Temperature dependence of solid state 1H NMR line shapes and M2 in polycrystalline BF4− salts of 1-propyltetrazole complexes of iron(II) and zinc(II)

Abstract

1H NMR spectra of the spin-crossover compound [Fe(ptz)6](BF4)2 and its zinc(II) analogue were recorded on polycrystalline samples as the function of decreasing temperature in order to study the connection of the change in the electronic state and the molecular motions present in both crystals. The temperature range for the zinc compound was 295–30K, with a cooling rate of ∼1Kmin−1, while in the case of the iron(II) compound this temperature range was 298–10K with the same cooling rate as in the case of [Zn(ptz)6](BF4)2. The analysis of the temperature dependence of the line shape parameters and calculations of 1H second moments showed the following. The temperature of the spin transition in the iron(II) compound is 132K on cooling. On the basis of the similarity of the temperatures at which the different molecular motions appear, the same motions are present in both compounds in the whole temperature range. The [Zn(ptz)6](BF4)2 probably goes through a structural transition between 90 and 60K, this transition manifests in the virtual increase of the average proton–proton distance, and the sign of a similar transition is also observed in [Fe(ptz)6](BF4)2.