Lattice-Dictated Conformers in Bis(pyrazolyl)pyridine-Based Iron(II) Complexes: Mössbauer, NMR, and Magnetic Studies

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 P1̄, and
the third, with PF6- anion in Ibca space group. The FeL2 complex ions in all lattices have similarly distorted
octahedral geometry. Variable-temperature Mössbauer 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, Mössbauer parameters, and temperature dependence.
But the variable-temperature Mössbauer 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 formsa 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)).