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/Fe(2+) 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 Fe(2+) by an excess of the ligand. The polymer therewith formed contains 100 or more Fe(2+) 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.