From field-induced to zero-field SMMs associated with open/closed structures of bis(ZnDy) tetranuclear complexes: a...

We have prepared a bis(compartmental) Mannich base ligand H₄L (1,4,8,11-tetraaza-1,4,8,11-tetrakis(2-hydroxy-3-methoxy-5-methylbenzyl)cyclotetradecane) specifically designed to obtain bis(TM^IILn^III) tetranuclear complexes (TM = transition metal). In this regard, we have succeeded in obtaining three new complexes of the formula [Zn₂(μ-L)(μ-OAc)Dy₂(NO₃)₂]·[Zn₂(μ-L)(μ-OAc)Dy₂(NO₃)(OAc)]·4CHCl₃·2MeOH (1) and [TM₂(μ-H₂L)₂(μ-succinate)Ln₂(NO₃)₂] (NO₃)₂·2H₂O·6MeOH (TM^II = Zn, Ln^III = Dy (2); TM^II = Co, Ln^III = Dy (3)). Compound 1 contains two different bis(ZnDy) tetranuclear molecules that cocrystallize in the structure, in which acetato bridging ligands connect the Zn^II and Dy^III ions within each ZnDy subunit. This compound does not exhibit slow magnetic relaxation at zero field, but it is activated in the presence of an applied dc magnetic field and/or by Dy/Y magnetic dilution, showing two relaxation processes corresponding to each of the two different bis(ZnDy) units found in the structure. As revealed by the theoretical calculations, magnetic relaxation in 1 is single-ion in origin and takes place through the first excited state of each Dy^III ion. When using the succinato dicarboxylate bridging ligand instead of acetate, compounds 2 and 3 were serendipitously formed, which have a closed structure with the succinate anion bridging two ZnDy subunits belonging to two different ligands. It should be noted that only compound 2 exhibits slow relaxation of magnetization in the absence of an external magnetic field. According to experimental and theoretical data, 2 relaxes through the second excited Kramers doublet (U_eff = 342 K). In contrast, 3 displays field-induced SMM behaviour (U_eff = 203 K). However, the Co/Zn diluted version of this compound 3Zn shows slow relaxation at zero field (U_eff = 347 K). Ab initio theoretical calculations clearly show that the weak ferromagnetic coupling between Co^II and Dy^III ions is at the origin of the lack of slow relaxation of this compound at zero field. Compound 2 and its diluted analogues 2Y and 3Zn show hysteresis loops at very low temperature, thus confirming their SMM behaviour. Finally, compounds 1 and 2 show Dy^III based emission even at room temperature that, in the case of 2, allows us to extract the splitting of the ground ⁶H_15/2 term, which matches reasonably well with theoretical calculations.