Magneto-structural investigations of calix[n]arene-supported metal clusters

Abstract

Thesynthesis of new molecular magnetic materials is not trivial. Ligand design,reaction conditions and serendipity all contribute to the synthesis of stable,tuneable magnetic molecules with interesting physical properties. Such clustershave been known to display fascinating magnetic behaviour, such assingle-molecule magnetism or enhanced magnetocaloric effects. Developing acomprehensive understanding of the structure-property relationships of thesemolecules proffers one strategy to attaining species with attractive magneticbehaviour. Calix[n]arenes are cyclic, usually bowl-shaped, polyphenolicmolecules which have been touted as excellent ligand candidates, in part due totheir capacity for metal complexation and subsequent cluster formation. Thedominant structure-directing role of the calix[n]arene also means that,in many cases, analogous structural topologies can be made with differentmetals and with metals in a variety of oxidation states, allowing detailedmagneto-structural studies.

Chapter1 reviews the coordination chemistry of polymetallic clusters synthesised from p-tert­-butylcalix[4]arene(H4TBC[4]) in literature. The coordination preferences and assemblybehaviours of these clusters show consistent trends, allowing prevailingarchitectures to be established.

Chapter2 describes a series of novel polymetallic clusters of nuclearities {MnIII₂MnII₂},{Mnᶦᶦᶦ₅Mnᶦᶦ₂}, {Mnᶦᶦᶦ₄Mnᶦᶦ₆}and {Cuᶦᶦ₁₆}, which are supported by 2,2’-bis-p-tert-butyl-calix[4]arene(BisTBC[4], H₈L), where two TBC[4] moieties are linked directly viaa methylene bridge. Examination of these structures reveals adherence to thecoordination bonding rules expected for TBC[4]. Magnetic studies show weakantiferromagnetic exchange between metal ions in most cases, with DFTcalculations revealing the importance of both relevant bond parameters andmetal oxidation states on the value of J.

Chapter3 reports the synthesis and characterisation of the high-nuclearity truncatedoctahedron {Cuᶦᶦ₂₄} and its {Cuᶦᶦ₈}‘building block’. Both are supported by p-tert-butyl-thiacalix[4]arene(TC[4]A), where the methylene bridges of TBC[4] have been replaced with sulfur,and magnetic studies reveal strong antiferromagnetic exchange interactions. Theproclivity of TC[4]A to house Mᶦᶦ ions prompted the investigation ofmixed-calix[n]arene syntheses, yielding clusters {Mnᶦᶦᶦ₂Mnᶦᶦ₄}and {Mnᶦᶦᶦ₆Mnᶦᶦ₄Cuᶦᶦ₄},supported by TBC[4] and TC[4]A, and {Coᶦᶦ₄}, supported byTBC[8] and TC[4]A. The favoured metalloligand motifs of distinct calix[n]arenesare upheld giving access to novel architectures, all of which display increasedcompetitive exchange with respect to the homo-calix CuIIclusters.

Chapter4 builds on the work of the previously yielded TBC[4] clusters, reporting novelclusters of familiar topologies across the 3d and 3d-4fseries’ ({Cuᶦᶦ₉}, {Cuᶦᶦ₇Gdᶦᶦᶦ₂},{Mnᶦᶦᶦ₃Na₆} and the relatedBisTBC[4]-supported {Mnᶦᶦᶦ₆Na₆}). Attempts toconstruct large, high-symmetry TBC[4]-supported clusters by applyinghigh-temperature, high-pressure conditions yielded {Feᶦᶦᶦ₁₈}and {Feᶦᶦᶦ₂₈}. Magnetic studies reveal significant spinfrustration effects arising from the high-symmetry, and dominantantiferromagnetic behaviour.

Chapter 5 presents the key conclusions andfuture outlooks derived from the body of this work.