Synthesis and mononuclear complexes of the bis-bidentate ligand2,5-di(2-pyridyl)-1,3,4-thiadiazole (dptd): spin crossover in [FeII(dptd)2(NCSe)2] and [FeII(dptd)2(NCBH3)2]·H2O

Abstract

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′,N¹)₂ coordination mode. Spin crossover behaviour has been found for [Fe^II(dptd)₂(NCSe)₂] (2) and [Fe^II(dptd)₂(NCBH₃)₂]·H₂O (3·H₂O) 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)₂L₂] 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·0.5H₂O (5·1.25DCM·0.5H₂O) 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-d₃ solution the latter two complexes, as well as [Fe^II(dptd)₃](ClO₄)₂·H₂O (6·H₂O), show counterion-independent stereoisomerisation, each yielding a ca. 3 : 1 mixture of mer and fac stereoisomers as identified by ¹H NMR spectroscopy.