Bi-stable spin-crossover in charge-neutral [Fe(R-ptp)2] (ptp = 2-(1H-pyrazol-1-yl)-6-(1H-tetrazol-5-yl)pyridine) complexes

Bi-stable charge-neutral iron(II) spin-crossover (SCO) complexes are a class of switchable molecular materials proposed for molecule-based switching and memory applications. In this study, we report on the SCO behavior of a series of iron(II) complexes composed of rationally designed 2-(1H-pyrazol-1-yl)-6-(1H-tetrazol-5-yl)pyridine (ptp) ligands. The powder forms of [Fe2+(R-ptp−)2]0 complexes tethered with less-bulky substituents—R = H (1), R = CH2OH (2), and R = COOCH3 (3; previously reported)—at the 4-position of the pyridine ring of the ptp skeleton showed abrupt and hysteretic SCO at or above room temperature (RT), whereas complex 5 featuring a bulky pyrene substituent showed incomplete and gradual SCO behavior. The role of intermolecular interactions, lattice solvent, and electronic nature of the chemical substituents (R) in tuning the SCO of the complexes is elucidated.