Low-Coordinate and Radical-Bridged Lanthanide Molecular Nanomagnets

The synthesis of complex magnetic materials from simple chemical building blocks encapsulates the intrinsic fascination of molecular magnetism. Molecular magnets are typically designed using bottom-up approaches that provide access to experimental testbeds for theoretical models such as ligand field theory. The molecular approach also enables rational approaches to applications, such as the use of transition metal complexes as qubits for quantum computing, the use of lanthanide complexes as magnetic refrigerants, and in NMR spectroscopy as shift reagents and in magnetic resonance imaging.
Considerable effort is currently focused on the phenomenon of magnetic bistability in molecular systems, whereby two different magnetic responses can be measured under identical physical conditions. Two important types of bistable molecular magnets are spin-crossover (SCO) compounds and single-molecule magnets (SMMs). The bistability of SCO materials based on 3d metals (predominantly FeII) has allowed these materials to be proposed for applications in displays, sensors and information storage devices. Similarly, SMMs, which are d- and f-block coordination compounds defined by an effective energy barrier to reversal of their magnetization, have also been suggested for use in information storage devices by virtue of their magnetic bistability.
Despite the bistability in SCO compounds and in SMMs, the two phenomena are observed in different temperature regimes; SCO tends to occur in the 50-350 K regime, whereas SMM behaviour usually occurs below 50 K. If systems displaying both phenomena could be obtained, they would provide a route to bifunctional molecular magnets in which the SCO and SMM phenomena influence each other in a synergic way. The first challenge confronting this ambition is that general synthetic routes to SCO-SMM materials are extremely rare: in this project supramolecular architectures consisting of lanthanide SMMs and SCO compounds will be developed in to explore the properties of bifunctional molecular magnets using a novel synthetic methodology. The second challenge will be to understand how the SCO and SMM phenomena influence each other via factors such as magnetic exchange. To achieve this, we will study SCO-SMMs on the molecular and supramolecular (rotaxane) scales, thus allowing through-bond and through-space interactions to be probed, understood and controlled. The following milestones will be achieved:
Milestone 1: a general method - based on the reactivity of 'masked' divalent lanthanide reagents - for the synthesis of heterobimetallic Fe-Ln complexes with interdependent SCO and SMM properties.
Milestone 2: synthetic route to a unique family of rotaxane molecular magnets - including photoactivated derivatives - in which communication between SCO and SMM subunits is mediated via the guest molecule.
Milestone 3: detailed characterization of SCO-SMMs based on a range of physical techniques.
Milestone 4: Theoretical understanding of the SCO-SMMs, with an ability to use this understanding to enhance the magnetic properties through controlled synthetic modifications.