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Title: Lattice-dictated conformers in bis(pyrazolyl)pyridine-based iron(II)complexes: M�ssbauer, NMR, and magnetic studies
Authors: Manikandan, P.
Padmakumar, K.
Justin Thomas, K.R.
Varghese, B.
Onodera, H.
Manoharan, P.T.
Keywords: 2,6 bis(3,5 dimethylpyrazol 1 ylmethyl)pyridine
iron complex
pyridine derivative
unclassified drug
chemical structure
complex formation
conformational transition
crystal structure
molecular dynamics
Mossbauer spectroscopy
nuclear magnetic resonance spectroscopy
proton nuclear magnetic resonance
temperature dependence
Issue Date: 2001
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)).
ISSN: 201669
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