Design, syntheses and magnetic properties of vanadium-lanthanide complexes

3d-4f coordination complexes have received considerable attention due to their potential applications in single molecule magnets (SMMs) and magnetic refrigerants. These complexes possess a combination of 3d ions and 4f ions, both individually renowned for their excellent magnetic properties. The intriguing properties of SMMs make them highly promising for a wide range of applications, including molecular spintronics, high-density information storage materials, and quantum computing. A total of forty-five V-Ln complexes with nuclearities ranging from tetranuclear to tridecanuclear were synthesized, some of which have localized heterovalent states of vanadium ions. The main work is divided into the following three parts: Chapter 2 of this thesis provides a comprehensive description of the molecular structures and magnetic properties of two distinct types of V-Ln complexes synthesized using Schlenk techniques. Different combinations of ligands and solvents resulted in diverse V-Ln complexes. The magnetic behaviors of each complex are explored in detail. These butterfly-type V${}^{III}$${}_2$Dy${}_2$ complexes exhibit slow relaxation of magnetization usually under an applied direct-current (dc) field. The magnetic properties of the complexes are quantified fitting the relaxation times. For the V${}^{III}$${}_2$Dy${}_2$ 2.2 complex, the obtained parameters are τ${}_0$ = 1.518×10${}^{-5}$ s, τ${}_{QTM}$ = 1.15×10${}^{-3}$ s, and U${}_{eff}$ = 14(2) K. On the other hand the V${}^{III}$${}_5$Dy${}_8$ 2.9 and V${}^{III}$${}_3$V${}^{IV}$Dy${}_5$ 2.10 clusters demonstrate SMM characteristics without the need for an applied magnetic field. In the case of the V${}^{III}$${}_5$Dy${}_8$ 2.9 complex, the results of micro-SQUID experiments confirm the presence of SMM behavior below 0.3 K, as seen from the hysteresis. Furthermore, the fitting of the Arrhenius equation yields an energy barrier of 8 K and a relaxation time of 10${}^{-13}$ s for the V${}^{III}$${}_5$Dy${}_8$ 2.9 complex. Compared with SMM complexes of other trivalent transition metal ions and lanthanide ions, this chapter confirms that V-Ln type complexes can also be candidates for SMMs, and also significantly expands the scope of research in the field of 3d-4f SMMs. Chapter 3 focuses on the crystal structures and magnetic properties of 14 V${}^{III}$${}_4$Ln${}_4$ complexes, all of which share the same square-in-square topology but differ in their ligands and lanthanide ions. The results of dc magnetic susceptibility experiments reveal that these V${}^{III}$${}_4$Ln${}_4$ complexes exhibit ferromagnetic properties. In contrast, the V${}^{III}$${}_4$Y${}_4$ complexes containing diamagnetic Y ions exhibit antiferromagnetism. Therefore, it can be concluded that there is a ferromagnetic interaction between the lanthanide and vanadium ions. Furthermore, this section discusses the magnetocaloric properties of two isostructural V${}^{III}$${}_4$Gd${}_4$ complexes with slight differences in the ligands. V${}^{III}$${}_4$Gd${}_4$ 3.3 has an entropy change of 16.08 J kg${}^{-1}$ K${}^{-1}$ at 3 K for H = 7 T, while V${}^{III}$${}_4$Gd${}_4$ 3.7 has an entropy change of 19.53 J kg${}^{-1}$ K${}^{-1}$ at 3 K for H = 7 T. Chapter 4 delves into the magnetostructural correlations of a series of ring-shaped V${}^{III}$-Ln complexes via dc susceptibility measurements. Additionally, this chapter examines the relationship between the metal-metal distances and the arrangement of vanadium and lanthanide ions in the ring. Furthermore, the effects of ligand changes on the dc magnetic susceptibility properties of these isostructural complexes are discussed. Notably, the alteration of the ligand does not significantly impact on the dc magnetic susceptibility properties. The findings contribute towards a better understanding of the magnetic behavior observed in these ring-shape V${}^{III}$-Ln complexes.