RESUMO
Phase transitions in molecular solids involve synergistic changes in chemical and electronic structures, leading to diversification in physical and chemical properties. Despite the pivotal role of hydrogen bonds (H-bonds) in many phase-transition materials, it is rare and challenging to chemically regulate the dynamics and to elucidate the structure-property relationship. Here, four high-spin CoII compounds were isolated and systematically investigated by modifying the ligand terminal groups (X=S, Se) and substituents (Y=Cl, Br). S-Cl and Se-Br undergo a reversible structural phase transition near room temperature, triggering the rotation of 15-crown-5 guests and the swing between syn- and anti-conformation of NCX- ligands, accompanied by switchable magnetism. Conversely, S-Br and Se-Cl retain stability in ordered and disordered phases, respectively. H-bonds geometric analysis and ab initio calculations reveal that the electronegativity of X and Y affects the strength of NY-ap-Hâ â â X interactions. Entropy-driven structural phase transitions occur when the H-bond strength is appropriate; otherwise, the phase stays unchanged if it is too strong or weak. This work highlights a phase transition driven by H-bond strength complementarity - pairing strong acceptor with weak donor and vice versa, which offers a straightforward and effective approach for designing phase-transition molecular solids from a chemical perspective.
RESUMO
Introducing magnetic switchability into artificial molecular machines is fascinating for precise control of magnetism via external stimuli. Herein, a field-induced CoII single-molecule magnet was found to exhibit the reversible switch of Jahn-Teller distortion near room temperature, along with thermal conformational motion of the 18-crown-6 rotor, which pulls the coordinated H2 O to rotate through intermolecular hydrogen bonds and triggers a single-crystal-to-single-crystal phase transition with Twarm =282â K and Tcool =276â K. Interestingly, the molecular magnetic anisotropy probed by single-crystal angular-resolved magnetometry revealed the reorientation of easy axis by 14.6°. Moreover, ON/OFF negative magnetodielectric effects were respectively observed in the high-/low-temperature phase, which manifests the spin-lattice interaction in the high-temperature phase could be stronger, in accompanied by the hydrogen bonding between the rotating 18-crown-6 and the coordinated H2 O.