Bridging the Gap between Single-Molecule Magnets and Remanent Magnetic Surface Sites

Abstract

The development of smaller electronic components to tackle the increasing demand for more efficient data storage and computing devices is one of the most pressing challenges of modern society. This demand results from the progress in the computer industry and has initiated the search for molecular or even atom-sized computing and data storage units. In this context, single-molecule magnets (SMMs) have shown promising magnetic properties that render them suitable candidates not only for data storage, but also quantum computing and spintronics applications, bearing the potential to revolutionise information technology. State-of-the-art SMMs are typically sandwich-type complexes, a class of compounds consisting of a lanthanide ion coordinated by two ring-type ligands. In particular, dysprosocenium cations -Dy(III) coordinated with two bulky cyclopentadienyl (CpR)- ligands - are among the best-performing SMMs so far and are the first compounds to exhibit slow magnetic relaxation above liquid nitrogen temperature. To harness their tremendous potential, these complexes have to be embedded in solid-state devices, where every magnetic centre can be individually addressed, and magnetic remanence can be observed. This requires efficient surface immobilisation strategies. However, the generation or preservation of magnetic remanence in supported SMMs has proven to be one of the key challenges in the field. Fast relaxation and loss of magnetisation upon immobilisation has often been attributed to the multitude of possible surface-precursor interactions. Moreover, understanding the origin of slow magnetic relaxation on surfaces is challenging, primarily because most surface immobilisation methodologies lead to a distribution of surface sites where the precise structure(s) are unknown, impeding the establishment of structure-property relationships. The research conducted within this work aims to bridge the gap between high-performing SMMs in molecular and supported systems, thereby facilitating the transition towards practical applications of SMMs. A three-step approach was applied. First, the computational exploration of favourable surface sites was performed to gain fundamental understanding of the relationship between the coordination of surface dispersed lanthanide ions and their magnetic behaviour. This is followed by an experimental approach to investigate subtle changes in the coordination environment of lanthanides in molecular systems and their effect on the magnetic performance. Lastly, the transition to surface-supported systems by applying two surface immobilisation methodologies was performed, leading to guidelines for generating magnetic sites on surfaces. In Chapter 2, computational methods are used to predict the magnetic properties of ten model systems assembled in silico, representing possible surface structures of Dy(III) on silica. Such a system has been shown to exhibit SMM behaviour while the origin of these properties remain unclear. The calculations provide a way to identify suitable surface coordination for high-performing SMMs. The analysis of the data reveal a prevalent structural motif in all models for which good SMM properties were predicted. This structural feature is a T-shape coordination of the Dy(III) by three anionic ligands, dominating the predicted properties, while neutral ligands play a minor role. Chapter 3 focuses on the investigation of molecular systems in order to deepen the understanding of how small changes in the coordination sphere affect SMM behaviour. The long-standing hypothesis concerning the presence of two rotamers in the [(COT)M(Cp*)] family, leading to distinct magnetic species and different relaxation behaviours, is addressed in this study. The application of steric tuning of the coordination sphere enabled the isolation of structural analogues [(COT)M(Cpttt)] in which the steric bulk of the (Cpttt)- ligand prevents the rotation of the larger (COT)2- ring. This structural constraint leads to a stream-lined magnetic behaviour with only one magnetic species and also enables the isolation of the first early lanthanide analogue (Nd(III)) of this family, which was not possible for the (Cp*)- analogues. These studies highlight the close relationship between structural features and magnetic performance of SMMs. Lastly, the surface immobilisation of lanthanide sandwich complexes via two approaches, namely Surface Organometallic Chemistry (SOMC) and Lewis-acid-base interaction, is investigated. In Chapter 4, SOMC type grafting of one of the best SMMs ([(Cpttt)2Dy]+-[B(C6F5)4]) on silica results in a material with strongly diminished SMM properties compared to the precursor and a wide distribution of relaxation times, indicating a wide distribution of Dy surface species. This is corroborated by solid-state NMR analysis of the diamagnetic Y(III) analogue, which provides evidence of different surface sites resulting from secondary reactions upon grafting of the precursor. Avoiding such secondary reactivity after grafting can be achieved by using Lewis acid-base interactions as the primary surface immobilisation mechanism as described in Chapter 5. Tailored Lewis acidic Al(III) sites on silica are used as anchoring sites for the non-magnetic compound [(Cpttt)2DyCl] resulting in a magnetic material exhibiting slow magnetic relaxation at highest reported temperature thus far for a supported SMM (51 K). Detailed spectroscopic and computational analyses show that the interaction of Al(III) sites with the chloride in the precursor elongates the M-Cl bond leading to a large increase of SMM performance. Overall, this work provides a more profound understanding of the relationship between the local coordination environment of lanthanide(III) ions and the resulting magnetic performance for lanthanide-based single-molecule magnets. The robust analysis methodology based on calculations, solid-state NMR of a Y(III) analogues, IR spectroscopy and magnetic measurements provide further detailed insights into the origin of magnetic behaviour of molecular and supported systems. Therefore, this thesis provides guidelines for further synthetic and analytic investigations to improve SMM behaviour on surfaces toward the goal of practical applications in devices.