The role of bis-calixarenes in the synthesis of multi-component polymetallic clusters
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The work presented in this thesis encompasses the synthesis of novel p-tert butylcalixarene-based ligands with the overall aim of exploring and understanding their coordination chemistry. The ligands described have been obtained via substitution and functionalisation of p-tert-butylcalixarene at one methylene bridge position, the mechanism of which is discussed in detail herein. Once isolated, these ligands were reacted with a selection of transition (TM), lanthanide metals (Ln) and 3d-4f metal ion mixtures in order to systematically study the formation of polymetallic clusters and their prevailing magnetic properties. This also provided the opportunity to gain insight into ligand influence over the associated coordination chemistry, all of which was possible through the growth and X-ray diffraction study of single crystals in each case. Chapter 1 contains an overview of calix[n]arene synthesis (and related derivatives), with a particular emphasis on calixarene, its conformational properties, and synthetic modifications at the upper-, lower-rim and methylene bridge positions. Discussion then moves to a qualitative description of single-molecule magnets and molecular refrigerants, highlighting some of the seminal achievements in the area. The final section of Chapter 1 discusses the coordination properties of calixarene, including the formation of metal clusters that represent the foundation of the work presented in Chapters 2 – 5. Chapter 2 presents the synthesis of 2,2’-biscalixarene prior to discussion of the use of this ligand in reactions with TM / Ln ion mixtures, the results of which were the isolation of a series of 2,2’-biscalixarene-supported 3d/4f metal clusters. This was carried out with a particular focus on stoichiometric control over cluster synthesis, which subsequently afforded novel cluster topologies that are closely related to species already obtained from previous research within the group. Chapter 3 covers exploratory cluster formation that was carried out using 2,2’ biscalixarene in the presence of complementary and competitive co-ligands. The co ligands employed were carefully chosen based on their known metal ion binding properties, resulting in dramatic changes to the topologies of the prevailing polymetallic clusters, the structures of which are described in detail. Chapter 4 describes a modified synthetic procedure that facilitated the isolation of a family of flexible alkyl chain tethered biscalixarenes. These ligands were then used in exploratory cluster formation and afforded a series of known cluster topologies when the length of the alkyl tether permitted this. Structural discussion highlights these similarities and draws comparison with the TBC-supported clusters presented in Chapter 1. Chapter 5 focuses on exploratory coordination chemistry that was carried out with rigidly tethered biscalixarenes. The chapter begins with a brief description of the synthesis of these ligands before moving on to discussion of the new polymetallic clusters isolated, highlighting important structural analogies and differences with the architectures described in Chapters 1 and 4. Chapter 6 presents a summary of the work included in this thesis, as well as an outlook of future work to be undertaken in this area.