Susanne Wöhlert

Coexistence of Metamagnetism and Slow Relaxation of the Magnetization in a Cobalt Thiocyanate 2D Coordination Network

Recently, strategies for the design of coordination polymers, hybrid compounds, or metal–organic frameworks (MOFs) that show cooperative magnetic phenomena have become of increasing interest. Because of their great potential for possible applications as storage materials or in molecular electronics, 1D materials with a large magnetic anisotropy, slow relaxation of the magnetization M, and a hysteresis of molecular origin, for example, “single-chain magnets” (SCMs) are of special interest. Moreover, for future applications multifunctional materials are needed, in which different physical properties can be tuned or switched as a function of external parameters. These criteria also apply to metamagnetic compounds, which show different magnetic properties below and above a critical field HC. [1c,4] Unfortunately, because of strong interchain interactions most of these compounds show only 3D ordering above HC. [5] Therefore, only a very few metamagnetic coordination compounds have been reported in which slow relaxation of the magnetization is observed. 6] In our research we have developed an alternative method for the synthesis of compounds that show cooperative magnetic interactions. In this approach transition-metal coordination compounds with terminally bound anions and neutral co-ligands are heated leading to a stepwise removal of the co-ligands and the formation of intermediates with bridging anions and modified magnetic interactions. We have found that a large number of different compounds can be prepared by this route and that the dimensionality of the networks can easily be adjusted. In this context we have reported on the directed synthesis of a compound that shows SCM behavior. Such a behavior usually occurs only in 1D coordination networks, but should, in principle, also be observed in 2D networks if the magnetic chains are separated by magnetically inactive ligands. To investigate this possibility, precursor compounds based on cobalt(II) thiocyanate and the bidentate co-ligand 1,2-bis(4-pyridyl)ethylene (bpe) were prepared, and the intermediates formed by thermal decomposition were characterized for their magnetic properties. The reaction of Co(NCS)2 with an excess of bpe leads to the formation of [Co(NCS)2(bpe)(bpe)]n (1). [10] In its crystal structure the cobalt cations are octahedrally coordinated by four bpe ligands and two terminal N-bonded thiocyanato anions (Figure 1, top). The metal cations are linked by the bpe ligands into chains that are further connected by the coligands into layers. This arrangement leads to the formation of cavities in which additional bpe ligands are trapped. In further experiments using slightly different reaction conditions the hydrate [Co(NCS)2(bpe)2(H2O)2] [10] (2) could be obtained, in which the cobalt(II) cations are surrounded by two bpe ligands, two water molecules, and two terminal N-bonded thiocyanato anions in an octahedral coordination environment (Figure 1, bottom). These complexes are linked into layers by O H···N hydrogen bonds. Compounds 1 and 2 represent potential precursors for the preparation of liganddeficient compounds and thus, were investigated by thermoanalytical methods. On heating compound 1, a single mass step is observed, which leads to the formation of [Co(NCS)2(bpe)]n (4). [10] If the hydrate 2 is heated, two mass steps are observed corresponding to the formation of the anhydrate 3 in the first step, which transforms into compound 4 in the second step (see Supporting Information). Based on this information, single crystals of 4 were prepared using hydrothermal conditions. In the crystal structure of 4 the cobalt cations are octahedrally coordinated by two Sand two N-bonded thiocyanato anions as well as two N-bonded bpe ligands. The cations are linked into chains by m-1,3 bridging thiocyanato anions, which are further connected into layers by the bpe ligands (Figure 1, middle). Magnetic measurements on all the compounds show significant differences between the ligand-rich precursors 1– 3 and the ligand-deficient compound 4. In compounds 1–3 the thiocyanato anions are only terminal N-bonded, so that paramagnetic behavior is observed (see Supporting Information). On cooling, decreasing cm T values are observed until at about 25 K from which point increasing cm T values are observed which decrease again at approximately 4 K. A small magnetic exchange through the bpe ligands cannot be completely excluded. In contrast, for 4 a ferromagnetic coupling is observed between neighbored Co centers at HDC = 1 kOe (DC = direct current). Moreover, in the hysteresis curve a step is observed indicating metamagnetic behavior (Figure 2). Magnetic measurements at HDC = 0.1 kOe show antiferromagnetic behavior (Figure 3). Additional field dependent alternating current (AC) measurements using an external static field (HDC = 2 kOe, HAC = 10 Oe) show a transition from antiferromagnetic to ferromagnetic behavior at H>HC [*] Dipl.-Chem. S. W hlert, Dipl.-Chem. J. Boeckmann, Dr. M. Wriedt, Prof. Dr. C. N ther Institut f r Anorganische Chemie Christian-Albrechts-Universit t zu Kiel Max-Eyth-Strasse 2, 24118 Kiel (Germany) E-mail: cnaether@ac.uni-kiel.de

Synthesis, Crystal Structure, and Magnetic Properties of the Coordination Polymer [Fe(NCS)2(1,2-bis(4-pyridyl)-ethylene)]n Showing a Two Step Metamagnetic Transition

Reaction of iron(II) thiocyanate with an excess of trans-1,2-bis(4-pyridyl)-ethylene (bpe) in acetonitrile at room temperature leads to the formation of [Fe(NCS)(2)(bpe)(2)·(bpe)] (1), which is isotypic to its Co(II) analogue. Using slightly different reaction conditions the literature known compound [Fe(NCS)(2)(bpe)(2)(H(2)O)(2)] (2) was obtained as a phase pure material. Simultaneous differential thermoanalysis and thermogravimetry prove that the hydrate 2 transforms into the anhydrate [Fe(NCS)(2)(bpe)(2)] (3), that decomposes on further heating into the new ligand-deficient 1:1 compound of composition [Fe(NCS)(2)(bpe)](n) (4), which can also be obtained directly by thermal decomposition of 1. Further investigations reveal that 4 can also be prepared under solvothermal conditions, and single crystal structure analysis shows that the iron(II) cations are linked via μ-1,3 bridging thiocyanato anions into chains, that are further connected into layers by the bpe ligands. Magnetic measurements, performed on powder samples, prove that 1 and 2 show only Curie-Weiss behavior, whereas in 4 antiferromagnetic ordering with a Néel temperature of 5.0 K is observed. At T < 4.0 K a two-step metamagnetic transition occurs at applied magnetic fields of 1300 and 1775 Oe. The magnetic properties are discussed and compared with those of related compounds.

Influence of the co-ligand on the magnetic and relaxation properties of layered cobalt(II) thiocyanato coordination polymers.

Reaction of Co(NCS)2 with 1,2-bis(4-pyridyl)-ethane (bpa) leads to the formation of [Co(NCS)2(bpa)2]n, which, on heating, transforms into the new layered coordination polymer [Co(NCS)2(bpa)]n. This compound can also be prepared in solution, but because no reasonable single crystals are available, its crystal structure was determined from X-ray powder data from scratch. In the crystal structure of [Co(NCS)2(bpa)]n, the cobalt(II) cations are coordinated by two S-bonded and two N-bonded thiocyanato anions and two N atoms of the bpa co-ligands in a distorted octahedral geometry. The cobalt(II) cations are linked into chains by pairs of μ-1,3 bridging thiocyanato anions. These chains are further connected into layers by the 1,2-bis(4-pyridyl)-ethane ligand. The compound was magnetically characterized, and, for comparative purposes, the complementary magnetic study of a known and very similar compound, [Co(NCS)2(bpe)]n (bpe = 1,2-bis(4-pyridyl)-ethylene), was also undertaken. The compounds differ in their interchain interactions, which are antiferromagnetic but significantly greater for [Co(NCS)2(bpe)]n. Magnetic measurements indicate that [Co(NCS)2(bpa)]n is a canted antiferromagnet with Néel temperature TN = 3.1 K and that Co(NCS)2(bpe) is an antiferromagnet with TN = 4.0 K. Both compounds show a metamagnetic transition with a critical field HC ∼ 40 Oe and ∼ 400 Oe, respectively. Magnetic relaxations were studied by means of dc and ac methods and analyzed using the Argand diagrams. Except for the thermally activated single chain and domain wall relaxations observed for both compounds, temperature-independent slow relaxations were observed for [Co(NCS)2(bpa)]n.

Structural and magnetic studies of a new Co(II) thiocyanato coordination polymer showing slow magnetic relaxations and a metamagnetic transition.

Reaction of Co(NCS)2 with 4-ethylpyridine leads to the formation of three new compounds of composition Co(NCS)2(4-ethylpyridine)4 (1), [(Co(NCS)2]2(4-ethylpyridine)6 (2), and [Co(NCS)2(4-ethylpyridine)2]n (3). In all compounds the coordination of the Co(II) ions is distorted octahedral. 1 consists of discrete monomeric complexes and in 2 two Co(II) cations are linked by pairs of μ-1,3-bridging thiocyanato ligands into dimers. In the crystal structure of 3 the Co(II) cations are connected into chains by the same bridge as in 2. Magnetic studies show that 1 and 2 are paramagnets down to a temperature of 2 K, while compound 3, which is the main object of this study, is an antiferromagnet with the Néel temperature T(N) = 3.4 K. Its magnetic structure is built from ferromagnetic chains, which are weakly antiferromagnetically coupled. With increasing magnetic field a metamagnetic transition starts at ~175 Oe, as observed for a polycrystalline sample. Magnetic relaxations, which were observed in the antiferromagnetic state, are retained at the metamagnetic transition. With decreasing field 3 remains in a state, in which except of the faster magnetic relaxation process in single chains also a slower process coexists resulting in the appearance of a magnetic hysteresis loop.

Synthesis of new stable and metastable thiocyanato coordination polymers by thermal decomposition and by solution reactions

Investigations on the thermal behavior of the 1 : 4 (ratio between metal and neutral co-ligand) compound [Cd(NCS)2(pyridine)4] (1-Cd) show that the pyridine ligands can be removed in three separate steps. In the first TG step a pyridine-deficient 1 : 2 compound of composition [Cd(NCS)2(pyridine)2]n (2-Cd/III) is obtained, which transforms into an 1 : 1 compound of composition [Cd(NCS)2(pyridine)]n (3-Cd) on further heating. Rietveld refinement of 2-Cd/III proves that this compound represents a new form, which is different from that of the 1 : 2 compounds 2-Cd/I and 2-Cd/II reported recently. In further experiments a fourth 1 : 2 compound of composition [Cd(NCS)2(pyridine)2]n (2-Cd/IV) was crystallized and characterized by single crystal X-ray diffraction. In the crystal structures of 2-Cd/II, 2-Cd/III and 2-Cd/IV the metal cations are linked by the anionic ligands into chains, in which the Cd cations are coordinated by two trans-oriented pyridine ligands and four thiocyanato anions within slightly distorted octahedra. For the literature known compound 2-Cd/I no atomic coordinates are given, but for 2-Cd/II an all-cis, for 2-Cd/III an all-trans and for 2-Cd/IV an alternating cis-cis-trans arrangement of the thiocyanato anions is observed. Solvent mediated conversion experiments prove that 2-Cd/III transforms into 2-Cd/IVvia2-Cd/II as intermediate and therefore, the cis-cis-trans coordinated compound 2-Cd/IV represents the thermodynamic stable form at room-temperature. The 1 : 1 compound 3-Cd can also be prepared from solution. In its crystal structure the Cd cations are coordinated by one pyridine ligand as well as two N-bonded and three S-bonded thiocyanato anions. Two neighboring octahedra are connected by two μ-1,1,3(S,S,N)-coordinating anions into double octahedra, which are further linked into chains by μ-1,1,3(S,S,N)- and μ-1,3-coordinating thiocyanato anions. IR investigations of the 1 : 4, 1 : 2 and 1 : 1 compounds reveal that the different coordination modes of the anionic ligands can be rationalized by analyzing the changes in the values of their asymmetric νas(CN) stretching vibration. Additional investigations on 2-Cd/II, 2-Cd/III and 2-Cd/IV using differential scanning calorimetry and X-ray powder diffraction reveal that on heating additional crystalline 1 : 2 compounds are formed in reversible and irreversible phase transitions.

Synthesis, structures and magnetic properties of Fe(II) and Co(II) thiocyanato coordination compounds: on the importance of the diamagnetic counterparts for structure determination

Reaction of Fe(NCS)2 and Co(NCS)2 with 2-methylpyrazine in different molar ratios and solvents at room temperature leads to the formation of five new coordination compounds of composition M(NCS)2(2-methylpyrazine)2(H2O)2 (M = Fe (1-Fe) , Co (1-Co)), Co(NCS)2(2-methylpyrazine)2(CH3OH)2 (2-Co) and Co(NCS)2(2-methylpyrazine)4·2-methylpyrazine solvate (3-Co). In all of these compounds, discrete complexes are found in which the metal cations are octahedrally coordinated by two terminal N-bonded thiocyanato anions and four N- or O-donor co-ligands. On heating compounds 1-3 in a thermobalance new coordination polymers of composition M(NCS)2(2-methylpyrazine)2 (M = Co (4-Co) , Fe (4-Fe)) are obtained in the first step, which transform into M(NCS)2(2-methylpyrazine) (M = Co (5-Co) , Fe (5-Fe)) in the second. Because of the low chalcophilicity of these cations, compounds 4 and 5 are not accessible from solution. Further investigations prove that 4-Co and 4-Fe obtained by thermal decomposition are of low crystallinity, might be isotypic and might consist of metal cations, in which the anionic ligands are only terminal N-bonded. In contrast, 5-Co and 5-Fe are of good crystallinity but their structure cannot be solved from X-ray powder data. However, a compound of the same composition (5-Cd) based on the more chalcophilic cadmium can easily be crystallized from solution and characterized by single crystal X-ray diffraction. This compound is isotypic to 5-Co and 5-Fe and therefore their structures were determined by Rietveld refinements. In their crystal structures the metal atoms are linked by μ-1,3-bridging thiocyanato anions into a 2D network. Magnetic measurements reveal that compounds 1-4 show only Curie-Weiss paramagnetism and that for 5-Fe antiferromagnetic ordering is observed. In contrast, 5-Co shows metamagnetic behavior with a very large critical field. Finally, it was shown that 4-Co and 4-Fe are hygroscopic and transform within minutes into the hydrates 1-Co and 1-Fe.

Exploration and synthesis of condensed coordination networks with modified magnetic properties

Eight new coordination polymers based on Ni(NCS)2 and 2-methylpyrazine were discovered, structurally characterized and most of them investigated for their thermal and magnetic properties. In the crystal structures of Ni(NCS)2(2-methylpyrazine)4 (1), Ni(NCS)2(2-methylpyrazine)4·2-methylpyrazine solvate (2) and Ni(NCS)2(2-methylpyrazine)4·ethanol solvate (3), the nickel(II) cations are always octahedral coordinated by two terminal N-bonded thiocyanato anions and four N-bonded 2-methylpyrazine ligands into discrete complexes. The crystal structures of Ni(NCS)2(2-methylpyrazine)2(H2O)2 (4) and Ni(NCS)2(2-methylpyrazine)2(CH3OH)2 (5) also consist of discrete complexes, but in contrast to compounds 1–3 two terminal bonded 2-methylpyrazine ligands are exchanged by two water and methanol molecules, respectively. In [Ni(NCS)2(2-methylpyrazine)3(H2O)]·[Ni(NCS)2(2-methylpyrazine)4] (6) two different complexes are observed, in which one nickel(II) cation is coordinated by two terminal N-bonded thiocyanato anions, three 2-methylpyrazine ligands and one water molecule, whereas the second cation is coordinated by two terminal N-bonded thiocyanato anions and four 2-methylpyrazine ligands. Compounds 2, 3, 4 and 5 transformed into a 2-methylpyrazine deficient 1 : 2 (1 : 2 = ratio between metal and 2-methylpyrazine) compound with the composition Ni(NCS)2(2-methylpyrazine)2 (7) on heating. On further heating this compound subsequently transformed into a 1 : 1 compound with the composition [Ni(NCS)2(2-methylpyrazine)]n (8). Compound 7 can only be prepared by thermal decomposition and therefore, its structure was solved ab initio from X-ray powder data. It consists of [Ni(NCS)]2 dimers that are μ-1,3-bridged by single thiocyanato anions into layers. Single crystal structure analysis of compound 8 proves that nickel(II) cations are linked by the thiocyanato anions into Ni–(NCS)2–Ni double chains, which are further connected by the 2-methylpyrazine ligands into layers. Magnetic measurements on compounds 2, 3, 4 and 5 show only Curie or Curie–Weiss paramagnetism, whereas in the ligand-deficient compound 7 metamagnetic behavior is observed. The results of these investigations are discussed and compared with those obtained for related coordination compounds with e.g. pyrazine and pyridine as neutral co-ligands.

Synthesis and crystal structures of Zn(II) and Co(II) coordination compounds with ortho substituted pyridine ligands: two structure types and polymorphism in the region of their coexistence

Reaction of cobalt(II) thiocyanate or zinc(II) thiocyanate with 2-methylpyridine, 2-bromopyridine and 2-chloropyridine at room-temperature lead to tetrahedral complexes of composition M(NCS)2L2 (M = Co, Zn; 1: L = 2-methylpyridine, 2: L = 2-bromopyridine, 3: L = 2-chloropyridine). For all compounds, X-ray diffraction showed that the M2+ cations are coordinated by two terminal N-bonded thiocyanato anions and two substituted pyridine ligands in a distorted tetrahedral geometry. In all cases, complexes derived from cobalt and zinc with the same ligands are isomorphous. Eight crystalline solids were studied, which belong to only two different structure types: two polymorphs were obtained for the compounds with 2-chloropyridine: 3-Mα is isotypic with the 2-methylpyridine complexes 1-M and crystallizes in the orthorhombic space group P212121, whereas 3-Mβ and the isotypic 2-bromopyridine complexes 2-M adopt space group Pbcn. The structure types differ with respect to molecular conformation and packing: the complexes crystallizing in the former type are less distorted with respect to metal coordination; in the latter structure type, the angle subtended by the two ligands is by far the largest in the coordination sphere. P212121 allows for a more efficient intermolecular arrangement, whereas the complex geometry in Pbcn features shorter non-bonding distances between the ortho substituents of the pyridine ligands and comes closer to the gas phase geometry. DSC measurements gave no hints of a transformation between these modifications but from a melt of 3-Coα, 3-Coβ was obtained. Solvent mediated conversion experiments revealed that 3-Mα represents the thermodynamically stable form at room-temperature whereas 3-Mβ is metastable, in agreement with its lower density. Therefore, the thermodynamically stable phases with 2-chloropyridine as ligand adopt the crystal structure of the compounds with 2-methylpyridine, whereas the metastable forms are isotypic to the compounds with 2-bromopyridine.

Crystal structure of tri­aqua­(2,6-di­methyl­pyrazine-κN4)bis­(thio­cyanato-κN)manganese(II) 2,5-di­methyl­pyrazine disolvate

In the crystal structure of the title complex, [Mn(NCS)2(C6H8N2)(H2O)3]·2C6H8N2, the MnII cation is coordinated by two terminally N-bonded thiocyanate anions, three water molecules and one 2,6-dimethylpyrazine ligand within a slightly distorted N3O3 octahedral geometry; the entire complex molecule is generated by the application of a twofold rotation axis. The asymmetric unit also contains an uncoordinating 2,5-dimethylpyrazine ligand in a general position. Obviously, the coordination to the 2,6-dimethylpyrazine ligand is preferred because coordination to the 2,5-dimethylpyrazine is hindered due to the bulky methyl group proximate to the N atom. The discrete complexes are linked by water-O—H⋯N(2,6-dimethylpyzazine/2,5-dimethylpyzazine) hydrogen bonding, forming a three-dimensional network. In the crystal, molecules are arranged in a way that cavities are formed in which unspecified, disordered solvent molecules reside. These were modelled employing the SQUEEZE routine in PLATON [Spek (2015 ▸). Acta Cryst. C71, 9–18]. The composition of the unit cell does not take into account the presence of the unspecified solvent.

Poly[bis-(acetonitrile-κN)di-μ-thio-cyanato-κN,S;κS,N-nickel(II)].

In the title compound, [Ni(NCS)2(CH3CN)2]n, the NiII cation is coordinated by two N-bonded and two S-bonded thiocyanate anions, as well as two acetonitrile molecules in an octahedral NiN4S2 coordination mode. The asymmetric unit comprises one nickel cation, two thiocyanate anions and two actonitrile molecules. In the crystal, the NiII cations are connected by bridging thiocyanate anions into a three-dimensional coordination network.