Structural Variety in Mn(NCS)2 4-Cyanopyridine Coordination Compounds: Synthesis, Structures, Isomerism, and Magnetic Properties

The reaction of Mn(NCS)(2) with 4-cyanopyridine (CNpy) leads to the formation of discrete complexes with the composition [Mn(NCS)(2)(CNpy)(4)] (1), [Mn(NCS)(2)(H2O)(2)(CNpy)(2)] (2-I and 2-II), and [Mn(NCS)(2)(H2O)(2)(CNpy)(2)].xCNpy (x = 4, 3; x = 2, 4), in which the Mn(II) centers are octahedrally coordinated by two terminal N-bonded thiocyanate anions and by four (1) or two CNpy coligands and two water molecules (2-I, 2-II, 3, and 4). If an excess of Mn(NCS)(2) is used, two additional compounds with the compositions [Mn(NCS)(2)(CNpy)(2)](n) (5) and [Mn-(NCS)(2)(CNpy)](n) (6-I and 6-II) are obtained. In all compounds the Mn(II) cations are octahedrally coordinated and linked into linear chains (5), into layers (6-I), or into a 3D network (6-II) by the thiocyanate anions. Investigations using TG-DTA and temperature-dependent powder X-ray diffraction prove that the discrete complexes 2-II, 3, and 4 decompose in several steps, leading to the formation of Mn(NCS) 2 via 5 and 6-I as intermediates. For compounds 2-II, 4, and 6-II only one batch was obtained, indicating that these compounds are metastable. Magnetic measurements for compounds 5 and 6-I reveal dominating antiferromagnetic interactions and maxima in the susceptibility curve at 20 K (5) and 24 K (6-I), which are reproduced by quantum Monte Carlo simulations. The specific heat proves magnetic ordering at 2.8 K (5) and 12.4 K (6-I). The ordering of 6-I is associated with a weak ferromagnetism.