Trinuclear Cyanido-Bridged [Cr2Fe] Complexes: To Be or not to Be a Single-Molecule Magnet, a Matter of Straightness

Trinuclear systems of formula [{Cr(L-N3O2Ph)(CN)(2)}(2)M(H2LN3O2R)] (M=Mn-II and Fe-II, L-N3O2R stands for pentadentate ligands) were prepared in order to assess the influence of the bending of the apical M-N equivalent to C linkages on the magnetic anisotropy of the Fe-II derivatives and in turn on their Single-Molecule Magnet (SMM) behaviors. The cyanido-bridged [Cr2M] derivatives were obtained by assembling trans-dicyanido Cr-III complex [Cr(L-N3O2Ph)(CN)(2)](-) and divalent pentagonal bipyramid complexes [M-II(H2LN3O2R)](2+) with various R substituents (R=NH2, cyclohexyl, S,S-mandelic) imparting different steric demand to the central moiety of the complexes. A comparative examination of the structural and magnetic properties showed an obvious effect of the deviation from straightness of the M-N equivalent to C alignment on the slow relaxation of the magnetization exhibited by the [Cr2Fe] complexes. Theoretical calculations have highlighted important effects of the bending of the apical C-N-Fe linkages on both the magnetic anisotropy of the Fe-II center and the exchange interactions with the Cr-III units.