New Candy-Shaped Hexanuclear CoII2CoIII2Gd2 Aggregate: Synthesis, Structures, and Magnetic Performance for Magnetocaloric Effect and Theoretical Understanding

The hexanuclear cobalt(II/III)–gadolinium(III) coordination aggregate [CoII2CoIII2Gd2(L)2(μ3-OMe)2(Me3CCOO)8]·H2O (1) has been synthesized and characterized, and structural analysis has been performed via single-crystal X-ray diffraction analysis. The bridging and aggregating potential of the deprotonated form of 3-[(2-hydroxy-3-methoxy-benzylidene)-amino]-propane-1,2-diol (H3L) toward cobalt and gadolinium ions was scrutinized during a coordination-driven cleavage of [Co2(μ-OH2)(O2CCMe3)4(HO2CCMe3)4] (Co2-Piv) by Gd(NO3)3·6H2O in methanol and a characteristic rearrangement of pivalate ions from Co2-Piv only. Thus, atom economy was preserved with respect to the pivalate ions as no outside addition was needed. The Co2Gd2 dicubane core of the aggregate was diagonally capped by two distorted trigonal bipyramid CoII centers (τ5 value of 0.617) to provide a unique candy-shaped CoII2CoIII2Gd2 complex. Complex 1 behaves like a paramagnet comprising weak antiferromagnetic exchange between adjacent CoII and GdIII ions. The magnetization changes significantly below 10 K depending on the magnetic field and temperature. Complex 1 shows a magnetocaloric effect with the maximum entropy change (−ΔSm) of 12.75 J kg–1 K–1 at 3 K for ΔH = 70 kOe. DFT- and CASSCF-based analyses impart detailed insights into the electronic structure and observed magnetic properties. Theoretical calculations corroborate the experimental findings.