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

Abstract

The family of compounds of general formula [LnTM(OH)(L)(OCR)(MeOH)](ClO) {[GdZn(OH)(hmp)(OCPr)](ClO) (1a); [YZn(OH)(hmp)(OCPr)](ClO) (1b); [GdCu(OH)(hmp)(OCPr)](ClO) (2a); [YCu(OH)(hmp)(OCPr)](ClO) (2b); [GdCu(OH)(hep)(OCPr)](ClO) (3a); [GdCu(OH)(Hpdm)(OCBu)](ClO) (4a); [GdCu(OH)(ea)(OCMe)](ClO) (5a); [GdNi(OH)(hmp)(OCEt)(MeOH)](ClO) (6a); [YNi(OH)(hmp)(OCEt)(MeOH)](ClO) (6b); [GdCo(OH)(hmp)(OCEt)(MeOH)](ClO) (7a); [YCo(OH)(hmp)(OCEt)(MeOH)](ClO) (7b)} can be formed very simply and in high yields from the reaction of Ln(NO)·6HO and TM(ClO)·6HO and the appropriate ligand blend in a mixture of CHCl and MeOH in the presence of a suitable base. Remarkably, almost all the constituent parts, namely the lanthanide (or rare earth) ions Ln (here Ln = Gd or Y), the transition metal ions TM (here TM = Zn, Cu, Ni, Co), the bridging ligand L (Hhmp = 2-(hydroxymethyl)pyridine; Hhep = 2-(hydroxyethyl)pyridine; Hpdm = 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 [LnCu(OH)(hmp)(OCPr)](ClO). 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 (1a) with Cu (2a), Ni (6a) or Co (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 spins per molecule contribute to the magnetocaloric properties.

Further synthetic investigation of the general lanthanoid(iii) [Ln(iii)]/copper(ii)/pyridine-2,6-dimethanol/carboxylate reaction system: {CuII5LnIII4} coordination clusters (Ln = Dy, Tb, Ho) and their yttrium(iii) analogue

“…Most of the to-date reported 3d/4f-metal complexes containing the doubly ( pdm 2− ) or singly (Hpdm − ) form of H 2 pdm as ligand are Cu II /Ln III clusters. 51,52,63,64 A common feature of all such complexes is that they possess a secondary (ancillary) carboxylate ligand. The general Cu II /Ln III /H 2 pdm/RCO 2 − reaction system is very fertile, and the identity of products has been found to depend on a number of synthetic parameters, the most important of which is the nature of R. Using rather similar reaction conditions ( Yttrium has radii (atomic, metallic, ionic) that fall close to those of Er and Ho, and all of its chemistry is in the trivalent state.…”

“…Complexes 1-3 are new members of the small family of 3d/ 4f-metal clusters containing H 2 pdm and its anionic forms as ligands. 51,52,[60][61][62][63][64] The previously characterized compounds are conveniently summarized in Table 1, together with diagnostic structural and magnetic information. It is clear that the nuclearity, metallic skeleton and core are all unique in the clusters of the present work.…”

“…With the above results in mind, we continue working in the area of the chemistry and magnetism of Cu II /Ln III cluster chemistry using the H 2 pdm/RCO 2 − ligand "blend", paying more attention to the study of the influence of R on the chemical and structural identity of the products. Although Cu II /Ln III / pdm 2− or Hpdm − clusters with R = Ph, 51 Bu t , 63,64…”

“…On the contrary, their use in 3d/4fmetal chemistry has been limited. 51,52,[60][61][62][63][64] The tridentate pdm 2− anion is not a compartmental ligand. However, it provides a stable tridentate chelating O,N,O environment to Cu II (formation of two 5-membered chelating rings), while each of the deprotonated alkoxide O atoms can further bridge a Ln III centre (Scheme 1).…”

Spin-1/2 XXZ Heisenberg cupolae: magnetization process and related enhanced magnetocaloric effect

“…Among them there could be mentioned the magnetic cooling based on the magnetocaloric effect [28][29][30][31], which is especially pronounced in frustrated quantum spin systems [32]. The magnetocaloric effect has attracted much attention in the study of low-dimensional magnetic systems both from theoretical [33][34][35][36][37] and experimental [38,39] points of view due to the fact that an enhanced magnetocaloric effect represents the basis for economically cheaper and environmentally friendly refrigeration technology compared to standard refrigeration technology based on vapor expansion [40]. An enhanced magnetocaloric effect was found in the vicinity of magnetization jumps, around which the rapid change of temperature was observed upon the variation of the magnetic field during the process of adiabatic demagnetization.…”