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|Title:||Lattice-dictated conformers in bis(pyrazolyl)pyridine-based iron(II)complexes: M�ssbauer, NMR, and magnetic studies|
Justin Thomas, K.R.
|Keywords:||2,6 bis(3,5 dimethylpyrazol 1 ylmethyl)pyridine|
nuclear magnetic resonance spectroscopy
proton nuclear magnetic resonance
|Citation:||Inorganic Chemistry, 40(27), 6930-6939|
|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 1, and the third, with PF6- anion in Ibca space group. The FeL2 complex ions in all lattices have similarly distorted octahedral geometry. Variable-temperature M�ssbauer 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, M�ssbauer parameters, and temperature dependence. But the variable-temperature M�ssbauer spectra of [FeL2](Bob4)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](C1O4)2 (L? = 2,6-(bis(pyrazol-1-ylmethyl)pyridine)).|
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