Coexistence of metamagnetism and slow relaxation of the magnetization in a cobalt thiocyanate 2D coordination network.

TLDR: This research has developed an alternative method for the synthesis of compounds that show cooperative magnetic interactions and 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.

Abstract: 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