Preparation of new ligand-deficient thiocyanato compounds with cooperative magnetic phenomena by thermal decomposition of their ligand-rich precursors

Abstract

The reaction of nickel thiocyanate with bipy (4,4′-bipyridine) in water at room temperature leads to the formation of the ligand-rich 1:2 (1:2 ratio between metal salt and organic ligand) hydrate [{Ni(NCS) 2(bipy)(H2O)2}·(bipy)]n (1-Ni), which is isotypic to its analogues 1-Mn and 1-Co reported recently. In their crystal structure, the metal cations are coordinated by two terminal N-bonded thiocyanato anions, two water molecules, and two bridging bipy ligands in an octahedral coordination mode. These building blocks elongate in linear M-bipy-M chains, which are further connected by hydrogen bonds between H 2O and noncoordinated bipy ligands into layers. On heating these precursor compounds, two-step decomposition is observed: New ligand-rich 1:2 anhydrous compounds [M(NCS)2(bipy)2]n [M = Mn (2-Mn), Co (2-Co), and Ni (2-Ni)] could be identified as intermediates in the first heating step, and new ligand-deficient 1:1 compounds [M(NCS) 2(bipy)]n [M = Mn (3-Mn), Co (3-Co), and Ni (3-Ni)] could be identified as intermediates in the second heating step. A smooth reaction pathway in their crystal structures is observed: First, water is removed, leading to directly coordinating bipy ligands, and second, half of the bipy ligands are removed, leading to μ-1,3-bridging thiocyanato anions. This structural transformation is accompanied by a dramatic change in their magnetic properties: Whereas the ligand-rich 1:2 hydrates and anhydrous compounds show only Curie-Weiss paramagnetism, the ligand-deficient 1:1 intermediates show either Curie-Weiss paramagnetism or antiferromagnetic ordering at lower temperatures, mediated by the bridging thiocyanato anions. These results are qualitatively compared with, those of the related ligand-rich and ligand-deficient compounds with pyrazine and pyrimidine. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.