New Candy-Shaped Hexanuclear Co^II₂Co^III₂Gd₂ Aggregate: Synthesis, Structures, and Magnetic Performance for Magnetocaloric Effect and Theoretical Understanding

Abstract

The hexanuclear cobalt(II/III)−gadolinium(III) c o o r d i n a t i o n a g g r e g a t e [ C o I I 2 C o I I I 2 G d 2 ( L ) 2 ( μ 3 -OMe) 2 (Me 3 CCOO) 8 ]•H 2 O (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 (H 3 L) toward cobalt and gadolinium ions was scrutinized during a coordination-driven cleavage of [Co 2 (μ-OH 2 )(O 2 CCMe 3 ) 4 (HO 2 CCMe 3 ) 4 ] (Co 2 -Piv) by Gd(NO 3 ) 3 •6H 2 O in methanol and a characteristic rearrangement of pivalate ions from Co 2 -Piv only. Thus, atom economy was preserved with respect to the pivalate ions as no outside addition was needed. The Co 2 Gd 2 dicubane core of the aggregate was diagonally capped by two distorted trigonal bipyramid Co II centers (τ 5 value of 0.617) to provide a unique candy-shaped Co II 2 Co III 2 Gd 2 complex. Complex 1 behaves like a paramagnet comprising weak antiferromagnetic exchange between adjacent Co II and Gd III 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 (−ΔS m ) 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.