Synthesis, Structures, Polymorphism, and Magnetic Properties of Transition Metal Thiocyanato Coordination Compounds

Reaction of manganese, iron, and nickel thiocyanate with 4-ethylpyridine leads to the formation of single crystals of compounds with composition M­(NCS)2(L)4 (1), M­(NCS)2(L)2(H2O)2 (2-Mn), and M­(NCS)2(L)2 (3) with M = Mn, Fe, and Ni and L = 4-ethylpyridine. For most compounds, different polymorphic modifications are observed, and their transition behavior and thermodynamic stability was investigated. Additionally, compounds of composition M­(NCS)2(L)2 (M = Mn or Ni) were prepared from solution and by thermal decomposition of compounds 1 and 2, which lead to different stable and metastable modifications. The crystal structures of most compounds were determined by single crystal X-ray diffraction and some of them by Rietveld refinements. Compounds 1 and 2 consist of octahedrally coordinated discrete complexes with terminal N-bonded thiocyanato anions. In compounds 3, the metal cations are linked by pairs of μ-1,3-bridging thiocyanato anions into chains. Surprisingly, thermal decomposition of Ni­(NCS)2(4-ethylpyridine)4 leads to the formation of a new compound of composition Ni­(NCS)2(4-ethylpyridine) (4-Ni). Magnetic measurements reveal that 3-Mn/II and 3-Mn/III show antiferromagnetic ordering at TN = 21.5 and 23.9 K and that 4-Ni is a metamagnet with a critical field of 1.4 kOe at 2 K. All other compounds show Curie or Curie–Weiss behavior with no magnetic anomalies.