Thermodynamically Metastable Thiocyanato Coordination Polymer That Shows Slow Relaxations of the Magnetization

Reaction of cobalt thiocyanate with 4-acetylpyridine leads to the formation of [Co(NCS)(2)(4-acetylpyridine)(2)](n) (3/I). In its crystal structure the Co cations are connected by pairs of mu-1,3-bridging thiocyanato ligands into dimers that are further connected into layers by single anionic ligands. DTA-TG measurements of Co(NCS)(2)(4-acetyl-pyridine)(4) (1) led to the formation of 3/I. In contrast, when the hydrate Co(NCS)(2)(4-acetyl-pyridine)(2)(H2O)(2) (2) is decomposed, a mixture of 3/I and a thermodynamically metastable form 3/II is obtained. Further investigations reveal that thermal annealing of 2 leads to the formation of 3/II, that contains only traces of the stable form 3/I. DSC and temperature dependent X-ray powder diffraction (XRPD) measurements prove that 3/II transforms into 3/I on heating. The crystal structure of 3/II was determined ab initio from XRPD data. In its crystal structure the Co cations are linked by pairs of bridging thiocyanato anions into a 1D coordination polymer, and thus, 3/II is an isomer of 3/I. Magnetic measurements disclose that the stable form 3/I only shows paramagnetism without any magnetic anomaly down to 2 K. In contrast, the metastable form 3/II shows ferromagnetic behavior. The phase transition into ordered state at T-c = 3.8 K was confirmed by specific heat measurements. Alternating current susceptibility measurements show frequency dependent maxima in chi' and chi '' which is indicative for a slow relaxation of the magnetization.