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}₂M(H₂L^N3O2R)] (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≡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(H₂L^N3O2R)]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≡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.