Structurally Flexible and Solution Stable [Ln₄TM₈(OH)₈(L)₈(O₂CR)₈(MeOH)_y](ClO₄)₄: A Playground for Magnetic Refrigeration

The family of compounds of general formula [Ln^III ₄TM^II ₈(OH)₈(L)₈(O₂CR)₈(MeOH)_y](ClO₄)₄ {[Gd₄Zn₈(OH)₈(hmp)₈(O₂CⁱPr)₈](ClO₄)₄ (1a); [Y₄Zn₈(OH)₈(hmp)₈(O₂CⁱPr)₈](ClO₄)₄ (1b); [Gd₄Cu₈(OH)₈(hmp)₈(O₂CⁱPr)₈](ClO₄)₄ (2a); [Y₄Cu₈(OH)₈(hmp)₈(O₂CⁱPr)₈](ClO₄)₄ (2b); [Gd₄Cu₈(OH)₈(hep)₈(O₂CⁱPr)₈](ClO₄)₄ (3a); [Gd₄Cu₈(OH)₈(Hpdm)₈(O₂C^tBu)₈](ClO₄)₄ (4a); [Gd₄Cu₈(OH)₈(ea)₈(O₂CMe)₈](ClO₄)₄ (5a); [Gd₄Ni₈(OH)₈(hmp)₈(O₂CEt)₈(MeOH)₆](ClO₄)₄ (6a); [Y₄Ni₈(OH)₈(hmp)₈(O₂CEt)₈(MeOH)₆](ClO₄)₄ (6b); [Gd₄Co₈(OH)₈(hmp)₈(O₂CEt)₈(MeOH)₆](ClO₄)₄ (7a); [Y₄Co₈(OH)₈(hmp)₈(O₂CEt)₈(MeOH)₆](ClO₄)₄ (7b)} can be formed very simply and in high yields from the reaction of Ln(NO₃)₃·6H₂O and TM(ClO₄)₂·6H₂O and the appropriate ligand blend in a mixture of CH₂Cl₂ and MeOH in the presence of a suitable base. Remarkably, almost all the constituent parts, namely the lanthanide (or rare earth) ions Ln^III (here Ln = Gd or Y), the transition metal ions TM^II (here TM = Zn, Cu, Ni, Co), the bridging ligand L (Hhmp = 2-(hydroxymethyl)pyridine; Hhep = 2-(hydroxyethyl)pyridine; H₂pdm = pyridine-2,6-dimethanol; Hea = 2-ethanolamine), and the carboxylates can be exchanged while maintaining the structural integrity of the molecule. NMR spectroscopy of diamagnetic complex 1b reveals the complex to be fully intact in solution with all signals from the hydroxide, ligand L, and the carboxylates equivalent on the NMR time scale, suggesting the complex possesses greater symmetry in solution than in the solid state. High resolution nano-ESI mass spectrometry on dichloromethane solutions of 2a and 2b shows both complexes are present in two charge states with little fragmentation; with the most intense peak in each spectrum corresponding to [Ln₄Cu₈(OH)₈(hmp)₈(O₂CⁱPr)₈](ClO₄)₂ ²⁺. This family of compounds offers an excellent playground for probing how the magnetocaloric effect evolves by introducing either antiferromagnetic or ferromagnetic interactions, or magnetic anisotropy, by substituting the nonmagnetic Zn^II (1a) with Cu^II (2a), Ni^II (6a) or Co^II (7a), respectively. The largest magnetocaloric effect is found for the ferromagnetically coupled complex 6a, while the predominant antiferromagnetic interactions in 2a yield an inverse magnetocaloric effect; that is, the temperature increases on lowering the applied field, under the proper experimental conditions. In spite of increasing the magnetic density by adding ions that bring in antiferromagnetic interactions (2a) or magnetic anisotropy (7a), the magnetocaloric effect is overall smaller in 2a and 7a than in 1a, where only four Gd^III spins per molecule contribute to the magnetocaloric properties.