Microscopic Approach to the Problem of Cooperative Spin Crossover in Polynuclear Cluster Compounds. Application to Tetranuclear Iron(II) Square Complexes

A new microscopic approach to the problem of cooperative spin crossover in molecular crystals containing polynuclear complexes as structural units has been developed. The cooperative intercluster interaction in the crystal is shown to arise from the coupling of the molecular modes to the acoustic phonons. The interaction between ions, which belong to the same cluster, is also taken into account, that leads to interesting features in the temperature dependence of the magnetic susceptibility, which are not observed in mononuclear molecular crystals. The developed general approach is adapted to the case of tetranuclear square complexes. In the latter, the effect of intracluster interactions on the type of the spin transition is analyzed. The applicability of the developed approach is illustrated by the interpretation of the experimental data on the spin transition in the tetranuclear [Fe(tpa){N(CN)(2)}(4)center dot(BF4)(4)center dot(H2O)(2) cluster compound. For this compound, the theory well reproduces the observed two-step spin transition with the plateau between the steps that corresponds to the cluster configuration with two high-spin Fe(II) ions located along the diagonal of the square.