Magnetism of multinuclear 3-d transition metal complexes of 2-hydroxymethylpyridine

Abstract

Magnetic materials are ubiquitous in modern technology. As we seek to expand our arsenal of functional magnetic materials, upon which to build the technology of the future, we must understand the fundamental relationship between molecular structure and magnetic properties. To achieve that goal, we can leverage synthetically accessible modification to investigate the impact of modular changes to iso-structural complexes. Magnetic susceptibility and magnetization measurements can then be supported by other experimental methodologies and computational theory to build the knowledge required to understand these fundamental relationships.

In order to generate families of iso-structural or structurally related compounds, diverse synthetic strategies are needed. One ligand used extensively in the coordination chemistry of the first-row transition metals is 2-hydroxymethylpyridine (hmpH). hmpH has been shown to exhibit multiple binding modes and facilitate the construction of heterometallic clusters. By exploiting different temperature and pressure regimes, including solvothermal methods, a large range of nuclearities and structural topologies can be accessed, allowing the fundamental magneto-structural relationships to be examined.

Each chapter of this thesis aims to contribute to our understanding of magneto-structural correlations in 3-d transition metal complexes, exploiting solvothermal methods to synthesise families of iso-structural complexes, magnetothermal measurements to characterise magnetic behaviour and computational techniques to rationalise the magnetic properties observed.

Chapter 1 discusses the diversity of structure types used to study magneto structural correlations in 3-d transition metal complexes. The discussion is focussed on heterometallic examples with a nuclearity of 10 or less, highlighting significant examples used in the quantitative study of fundamental magnetic properties.

Chapter 2 describes a family of heterometallic Anderson wheels of formula [(VᶦⱽO)₂Mᶦᶦ₅(hmp)₁₀X₂](ClO₄)₂·2MeOH (M = Ni, Co and X = Cl, N₃, NCS, OCN, OMe) which have been synthesised from the solvothermal reaction of M(ClO₄)₂·6H₂O and VCl₃ with hmpH (2-(hydroxymethyl)pyridine) and subsequent reaction with excess sodium salts of the pseudohalides. The metallic skeleton describes a centred hexagon, with the two vanadyl ions sitting on opposing sides of the outer ring. Magnetic susceptibility and magnetisation measurements indicate the presence of both ferromagnetic and antiferromagnetic exchange interactions. Theoretical calculations based on density functional methods reproduce both the sign and strength of the exchange interactions found experimentally, and rationalise the parameters extracted.

Chapter 3 describes a family of 11 heterometallic butterfly complexes of formula [Crᶦᶦᶦ₂(MᶦᶦX₂)₂(OMe)₂(hmp)₄] (M = Mn, Fe, Co, Zn and X = Cl, Br, I) that have been synthesised from the solvothermal reaction of the metal halide salts in methanol/acetonitrile with Cr(ClO₄)₃∙6H₂O and hmpH. The metallic skeleton describes a butterfly shape with two Crᶦᶦᶦ ions bridged to one another through OMe moieties and bridged to the halide-capped tetrahedral Mᶦᶦ. Magnetic susceptibility and magnetisation measurements, supported by theoretical calculations, indicate the presence both ferromagnetic and antiferromagnetic exchange interactions. Calculations suggest the dominant antiferromagnetic exchange is that mediated by the alkyl arms of the four hmp ligands between the Mᶦᶦ and Crᶦᶦᶦ ions, resulting in unusual ferromagnetic exchange between the methoxy bridged Crᶦᶦᶦ groups.

Chapter 4 seeks to characterise the magnetic exchange and anisotropy found in [Niᶦᶦ₇] Anderson Wheels, which is the subject of disagreement in the literature. The synthesis and characterization of a [Niᶦᶦ₇] Anderson wheel is reported. This is achieved through a combination of first principles calculations, magnetic susceptibility and magnetisation measurements, and neutron scattering. To consistently describe the magnetic parameters and neutron response an excitonic formalism using a Green's function response theory is applied. This allows, for the first time, extraction of accurate exchange interactions and anisotropies in a Ni-based Anderson wheel showing the presence of both ferro- and antiferromagnetic exchange and easy-plane and easy axis anisotropy.

Chapter 5 summarizes the findings of the preceding chapters and outlines potential next steps to expand upon them. This includes discussion of opportunities to expand the families of complexes discussed herein.