A systematic study on the thermal decomposition reactions of coordination polymers based on metal(II) selenocyanates and 4,4′-bipyridine: Directed synthesis of a μ-1,3 bridging selenocyanate

Reaction of manganese(II), iron(II), cobalt(II) and nickel(II) selenocyanate with 4,4′-bipyridine (bipy) in water at room temperature leads to the formation of the ligand-rich 1:2 hydrates [{M(bipy)(NCSe)2(H2O)2}·bipy]n (bipy = 4,4′-bipyridine) with M = Mn (1-Mn), Fe (1-Fe), Co (1-Co) and Ni (1-Ni). In their crystal structures, the metal cations are coordinated by two terminally N-bonded selenocyanato anions, two water molecules, and two bridging bipy ligands in an octahedral coordination mode. These building blocks are connected into linear M–bipy–M chains, which are further linked by hydrogen bonds between the water molecules and non-coordinated bipy ligands into layers. On heating these precursor compounds, they decompose into ligand-rich 1:2 anhydrates [M(NCSe)2(bipy)2]n with M = Mn (2-Mn), Fe (2-Fe), Co (2-Co) and Ni (2-Ni). After water removal the coordination spheres of the metal cations are completed by N-coordination of the bipy ligand which formerly was involved in OH···N hydrogen bonding. On further heating, the manganese(II) compound loses half of its bipy ligands leading to a new ligand-deficient 1:1 intermediate [Mn(NCSe)2(bipy)]n (3-Mn) with μ-1,3-bridging selenocyanato anions. In contrast, all other compounds decompose without the formation of ligand-deficient intermediates. These structural changes are accompanied with a dramatic change in their magnetic properties: Whereas all ligand-rich 1:2 compounds 1-M and 2-M (M = metal) show only Curie–Weiss paramagnetism, in the ligand-deficient 1:1 intermediate 3-Mn an antiferromagnetic long-range ordering at TN = 10.5 K is found. The thermal and magnetic properties are qualitatively compared with those of the related ligand-rich and ligand-deficient selenocyanato and thiocyanato compounds based on bipy, pyrazine and pyrimidine as ligand.

Selenocyanato hydrates [{M(4,4′-bipy)(NCSe)2(H2O)2}·4,4′-bipy]n (1-M) decompose on heating into [M(NCSe)2(4,4′-bipy)2]n (2-M). Only 2-Mn transforms into [M(NCSe)2(4,4′-bipy)]n (3-Mn) on further heating, in which the SeCN anions become μ-1,3 bridging, leading to an antiferromagnetic ordering. A comparison with related SCN and SeCN compounds show interesting differences in their thermal and magnetic behavior.Figure optionsDownload as PowerPoint slideHighlights
► Selenocyanato hydrates 1-M decompose on heating into anhydrates 2-M.
► Only the Mn(II) compound transforms into an intermediate 3-Mn on further heating.
► The pristine terminally bonded selenocyanato anions become μ-1,3 bridging in 3-Mn.
► A comparison with related SCN and SeCN compounds show interesting differences.