Extended NMR Study of Spin-Crossover Compounds [Fe(1-alkyl-1H-tetrazole)6](BF4)2 and Their ZnII Analogs

Abstract

A systematic NMR study was performed on several alkyl–tetrazole complexes of iron(II) and zinc(II) in the 10–300 K temperature range. The experiments were designed to separate the electronic and reorientational phase transitions caused by the spin crossover of the iron compounds from those independent of unpaired electrons. The 19F spectral data on the propyl-tetrazole compounds show that the electronic spin-transition has a strong effect on the spectra, and their behavior can be explained as a combined response to molecular reorientations and the spin transition. For these complexes, second-moment calculations revealed the strength of the interaction between resonant and nonresonant nuclei. Both of the applied NMR methods show irregularities at the temperature region between 70 and 120 K, suggesting the presence of a phase transition. The data also suggest two kinds of reorientational behavior for the BF4 − counter ions. In the iron–ethyl–tetrazole compound, unlike in the propyl–tetrazole complex, a significant amount of unpaired electrons remains in their original high-temperature HS state. Above their effect, the behavior of the nuclear spins of the iron compound is basically governed by the same structural factors as in its zinc analog. The two-exponential behavior of the 1H-T 1 in case of the zinc–methyl–tetrazole compound can be explained on the basis of cross relaxation with the 19F nuclei due to the low 1H/19F ratio. The presence of the two types of methyl reorientation is assumed to be the sign of the two different lattice sites known to be present in the FeII compound. The single-exponential T 1 above T c in the case of [Fe(mtz)6](BF4)2 is consistently the sign of the strength of the paramagnetic relaxation observed in the ethyl and propyl compounds.