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Polynuclear Iron(II)āAminotriazole Spincrossover Complexes (Polymers) In Solution
Polynuclear spincrossover (SCO) complexes
prepared by the combination of [FeĀ(DMF)<sub>6</sub>]<sup>2+</sup> and
NH<sub>2</sub>trz (NH<sub>2</sub>trz = 4-amino-1,2,4-triazole) were
studied (2ns<sup>ā</sup> = counterion 2-naphthalenesulfonate).
It is demonstrated that these [FeĀ(NH<sub>2</sub>trz)<sub>3</sub>]Ā(2ns)<sub>2</sub> complexes can be dissolvedīøcontrary to common reported
experienceīøin <i>N</i>,<i>N</i>-dimethylformamide
(DMF) and, therefore, can be conveniently processed by simple means.
The resulting solutions were examined with UV/vis and X-ray absorption
spectroscopy (XANES and EXAFS) as well as with small-angle X-ray scattering
(SAXS). At a molar NH<sub>2</sub>trz/Fe<sup>2+</sup> ratio of 3/1,
corresponding to the stoichiometric ratio of the ideal coordination
compound, [FeĀ(NH<sub>2</sub>trz)<sub>3</sub>]<sup>2+</sup> in the
low-spin state was found to be in equilibrium with polynuclear species
in the high-spin state. The equilibrium can be shifted virtually completely
to the side of low-spin Fe<sup>2+</sup> by an excess of the ligand.
The polymer therewith formed contains 100 or more Fe<sup>2+</sup> ions
and is of a pronounced rigid-rod structure, with FeāFe distances
around 3.32 Ć
(in comparison to 3.94 Ć
of the polynuclear
species in the high-spin state). Reversible spin crossover takes place
in solution upon a temperature increase to around 60 Ā°C; this
process is associated with a shift in equilibrium toward species shorter
than the initial polynuclear species