Easy Excited-State Trapping and Record High TTIESST in a Spin-Crossover Polyanionic FeII Trimer

Reaction of the polysulfonated triazole ligand L = 4-(1,2,4-triazol-4-yl)­ethane­disulfonate) with iron­(II) salts in water yields the trimeric species [Fe3­(μ-L)6­(H2O)6]6–. This polyanion, as the dimethyl­ammonium salt, shows a thermally induced spin transition above room temperature for the central Fe position in the trimer with a large hysteresis cycle (>85 K) and remarkably slow dynamics. This allows easy quenching of the metastable high-spin (HS) state via gradual cooling (5 K min–1). Once it is trapped, the HS state remains metastable. Thermal energy is not able to promote relaxation into the low-spin ground state below 215 K, with a characteristic TTIESST = 250 K, the highest temperature ever observed for thermal trapping of an excited spin state in a switchable molecular material.