Thermodriven, Acidity-Driven, and Photodriven Spin-State Switching in Pyridylacylhydrazoneiron(II) Complexes at or above Room Temperature

The magnetic bistability of spin-crossover (SCO) materials is highly appealing for applications as molecular switches and information storage. However, switching of the spin state around room temperature remains challenging. In this work, we reported the successful manipulation of the spin states of two iron(II) complexes (1-Fe and 2-Fe) based on pyridylacylhydrazone ligands in manifold ways. Both complexes are stabilized in the low-spin (LS) state at room temperature because of the strong ligand-field strength imposed by the ligands. 2-Fe shows thermoinduced SCO above room temperature with a very large and reproducible hysteresis (>50 K), while 1-Fe remains in the LS state up to 400 K. Acidity-driven spin-state switching of the two complexes was achieved at room temperature as a result of the complex dissociation and release of iron(II) in its high-spin (HS) state. Recovery of the complex is feasible upon further alkalization treatment in the case of 1-Fe, allowing bidirectional modulation of the spin state of the metal center. Light-driven one-way switching from LS to HS is also achieved by virtue of E-to-Z isomerization at the C═N double bond, which results in dissociation of the complex because of the poor binding affinity in the Z configuration.