Spin-Transition-Like Behavior on One Side in a Nitroxide-Copper(II)-Nitroxide Triad System

The ground spin-state of [Cu(phpyNO)₂(H₂O)₂](BF₄)₂ was switched between S_total = 1/2 and 3/2 across 175 K. On warming, the space group was changed from P2₁2₁2₁ to C222₁ in a single-crystal-to-single-crystal manner, and the transient structure could be monitored by means of the crystallographic analysis. The copper-radical exchange coupling changed from 2J/k_B = −463(3) to +312(6) K with rather small Cu−O−N−C_2py twisting deformation on one side, while practically no distortion occurred on the other.