Self-assembly of tetra-nuclear lanthanide clusters via atmospheric CO2 fixation: interesting solvent-induced structures and magnetic relaxation conversions

Abstract

Two systems of carbonate complexes were constructed by employing a newly prepared Schiff based ligand (H₂L; H₂L = 2-(hydroxyimino)-2-[(3-methoxyl-2-hydroxyphenyl)methylene]hydrazide) and Ln(acac)₃·H₂O (Hacac = acetylacetone) in different reaction solutions, namely [Ln₄(CO₃)(L)₄(acac)₂(H₂O)₄]·2CH₃CN (Ln = Gd(1), Dy(2)) and [Ln₄(CO₃)(L)₄(acac)₂(CH₃OH)₂(H₂O)₂]·CH₃OH·H₂O (Ln = Gd(3), Dy(4)). Interestingly, complexes 1–4 are nearly structurally identical except for the coordinated solvent molecules. All of them consist of a Ln₄ cluster in the center, in which four Ln(III) ions are bridged by two μ₂-O atoms from the in situ formed CO₃²⁻ through spontaneous fixation of atmospheric CO₂. Magnetic investigation suggests that complexes 1 and 3 display magnetic refrigeration behaviors with maximum values of magnetic entropy changes of 31.23 J kg⁻¹ K⁻¹ and 27.06 J kg⁻¹ K⁻¹, respectively. Additionally, the energy barriers of the magnetization reversal were significantly improved from 2.73 K for 2 to 23.79 K for 4 by deliberately using methanol to replace the coordinated H₂O molecule. It should be noted that the divergence in structures and magnetic properties could be mainly ascribed to the different solvent effects in the synthesis process.