Lattice-dictated conformers in bis(pyrazolyl)pyridine-based iron(II) complexes: Mossbauer, NMR, and magnetic studies

Abstract

Iron(II) complexes [FeL₂](ClO₄)₂·CH₃CN, [FeL₂](BPh₄)₂·2CH₃CN, and [FeL₂](PF₆)₂ with an FeN₆ chromophore of the same ligand L (2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine) and differing counterions have been made and their crystal and molecular structures determined. The first two crystallized in triclinic space group P↑, and the third, with PF₆- anion in Ibca space group. The FeL₂ complex ions in all lattices have similarly distorted octahedral geometry. Variable-temperature Mossbauer spectra of [FeL₂](ClO₄)₂·CH₃CN and [FeL₂](PF₆)₂ measured in the temperature range 1.7-300 K reveal temperature-dependent populations of two different spin states with increased amount of low-spin form at high temperatures, a phenomenon unlike the normal spin crossover behavior; this abnormal behavior is interpreted here as due to the presence of two different conformations. It is very interesting to note that the two different compounds have similar spectra, Mossbauer parameters, and temperature dependence. But the variable-temperature Mossbauer spectra of [FeL₂](BPh₄)₂·2CH₃CN in the range 20-300 K do not show the presence of such different species but exhibit a clear phase transition at 200 K. This phase transition is further supported by SQUID measurements. The results of variable-temperature ¹H NMR in CD₃CN and the solution susceptibility measurement of all complexes also support the presence of high-spin and low-spin forms in solution. Hence, the complex ion [FeL₂]²⁺ exhibits a thermally driven interconversion between low-spin and a high-spin structural formsa phenomenon observed in the solid and solution states due to ligand dynamics. This is not due to the well-known spin crossover phenomenon. These results are compared with the case of normal spin crossover seen in [FeL'₂](ClO₄)2 (L' = 2,6-(bis(pyrazol-1-ylmethyl)pyridine)).