Synthesis and characterisation of new ditetrazole-ligands as more rigid building blocks of envisaged iron(II) spin-crossover coordination polymers

As the ligand system plays the most important role in the behaviour of the spin-transition of iron(II) spin-crossover compounds a series of eight new mainly bridging di-tetrazole ligands were synthesised and produced new insights into spacer modifications as well as geometric prerequisites of the ligand and their impact on spin-crossover behaviour. The focus laid on aryl-spaced tetrazole ligands, which were interesting analogues to the well-known alkyl-di-tetrazoles due to expected enhanced interaction within the molecular structure through π-π-stacking. The results of this fundamental study yielded further guidelines to optimize and fine-tune the ligand design, which are envisaged to be used for spin-crossover iron(II) coordination polymers of high T½-values with abrupt spin transition behaviour. Additionally, one new SCO compound [μ-Tris(1-[1,1-dimethyl-2-(1H-tetrazol-1-yl)ethyl]-1H-tetrazole-N4,N4′)iron(II)] bis(tetrafluoroborate)-[Fe(dtmp)3](BF4)2-is presented. The compound features a spin transition around 160 K with a small thermal hysteresis of 5 K.