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 μ-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 Tc = 3.8 K was confirmed
by specific heat measurements. Alternating current susceptibility
measurements show frequency dependent maxima in χ′ and
χ″, which is indicative for a slow relaxation of the
magnetization.