Exploration and synthesis of condensed coordination networks with modified magnetic properties

Abstract

Eight new coordination polymers based on Ni(NCS)₂ and 2-methylpyrazine were discovered, structurally characterized and most of them investigated for their thermal and magnetic properties. In the crystal structures of Ni(NCS)₂(2-methylpyrazine)₄ (1), Ni(NCS)₂(2-methylpyrazine)₄·2-methylpyrazine solvate (2) and Ni(NCS)₂(2-methylpyrazine)₄·ethanol solvate (3), the nickel(II) cations are always octahedral coordinated by two terminal N-bonded thiocyanato anions and four N-bonded 2-methylpyrazine ligands into discrete complexes. The crystal structures of Ni(NCS)₂(2-methylpyrazine)₂(H₂O)₂ (4) and Ni(NCS)₂(2-methylpyrazine)₂(CH₃OH)₂ (5) also consist of discrete complexes, but in contrast to compounds 1–3 two terminal bonded 2-methylpyrazine ligands are exchanged by two water and methanol molecules, respectively. In [Ni(NCS)₂(2-methylpyrazine)₃(H₂O)]·[Ni(NCS)₂(2-methylpyrazine)₄] (6) two different complexes are observed, in which one nickel(II) cation is coordinated by two terminal N-bonded thiocyanato anions, three 2-methylpyrazine ligands and one water molecule, whereas the second cation is coordinated by two terminal N-bonded thiocyanato anions and four 2-methylpyrazine ligands. Compounds 2, 3, 4 and 5 transformed into a 2-methylpyrazine deficient 1 : 2 (1 : 2 = ratio between metal and 2-methylpyrazine) compound with the composition Ni(NCS)₂(2-methylpyrazine)₂ (7) on heating. On further heating this compound subsequently transformed into a 1 : 1 compound with the composition [Ni(NCS)₂(2-methylpyrazine)]_n (8). Compound 7 can only be prepared by thermal decomposition and therefore, its structure was solved ab initio from X-ray powder data. It consists of [Ni(NCS)]₂ dimers that are μ-1,3-bridged by single thiocyanato anions into layers. Single crystal structure analysis of compound 8 proves that nickel(II) cations are linked by the thiocyanato anions into Ni–(NCS)₂–Ni double chains, which are further connected by the 2-methylpyrazine ligands into layers. Magnetic measurements on compounds 2, 3, 4 and 5 show only Curie or Curie–Weiss paramagnetism, whereas in the ligand-deficient compound 7 metamagnetic behavior is observed. The results of these investigations are discussed and compared with those obtained for related coordination compounds with e.g. pyrazine and pyridine as neutral co-ligands.