Metamagnetism and long range ordering in μ-1,3 bridging transition metal thiocyanato coordination polymers

Reaction of M(SCN)2 (M = Mn, Fe, Ni) with pyridine (pyr) in aqueous solution at room temperature leads to the formation of the literature known pyridine-rich 1:4 compounds of composition [M(SCN)2(pyridine)4] (M = Mn (1-Mn), Fe (1-Fe), Ni (1-Ni)) reported recently. On heating, the 1:4 compounds decompose into their corresponding pyridine-deficient 1:2 compounds of composition [M(SCN)2(pyridine)2]n (M = Mn (2-Mn), Fe (2-Fe), Ni (2-Ni)) which decompose on further heating. In the crystal structure of the pyridine-deficient 1:2 compounds the metal cations are coordinated by four N-atoms of two pyridine ligands and two N-bonded thiocyanato anions, each in mutually trans orientation, and by two S-atoms of two adjacent thiocyanato anions in a slightly distorted octahedral geometry. The thiocyanato anions bridge the metal cations into one-dimensional (1D) polymeric chains. IR spectroscopic investigations on the pyridine-deficient 1:2 compounds are in agreement with the presence of μ-1,3 bridging thiocyanato anions. Magnetic measurements of the pyridine-rich 1:4 compounds show only Curie–Weiss paramagnetism whereas for the pyridine-deficient 1:2 compounds an antiferromagnetic ordering for [Mn(NCS)2(pyridine)2]n (2-Mn) and metamagnetic behavior for [Ni(NCS)2(pyridine)2]n (2-Ni) is found. For [Cu(NCS)2(pyridine)2]n (2-Cu) Curie–Weiss paramagnetic behavior is observed. [Fe(NCS)2(pyridine)2]n (2-Fe) shows metamagnetic behavior, which was already investigated but remeasured for a more detailed characterization.

Solid-state transformation of ligand-rich precursor complexes based on transition metal thiocyanates with Mn, Fe and Ni and pyridine as co-ligand lead to the formation of ligand-deficient 1D polymeric chains with μ-1,3 bridging thiocyanato anions. Magnetic measurements of the ligand-deficient compounds reveal different magnetic properties like metamagnetism or antiferromagnetic ordering, whereas the discrete precursor compounds only show Curie- or Curie–Weiss paramagnetism.Figure optionsDownload as PowerPoint slide