LAUSR

The development of molecule-based switchable materials remains an important challenge in the field of molecular science. Achievement of a structural phase transition induced by adsorption/desorption of guest molecules in spin crossover (SCO) Co(II) compounds is of significant interest because of the possibility that the spin state of the magnetic anisotropic high-spin (HS, S = 3/2) and low-spin (LS, S = 1/2) states can be switched via the induced changes in associated intermolecular interactions. In this study, we demonstrated a reversible magnetic switching associated with spin state conversion, along with a single-crystal to single-crystal (SCSC) phase transition induced by dehydration/rehydration. [Co(terpy)2](BF4)2·H2O (1·H2O; terpy = 2,2':6',2''-terpyridine) assembles in the solid state via π-π and CH-π interactions involving adjacent terpyridine cores along the ab direction to form two-dimensional (2D) layered domains. 1·H2O exhibits gradual and incomplete SCO, from fully HS to ca. 0.5 HS, and the field-induced single-molecule magnet (SMM) behavior attributed to the presence of the anisotropic partial high-spin Co(II) species. 1·H2O undergoes a SCSC transformation accompanied by a change from the tetragonal space group I41/a to P42/n via a dehydration process. Dehydrated 1 exhibits a reverse thermal hysteresis behavior (T1/2↑ = 287 K; T1/2↓ = 270 K) in the gradual SCO region from fully HS to ca. 0.5 HS, followed by an ordinary thermal hysteresis (T'1/2↑ = 195 K; T'1/2↓ = 155 K) to fully LS Co(II). A temperature-dependent single-crystal X-ray structural analysis revealed that the reverse hysteresis can be attributed to an order/disorder structural phase transition of the BF4- anions involving a symmetry breaking to yield the monoclinic space group P21/n and orbital (angular momentum) transition (LT). Both the SCSC phase transition and magnetic behavior are switchable by dehydration/rehydration processes; thus 1 again adsorbs water at room temperature to give both the original structure and its magnetic behavior.