Crystal Design for Tuning the Mechanical Flexibilities of M(salophen) Complexes

Elastic crystals are a novel class of materials for use in optoelectronics and solar cells. Although controlling mechanical flexibility is critical when developing functional soft crystals, the number of reported metal-coordination-complex-based elastic crystals is limited. When developing such crystals, the flexibility of the crystals may be tuned via ligand substitution of the central metals or by exchange of the central metal ions; however, this has not been demonstrated experimentally to date. Thus, we herein report the preparation of mechanically flexible compounds using a synthetic procedure based on metal coordination complexes. It was found that the packing structure was considerably affected by the crystallization solvent, and control over the packing process was critical in obtaining complex molecules with designed flexibilities and structures. Furthermore, we demonstrate new Pd and Pt complexes with the salophen ligand, which exhibit the second and third highest elastic strains (ε values) reported to date, respectively, and the highest and second highest ε values (ε = 11.95% for Pd; 10.65% for Pt), respectively, among metal coordination complexes. These effects can be attributed to the microscopic changes in bond distance between the metals and ligands. Our findings are expected to inspire the tuning and development of metal-complex-based flexible crystals.