Structural basis for the phase switching of bisaminecopper(II) cations at the thermal limits of lattice stability

The structural grounds of the decrease of point and lattice symmetries coupled with switching of the exchange interaction in single crystals of a highly strained, coordinationally unsaturated bisdiaminecopper(II) cation are described. The combined magnetic susceptibility and X-ray diffraction results indicate that the interplay between the inherent vibronic instability and ligand-field strain imposed by moderately flexible, coordinationally shielding ligands enables effective switching of the pseudo-Jahn-Teller d⁹ centers between states with different exchange interaction in the low-temperature regime and valence orbital orientation and coordination geometry in the high-temperature regime. Within the low-temperature hysteresis region, the phase transition can also be induced by excitation of the ligand-to-metal charge-transfer bands, resulting in overall shrinkage of the lattice. The compound is a prototype of weakly electronically coupled one-dimensional Jahn-Teller systems, which can undergo phase transitions induced by light, in addition to heating, cooling, and change of pressure, and it represents a prospective basis for the design of switching materials capable of multimode external control.