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 (T 1/2↑ = 287 K; T 1/2↓ = 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.

Ferroelectric spin crossover (SCO) behavior is demonstrated to occur in the cobalt(II) complex, [Co(FPhterpy) 2 ](BPh 4 ) 2 •3ac (1•3 ac; FPh-terpy = 4'-((3-fluorophenyl)ethynyl)-2,2':6',2''-terpyridine) and is dependent on the degree of 1808 flip-flop motion of the ligands polar fluorophenyl ring. Single crystal X-ray structures at several temperatures confirmed the flip-flop motion of fluorobenzene ring and also gave evidence for the SCO behavior with the latter behavior also confirmed by magnetic susceptibility measurements. The molecular motion of the fluorobenzene ring was also revealed using solid-state 19 F NMR spectroscopy. Thus the SCO behavior is accompanied by the flip-flop motion of the fluorobenzene ring, leading to destabilization of the low spin cobalt(II) state; with the magnitude of rotation able to be controlled by an electric field. This first example of spin-state conversion being dependent on the molecular motion of a ligand-appended fluorobenzene ring in a SCO cobalt(II) compound provides new insight for the design of a new category of molecule-based magnetoelectric materials.

We report for the first time ferroelectricity induced by spin transition associated with a crystal – liquid crystal phase transition in achiral spin crossover molecules.

Magnetoelectric (ME) materials exhibiting coupled electric and magnetic properties are of significant interest because of their potential use in memory storage devices, new sensors, or low‐consumption devices. Herein, we report a new category of ME material that shows liquid crystal (LC), ferroelectric (FE), and field‐induced single molecule magnet (SMM) behaviors. Co(II) complex incorporating alkyl chains of type [Co(3C16‐bzimpy)2](BF4)2 (1; 3C16‐bzimpy=2,2’‐(4‐hexadecyloxy‐2,6‐diyl)bis(1‐hexadecyl‐1H‐benzo[d]imidazole)) displayed a chiral smectic C mesophase in the temperature range 321 K–458 K, in which distinct FE behavior was observed, with a remnant polarization (88.3 nC cm−2). Complex 1 also exhibited field‐induced slow magnetic relaxation behavior that reflects the large magnetic anisotropy of the Co(II) center. Furthermore, the dielectric property of 1 was able to be tuned by an external magnetic field occurring from both spin‐lattice coupling and molecular orientational variation. Clearly, this multifunctional compound, combining LC, FE, and SMM properties, represents an entry to the development of a range of next‐generation ME materials.