The Role of supramolecular interactions in magnetic systems: 1,8-Napthalimides and other π-containing species

Abstract

Supramolecular chemistry is a rapidly expanding area of research having seen the award of two Nobel prizes within the last 30 years. Whilst this means the phenomenon is well known, the important role it plays within inorganic structural chemistry is only beginning to be fully appreciated. The most common supramolecular interaction utilized is hydrogen bonding, which is fundamental for all life by allowing liquid water to exist over a wide range of temperatures and pressures. In addition to hydrogen bonding, there are other intermolecular interactions such as π-π, van der Waals, and the less common halogen, chalcogen, tetrel, or pnictogen bonds. The prevalence of hydrogen bond donors and acceptors in most systems can lead to unpredictability of the long-range structure, making rational design of specific supramolecular architectures a complex task. In a similar vein, halogen, chalcogen, tetrel, and pnictogen bonds can often prove unwieldy or challenging to work with as they typically require specialist design. The use of π-π interactions is however an attractive idea; these systems are typically easy to work with, stable, and able to be incorporated into many systems.

Magnetically interesting compounds have had a large success over the same time frame, with the first single molecule magnet (SMM) being produced in the early 1990s. This field has continued to grow and recently, in 2018, reached a major milestone with the first reported system which retains the slow magnetic relaxation required of an SMM compound above the temperature of liquid nitrogen. The result has long been thought to be the first step on the route to incorporating SMMs into working components such as data processing and storage devices, as current SMM devices require expensive and finite liquid helium.

Whilst the definition of a single molecule magnet is “a molecule that shows slow relaxation of the magnetisation of purely molecular origin”, researchers have recently begun to believe that the SMM properties can be modulated by the non-local environment in a similar manner to another magnetic phenomenon, spin crossover.

This research explores the use of supramolecular design and control within magnetochemistry by producing a series of 1,8-naphthalimide based complexes to investigate their structure directing properties, with a view for inclusion into existing SMM architectures. The 1,8-naphthalimide system allows for a wide variety of components and simple synthesis resulting in a wide library of available ligands for almost any purpose. This organic species also provides a rigid framework for building πbased supramolecular architectures given the predictability of the interactions between the naphthalimide molecules. We detail how the predictability of the system allowed us to build layered networks which demonstrate SMM characteristics, although not in the frequency region of a standard SQUID magnetometer.

This research also investigates stronger π-based interactions using radical enhanced pancake-like bonds. The 9,10-phenanthrenequinone system used here provided a facile route to the 9,10-phenanthrenesemiquinone radical via an in situ reduction. When combined with the presence of Ln(III) ions, the radical ligands form intramolecular π-bonds with an exceptionally short distance encapsulating two lanthanide ions in a cage consisting of four ligands. The exceptional strength of this pancake bond effectively forces the two lanthanide ions together, resulting in a strong electrostatic interaction which appears to quench the quantum tunnelling of magnetisation relaxation pathway. Further demonstrating the use of π-based interactions in SMM research. The future roadmap for modification to include supramolecular pancake-like bonds is provided in the future work section.

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Identifiers

ORCID for Simon Coles: orcid.org/0000-0001-8414-9272

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