Exploring Magnetocaloric Effects and Magnetic Properties in Lanthanide-Organic Frameworks

In this thesis the synthesis and characterisation of novel lanthanide-organic frameworks are investigated, with a particular focus on their magnetocaloric effect (MCE) and single-molecule magnet (SMM) behaviour. By utilising lanthanide ions in both Ln(II) and Ln(III) oxidation states, this research aims to develop advanced materials with adjustable magnetic and electronic properties suitable for applications in very-low temperature refrigeration, information storage, and molecular spintronics. To achieve the desired properties, the materials are designed to feature triangular motifs in both discrete complexes and extended systems, employing either ditopic N-donor ligands, tert-butoxide ligands or both.

The initial chapter offers a comprehensive introduction to magnetic refrigeration and the design principles for optimal materials, focusing on inducing magnetic frustration through the arrangement of metal ions in triangular motifs with organic bridging ligands.

Chapter 2 presents the novel 2D coordination networks formed by the self-assembly reaction of Eu(II) with pyrazine (pyz) and 4,4’-bipyridine (bipy). These structures feature Eu(II) ions in a distorted elongated triangular Archimedean tessellation network with intra-triangle antiferromagnetic interactions. Despite the absence of long-range magnetic ordering down to 0.5 K, these materials do not exhibit clear signs of magnetic frustration and show only a moderate MCE at large magnetic field changes.

Chapter 3 discusses the noteworthy discovery of the frustrated 2D triangular coordination network BaI₂(pyz)_6/2, the only simple structure of its type, which maintains its structure with up to 92 % paramagnetic Eu(II) doping. This material exhibits exceptionally long Eu—N bonds, promoting weak exchange interactions that result in a substantial MCE at 0.4 K. Its effective performance at low magnetic fields positions it as a promising candidate for low-temperature magnetic refrigeration, with no magnetic ordering observed down to 0.3 K. The successful synthesis and characterisation of these 2D networks not only pave the way for advanced cooling technologies but also open new avenues for exploring frustrated magnetism and its quantum properties.

Chapter 4 investigates molecular triangular lanthanide motifs based on Ln(III) ions and tert-butoxide ligands, yielding the molecular complexes [Ln₃(OtBu)₈(MeCN)₃][BPh₄]·xMeCN (Ln = Gd(III), Tb(III), Dy(III), Ho(III)) with short Ln—Ln bonds indicative of strong magnetic interactions and potential frustration. Although magnetic characterisation does not reveal clear signs of frustration, the Dy(III) complex demonstrates slow magnetisation relaxation, with U_eff = 12.2 cm⁻¹, comparable to related triangles.

Chapter 5 explores the phenomenon of valence tautomerism in lanthanide-based materials, with focus on Sm and Yb complexes. The phenomenon of valence tautomerism can be explored in cases where both Ln(II) and Ln(III) oxidation states are accessible together with a redox-active ligand. This research reveals that the pseudo-1D triangle-based chain SmI₂(pyz)₃ undergoes valence transitions with reversible first-order phase transitions coupled with Sm(II)(pyz) ⇄ Sm(III)(pyz·−) valence tautomerism below 190 K. Introducing Yb(II) into the Sm framework shifts the transition temperature, demonstrating the tunability of these systems.

Lastly, with the aim of incorporating SMM behaviour into extended networks, Chapter 6 discusses the synthesis of 2D lanthanide networks as SMMs using Dy(III) and axial tert-butoxide ligands, linked by the bipyridine ligand, is explored. The 2D triangle-based network [Dy(O^tBu)₂(bipy)_4/2(THF)] [BPh₄] features Dy(III) nodes with axial tert-butoxide ligands linked by four bipyridine ligands, with a pendant THF completing the coordination sphere. Despite synthetic challenges, this material exhibits promising SMM behaviour with notable hysteresis and slow relaxation of magnetisation. However, reproducibility issues and challenges in structural characterisation make the presented results preliminary.

Overall, the research presented in this thesis advances the understanding of lanthanide-organic frameworks based on ditopic linkers in triangular patterns, providing valuable insights into the design of materials with customisable magnetic and electronic properties. These findings open new avenues for research in magnetocaloric systems and SMMs, with potential applications in very-low temperature refrigeration and spintronics.