Polynuclear Iron(II)–Aminotriazole Spincrossover Complexes (Polymers) In Solution

Polynuclear spincrossover (SCO) complexes
prepared by the combination of [Fe­(DMF)6]2+ and
NH2trz (NH2trz = 4-amino-1,2,4-triazole) were
studied (2ns– = counterion 2-naphthalenesulfonate).
It is demonstrated that these [Fe­(NH2trz)3]­(2ns)2 complexes can be dissolvedcontrary to common reported
experiencein N,N-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 NH2trz/Fe2+ ratio of 3/1,
corresponding to the stoichiometric ratio of the ideal coordination
compound, [Fe­(NH2trz)3]2+ 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 Fe2+ by an excess of the ligand.
The polymer therewith formed contains 100 or more Fe2+ ions
and is of a pronounced rigid-rod structure, with Fe–Fe distances
around 3.32 Å (in comparison to 3.94 Å of the polynuclear
species in the high-spin state). Reversible spin crossover takes place
in solution upon a temperature increase to around 60 °C; this
process is associated with a shift in equilibrium toward species shorter
than the initial polynuclear species.