Ligand design strategies for molecular nanomagnets

This thesis describes the synthesis and magnetic characterisation of a series of polynuclear 3d and 3d/4f complexes built using phenolic oxime type ligands. Chapter two describes the reaction of salicylaldoxime and its derivatised analogues (R-saoH2) with the alkoxide containing co-ligands triethanolamine (TeaH3) and 2-(hydroxymethyl)pyridine (Hhmp), in the presence of Mn(II)/Ln(III) salts. This results in the formation of a family of sixteen [MnIII4LnIII2] clusters, which are structurally related to a previously studied [MnIII6] family of Single-Molecule Magnets (SMMs). The magnetic properties of the Ln = Y, Gd and Lu analogues can be qualitatively rationalised on the basis of a magneto-structural correlation (MSC), previously developed for MnIII alkoxide/oxime bridged dimers. Chapter three describes how the combination of two complimentary ligands, the phenolic oximes (R-SaoH2) and the diethanolamines (DeaH3), into one organic framework, creates new ligand types (H3L1 and H4L2) which can be used to construct a hexametallic MnIII wheel; [MnIII6Na(L1)6]Cl, the first example of a ferromagnetically coupled dodecametallic MnIII wheel;[MnIII 12(OMe)16(L2)4(O2CCMe3)4(MeOH)4], and the first example of a dodecametallic MnIII truncated tetrahedron; [MnIII12O4(H3L2)8(H2L2)4(TMA)4 (TMA = trianion of trimesic acid). Single crystal hysteresis measurements reveal both 3.2 and 3.3 to be SMMs at low temperature. Chapter four deals with the use of H4L2 in Cu coordination chemistry. Phenolic oximes are known to form monometallic complexes with CuII ions, as are the diethanolamines. However, the deliberate incorporation of one ligand onto the organic framework of the other permits the preparation of a family of [CuIIn] wheels (n = 4, 6, 8). In each case nearest neighbour interactions between CuII ions are shown to be strongly antiferromagnetic. DFT calculations suggest the origin of this interaction is related to the Cu-O-N-Cu dihedral angle, an observation which allows for the development of a theoretical MSC, that suggests a switch from antiferro- to ferro-magnetic exchange is possible at Cu-O-N-Cu angles > 60o