Face-sharing heterotrinuclear M^II-Ln^III-M ^II (M = Mn, Fe, Co, Zn; Ln = La, Gd, Tb, Dy) complexes: Synthesis, structures, and magnetic properties

Trinuclear linear 3d-4f-3d complexes (3d = Mn^II, Fe ^II, Co^II, Zn^II and 4f = La^III, Gd^III, Tb^III, Dy^III) were prepared by using a tripodal nonadentate Schiff base ligand, N,N',N"-tris(2-hydroxy-3- methoxybenzilidene)-2-(aminomethyl)-2-methyl-1,3-propanediamine. The structural determinations showed that in these complexes two distorted trigonal prismatic transition metal complexes of identical chirality are assembled through 4f cations. The Mn and Fe entities crystallize in the chiral space group P2 ₁2₁2₁ as pure enantiomers; the cobalt complexes exhibit a less straightforward behavior. All Mn, Fe, and Co complexes experience M^II-Ln^III ferromagnetic interactions. The Mn-Gd interaction is weak (0.08 cm^-1) in comparison to the Fe-Gd (0.69 cm^-1) and Co-Gd (0.52 cm^-1) ones while the single ion zero field splitting (ZFS) term D is larger for the Fe complexes (5.7 cm ^-1) than for the cobalt ones. The cobalt complexes behave as single-molecules magnets (SMMs) with large magnetization hysteresis loops, as a consequence of the particularly slow magnetic relaxation characterizing these trinuclear molecules. Such large hysteresis loops, which are observed for the first time in Co-Ln complexes, confirm that quantum tunnelling of the magnetization does not operate in the Co-Gd-Co complex.