Synthesis and mononuclear complexes of the bis-bidentate ligand 2,5-di(2-pyridyl)-1,3,4-thiadiazole (dptd): spin crossover in [Fe(II)(dptd)2(NCSe)2] and [Fe(II)(dptd)2(NCBH3)2]·H2O.

A simple and convenient protocol for the synthesis of the ligand 2,5-di(2-pyridyl)-1,3,4-thiadiazole (dptd) has been developed. Five new 2 : 1-type iron(II), cobalt(II), nickel(II) and copper(II) complexes have been prepared and structurally characterised, all of which feature the mononuclear trans-(N',N(1))(2) coordination mode. Spin crossover behaviour has been found for [Fe(II)(dptd)(2)(NCSe)(2)] (2) and [Fe(II)(dptd)(2)(NCBH(3))(2)]·H(2)O (3·H(2)O) with T(1/2) = 192 and 285 K, respectively, reflecting the increasing ligand field strength of the respective co-ligands and showing that the [Fe(II)(dptd)(2)L(2)] unit is suitable for the investigation of the co-ligand field effects on T(1/2). In addition, the 3 : 1-type low-spin complexes [Fe(II)(dptd)(3)]{B(CN)(4)}(2)·3MeOH (4·3MeOH) and [Fe(II)(dptd)(3)](OTf)(2)·1.25DCM·0.5H(2)O (5·1.25DCM·0.5H(2)O) have been studied by X-ray diffraction and have been found to exist exclusively as the mer or fac stereoisomer, respectively, in the solid state. In MeCN-d(3) solution the latter two complexes, as well as [Fe(II)(dptd)(3)](ClO(4))(2)·H(2)O (6·H(2)O), show counterion-independent stereoisomerisation, each yielding a ca. 3 : 1 mixture of mer and fac stereoisomers as identified by (1)H NMR spectroscopy.