A {Cr₄Ln₂} Complex with Exchange Coupled {Cr₂} Units: Structural Description and Magnetic Study

We have prepared and structurally characterized pivalate based {Cr^III ₄Ln^III ₂} complexes with Ln = Dy and Gd as well as the Y analogue, with the overall formula [Cr^III ₄Ln^III ₂(mdea)₂(piv)₁₀(OH)₄], Ln = Gd, Dy and Y. We are reporting a detailed experimental magnetic properties study, including magnetization relaxation dynamics and calorimetric data, supported with quantum chemical calculations. The synthesis of the Y derivative, allowed to precisely identify the Cr(III)-Cr(III) exchange interaction magnitude which proved moderately strong and in agreement with known magneto-structural correlations. This result agrees with the observed double bridged {Cr₂-μOH-μOR} units within the {Cr₄Ln₂} complexes. The Gd(III) complex magnetic properties can be properly described with the already established exchange coupled {Cr₂} units, and a unique Gd(III)-Cr(III) exchange coupling parameter which proved anti-ferromagnetic in nature. The MCE characterization of this complex based on magnetization and calorimetric data down to 2 K affords a moderate entropy change value, mainly affected by the strong Cr-Cr exchange interaction. The complex with Ln = Dy, showed SMM behaviour below 10 K under 0 DC applied field with negligible field dependence up to 3000 Oe and 10 kHz of AC field frequency. Two relaxation processes are clearly distinguished, with thermal barriers for an Orbach mechanism of ca. 20 cm^-1 and 40 cm^-1.