Summary: | Herein we report the synthesis and characterization of a novel bis-tridentate 1,3,4-thiadiazole ligand (<b>L</b> = 2,5-bis[(2-pyridylmethyl)thio]methyl-1,3,4-thiadiazole). Two new mononuclear complexes of the type [M<sup>II</sup>(<b>L</b>)<sub>2</sub>](ClO<sub>4</sub>)<sub>2</sub> (with M = Fe<sup>II</sup> (<b>C1</b>) and Co<sup>II</sup> (<b>C2</b>)) have been synthesized, containing the new ligand (<b>L</b>). In both complexes the metal centers are coordinated by an N<sub>4</sub>S<sub>2</sub>-donorset and each of the two ligands is donating to the metal ion with just one of the tridentate pockets. The iron(II) complex (<b>C1</b>) is in the low spin [LS] state below room temperature and shows an increase in the magnetic moment only above 300 K. In contrast, the cobalt(II) complex (<b>C2</b>) shows a gradual spin crossover (SCO) with <i>T</i><sub>1/2</sub> = 175 K. To our knowledge, this is the first cobalt(II) SCO complex with an N<sub>4</sub>S<sub>2</sub>-coordination.
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