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Thermal transformation of a 0D thiocyanate precursor into a ferromagnetic 3D coordination network via a layered intermediate Stefan Suckert, Micha# Rams, Luzia S. Germann, Daria M. Cegie#ka, Robert E. Dinnebier, and Christian Näther Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 08 Jun 2017 Downloaded from http://pubs.acs.org on June 9, 2017

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Crystal Growth & Design

Thermal transformation of a 0D thiocyanate precursor into a ferromagnetic 3D coordination network via a layered intermediate Stefan Suckert[a], Michał Rams[b], Luzia S. Germann[c], Daria M. Cegiełka[b], Robert E. Dinnebier[c] and Christian Näther*[a] [a]

Institut für Anorganische Chemie, Christian-Albrechts-Universität zu Kiel, Max-Eyth-Straße 2, 24118 Kiel. [b]

[c]

Institute of Physics, Jagiellonian University, Łojasiewicza 11, 30-348 Kraków, Poland.

Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany.

Reaction of Co(NCS)2 with 2,6-dimethylpyrazine (2,6-DMPy) leads to the formation of a compound with the composition Co(NCS)2(2,6-DMPy)4 (Co-0D) that consists of discrete complexes, in which the Co cations are octahedrally coordinated. Upon heating half of the 2,6DMPy ligands are removed leading to the 2D coordination polymer [Co(NCS)2(2,6-DMPy)2]n (Co-2D), shown by thermogravimetry and X-ray powder diffraction. On further heating the intermediate Co-2D loses next 2,6-DMPy ligand and transforms into [Co(NCS)2(2,6-DMPy)]n (Co-3D). The crystal structures of Co-2D and Co-3D were solved and refined using XRPD data. The crystal structure of Co-2D consists of octahedrally coordinated Co cations that are linked by pairs of anionic ligands into dimers, which are further connected by single thiocyanate anions

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into layers. In Co-3D octahedrally and tetrahedrally coordinated Co cations are present, that are linked by µ-1,3-bridging single thiocyanate anions into a novel 3D network. Magnetic measurements for Co-0D show paramagnetic behavior and slow relaxations of the magnetization in applied field due to single ion magnet behavior. For Co-2D the antiferromagnetic exchange interaction leads to the maximum of susceptibility at 6.0 K, but a long range ordering is observed at 2.2 K. In contrast, the ferromagnetic order is observed in Co-3D below 3.75 K. The phase transitions are investigated by specific heat measurements.

INTRODUCTION The synthesis of new coordination polymers is still a major topic in coordination chemistry, because of their promising physical properties and potential applications.1-8 In most cases such compounds were prepared in solution, which can lead to the formation of compounds of different stoichiometry or of different thermodynamically stable or metastable stable isomeric or polymorphic modifications.9-18 However, some special compound might not form or can easily be overlooked if the synthesis is performed in solution. In this context we have reported on an alternative route for the preparation of new coordination polymers, which is based on thermal decomposition of suitable precursor compounds.19-21 On heating, the additional co-ligands are stepwise removed, which enforces the formation of ligand deficient compounds with more condensed coordination networks. In several cases thermodynamic metastable modifications were obtained following this route or compounds will form, to which no access in solution was found.10-11 In this context it is noted, that the synthesis of new coordination compounds via typical solid state routes is a common procedure and other approaches were reported.22-24 In view of potential applications, compounds based on paramagnetic metal cations are of special interest, because cooperative magnetic properties can be observed.1-2,

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Thio- and


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selenocyanate anions are promising ligands, as they coordinate to metal cations in different ways, with the bridging modes as the most important, because magnetic exchange can be mediated.28-35 Dependent on the nature of the metal cation, the coordination behavior of the coligand and the reaction conditions, compounds with different thio- or selenocyanate coordination networks can be obtained, in which transition metal cations are usually linked by the anionic ligands into M(NCX)2 substructures (M = metal cation; X = S, Se) of different dimensionalities. In this context we have reported on a number of compounds based on 3d metal cations which show a variety of magnetic properties including a slow relaxation of the magnetization.36-42 M(NCX)2 chains are frequently found with monodentate co-ligands, in which the metal cations are linked by pairs of µ-1,3-bridging anionic ligands. In some other compounds layered thiocyanate networks occur, in which the metal cations are either connected by only single anionic ligands or by pairs of thiocyanato anions into dimers, which are further linked into layers by single thiocyanate anions. With 4-acetylpyridine, e.g., we have obtained two isomers with Co(II) as cation of which the thermodynamically stable isomer forms a layered structure, whereas the metastable one consists of chains.11 Compounds with the same layer topology were also obtained with ethylisonicotinate as co-ligand.43 These two layered compounds show dominating ferromagnetic exchange and ordering at low temperatures, which indicates that this layer topology generally exhibits ferromagnetic exchange.18 To investigate the influence of the N-donor co-ligand in more detail we became interested in the co-ligand 2,6-dimethylpyrazine (2,6-DMPy), which will predominantly act as monodentate ligand because of the two bulky methyl groups neighbored to one of the two N atoms. In this case, the question arises what kind of thiocyanate networks will form and which magnetic properties possess. In the course of our investigations we obtained a compound with an identical


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topology of the coordination network as those observed with ethylisonicotinate and 4acetylpyridine as ligands, but with distinct different magnetic properties. Interestingly, [Co(NCS)2(2,6-DMPy)2]n, transforms into a 2,6-DMPy deficient compound with the composition [Co(NCS)2(2,6-DMPy)]n upon heating.

The thermal product possess a new

Co(NCS)2 coordination network, which was hitherto never observed in literature. Remarkably, only two thiocyanate coordination polymers are reported with 2,6-DMPy in literature. In Hg(NCS)2(2,6-dimethylpyrazine)2 the metal cations are square pyramidal coordinated by two µ1,3-S- and two µ-1,3-N-coordinating thiocyanate anions as well as one 2,6-DMPy ligand, into a 3D network.44 In the crystal structure of Cu(I)Cu(II)2(CN)4(NCS)8(2,6-Dimethylpyrazine)7 the Cu cations are either tetrahedral or square pyramidal coordinated by thiocyanate and cyanide anions into a three-dimensional network.45 There are also three discrete complexes reported in literature that contain both, 2,6- and 2,5-DMPy.46-48 Here we report on our investigations.

EXPERIMENTAL SECTION Co(NCS)2 and 2,6-Dimethylpyrazine were obtained from Alfa Aesar. 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 Co-0D: Co(NCS)2(2,6-DMPy)4. Single crystals were obtained by reacting 0.05 mmol (8.8 mg) Co(NCS)2 and 0.20 mmol (21.6 mg) 2,6-dimethylpyrazine in 2.0 mL methanol. The reaction vessel was heated to 65°C and then slowly cooled down to room temperature. After 3d suitable crystals for single crystal X-ray diffraction were obtained. A crystalline powder was synthesized by stirring 0.25 mmol (43.8 mg) Co(NCS)2 and 1.00 mmol (108.2 µL) 2,6-dimethylpyrazine in 1.5 mL ethanol for 3d at room-temperature. Yield: 88.4%.


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C26H32CoN10S2 (607.67 g/mol): calcd. C 51.39, H 5.31, N 23.05, S 10.55; found C 50.37, H 5.70, N 24.03, S 10.46. IR (ATR): νmax = 3058 (w), 2967 (w), 2921 (w), 2053 (s), 1531 (m), 1418 (m), 1371 (m), 1253 (s), 1158 (s), 1024 (m), 944 (m), 870 (m), 818 (m), 736 (m), 564 (s). Synthesis of Co-2D: Co(NCS)2(2,6-DMPy)2 and Co-3D: Co(NCS)2(2,6-DMPy)2. Co-2D and Co-3D were synthesized by thermal degradation of Co-0D. Co-2D forms in the range between 140 and 150°C and Co-3D around 170°C. Both transformations are also optically visible; Co-0D is a pink, Co-2D a beige and Co-3D a turquoise colored powder. Co-2D: C14H16CoN6S2 (391.39 g/mol): calcd. C 42.96, H 4.12, N 21.47, S 16.39; found C 42.82, H 4.10, N 21.27, S 16.28. IR νmax = 3073 (w), 2920 (w), 2107 (s), 1592 (w), 1534 (m), 1420 (m), 1399 (m), 1379 (m), 1286 (w), 1255 (m), 1162 (s), 1022 (m), 979 (w), 943 (m), 921 (m), 870 (m), 788 (m), 737 (s), 616 (w), 566 (w), 533 (w). Co-3D: C8H8CoN4S2 (283.24 g/mol): calcd. C 33.92, H 2.85, N 19.78, S 22.64; found C 33.40, H 2.76, N 19.53, S 22.48. IR νmax = 2154 (s), 2141 (s), 1792 (w), 1742 (w), 1583 (w), 1534 (m), 1446 (w), 1419 (m), 1399 (m), 1370 (m), 1286 (w), 1255 (m), 1160 (m), 1097 (m), 1024 (m), 977 (w), 944 (m), 872 (m), 739 (m), 602 (w), 564 (w). Elemental Analysis. CHNS analysis was performed using an EURO EA elemental analyzer, fabricated by EURO VECTOR Instruments and Software. Scanning electron microscopy (SEM). SEM images were measured in a secondary electron mode on a XL30 S-FEG instrument produced by FEI. All three samples were coated with gold before the measurements. Spectroscopy. All IR data were obtained using an ATI Mattson Genesis Series FTIR Spectrometer, control software: WINFIRST, from ATI Mattson.


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Differential Thermal Analysis and Thermogravimetry (DTA-TG). The thermal decomposition reactions were performed in a dynamic nitrogen atmosphere (purity: 5.0) in Al2O3 crucibles using a STA-409CD thermobalance from Netzsch. The instrument was calibrated using standard reference materials. All measurements were performed with a flow rate of 75 mL⋅min−1 and were corrected for buoyancy. Different heating rates in the range of 1-16 K/min were used. Magnetic measurements. Magnetic measurements were performed using a MPMS squid magnetometer from QuantumDesign. Powders were covered with nujol or lightly 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. 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 SHELXT49 and structure refinement was performed against F2 using SHELXL2014.50 A numerical absorption correction was applied using programs X-RED and X-SHAPE of the program package X-Area.51-53 All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were positioned with idealized geometry and were refined isotropic with Uiso(H) = -1.2 Ueq(C) (1.5 for methyl H atoms) using a riding model. Selected crystal data and details on the structure refinements can be found in Table 1.


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CCDC-1548633 (Co-0D), CCDC-1548632 (Co-2D) and 1548634 (Co-3D) 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.

Table 1. Selected crystal data and details on the single crystal structure refinements for Co-0D. compound Formula MW / g mol−1 crystal system space group a/Å b/Å c/Å α/° β/° γ/° V / Å3 T/K Z Dcalc / g cm−3 µ / mm−1 θmax / deg measured refl. unique refl. refl. [F0>4σ(F0)] parameters Rint R1 [F0 > 4σ(F0)] wR2 [all data] GOF ∆ρmax/miin / e Å−3

Co-0D C26H32N10CoS2 607.66 Triclinic P-1 10.717(2) 12.562(3) 12.964(3) 76.36(3) 76.42(3) 68.71(3) 1558.7(7) 170(2) 2 1.295 0.717 28.552 27418 7908 6849 360 0.0605 0.0384 0.1066 1.040 0.818/ -0.646

X-Ray Powder Diffraction (XRPD). The temperature dependent XRPD measurements were performed using a PANalytical X’Pert Pro MPD Reflection Powder Diffraction System with CuKα1 radiation (λ = 1.546 Å, equipped with an Anton Paar HTK 1200N high temperature


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chamber and a PIXcel semiconductor detector from PANalytical. Phase analysis and measurements for indexing were performed a Stoe Transmission Powder Diffraction System (STADI P) with CuKα1 radiation that was equipped with a linear position-sensitive MYTHEN 1K detector and a Ge(111) monochromator from STOE & CIE. Measurements used for crystal structure solution and Rietveld refinements were performed at room temperature on a Stoe Stadi P machine (Mo Kα1, λ = 0.7093 Å, equipped with a Johann Ge(111) monochromator) using a Mythen Dectris 1K detector. All samples were gently ground, filled into ϕ 0.5 mm glass capillaries (Hilgenberg) and spun during the experiments for better particle statistics. The patterns were collected in the range of 2 – 80 °2θ with Cu and 2 – 50 ° 2θ with Mo-radiation within 8 h 40 min and 12 h, respectively. All Pawley54 and Rietveld55 refinements were performed using TOPAS V5.56 Indexing of Co-2D was carried out by an interative use of singular value decomposition leading to a monoclinic space group with the lattice parameters reported in Table 2.57 Extinction led to the unique space group P21/c. The peak profile was determined by Pawley refinements54 using the fundamental parameter approach as implemented in TOPAS.58 The background was modelled using Chebychev polynomials. The crystal structure of Co-2D was determined by applying the global optimization method of simulated annealing (SA).57 The cobalt cation was placed on a general position. The two thiocyanate anions and the 2,6-dimethylpyrazine ligands were described using rigid bodies in z-matrix notation with optimized bond lengths and angles, taken from related single crystal structure data. During the final Rietveld refinement the background, peak profile, all lattice parameters, rotations and translations of the rigid body as well as coordinates of Co were subsequently refined without any constraints.55


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The procedure for Co-3D was similar as for Co-2D. Indexing led to a C-centered monoclinic unit cell. Structure solution was performed in the centrosymmetric space group C2/c. The coordinates of cobalt ions, thiocyanate linkers and the 2,6-DMPy ligands were determined gradually. The two symmetry independent Co ions were found using the “occ_merge” command in TOPAS and releasing all atomic positions freely during the simulated annealing process. The position of the two thiocyanate anions and the 2,6-DMPy ligands were found by describing them by rigid bodies using the z_matrix notation and fixing the atomic coordinates of both Co(II) ions during the SA process. During the final Rietveld refinement the background, peak profile, all lattice parameters, rotations and translations of the rigid body were subsequently refined without any constraints. Selected crystal data and details on the Rietveld refinements are listed in Table 2. Table 2. Selected crystal data and details of the Rietveld refinements for Co-2D and Co-3D. Compound Formula MW / g mol−1 crystal system space group Wavelength / Å a/Å b/Å c/Å α/° β/° γ/° V / Å3 T/K Z Dcalc / g cm−3 µ / mm−1 θmax / deg Rwp / %[a] Rp / %[a] Rexp / %[a]

Co-2D C14H16N6CoS2 391.38 Monoclinic P21/c 0.7093 14.0536(6) 9.2427(4) 15.4917(7) 90 116.120(4) 90 1806.74(15) 298 4 1.439 1.216 50 3.034 2.439 2.58

Co-3D C8H8N4CoS2 283.24 Monoclinic C2/c 0.7093 12.5183(8) 13.8433(9) 13.7533(8) 90 100.137(4) 90 2346.2(2) 298 8 1.604 1.833 42 3.22 2.51 3.48


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RBragg / %[a] 0.797 [a] as defined in TOPAS 5.0

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0.983

RESULTS AND DISCUSSION Synthetic investigations and thermal properties. Preliminary synthetic investigations at room-temperature in water, methanol and ethanol using different stoichiometric amounts of Co(NCS)2 as well as 2,6-dimethylpyrazine led to the formation of only one pure crystalline phase with the composition Co(NCS)2(2,6-dimethylpyrazine)4 (Co-0D). The CN stretching vibration is observed at 2053 and 2066 cm−1 in the IR spectra, which indicates that a discrete octahedral complex has formed, in which the metal cations are octahedrally coordinated by two terminal N-bonded anionic ligands and four 2,6-DMPy co-ligands (Figure S1 in the Supporting Information). Even if an excess of the metal salt is used during the synthesis, no other 2,6DMPy-deficient phase was obtained. For Co-0D single crystals were obtained and characterized by single crystal X-ray diffraction (see below). Comparison of experimental and calculated powder pattern proves that a pure crystalline phase was obtained (Figure S2). However, Co-0D consists of discrete complexes and thus might be a suitable precursors for the preparation of a 2,6-DMPy-deficient phase by thermal annealing. Therefore, this compound was investigated by differential thermoanalysis and thermogravimetry (DTA-TG). Co-0D shows three poorly resolved mass loss steps upon heating to 250°C, which are each accompanied by endothermic events in the DTA curve (Figure 1). The best resolution was obtained with a heating rate of 1°C/min, shown by heating rate dependent measurements (Figure S3). The experimental mass loss of the first step of 34.7 % is in reasonable agreement with that calculated for the removal of two 2,6-DMPy ligands (∆mcalc = 35.6 %) and in the second and third step an additional co-ligand is removed. Based on the observed mass loss it can be assumed that Co-0D transforms into a compound with the composition Co(NCS)2(2,6-DMPy)2 (Co-2D),


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that on further heating forms an even more 2,6-DMPy deficient compound with the composition Co(NCS)2(2,6-DMPy) Co-3D. The formation of additional crystalline phases is also supported by temperature dependent XRPD measurements (Figure S4). Scanning electron microscopy showed that the particle size decreased significantly during the synthesis of the two polycrystalline phases formed upon thermal treatment (Figure S5).

Figure 1. DTG, TG and DTA curve for Co-0D measured with a heating rate of 1°C/min. For Co-2D the asymmetric CN stretching vibration occur at 2107 cm−1, whereas for Co-3D, two values at 2141 cm−1 and 2154 cm−1 are observed, indicating that a more condensed Co(NCS)2 coordination network might have formed that consists of more than one crystallographically independent thiocyanate anion (Figure S6 and S7). Several experiments were made to obtain the new compounds from solution but without any success. Therefore, their crystal structures were solved from XRPD data and refined using the Rietveld method (Figure S8


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and S9). Based on the structural data XRPD patterns were calculated and compared with those measured, which proves that both compounds were obtained as pure phases (Figure S10 and S11). Crystal Structures. Compound Co-0D crystallizes in the triclinic space group P-1 with 2 formula units in the unit cell. The asymmetric unit consists of one metal cation, two thiocyanato anions and four 2,6-dimethylpyrazine ligands in general positions (Figure S12). The metal cations are coordinated by two terminal N-bonding thiocyanato anions as well as four N-bonding 2,6-DMPy ligands. This leads to discrete slightly distorted octahedral complexes (Figure 2). The Co···N bond lengths are comparable to those in related octahedrally coordinated thiocyanate complexes (Table S1).

Figure 2. View of the Co(II) coordination in the crystal structure of Co-0D. An ORTEP plot is shown in Figure S12. Co-2D crystallizes in the monoclinic space group P21/c with Z = 4 and all atoms located in general positions. The metal cations are coordinated by two N and two S bonded thiocyanate anions and two 2,6-DMPy ligands, within slightly distorted octahedra. In the crystal structure the


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metal cations are linked into dimers by pairs of µ-1,3-bridging anionic ligands (Figure 3: top). These dimers are further linked by µ-1,3-bridging single thiocyanate anions into layers, that are parallel to the b-c plane (Figure 3: bottom). It is noted, that this M(NCS)2 layer topology is identical to that already observed in the ethylisonicotinate and 4-acetylpyridine compounds.18, 43 However, in Co-2D and in [Co(NCS)2(ethylisonicotinate)2]n all layers are eclipsed, whereas they are shifted in [Co(NCS)2(4-acetylpyridine)2]n (Figure S13).

Figure 3. View of the Co(II) coordination in the crystal structure of Co-2D (top) and of the thiocyanate layers along the a-axis (bottom). In the bottom figure the 2,6-DMPy ligands are omitted for clarity. Compound

Co-3D

crystallizes

in

the

monoclinic

space

group

C2/c

with

two

crystallographically independent Co(II) cations in special positions and two thiocyanate anions and one 2,6-DMPy ligand that occupy both general positions. One of the two cations is located


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on a center of inversion and is coordinated by two trans-oriented 2,6-DMPy ligands and four Nbonded thiocyanate anions into a slightly distorted octahedral coordination geometry (Figure 4: top). The second Co(II) cation is located on a 2-fold rotation axis and is tetrahedral coordinated by four S-bonded anionic ligands (Figure 4: top). Each of the octahedral coordinated Co(II) cations is linked to four tetrahedral Co(II) cations by single µ-1,3-bridging anionic ligands (Figure 4: bottom).

Figure 4. View of the Co(II) coordination in the crystal structure of Co-3D (top) and of the thiocyanate network along the b-axis (bottom). 2,6-DMPy ligands are omitted for clarity in the bottom figure.


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In this way a three-dimensional M(NCS)2 coordination network is formed, that consists of channels that proceeds along the c-axis and in which the organic ligands are located (Figure S14). The 4-fold coordination of Co(II) is in good agreement with the observed color (turquoise) which suggests a tetrahedral coordination geometry. To the best of our knowledge this topology has never been reported in thio- or selenocyanate chemistry. It is noted that some compounds are reported in which a Co(II) cation is coordinated by four S atoms,59-63 but this was never observed before for thio- or selenocyanate anions. Magnetic investigations. The effective magnetic moment at 240 K is 5.15, 4.96, and 4.8(1)μB for Co-0D, -2D, and -3D, respectively. These values are in the range typically observed around room temperature for octahedrally coordinated Co(II) cations (4.7-5.2μB) and sligthly higher than values for Co(II) in tetrahedral coordination (4.6-4.8μB).64 Lowering the temperature, χT decreases for Co-0D (Figure 5), due to spin-orbit coupling and crystal field effects in well separated Co(II) cations. Taking this as a reference, antiferromagnetic interactions dominate in Co-2D. This is in contrast to Co(NCS)2(ethylisonicotinate)2]n and [Co(NCS)2(4-acetylpyridine)2]n that are built up of very similar layers where a ferromagnetic exchange prevails.43 Ferromagnetic interactions dominate also for Co-3D. The sample of Co-0D remains paramagnetic down to 1.8 K. In applied field of 1 kOe the slow relaxation of magnetization is observed (Figures S15 and S16), but relaxation times are short in comparison to data reported for numerous single ion magnets (SIM) based on Co(II) ions. The Arrhenius energy barrier estimated for Co-0D amounts to Ueff = 27(2) K at 1 kOe.65-66


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Figure 5. Temperature dependence of magnetic susceptibility χ, measured at H = 1 kOe for Co0D, 2D, and 3D, shown as χT product. For Co-2D the low field susceptibility has a broad maximum around Tmax = 6.0 K, identical for dc measurement at 1 kOe, and ac measurement at Hac = 3 Oe (Figures 6 and S18). It is noted, that no χ'' signal is observed around this maximum of χ'. The specific heat C(T) shows a maximum at Tc = 2.20 K clearly demonstrating that a long range ordering takes place, but at temperature much lower than Tmax observed in magnetic measurements (Figure 6 and Figure S19). A closer investigation of the susceptibility around 2.2 K reveals a small bifurcation between the χ(T) curves measured in zero-field cooled and field cooled modes. All together, these data suggest that all Co(II) spins in Co-2D are coupled by an antiferromagnetic exchange into pairs or chains, and that this leads to the χ(T) maximum, but without any long range order. A weaker interaction linking these objects manifests itself at lower temperature, producing a long range ordered structure, most probably a canted AF structure.


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Figure 6. Zero-field cooled and field cooled susceptibility χ measured at 100 Oe (circles, left axis) and specific heat (squares, right axis) for Co-2D. The dashed line at 2.20 K is to guide the eye. It is not clear if the dominant AF interaction originates from the double NCS bridge or from the single bridge in the structure of Co-2D. As already mentioned the topology of the coordination network is identical to that in Co(NCS)2(ethylisonicotinate)2]n and [Co(NCS)2(4acetyl-pyridine)2]n, in which ferromagnetic exchange dominates. It can be assumed that the sign of this exchange depends on the actual geometry of the metal centers and briges. The Co···Co distance for single thiocyanate bridges are similar in all three compounds, but the double bridge in Co-2D is elongated compared to that in the ethylisonicotinate and 4-acetylpyridine compound (Table 3). There are also some differences in the coordination sphere of Co cations, from which no general trend can be extracted (Table S2). More importantly, the crystal structures of Co(NCS)2(ethylisonicotinate)2 and Co-2D were determined from XRPD data, where several restraints were used in the structure refinement. For Co-2D prepared by thermal decomposition the crystallinity is very poor, which makes an accurate refinement more difficult (Figure S5 and S8). Therefore, even if the Co···Co distances might be precise, observed differences in other geometrical parameters might be insignificant and cannot be discussed.


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Table

3.

Intrachain

and

interlayer

distances

(Å)

Page 18 of 27

for

Co-2D

as

well

as

for

Co(NCS)2(ethylisonicotinate)2]n and [Co(NCS)2(4-acetyl-pyridine)2]n taken from literature. Compound

M···M

M···M

M···M

double bridge

single bridge

interlayer

Co-2D

5.772(6)

5.877(6)

14.946(1)

[Co(NCS)2(ethyliso-

5.643(9)

5.899(9)

14.946(1)

5.678(1)

5.918(1)

12.811(1)

nicotinate)2]n [Co(NCS)2(4-acetylpyridine)2]n

The Co(II) electron structure in a similar N4S2 crystal field was recently calculated by ab-initio method for [Co(NCS)2(ethylisonicotinate)2]n.43 The excited Kramers doublet of Co(II) is about 250 cm-1 above the ground doublet. For this reason it is safe to assume that in Co-2D only the ground Kramers doublet is populated at 20 K, and can be treated as effective spin s = 1/2 state. The entropy related to the magnetic ordering of such spins should be Rln(2s + 1) = 5.76 Jmol−1K−1. The entropy change estimated for Co-2D in the range 0.5-10 K is ∆S = 5.36 Jmol−1K−1, and 65% of this is above Tc = 2.20 K (Figure S19). This is in agreement with suggested low-dimensional spin correlations above Tc. The smaller peak in C(T) around 1.0 K has the shape which resembles more another transition of the same phase, rather than an independent impurity phase. On the other hand, the height of this peak is different for different sample batches.


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The magnetization M(H) curve for Co-2D (Figure 7) has the shape characteristic for AF coupled spins, with the upturn which starts around Hc = 15 kOe. At 4 K, i.e. above Tc, the same effect is observed at 14 kOe. For two s = 1/2 spins with the Ising exchange interaction Hexch = −2Js1,zs2,z , J < 0, the χz(T) maximum is at Tmax/J = −0.782, while gμBHc = −J. This leads to J = −7.6 K using Tmax, or similarly J = −7.2 K using the Hc value and gz = 7.2 as calculated for Co(NCS)2(ethylisonicotinate)2.43 The second involved exchange constant cannot be estimated directly.

Figure 7. Magnetization measured for Co-0D (triangles), Co-2D (dots) and Co-3D (circles) at 1.8 K. For Co-3D, dominant ferromagnetic interactions visible in Figure 5 lead to a long range ferromagnetic ordering. The susceptibility measured in the low temperature range in zfc/fc regime is shown in figure 8. The rise of χ at 4.0 K and the difference between zfc and fc curves point to the ferromagnetic state. The presence of the bulk phase transition is confirmed by the visible peak of specific heat with the maximum at 3.75 K. Magnetization quickly rises to about 1.3μB, then slowly increases and at 50 kOe is still not saturated (Figure 7). The tetrahedral site Co(T) in Co-3D is strongly distorted and similar to that in discrete complexes reported earlier67-68 In those cases Co(II) S = 3/2 state with zfs splitting (D from −30


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Page 20 of 27

to −70 cm−1) led to the ground Kramers doublet with strong easy-axis anisotropy and a similar situation can be anticipated for Co(T) in Co-3D. The octahedral site Co(O) in CoN6 is very different from Co-2D, but also may have a significant anisotropy. This high anisotropy of Co-3D is visible in the almost linear change of M(H) from 2 to 40 kOe. However, because both Co coordinations are completely different, one can assume that easy-axes of both cations are not parallel and therefore, the ferromagnetic structure can be non-collinear.

Figure 8. Zero-field cooled and field cooled magnetic susceptibility

χ

measured at 30 Oe and

specific heat C (squares, right axis) for Co-3D. Finally, the estimated entropy change from 2 to 10 K in Co-3D is ∆S = 7.1 J/molK per single Co ion, and 28% of this is above Tc. This proportion, much smaller than for Co-2D, is related with the presence of the 3-dimensional structure mediating the exchange interaction.64

CONCLUSION In the present contribution we demonstrated the potential of thermal decomposition reactions for the directed synthesis of new coordination polymers with novel coordination networks. This method is especially useful for the synthesis of bridging thio- or selenocyanate coordination polymers with less chalcophilic metal cations, where the equilibrium is usually on the site of the


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compounds with only terminal bonded ligands but can also be used for the synthesis of a wide range of other coordination polymers. One of the major advantages of this approach is the fact, that on heating an irreversible and frequently stepwise ligand removal is enforced, which leads to an increase of the dimensionality of the coordination networks and consequently modified magnetic properties. Following these ideas, a hitherto unknown 3D network based on cobalt(II) in an alternating tetrahedral and octahedral coordination geometry was prepared that shows ferromagnetic ordering. Interestingly, this compound formed via a layered intermediate that shows a topology of the coordination network already observed for similar compounds with different co-ligands. In contrast to the reported compounds the new layered intermediate shows dominating antiferromagnetic interactions, for which small changes of the geometrical parameter can be responsible. Co-2D and Co-3D were synthesized as polycrystalline materials upon heating and their structures were therefore solved from XRPD data.

ACKNOWLEDGEMENTS This project was supported by the Deutsche Forschungsgemeinschaft (Project No. Na 720/5-1) and the State of Schleswig-Holstein. We thank Prof. Dr. Wolfgang Bensch for access to his experimental facilities and Viola Duppel for the SEM images. LSG thanks Dr. Dubravka Šišak Jung and Dr. Anthanassios Katsenis for helpful discussions. MR thanks for support of National Science Centre Poland (2011/01/B/ST3/00436) and support of European Regional Development Fund within the Polish Innovation Economy Operational Program (POIG.02.01.00-12-023/08).

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI:


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AUTHOR INFORMATION Corresponding Author *Email: [email protected] Fax: +49-431-8801520 Notes The authors declare no competing financial interest.

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Entry for the Table of Contents

Thermal transformation of a 0D thiocyanate precursor into a ferromagnetic 3D coordination network via a layered intermediate Stefan Suckert, Michał Rams, Luzia S. Germann, Daria M. Cegiełka, Robert E. Dinnebier and Christian Näther

Thermal annealing of a discrete precursor complex leads to the formation of a layered Co(II) thiocyanato coordination polymer, that transforms into a novel 3D network on further heating in which octahedral and tetrahedral cations coexist. All compounds were structurally characterized by X-ray single crystal or powder diffraction and investigated for their thermal and magnetic properties.


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Thermal Transformation of a Zero-Dimensional ... - ACS Publications

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