Interplay between lattice, orbital, and magnetic degrees of freedom in the chain-polymer Cu(II) breathing crystals

The chain-polymer Cu(II) “breathing crystals” C${}_{21}$H${}_{19}$CuF${}_{12}$N${}_4$O${}_6$ were studied using the x-ray diffraction and ab initio band structure calculations. We show that the crystal structure modification at $T=146$ K, associated with the spin crossover transition, induces the changes of the orbital order in half of the Cu sites. This in turn results in the switch of the magnetic interaction sign in accordance with the Goodenough-Kanamori-Andersen theory of the coupling between the orbital and spin degrees of freedom.