• High-Nuclearity Lanthanide(III) Complexes as Single-Molecule Magnets and Luminescent Materials

      Mazarakioti, Eleni; Department of Chemistry
      The employment of the bridging/chelating Schiff base ligands, N-salicylidene-o-aminophenol (saphH2), N-salicylidene-o-aminocyclohexanol (sachH2) and N-salicylidene-2-amino-5-chlorobenzoic acid (sacbH2), in lanthanide (LnIII) cluster chemistry has afforded four families of polynuclear and dinuclear complexes with new structural motifs, and interesting magnetic and optical properties. Chapter 1 deals with most of the fundamental aspects within the areas of polynuclear metal complexes, molecular magnetism and optics as these are applied to 4f-metal based systems, while the research results are reported in Chapters 2, 3 and 4. In the first project (Chapter 2), the coordination chemistry of the organic chelating/bridging ligand, N-salicylidene-o-aminophenol (saphH2) in lanthanide cluster chemistry was investigated. The general LnIII/X-/saphH2/base reaction system has led to a family of (NHEt3)[Ln7(OH)2(saph)10(Me2CO)2] (Ln = Gd (1); Tb (2); Dy (3)) clusters with a new core topology that comprises two {Ln4} butterflies sharing a common metal vertex. The {DyIII7} analogue exhibits slow magnetization relaxation, whereas all heptanuclear compounds show ligand-centered blue-green emissions. The second project of this thesis, which is discussed in Chapter 3, comprises the first use of the Schiff base ligand N-salicylidene-2-aminocyclohexanol (sachH2; mixture of cis- and trans-analogue) in metal cluster chemistry which has afforded a new family of [Ln7(OH)6(CO3)3(sach)3(sachH)3(MeOH)6] (Ln = Gd (4); Tb (5); Dy (6)) clusters with ideal D3h point group symmetry and metal-centered trigonal prismatic topology. Solid-state and solution studies revealed single-molecule magnetism (SMM) and photoluminescence behaviors. Moreover, in order to investigate the steric and stereoisomerism effects of the ligand on the chemical and structural identity of the {Ln7} clusters, the pure trans-analogue of the sachH2 ligand was utilized. As a result, a new family of octanuclear [Ln8(OH)4(CO3)2(trans-sach)8(EtOH)4] (Ln = Gd (7); Tb (8); Dy (9); Eu (10)) clusters were obtained, while the solid-state studies revealed SMM behavior and lanthanide-centered emissions. In the last chapter of this thesis (Chapter 4), the Schiff base ligand N-salicylidene-2-amino-5-chlorobenzoic acid (sacbH2) was introduced for a first time in lanthanide cluster chemistry. This has afforded a family of dinuclear [Ln2(NO3)4(sacbH)2(H2O)2(MeCN)2] compounds (Ln = Gd (11); Tb (12); Dy (13)) with the Dy-analogue exhibiting SMM behaviour with a high-energy barrier for the magnetization reversal and interesting magnetization dynamics. All research-based Chapters (Chapters 2-4) are divided into subsections in order to facilitate the understanding of the research concepts by the familiar and non-familiar readers and contextualize the messages, goals and conclusions of each individual project. I felt it prudent to always begin with a short preface of the work that summarizes the most important aspects of the specific project, followed by the complete experimental part and discussion of the results, and finishing up with conclusions and some future perspectives.
    • High-Nuclearity Metal Complexes and Single-Molecule Magnets from the Employment of Oximato- and Alkoxido-based Ligands

      Giannopoulos, Dimosthenis; Department of Chemistry
      The employment of 2-pyrrolyloximes, pyridine-2,6-diketones and 3-hydroxy-2-naphthohydroxamic acid in homometallic 3d- and heterometallic 3d/4f-metal cluster chemistry has yielded new families of Fe, Mn and Mn/Dy clusters. These complexes were shown to possess interesting structural motifs and single-molecule magnetism (SMM) behaviour. The introductory chapter discusses the fundamentals of molecular magnetism, polynuclear metal complexes, as well as the approaches used for the synthesis of new polynuclear metal complexes and the selection criteria for the chelating/bridging ligands. Chapters 2, 3 and 4 report the results of the current thesis. In Chapter 2, the synthesis and characterization of a family of complexes resulting from the employment of 2-pyrrolyloximes in high-nuclearity transition metal cluster chemistry is reported. Complexes {Fe10} (1) and {Fe12} (2) are two of the highest nuclearity iron clusters containing an oximate ligand, while complex 3 is a barrel-like {Mn25Na} complex that exhibits SMM behaviour. Although there are previously reported examples of discrete {Mn25} barrel-like SMMs, complex 3 is the highest nuclearity Mn cluster organized into a 1D polymer through chelation with diamagnetic metal centers. Chapter 3 includes the synthesis and characterization of new Mn complexes featuring ligands that result from the metal-assisted reactivity of pyridyl- and pyrazine-based diketones. Complexes {Mn6} (4) and {Mn10} (5) are the highest nuclearity Mn clusters containing any form of the ligand 2,6-di-(2-pyridylcarbonyl)pyridine [(py)CO(py)CO(py)]. Despite the large number of {Mn6} and {Mn10} complexes reported in the literature, both complexes 4 and 5 possess unique topologies in their respective oxidation state levels. Complex {Mn3Na2} (6) possesses a iii unique metal stoichiometry and is the only compound containing any form of the ligand pyridine-2,6-diylbis(pyrazine-2-ylmethanone) [(pz)CO(py)CO(pz)]. More interestingly, complex 6 contains the first {MnIII3(μ3-O2−)}7+ triangular core where the Mn centers are solely bridged by an oxido group, essentially being a unique ‘edge-naked’ equilateral triangle. In Chapter 4, the synthesis and characterization of complexes bearing the ligand 3-hydroxy-2-naphthohydroxamic acid are presented. The {Mn10} complexes 7 and 8 are the highest nuclearity 3d-metal and the first homometallic Mn clusters containing the hydroxime form of the ligand. Both compounds possess unique metal topologies, which are affected by the nature of the carboxylate ligand present in the reaction mixture, and they behave as SMMs. The use of 3-hydroxy-2-naphthohydroxamic acid in Mn/Dy cluster chemistry has afforded the {Mn4Dy} complexes 9 and 10, as well as a family of {Mn8Dy2} complexes (11 and 12). These compounds are the first Mn/Dy complexes containing this particular hydroxime ligand and they also possess unique metal stoichiometries and topologies. The reported heterometallic products resulted from our efforts to deliberately replace the divalent Mn atoms located in 7 and 8 with DyIII as a means of enhancing the magnetic properties of the former. Complexes 11 and 12 were found to be single-molecule magnets.