Crystal Structures, Magnetic Relaxation, Magnetic Refrigeration, Hirshfeld Surface Analysis, and Theoretical Calculations for Three High-Nuclear Co ^II ₆ Ln ^III ₄ Complexes (Ln = Gd, Dy, and Er)

Magnetic coupling is one of the most critical factors that reduce the magnetic relaxation of single-molecule magnets (SMMs) and the magnetocaloric effect (MCE) of molecular refrigerants, and studying the magnetic couplings within high nuclearity is a huge challenge. In this work, three novel high-nuclear CoII6LnIII4 complexes with the molecular formula of [Co6Ln4(HL)4(pdmH)2(pdm)8Cl2]·xCH3CN·yH2O (Ln = Gd (1Gd), x = 3, y = 9; Ln = Dy (2Dy), x = 0, y = 8; Ln = Er (3Er), x = 1, y = 6) have been synthesized by employing 2-[[2-(2-hydroxy-ethoxy)-ethylimino]-methyl]-6-methoxyphenol (H2L) and 2,6-pyridinedimethanol (pdmH2) to react with Ln(NO3)3·6H2O and CoCl2·6H2O in the CH3CN solution containing triethylamine. Structural analysis revealed that 1Gd, 2Dy, and 3Er are all isostructural, and the cores of the three complexes can be viewed as two Co3Ln2O8 subunits linked by two alkoxido-type O atoms from two pdm2– ligands. In each subunit, three CoII atoms, two Ln atoms, and eight O atoms form three defective cubanes. Magnetic studies indicate that only 1Gd and 2Dy show slow magnetic relaxation behavior under a zero dc field. However, complex 3Er cannot exhibit any out-of-phase (χ″) ac susceptibility signals. The discrepancies of the temperature-dependent χ″ signals of 1Gd, 2Dy, and 3Er revealed that their slow magnetic relaxation behavior would decrease from oblate DyIII to prolate ErIII. Additionally, 1Gd exhibits the MCE with a magnetic entropy change −ΔSm of 26.68 J kg–1 K–1 at ΔH = 7 T and T = 3.0 K, to our knowledge, which is high among the reported polynuclear complexes. The direct current (dc) data of the three complexes have been simultaneously fitted well by POLY_ANISO calculations. Moreover, the anisotropies of Co3 and Dy2, larger than Co2 and Dy1 from the ab initio calculations, are almost consistent with the results from the Hirshfeld surface analysis.