Jan Boeckmann

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

A rational route to SCM materials based on a 1-D cobalt selenocyanato coordination polymer

Thermal annealing of a discrete complex with terminal SeCN anions and monodentate coligands enforces the formation of a 1D cobalt selenocyanato coordination polymer that shows slow relaxation of the magnetization. Therefore, this approach offers a rational route to 1D materials that might show single chain magnetic behaviour.

Metamagnetism and Single-Chain Magnetic Behavior in a Homospin One-Dimensional Iron(II) Coordination Polymer

Reaction of FeCl(2)·4H(2)O with KNCSe and pyridine in ethanol leads to the formation of the discrete complex [Fe(NCSe)(2)(pyridine)(4)] (1) in which the Fe(II) cations are coordinated by two N-terminal-bonded selenocyanato anions and four pyridine co-ligands. Thermal treatment of compound 1 enforces the removal of half of the co-ligands leading to the formation of a ligand-deficient (lacking on neutral co-ligands) intermediate of composition [Fe(NCSe)(2)(pyridine)(2)](n) (2) to which we have found no access in the liquid phase. Compound 2 is obtained only as a microcrystalline powder, but it is isotypic to [Cd(NCSe)(2)(pyridine)(2)](n) and therefore, its structure was determined by Rietveld refinement. In its crystal structure the metal cations are coordinated by two pyridine ligands and four selenocyanato anions and are linked into chains by μ-1,3 bridging anionic ligands. Magnetic measurements on compound 1 show only paramagnetic behavior, whereas for compound 2 an unexpected magnetic behavior is found, which to the best of our knowledge was never observed before for a iron(II) homospin compound. In this compound metamagnetism and single-chain magnetic behavior coexist. The metamagnetic transition between the antiferromagnetically ordered phase and a field-induced ferromagnetic phase of the high-spin iron(II) spin carriers is observed at a transition field H(C) of 1300 Oe and the single-chain magnetic behavior is characterized by a blocking temperature T(B), estimated to be about 5 K.

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.

New nickel(II) thiocyanato coordination compounds: synthetic aspects, polymorphism, thermal reactivity and magnetic properties

Reaction of Ni(NCS)2 with bis(3-aminopropyl)amine (dpt) and pyridine (pyr) in water and molar ratios leads to the formation of the new polymorphic modifications [Ni(NCS)2(dpt)(pyr)] (1a and 1b). Slight variation of the reaction conditions results in the formation of the new nickel(II) thiocyanato coordination compounds {[Ni(dpt)2]2+·[(NCS)2]2−} (2), {[Ni(SCN)3(dpt)]−·[Ni(dpt)2·(NCS)]+·H2O} (3), {[Ni(μ-1,3-NCS)(dpt)(NCS)]2·[Ni(NCS)2(dpt)(H2O)]2·(H2O)4} (4) and [Ni(μ-1,3-NCS)(dpt)(NCS)]2 (5). All compounds were structurally characterized and investigated for their thermal, spectroscopic and magnetic properties. The polymorphic modifications 1a and 1b were additionally investigated by solvent mediated conversion experiments and DSC measurements for their thermodynamic stability, which show that form 1a represents the thermodynamically most stable over the whole temperature range. Investigations on the thermal degradation behavior of compounds 1a–4 show that only for compounds 1a and 4 ligand-deficient intermediates can be found whereas for all other compounds only complete decomposition is observed. Compound 4 decomposes to the literature known ligand-deficient compound {[Ni(μ-1,3-NCS)(dpt)(NCS)]2·[Ni(μ-1,3-NCS)(dpt)(NCS)]4} (Refcode MUYRAZ), whereas the intermediate of compound 1a could not be structurally characterized. On the basis of IR and Raman spectroscopic investigations it could be shown that in the intermediate of 1a bridging anions could be found and a tetrameric structure can be assumed. Magnetic measurements of compounds 1a–4 reveal only Curie and Curie-Weiss paramagnetism.

Bis(3-methyl­pyridine-κN)bis­(thio­cyanato-κN)zinc

The asymmetric unit of the title compound, [Zn(NCS)2(C6H7N)2], consists of one Zn2+ cation and two thiocyanate anions, all situated on special positions with site symmetry .m., and one 3-methylpyridine ligand. The zinc cation is coordinated by four N atoms of two terminal N-bonded thiocyanate anions and of two symmetry-related 3-methylpyridine co-ligands, defining a slightly distorted tetrahedral coordination polyhedron.

catena-Poly[[bis-(pyridine-κN)cadmium]-di-μ(2)-thio-cyanato-κN:S;κS:N].

The asymmetric unit of the title compound, [Cd(NCS)2(C5H5N)2]n, consists of two crystallographically independent CdII cations, four thiocyanato anions and four pyridine ligands. The CdII atoms are each coordinated by four N atoms from two pyridine ligands and two thiocyanato anions, each in a mutually cis orientation, and by two S atoms from two adjacent thiocyanato anions within a slightly distorted octahedral coordination environment. The CdII atoms are μ-1,3-bridged via the thiocyanato anions into polymeric chains parallel to [001]. The CdII⋯CdII intrachain separations range between 5.9688 (6) and 6.0195 (6) Å, whereas the shortest CdII⋯CdII interchain separations are 7.8272 (7) and 8.6312 (6) Å.

Poly[(μ-4,4′-bipyridine)(μ-naph­tha­lene-1,4-dicarboxyl­ato)iron(II)]

The asymmetric unit of the title compound, [Fe(C12H6O4)(C10H8N2)], consists of two independent Fe(II) atoms, two naphthalene-1,4-dicarboxylate anions and two 4,4′-bipyridine ligands. The Fe(II) atoms are each coordinated by four O atoms of the naphthalene-1,4-dicarboxylate anions and two N atoms of the 4,4′-bipyridine ligands within a distorted octahedron. Two Fe(II) atoms are bridged via the carboxylate groups of two symmetry-related anions into dimers, which are further connected into chains. These chains are linked by additional anions into layers that are finally connected by the 4,4′-bipyridine ligands into a three-dimensional coordination network.

Poly[(μ-4,4′-bipyridine)(μ-naphthalene-1,4-dicarboxyl­ato)manganese(II)]

In the crystal structure of the title compound, [Mn(C12H6O4)(C10H8N2)]n, the Mn atoms are each coordinated by four O atoms of naphthalene-1,4-dicarboxylate anions and two N atoms of two symmetry-related 4,4′-bipyridine ligands within a strongly distorted octahedra. Two of the O atoms originate from one naphthalene-1,4-dicarboxylate anion, whereas the remaining two O atoms derive from two symmetry-equivalent naphthalene-1,4-dicarboxylate anions. Two Mn atoms are connected via the anions into dimers, which are further linked by the anions and the N-donor ligands into a three-dimensional coordination network.

catena-Poly[[bis­(3-acetyl­pyridine-κN)cadmium]-di-μ-seleno­cyanato-κ2N:Se;κ2Se:N]

In the crystal structure of the title compound, [Cd(NCSe)2(C7H7NO)2]n, the Cd2+ cation is coordinated by two 3-acetylpyridine ligands and four μ-1,3-bridging selenocyanate anions within a slightly distorted CdN4Se2 octahedron. The asymmetric units consists of one Cd2+ cation, which is situated on a center of inversion, as well as one selenocyanate anion and one 3-acetylpyridine ligand in general positions. The metal cations are μ-1,3-bridged via the selenocyanate anions into chains along the a axis.