On the controversial fitting of susceptibility curves of ferromagnetic Cu~(II) cubanes: Insights from theoretical calculations

摘要

This paper reports a theoretical analysis of the electronic structure and magnetic properties of a tetranuclear Cu~(II) complex, [Cu _4(HL)_4], which has a 4+2 cubane-like structure (H _3L=N,N′-(2-hydroxypropane-1,3-diyl)bis(acetylacetoneimine)). These theoretical calculations indicate a quintet (S=2) ground state; the energy-level distribution of the magnetic states confirm Heisenberg behaviour and correspond to an S_4 spin-spin interaction model. The dominant interaction is the ferromagnetic coupling between the pseudo-dimeric units (J_1=22.2 cm~(-1)), whilst a weak and ferromagnetic interaction is found within the pseudo-dimeric units (J_2=1.4 cm ~(-1)). The amplitude and sign of these interactions are consistent with the structure and arrangement of the magnetic Cu 3d orbitals; they accurately simulate the thermal dependence of magnetic susceptibility, but do not agree with the reported J values (J_1=38.4 cm~(-1), J _2=-18.0 cm~(-1)) that result from the experimental fitting. This result is not an isolated case; many other polynuclear systems, in particular 4+2 Cu~(II) cubanes, have been reported in which the fitted magnetic terms are not consistent with the geometrical features of the system. In this context, theoretical evaluation can be considered as a valuable tool in the interpretation of the macroscopic behaviour, thus providing clues for a rational and directed design of new materials with specific properties.