reaction of Ni(NCS)₂ with 4-aminopyridine in different solvents leads to the formation of compounds with the compositions Ni(NCS)₂(4-aminopyridine)₄ (1), Ni(NCS)₂(4-aminopyridine)₂(H₂O)₂ (2), [Ni(NCS)₂(4-aminopyridine)₃(MeCN)]·MeCN (3), and [Ni(NCS)₂(4-aminopyridine)₂]_n (5-LT). Compounds 1, 2, and 3 form discrete complexes, with octahedral metal coordination. In 5-LT the Ni cations are linked by single thiocyanate anions into chains, which are further connected into layers by half of the 4-aminopyridine coligands. Upon heating, 1 transforms into an isomer of 5-LT with a 1D structure (5-HT), that on further heating forms a more condensed chain compound [Ni(NCS)₂(4-aminopyridine)]_n (6) that shows a very unusual chain topology. If 3 is heated, a further compound with the composition Ni(NCS)₂(4-aminopyridine)₃ (4) is formed, which presumably is a dimer and which on further heating transforms into 6 via 5-HT as intermediate. Further investigations reveal that 5-LT and 5-HT are related by enantiotropism, with 5-LT being the thermodynamic stable form at room-temperature. Magnetic and specific heat measurements reveal ferromagnetic exchange through thiocyanate bridges and magnetic ordering due to antiferromagnetic interchain interactions at 5.30(5) K and 8.2(2) K for 5-LT and 6, respectively. Consecutive metamagnetic transitions in the spin ladder compound 6 are due to dipolar interchain interactions. A convenient formula for susceptibility of the ferromagnetic Heisenberg chain of isotropic spins S = 1 is proposed, based on numerical DMRG calculations, and used to determine exchange constants.

Full text of DOI:10.1021/acs.inorgchem.8b00092

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Structures, Thermodynamic Relations, and Magnetism of Stable and
Metastable Ni(NCS)2 Coordination Polymers
Tristan Neumann,† Magdalena Ceglarska,‡ Luzia S. Germann,§ Michał Rams,‡ Robert E. Dinnebier,§
Stefan Suckert,† Inke Jess,† and Christian Nather*,†
̈
†Institut fur Anorganische Chemie, Christian-Albrechts-Universitat zu Kiel, Max-Eyth-Straße 2, 24118 Kiel, Germany
̈
̈
‡Institute of Physics, Jagiellonian University, Łojasiewicza 11, 30348 Krakow
́
, Poland
§Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany
S
* Supporting Information
ABSTRACT: Reaction of Ni(NCS)2 with 4-aminopyridine in
different solvents leads to the formation of compounds with the
compositions Ni(NCS)2(4-aminopyridine)4 (1), Ni(NCS)2(4-
aminopyridine)2(H2O)2 (2), [Ni(NCS)2(4-aminopyridi-
ne)3(MeCN)]·MeCN (3), and [Ni(NCS)2(4-aminopyridi-
ne)2]n (5-LT). Compounds 1, 2, and 3 form discrete
complexes, with octahedral metal coordination. In 5-LT the
Ni cations are linked by single thiocyanate anions into chains,
which are further connected into layers by half of the 4-
aminopyridine coligands. Upon heating, 1 transforms into an
isomer of 5-LT with a 1D structure (5-HT), that on further
heating forms a more condensed chain compound [Ni-
(NCS)2(4-aminopyridine)]n (6) that shows a very unusual
chain topology. If 3 is heated, a further compound with the composition Ni(NCS)2(4-aminopyridine)3 (4) is formed, which
presumably is a dimer and which on further heating transforms into 6 via 5-HT as intermediate. Further investigations reveal that
5-LT and 5-HT are related by enantiotropism, with 5-LT being the thermodynamic stable form at room-temperature. Magnetic
and specific heat measurements reveal ferromagnetic exchange through thiocyanate bridges and magnetic ordering due to
antiferromagnetic interchain interactions at 5.30(5) K and 8.2(2) K for 5-LT and 6, respectively. Consecutive metamagnetic
transitions in the spin ladder compound 6 are due to dipolar interchain interactions. A convenient formula for susceptibility of
the ferromagnetic Heisenberg chain of isotropic spins S = 1 is proposed, based on numerical DMRG calculations, and used to
determine exchange constants.
removed, which leads to metastable compounds in several cases
that are different from those obtained from solution under
thermodynamic control. The latter approach is of extreme
importance in our own work, which focuses on the synthesis of
magnetic coordination polymers based on thio- and selenocya-
nates of 3d transition metal cations that are additionally
coordinated by N-donor coligands that predominantly consist
of pyridine derivatives.30,31 In most cases compounds with the
general composition [M(NCS)2(coligand)2]n (M = Mn, Fe,
Co, Ni) were obtained, in which the metal cations are linked by
pairs of anionic ligands into chains, but we also found a few
examples where the cations are linked by single and double
thiocyanate anions into 2D networks.32−34 In most chain
compounds the thiocyanate N and S atoms as well as the
coligands are in the trans-position leading to linear chains,
which might correspond to a very stable arrangement. In
contrast, in [Ni(NCS)2(4-(boc-amino)pyridine)2]n (4-boc-
INTRODUCTION
■
The synthesis of new coordination compounds and polymers is
still a major topic in coordination chemistry, because of their
diverse physical properties and the possibility for structure
control.1−11 In most cases their synthesis is performed in
solution leading to thermodynamically stable structures.12 In
contrast, reports where metastable compounds were prepared
are relatively rare and some selected examples are given in the
reference list.11−27 This is a pity, because, metastable
polymorphic modifications or isomers will have different
structures and consequently different physical properties,
which can be of importance in material development. For the
synthesis of metastable coordination compounds, different
methods can be applied. This includes, e.g., crystallization
under kinetic control, which means that the reactants are mixed
in solution and the product is immediately filtered off. If the
product is completely dissolved, an antisolvent can be added,
leading to fast precipitation under kinetic control. In some
cases, also mechanochemical reactions can be used.28,29
Moreover, the solvents or part of the ligands can be thermally
Received: January 10, 2018
© XXXX American Chemical Society
A
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Elemental analysis for C34H36N16Ni2S4 (914.4110 g/mol): calc. N
24.51, C 44.66, H 3.97, S 14.03. Exp.: N 24.32, C 44.43, H 3.968, S
13.89.
amino)pyridine = 4-(tert-butoxycarbonylamino)pyridine) the
thiocyanate N atoms are trans, whereas the thiocyanate S and
the 4-(boc-amino)pyridine N atoms are in the cis-position,
leading to zigzag chains.18 It is noted that this compound
contains channels, in which acetonitrile molecules are
embedded, that can be removed in a topotactic reaction. For
this compound a value for the intrachain interaction of J = 6.1
K for the solvate and of J = 3.9 K for the ansolvate were
determined.
In the course of our project we became interested in 4-
aminopyridine as coligand, in which the boc group is absent.
With this ligand a discrete complex with the composition
Ni(NCS)2(4-aminopyridine)4 as well as some solvates of this
compound were already reported by Nassimbeni and
Kilkenny.35 Interestingly, they also investigated the thermal
properties of this complex by hot-stage microscopy and TG-
DSC measurements, from which they concluded that a 4-
aminopyridine deficient compound might exist, but surpris-
ingly, no further investigations were performed. Therefore, we
investigated this system in more detail. In the course of these
investigations we obtained a compound with a novel Ni(NCS)2
2D network, which is thermodynamically stable at room-
temperature, but on thermal decomposition of Ni(NCS)2(4-
aminopyridine)4 two additional compounds were detected.
This includes an isomeric Ni(NCS)2 chain compound with the
same composition as that of the layered material, which
becomes thermodynamically stable at higher temperatures, and
a more 4-aminopyridine deficient compound that exhibits a
condensed 1D network that, to the best of our knowledge, is
without any precedence in the literature. Here we report on our
investigations.
Synthesis of [Ni(NCS)2(4-aminopyridine)2]n (5-HT). Single
crystals suitable for single crystal X-ray diffraction were obtained
from the reaction of Ni(NCS)2 (0.5 mmol, 87.5 mg) and 4-
aminopyridine (0.5 mmol, 47.1 mg) in 3.0 mL MeCN by annealing at
140 °C. A light green crystalline powder was obtained by the reaction
of Ni(NCS)2 (0.5 mmol, 87.5 mg) with 4-aminopyridine (1.0 mmol,
94.1 mg) in 3.0 mL MeCN, which was stirred for 3d at 140 °C and
immediately filtered off. Elemental analysis for C12H12N6NiS2
(363.0895 g/mol): calc. N 23.15, C 39.70, H 3.33, S 17.66. Exp.: N
22.92, C 39.55, H 3.15, S 17.61
Synthesis of [Ni(NCS)2(4-aminopyridine)2]n (5-LT). Single
crystals suitable for single crystal X-ray diffraction were obtained
from the reaction of Ni(NCS)2 (0.05 mmol, 8.8 mg) and 4-
aminopyridine (0.05 mmol, 4.7 mg) in 1.5 mL EtOH in a sealed test
vessel, which was left to stand under air and ambient temperature, after
a few days. A light blue crystalline powder was obtained from the
reaction of Ni(NCS)2 (1.0 mmol, 174.9 mg) with 4-aminopyridine
(1.0 mmol, 94.1 mg) in 4.0 mL EtOH after stirring the reactants for 1
day. Elemental analysis for C12H12N6NiS2 (363.0895 g/mol): calc. N
23.15, C 39.70, H 3.33, S 17.66. Exp.: N 22.81, C 39.37, H 3.21, S
17.58.
Synthesis of [Ni(NCS)2(4-aminopyridine)]n (6). A lime green
crystalline powder was obtained from the reaction of Ni(NCS)2 (1.0
mmol, 174.9 mg) with 4-aminopyridine (1.0 mmol, 94.1 mg) in 3 mL
EtOH at a temperature of 140 °C within 3d. Elemental analysis for
C7H6N4NiS2 (268.9734 g/mol): calc. N 20.83, C 31.26, H 2.25, S
23.84. Exp.: N 20.65, C 31.19, H 2.18, S 23.67.
Elemental Analysis. CHNS analysis was performed using a “vario
MICRO cube elemental analyzer”, fabricated by Elementar.
Spectroscopy. All IR data were obtained using an ATI Mattson
Genesis Series FTIR Spectrometer, control software: WINFIRST,
from ATI Mattson.
Differential Thermal Analysis and Thermogravimetry (DTA-
TG). DTA-TG measurements were performed in a dynamic nitrogen
atmosphere in Al2O3 crucibles using a STA PT 1000 thermobalance
from Linseis. Different heating rates in the range of 1−8 K/min were
used. The instrument was calibrated using standard reference
materials.
Differential Scanning Calorimetry (DSC). The DSC measure-
ments were performed with a DSC 1 Star System with STARe
Excellence Software from Mettler-Toledo AG. The instrument was
calibrated using standard reference materials.
Magnetic Measurements. Magnetic measurements were per-
formed using a MPMS squid magnetometer from Quantum Design.
Powders were pressed with some PTFE to immobilize sample grains.
The diamagnetic contributions of sample holders and the core
diamagnetism were subtracted, using measured and calculated values,
respectively.
Scanning Electron Microscopy (SEM). SEM images were
measured in a secondary electron mode on a XL30 S-FEG instrument
produced by FEI.
Specific Heat Measurements. Specific heat was measured by the
relaxation technique using a Quantum Design PPMS. Powders were
pressed into pellets with no binder. ApiezonN grease was used to
ensure thermal contact of the samples with the microcalorimeter. The
heat capacity of the grease was measured before each run and
subtracted from the measured data.
Single-Crystal Structure Analyses. Data collections were
performed with an imaging plate diffraction system (IPDS-2) from
STOE & CIE using Mo Kα-radiation. Structure solution was
performed with SHELXT36 and structure refinement was performed
against F2 using SHELXL-2014.37 A numerical absorption correction
was applied using programs X-RED and X-SHAPE of the program
package X-Area.38−40 All non-hydrogen atoms were refined with
anisotropic displacement parameters. All C−H hydrogen atoms were
positioned with idealized geometry and were refined isotropic with
Uiso(H) = −1.2 Ueq(C) using a riding model. The N−H and O−H H
EXPERIMENTAL SECTION
■
4-Aminopyridine was obtained from Acros. Ni(NCS)2 was obtained
from the reaction of equimolar amounts of NiSO4·6H2O with
Ba(NCS)2·3H2O in water. The white residue of BaSO4 was filtered
off and the filtrate was concentrated until complete dryness. The purity
was checked by X-ray powder diffraction (XRPD). The crystalline
powders were synthesized by stirring the reactants in the respective
solvents at room temperature. The residues were filtered off and
washed with the used solvent and dried in air. The purity of all
compounds was checked by XRPD.
Synthesis of Ni(NCS)2(4-aminopyridine)4 (1). A crystalline pale
violet powder was formed by the reaction of Ni(NCS)2 (0.5 mmol,
87.5 mg) with 4-Aminopyridine (2.0 mmol, 188.2 mg) in 3.0 mL
ethanol after 1d. Elemental analysis for C22H24N10NiS2 (551.3216 g/
mol): calc. N 25.41, C 47.93, H 4.39, S 11.63. Exp.: N 25.22, C 47.77,
H 4.30, S 11.49.
Synthesis of Ni(NCS)2(4-aminopyridine)2(H2O)2 (2). Single
crystals were obtained by reacting Ni(NCS)2 (1.0 mmol, 149.1 mg)
and 4-aminopyridine (1.0 mmol, 94.1 mg) in 1.5 mL water. The
precipitate was filtered off after 5 min and the filtrate was left to stand
in a closed test tube for a few days. No pure crystalline powder was
obtained.
Synthesis of Ni(NCS)2(4-aminopyridine)3(MeCN)·MeCN (3).
Single crystals suitable for single crystal X-ray diffraction were obtained
from the reaction of Ni(NCS)2 (0.5 mmol, 87.5 mg) with 4-
aminopyridine (0.5 mmol, 47.1 mg) in 3.0 mL MeCN at 140 °C. A
light blue crystalline powder was obtained by the reaction of
Ni(NCS)2 (0.5 mmol, 87.5 mg) with 4-aminopyridine (1.5 mmol,
140.1 mg) in 4.0 mL MeCN after stirring for 1 day. Elemental analysis
for C21H24N10NiS2 (539.3106 g/mol): calc. N 25.97, C 46.77, H 4.49,
S 11.89. Exp.: N 25.5, C 46.12, H 4.302, S 12.12.
Synthesis of Ni(NCS)2(4-aminopyridine)3 (4). This compound
can only be obtained by thermal removal of MeCN from compound 3
and was only characterized by elemental analysis and IR spectroscopy.
B
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formed (Figure S2 in the ESI).35 The asymmetric CN
stretching vibration is observed at 2094 cm−1 with IR
spectroscopy measurements, which is in agreement with the
presence of terminal N-bonded anionic ligands (Figure S3). For
5-LT the CN stretch is found at 2104 cm−1, indicating that μ-
1,3-bridging anionic ligands are present (Figure S4 in the ESI).
From acetonitrile as solvent, a further compound with the
composition Ni(NCS)2(4-aminopyridine)3(MeCN)2 (MeCN
= acetonitrile) (3) was obtained, for which the CN-stretch is
observed at 2091 cm−1, indicating that a discrete complex with
terminal anionic ligands has been obtained (Figure S5). For 3
and 5-LT single crystals were obtained and characterized by
SC-XRD, which show that 3 consists of discrete complexes and
that 5-LT forms a novel layered coordination network (see
below). Within these investigations a few crystals of a further
compound with the composition of Ni(NCS)2(4-
aminopyridine)2(H2O)2 (2) consisting of discrete complexes
were obtained, but no pure samples could be prepared.
The formation of the 2D network is somehow surprising
because as mentioned above, in most cases, compounds with
linear M(NCS)2 double chains (M = Mn, Fe, Co, or Ni) were
obtained. A different phase might be available by thermal
decomposition of precursors with terminal N-bonded thio-
cyanate anions as previously shown by us.47 Therefore,
compound 1 was investigated by thermoanalysis. In this
context also compound 3 is of interest, because one can
assume that a compound with the composition Ni(NCS)2(4-
aminopyridine)3 might be prepared, if the MeCN molecules
can be removed separately.
Measurements of 1 using simultaneous differential thermoa-
nalysis and thermogravimetry (DTA-TG) with a heating rate of
8 °C/min show two mass steps until 400 °C, which are each
accompanied by endothermic events in the DTA curves (Figure
S6). The experimental mass loss in both steps of 33.1% and
32.5% is in reasonable agreement with the removal of each of
the two 4-aminopyridine ligands. To investigate the inter-
mediate obtained in the first TG step, XRPD measurements
were performed, which proves that a new crystalline phase with
the composition [Ni(NCS)2(4-aminopyridine)2]n (5-HT) has
formed, that is different from 5-LT (Figure S7). The CN
stretching vibrations occur at 2094 and 2129 cm−1, which
indicates that μ-1,3-bridging anionic ligands are present (Figure
S8). Moreover, from the DTA and DTG curve it is obvious that
the second TG step might consist of two successive reactions
that are not completely resolved. Therefore, temperature
dependent XRPD measurements were performed, which clearly
show that a further crystalline phase (6) has formed before
decomposition occurs (Figure S9). To obtain large amounts of
this new crystalline phase, heating rate dependent measure-
ments were performed, leading to a much better resolved TG-
curve. The one with a heating rate of 1 °C/min shows three
separate TG steps (Figure 1).
atoms were located in difference map, their bond lengths were set to
ideal values and finally they were refined isotropic with Uiso(H) = −1.5
Ueq(N, O) using a riding model. The crystal of 2 is racemically twinned
and therefore, a twin refinement was performed leading to an BASF
parameter of 0.44(2). This compound shows pseudosymmetry and
this might be the reason why the absolute structure cannot be
determined. If the structure is refined neglecting the twinning,
significant poorer reliability factors were obtained (R1 = 3.13%), which
support the correctness of the space group. Selected crystal data and
details on the structure refinements can be found in Table S1.
CCDC-1815975 (2), CCDC-1815976 (3), CCDC- 1815977 (5-
LT), CCDC-1815979 (5-HT), CCDC-1815978 (6) and contain the
supplementary crystallographic data for this paper. These data can be
obtained free charge from the Cambridge Crystallographic Data
Centre via http://www.ccdc.cam.ac.uk/data_request/cif.
X-ray Powder Diffraction (XRPD). The temperature dependent
XRPD measurements of 1 were performed with an PANalytical X’Pert
Pro MPD Reflection Powder Diffraction System with CuKα radiation
(λ= 154.0598 pm) equipped with an Anton Paar HTK 1200 N high
temperature chamber and with a PIXcel semiconductor detector from
PANalytical.
Phase analysis and measurements for indexing were performed on a
Stoe Transmission Powder Diffraction System (STADI P) with CuKα1
radiation that was equipped with a linear position-sensitive MYTHEN
1K detector (Dectris Ltd.) and a Johann-type Ge(111) mono-
chromator from STOE & CIE. Measurements for ab initio crystal
structure solution and Rietveld refinement were performed at room
temperature on another Stoe Stadi P machine (Mo Kα1, λ = 0.7093 Å,
equipped with a Johann Ge(111) monochromator) using a Mythen
Dectris 1K detector (Dectris Ltd.). All samples were gently ground,
filled into ϕ 0.5 mm glass capillaries for ambient measurements and in
quartz capillaries for high temperature measurements (Hilgenberg).
The capillaries were spun during the experiments for better particle
statistics. The XRPD pattern for ab initio structure solution of 6 was
collected in the range of 2−60° 2θ within 40 h. All Pawley41 and
Rietveld42 refinements were performed using TOPAS V5.43
Indexing of 6 was carried out by an iterative use of singular value
decomposition leading to a triclinic space group with the lattice
parameters reported in Table S2.44 The peak profile was determined
by a Pawley refinement41 using the fundamental parameter approach
as implemented in TOPAS.45 The background was modeled using
Chebychev polynomials of 10th order. The crystal structure of
[Ni(NCS)2(4-aminopyridine)]n 6 was determined by applying the
global optimization method of simulated annealing (SA).44 The two
thiocyanate anions and the 4-aminopyridine ligand were described
using rigid bodies in z-matrix notation with optimized bond lengths
and angles, taken from related single crystal structure data. The
position of the two nickel atoms and all ligands were gradually fixed
during the simulated annealing process, once the coordination
geometry was reasonable. The background, peak profile, all lattice
parameters, rotations and translations of the rigid body as well as
fractional coordinates of Ni were subsequently refined without any
constraints during the final Rietveld refinement.42 Light preferred
orientation, originating from the needle habitus of the crystallites, was
successfully modeled by a spherical harmonics correction function of
fourth order (Figure S1).46
RESULTS AND DISCUSSION
■
The XRPD pattern of the intermediate formed in the second
TG step is different from that of 5-HT but corresponds to that
of 6, already detected in the temperature dependent XRPD
measurements (Figure S9 and Figure S10). In the IR spectra
the CN-stretch is observed at 2105 and 2139 cm−1, which
proves that μ-1,3-bridging anionic ligands are present (Figure
S11).
DTA-TG measurements on the acetonitrile solvate 3 showed
three mass steps, of which the first one is in reasonable
agreement with that calculated for the removal of two
molecules of acetonitrile, whereas the second and third steps
Synthetic Investigations and Thermoanalytical Meas-
urements. Reaction of Ni(NCS)2 with 4-aminopyridine in
different ratios in water, methanol, or ethanol leads to the
formation of two crystalline phases, for which an elemental
analysis indicates a composition of Ni(NCS)2(4-aminopyr-
idine)4 (1) and [Ni(NCS)2(4-aminopyridine)2]n (5-LT).
Comparison of the experimental X-ray powder pattern of 1
with that calculated for the discrete complex bis(thiocyanato)-
tetrakis(4-aminomethylpyridine) nickel(II) reported by Nas-
simbeni and Kilkenny proves that this crystalline phase has
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Investigations on the Thermodynamic Relation
between Both Isomers. The synthetic investigations
presented above indicate that 5-HT might be thermodynami-
cally stable at higher temperatures. To investigate the stability
of the isomers in more detail, a suspension of both isomers was
stirred in acetonitrile at room-temperature and the residue
formed was investigated as a function of time by XRPD (Figure
S23). This shows that within 2 days all reflections of 5-HT
disappeared, and thus, the layered isomer 5-LT represents the
thermodynamic stable form at room-temperature, where 5-HT
is metastable. Similar experiments at different temperatures
revealed that 5-LT is also the more stable form at elevated
temperatures up to about 112 °C and that 5-HT becomes more
stable above 135 °C (Figure S24). This clearly shows that 5-LT
is the low- and 5-HT the high-temperature phase, which is also
in agreement with the densities of both forms, because the
density of 5-LT is larger than that of 5-HT (Table S1). To
prove if the LT form can be transformed into the HT form,
measurements using differential scanning calorimetry (DSC)
were performed (Figure 2). 5-HT decomposes at 302 °C upon
Figure 1. Heating rate dependent TG and DTG curves for 1.
correspond to the stepwise removal of the 4-aminopyridine
ligands (Figure S12). The XRPD pattern of the compound
obtained in the first TG step shows that a new phase (4) of
very poor crystallinity has formed that is different from all other
compounds (Figure S13). Elemental analysis indicates a
composition of Ni(NCS)2(4-aminopyridine)3 (4) in agreement
with the results of the TG measurements (see Experimental
Section). It is noted that in some cases the residue obtained
after MeCN removal is amorphous. Therefore, it can be
assumed that the solvent removal leads to pure amorphous or
semicrystalline samples that can recrystallize on further heating,
which might explain the low crystallinity of 4. Measurements
using differential scanning calorimetry show an exothermic
event for some samples after MeCN removal that might
correspond to the recrystallization of 4 (Figure S13). Indexing
of the powder pattern failed and all attempts to crystallize this
compound were unsuccessful, and thus, its structure remains
unknown. However, based on simple structural considerations,
this compound should consist of dimers, in which each Ni
cation is coordinated by one terminal and two bridging
thiocyanate anions, as well as three 4-aminopyridine ligands.
This is supported by IR measurements of 4, where the CN
stretching vibration is observed at 2085 and 2125 cm−1,
indicating that terminal and bridging anionic ligands are present
(Figure S15).
All attempts to obtain single crystals of 5-HT and 6 at room-
temperature failed, but under solvothermal conditions crystals
of 5-HT were obtained and characterized by single crystal X-ray
diffraction, which proves that a new chain isomer has formed
(see below). Additional synthetic investigations reveal that
crystalline powders obtained at elevated temperatures using
Ni(NCS)2 and 4-aminopyridine in ratio 1:2 only show
reflections of 5-HT and that in the reaction of Ni(NCS)2
and 4-aminopyridine in ratio 1:1, obviously 6 is obtained as a
pure phase (Figure S16 and S17). For 6, no single crystals were
obtained, and therefore, its crystal structure was solved from
XRPD data (Figure S18 and Table S2), which revealed that 6
forms an unusual 1D structure that to the best of our
knowledge has never been observed before (see below). Based
on the structural results, XRPD patterns were calculated and
compared with the experimental pattern, which proves that 3,
5-LT, 5-HT, and 6 were obtained as pure phases (Figures
S19−S22).
Figure 2. DSC curves for 5-HT and 5-LT at 4 °C/min.
heating, without any sign of a previous transformation. In
contrast, for 5-LT an endothermic signal occurs at 256 °C,
before decomposition is observed at 298 °C. The heat of
transition ΔHtrs of about 7.3 kJ/mol is in the range expected for
such transitions. To prove the reversibility of this event, a
further sample was heated until 266 °C, cooled down, and
heated again, which shows that in the second heating cycle this
signal is absent (Figure S25). If the residue formed at 256 °C is
investigated by XRPD, it is proven that 5-LT has been
transformed into 5-HT (Figure S26). According to the heat of
transition rule, the endothermic nature of this transition proves
that both forms are related by enantiotropism, with a
thermodynamic transition temperature somewhere between
112 and 135 °C, as determined by the solvent mediated
conversion experiments (see above).48 A semiempirical energy
temperature diagram can be found in Figure S27.49
The reason why in the DSC measurement the transition
temperature is shifted to much higher values can be traced back
to the kinetics of such reactions. We also tried to suppress this
transition by fast heating to prove if also the 2D structure (5-
LT) transforms into 6, but even at a heating rate of 100 °C/
min the transition remains present (Figure S28). Finally, it is
noted that on further heating an endothermic signal is
observed, followed by an exothermic transition. If the residue
formed at the peak of the endothermic signal is investigated by
XRPD a mixture of 5-HT and 6 is obtained, whereas the
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exothermic events lead to the formation of sulfides, which
indicates that even 6 might be metastable (Figure S29).
Crystal Structures. Compounds 2 and 3 crystallize in the
monoclinic space group P21 with all atoms in general positions
(Figure S30 and S32 and Table S1). In compound 2 the metal
cations are octahedrally coordinated by two terminal N-bonded
thiocyanate anions, two water molecules, and two 4-amino-
pyridine coligands, all of them in trans-position (Figure 3: top).
Figure 4. Crystal structure of 5-HT with view of a single chain. An
Ortep plot of 5-HT can be found in Figure S34.
alternates with all ligands in trans position for one cation (Ni2),
and the thiocyanate N atoms in trans and the thiocyanate S
atoms as well as the 4-aminopyridine N atoms in cis position for
the second (Ni1; Figure 4). However, the Ni cations are linked
by pairs of anionic ligands into chains, which because of the
different coordination are not linear but zigzag like (Figure 4).
This means that two neighbored Ni(NCS)2Ni units are
coplanar, whereas the following two units are rotated by 81°
relative to the former plane. According to a search in the
CCDC database this chain geometry was never observed
before. It is noted that in [Ni(NCS)2(4-(boc-amino)-
pyridine)2]n, reported recently, each of the Ni cations shows
only a cis-cis-trans coordination, and thus, these chains are
different.18 The Ni−N distances are a bit longer for the Ni
cation with an all-trans coordination, whereas the Ni−N as well
as the Ni−S distances of both cations are comparable (Table
S7). The Ni···Ni intrachain distance is 5.6106(2) Å.
The low-temperature form of [Ni(NCS)2(4-aminopyridi-
ne)2]n (5-LT) crystallizes in the triclinic space group P-1, with
two crystallographically independent Ni cations that are located
on centers of inversion (Figure S35 and Table S1). Even this
compound shows a very unusual Ni coordination that
alternates. One of the two Ni cations (Ni1) shows the usual
octahedral coordination with the two thiocyanate N and S
atoms as well as the two pyridine N atoms in trans-position,
whereas the second Ni cation (Ni2) is coordinated by two
thiocyanate N atoms and four amino N atoms of the coligands
(Figure 5: top and Figure S35). The Ni−N distances to the
amino groups are significantly shorter than those to the
pyridine N atoms, whereas the Ni−Npyridine and Ni−S bond
lengths are comparable for both cations (Table S8).
Only the two thiocyanate anions that coordinate with the S
atoms to the metal centers act as bridging ligand, whereas the
other two anionic ligands are only terminally N-bonded (Figure
5: top). The Ni cations are linked via single μ-1,3-bridging
thiocyanate anions into chains that are further linked via some
of the 4-aminopyridine ligands into layers with a network
topology never observed before. In this context it is mentioned
that a Ni compound with Ni thiocyanate single chains was
already reported by Palion-Gazda et al., but the Ni coordination
was different and the chains were not linked into a 2D
network.50 However, only two of the four 4-aminopyridine
ligands that coordinate to Ni2 act as bridging ligands, whereas
the other two are only terminal bonded via the pyridine N atom
(Figure S22). The network is additionally stabilized and linked
into a 3D network by a number of N−H···S and C−H···S
Figure 3. Crystal structure of 2 (top) and 3 (bottom) with view of the
Ni coordination. Ortep plots of both compounds can be found in
Figures S30 and S31. Solvate molecules are not shown for clarity.
In Compound 3 the cations are octahedral coordinated by two
N-bonded thiocyanate anions in cis-position, three 4-amino-
pyridine, and one MeCN ligand (Figure 3). Bond lengths and
angles are comparable and in agreement with values reported in
the literature (Table S3 and S5).
In 2, the discrete complexes are linked by intermolecular O−
H···O and O−H···N hydrogen bonding into layers, that are
parallel to the b-c-plane (Figure S31). The amino H atoms are
additionally linked to the thiocyanate S atoms via N−H···S
hydrogen bonding, forming a three-dimensional network. In 3-
Ni the complexes are linked into chains along the a-axis, via
pairs of intermolecular N−H···S hydrogen bonds between the
two amino H atoms of one complex and the two thiocyanate S
atoms of a neighbored complex (Figure S33). These chains are
connected by N−H···N hydrogen bonding into layers parallel
to (010), that are further linked into a three-dimensional
network via N−H···S hydrogen bonding. By this arrangement
channels are formed along the b-axis, in which the acetonitrile
solvate molecules are embedded (Figure S33).
The high-temperature form of [(Ni(NCS)2(4-aminopyridi-
ne)2]n (5-HT) crystallizes in the monoclinic space group C2/c,
with two crystallographically independent Ni cations, of which
one is located on the 2-fold rotation axis, whereas the second
occupies a center of inversion (Figure S34 and Table S1). The
Ni cations are octahedrally coordinated by two thiocyanate N
and two S atoms as well as two 4-aminopyridine N atoms,
within slightly distorted octahedra (Figure 4 and Table S7). In
contrast to most other compounds with the same ratio between
Ni(NCS)2 and the pyridine derivative, the Ni coordination
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Figure 6. Crystal structure of 6 with view of a part of a single chain.
polymers. In this context it is noted that at least two magnetic
exchange pathways exist, with one via the S atoms and one
along the NCS chains, which is indicative for a spin ladder. The
coordination geometry is not discussed in detail as this
structure was solved and refined using XRPD data using
geometrical restraints.
Magnetic Investigations. The temperature dependence of
magnetic susceptibility measured at 1 kOe for compounds 5-
HT, 5-LT, and 6 is shown in Figure 7. In all three cases the
Figure 5. Crystal structure of 5-LT with view along the crystallo-
graphic a-axis and N−H···S hydrogen bonding shown as dashed lines.
Please note that in the top of this figure two of the terminal
coordinated coligands are omitted to show that the thiocyanate chains
are linked into layers by any second 4-aminopyridine ligand. An Ortep
plot of 5-LT can be found in Figure S35.
hydrogen bonds between the amino H atoms and the
thiocyanate S atoms (Figure 5: bottom, Figure S36 and Table
S9). There is also one N−H···S hydrogen bond to the
thiocyanato anion that is only terminally N-bonded to the Ni
cation, and this might be one of the reason why the value for
the CN-stretching vibration is shifted to a value expected for μ-
1,3-briding thiocyanate anions.
Figure 7. Magnetic susceptibility χ for 5-HT, 5-LT, and 6, measured at
1 kOe, presented as temperature dependence of the χT product. Solid
lines were fitted using the model of the Heisenberg chain of spins S =
1 (see text).
The most 4-aminopyridine deficient compound [Ni(NCS)2-
(4-aminopyridine)]n (6) crystallizes in the triclinic space group
P-1, with two crystallographically independent Ni cations in
general positions. The asymmetric unit consists, besides of the
two Ni cations, of four symmetry independent thiocyanates and
two 4-aminopyridine coligands. Each Ni cation is coordinated
by two trans-coordinating N atoms and three S atoms of five
thiocyanate anions as well as one N atom of the 4-
aminopyridine ligand (Figure 6). The symmetry independent
nickel cations are connected over two S-ends of two
thiocyanates, creating a rhombus, which is further linked by
pairs of anionic ligands into linear chains, in which the two
trans-S and two-trans N atoms are involved and which
corresponds to the usual chain geometry observed in most
other such compounds (Figure 6). Adjacent nickel distances are
Ni(1)−Ni(2) 3.904 Å (rhombus) and 5.575 Å (linear chains).
To the best of our knowledge, this chain geometry was never
observed before in paramagnetic thiocyanate coordination
room temperature value of the χT product corresponds to S = 1
spin of NiII with the g value 2.17−2.2. Decreasing the
temperature makes χT higher, which points to dominant
ferromagnetic exchange interactions in all three cases.
The NiII ion in a crystal may have a significant zero-field
splitting (zfs); however, its influence on magnetic properties
usually manifests itself only below 20 K.51 For 5-HT the
coordination chain comprises two different Ni sites that have
different local anisotropy but are alternatively linked by (NCS)2
bridges of identical geometry. Similarly in the structure of 5-LT,
two different Ni sites are linked by identical NCS bridges. For
analysis of the magnetic behavior at higher temperature, it is
possible to neglect different zero-field splitting of all Ni sites
and treat Ni spins as isotropic. Therefore, we use the
Heisenberg chain Hamiltonian with isotropic spins
F
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Article
̂
H = −J
SS + gμ H
S
i
∑
∑
i
i i+1
B
i
that includes exchange interaction J along respective (NCS)m
bridges and the Zeeman term. In this analysis all interchain
interactions are also omitted. To calculate the susceptibility for
such a Hamiltonian, we applied density matrix renormalization
group and quantum Monte Carlo methods, as presented in ref
18, where also zero-field splitting was included. Within the
present work, to facilitate the usage of such numerically
computed curves for experimental data analysis, we approxi-
mated the χT(T) curve (see Figure S37) as a function of J and g
parameters using Pade
exponent c.
́
approximant with an additional
c
N μ2
kB
a0 + a1x + a2x2
⎛
⎜
⎝
⎞
A
B g2
⎟
⎠
χT =
1 + b x + b x2 + b x3
1
2
3
x = J/T,
c = −0.81086,
a0 = 1.65273, a1 = 2.05175, a2 = 2.84018,
b1 = 2.88478, b2 = 4.65762, b3 = 4.30349
Figure 8. Top: zero-field cooled (circles) and field cooled (lines)
susceptibility χ measured for 5-HT, 5-LT, and 6. Bottom: Temper-
ature dependence of specific heat, C, shown as C/T.
Such formula provides predictable behavior in low and high
limits of T/J and introduces additional relative error below
0.05% (see Figure S38). It is valid for a quantum, isotropic S =
1 spin chain with ferromagnetic Heisenberg exchange (J > 0),
in the limit of small applied fields. The critical exponent of
susceptibility at the T = 0 limit, χT ∼ T−γ, in this approximation
is γ = 0.811. This value differs slightly from classical spin chain
behavior, γ = 1, but is identical to the value γ = 0.811 estimated
by extrapolation of results obtained by exact diagonalization for
rings of up to 10 spins S = 1.52
Fitting the parameters g and J in the above formula (and also
including a small diamagnetic correction) to the χT data above
10 K leads to values J = 3.0(1) K, g = 2.18(3) for 5-HT. A
similar procedure applied to 5-LT data above 40 K leads to
values J = 9.5(3) K, g = 2.23(3). At lower temperature
experimental points deviate from calculated curves, which is the
effect of anisotropy and interchain interactions. It is noted that
the J value for 5-HT is similar to that for the ansolvate of
[Ni(NCS)2(4-(boc-amino)pyridine)2]n (J = 3.9 K). Even if this
compound exhibits a slightly different Ni coordination, it also
consists of zigzag chains.18 In contrast, for 5-LT a significant
higher J value is obtained, indicating that Ni(II) compounds
with linear NCS chains will lead to an increase in the magnetic
exchange.
peak of C/T(T) is observed at temperatures 1.06(3) K, 5.30(5)
K, and 8.2(2) K for 5-HT, 5-LT, and 6, respectively. The χ(T)
also shows a peak at 5.35(5) K and 8.5(1) K for 5-LT and 6,
respectively, while for 5-HT the χ(T) curve increases at least
down to 1.8 K. The C(T) peak in the case of 5-HT suggests
that this compound orders magnetically around 1 K. The
presented data allow us to conclude that compounds 5-LT and
6 order in an antiferromagnetic structure. An interchain
interaction is essential to induce the observed magnetic
ordering, as strictly one-dimensional systems of spins cannot
order.
The ordering of 5-LT is antiferromagnetic. This, together
with ferromagnetic intrachain interaction, points to antiferro-
magnetic interaction between the chains. One probable
exchange path is through the coordination bonds of the
bridging 4-aminopyridine ligands in-between the chains, but
also an influence of dipolar interaction cannot be excluded. The
magnetization curve measured at 1.8 K (Figure 9) supports
such a scenario, as a metamagnetic transition starts around Hsf
In the structure of 6 the exchange interactions between NiII
spins create a spin ladder, with two different Ni sites and
different exchange paths via Ni1-(NCS)2-Ni2, Ni1-(NCS)2-
Ni1, Ni2-(NCS)2-Ni2, and Ni1-S2-Ni2 bridges. Even though it
is possible to calculate susceptibility for such an isotropic
system for given exchange values, e.g. using the quantum
Monte Carlo method like in ref 32, it is not possible in the case
of 6 to unambiguously determine all these values using a
limited range of high temperature susceptibility data. For sure,
the exchange through NCS and also the exchange through S2
have to be ferromagnetic to ensure a monotonous increase of
χT(T), faster than for 5-LT and 5-HT. The ferromagnetic
exchange Ni1-S2-Ni2 is in accord with reported ferromagnetic
exchange in, e.g., S = 1 Ni chains built up of Ni2S2 dimers.53
Magnetic ordering is detected by low-field susceptibility and
specific heat (C) temperature dependences (Figure 8). The
Figure 9. Magnetization of 5-HT and 5-LT measured at 1.8 K, with
decreasing field. Inset: zoomed low-field region.
G
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Article
= 300 Oe. Based on this value, the estimated interchain
interaction is zJ′ ≈ − 0.022 K, which is indeed much smaller
than J.
the synthesis of metastable phases. On further heating, the 1D
compound loses half of the 4-aminopyridine ligands and
transforms into a novel chain compound, whose structure was
solved ab initio from XRPD data. This compound shows an
unusual coordination topology that to the best of our
knowledge was never observed before in thiocyanate chemistry.
The magnetic properties of 5 and 6 are governed by
ferromagnetic exchange interaction along the Ni(NCS) chains.
A convenient formula for the susceptibility of the ferromagnetic
Heisenberg chain of isotropic spins S = 1 is proposed, based on
numerical DMRG calculations, and used to determine the
exchange constants. This leads to different J values for the
zigzag chain in 5-HT and the linear chain in 5-LT, indicating
that the magnetic exchange depends strongly on the geometry
of the chains. Much weaker AF interactions between chains
produce an antiferromagnetic ground state, observed for 5-LT
and 6. Consecutive metamagnetic transitions in 6 are explained,
as a result of ordering induced by dipolar interchain
interactions. Finally, it is noted that for all three compounds
AC magnetic measurements were performed, which as expected
show that the AC susceptibility follows the dc low-field
susceptibility, with no out of phase component (Figure S40).
The magnetic structure of 6 is more complicated. All Ni
spins within a single coordination chain are ferromagnetically
coupled. The anisotropy of Ni1 and Ni2 decides which are the
easy axes. As Ni1 and Ni2 may have different orientations, the
magnetic moments inside the single chain may be slightly
canted. The magnetization at 50 kOe reaches 1.8 μB and is still
not saturated, which is related to overcoming of the local NiII
ion anisotropy by an external field. The ground state is AF, due
to interchain interactions, but the M(H) curve reveals two
metamagnetic transitions, presented in Figure 10. The first
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00092.
Figure 10. Magnetization hysteresis loops measured for 6 at different
temperatures, revealing two metamagnetic transitions.
Crystal data and refinement results; SEM image; IR
spectra; DTA-TG and DSC curves; XRPD pattern
(PDF)
(MT1) is centered around 750 Oe, and is accompanied by a
pronounced hysteresis loop, which is wider the lower the
temperature. Following this transition, the magnetization
reaches a plateau, around 0.35 μB per Ni ion at 2 kOe. To
estimate the fraction of spins that undergo this MT1 transition,
we compare this value with the M(H) curve extrapolated
linearly to zero field from the data in the range 10−40 kOe, i.e.
with the value 1.05 μB. This agrees exactly if only one per six
spins flips during MT1. Several possible magnetic structures
can accommodate such metamagnetic transition, and one
possible scenario is shown in Figure S39. The second
metamagnetic transition (MT2), centered around 3 kOe,
displays much narrower hysteresis and is caused by flipping
of the remaining fraction of spins. Such consecutive
metamagnetic transitions are possible if long-range magnetic
interactions, i.e. the dipolar interactions, are responsible for the
AF ordering.54
Accession Codes
CCDC 1815975−1815979 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: cnaether@ac.uni-kiel.de. Fax: +49-431-8801520.
■
ORCID
Christian Nather: 0000-0001-8741-6508
̈
Notes
The authors declare no competing financial interest.
CONCLUSION
■
ACKNOWLEDGMENTS
In the present contribution new thiocyanate coordination
polymers were synthesized and structurally characterized
including two isomers with a novel 1D structure and a 2D
network. Detailed investigations using solvent mediated
conversion experiments and thermoanalytical measurements
prove that the 2D compound (5-LT) is thermodynamically
stable at lower temperatures up to 112 °C and that the 1D
isomer (5-HT) becomes thermodynamically stable above 135
°C. On heating, an endothermic transition of 5-LT into 5-HT
is observed, which proves that both forms are related by
enantiotropism with the chain compound as the HT phase.
Interestingly, the HT form was detected in the thermal
decomposition reaction of a simple discrete precursor complex,
which emphasizes the power of such solid state reactions for
■
This project was supported by the Deutsche Forschungsge-
meinschaft (Project No. Na 720/6-1) and the State of
Schleswig-Holstein and by National Science Centre, Poland
(Project no. 2017/25/B/ST3/00856). We thank Prof. Dr.
Wolfgang Bensch for access to his experimental facilities.
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