Putting the Squeeze on Molecule-Based Magnets: Exploiting Pressure to Develop Magneto-Structural Correlations in Paramagnetic Coordination Compounds

Abstract

The cornerstone of molecular magnetism is a detailed understanding of the relationship between structure and magnetic behaviour, i.e., the development of magneto-structural correlations. Traditionally, the synthetic chemist approaches this challenge by making multiple compounds that share a similar magnetic core but differ in peripheral ligation. Changes in the ligand framework induce changes in the bond angles and distances around the metal ions, which are manifested in changes to magnetic susceptibility and magnetisation data. This approach requires the synthesis of a series of different ligands and assumes that the chemical/electronic nature of the ligands and their coordination to the metal, the nature and number of counter ions and how they are positioned in the crystal lattice, and the molecular and crystallographic symmetry have no effect on the measured magnetic properties. In short, the assumption is that everything outwith the magnetic core is inconsequential, which is a huge oversimplification. The ideal scenario would be to have the same complex available in multiple structural conformations, and this is something that can be achieved through the application of external hydrostatic pressure, correlating structural changes observed through high-pressure single crystal X-ray crystallography with changes observed in high-pressure magnetometry, in tandem with high-pressure inelastic neutron scattering (INS), high-pressure electron paramagnetic resonance (EPR) spectroscopy, and high-pressure absorption/emission/Raman spectroscopy. In this review, which summarises our work in this area over the last 15 years, we show that the application of pressure to molecule-based magnets can (reversibly) (1) lead to changes in bond angles, distances, and Jahn–Teller orientations; (2) break and form bonds; (3) induce polymerisation/depolymerisation; (4) enforce multiple phase transitions; (5) instigate piezochromism; (6) change the magnitude and sign of pairwise exchange interactions and magnetic anisotropy, and (7) lead to significant increases in magnetic ordering temperatures.

Using internal strain and mass to modulate Dy⋯Dy coupling and relaxation of magnetization in heterobimetallic metallofullerenes DyM2N@C80and Dy2MN@C80(M = Sc, Y, La, Lu)

“…The use of external stimuli to modulate the properties of single molecule magnets attracted significant attention as it opens the way for multifunctional magnetic systems. 39,40 Mechanical pressure was found to change both the ligand field and magnetic interactions in several transition-metal single-molecule magnets (SMMs), [41][42][43][44][45][46][47][48] but the studies of pressure-induced variations in the SMM behavior are rather involved and remain very scarce for lanthanide SMMs. [49][50][51][52] One of the main effects of mechanical pressure on molecular systems is the internal strain, which not necessarily requires the use of external pressure as it can be addressed by a judicious molecular design.…”

Tandem High-Pressure Crystallography–Optical Spectroscopy Unpacks Noncovalent Interactions of Piezochromic Fluorescent Molecular Rotors

“…A single crystal can be compressed in a diamond anvil cell and its crystal structure and spectroscopic properties followed in situ . This technique has been used to elucidate structure–property relationships in functional organic crystals with luminescence and negative linear optical properties, , inorganic complexes, − and flexible framework materials, − to name a few.…”

High-pressure single-crystal diffraction at the Australian Synchrotron

“…High-pressure single-crystal diffraction techniques have been well developed for the investigation of materials over a wide range of scientific disciplines. Recent examples of materials where structure elucidation has been performed at pressure include spin-crossover complexes (Turner et al, 2020), molecular magnets (Etcheverry-Berrios et al, 2020), pharmaceuticals (Oswald et al, 2010), molecular and framework porous materials (McKellar & Moggach, 2015), gases (Lundegaard et al, 2009), pure metals (McMahon & Nelmes, 2006), alloys (Perez-Albuerne et al, 1966), proteins (Librizzi et al, 2018) and molecules associated with planetary science (Cable et al, 2021). A range of high-pressure single-crystal diffraction apparatuses have been developed over the years for investigating these materials, including high-pressure capillary pressure cells for both laboratory diffractometers (Yufit & Howard, 2005) and for use at central facilities (McMonagle et al, 2020) for exploring the effect of lower pressures (< 2000 atm) on soft materials (McMonagle et al, 2022).…”