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.