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.
Highlights
Over the there hashas been a significant increase in theinnumber of high-pressure singleOver thelast last5050years, years, there been a significant increase the number of high-pressure crystal and powder diffraction studies performed on molecular systems, with several comprehensive single-crystal and powder diffraction studies performed on molecular systems, with several reviews on the subject, the effect of pressure on amino acids [1], complexes and comprehensive reviewsincluding on the subject, including the effect of pressure onmetal amino acids [1],[2]
Despite HP single-crystal X-ray diffraction (XRD) being available since the 1950s, reports of its combination with other HP techniques to examine magneto-structural relationships in molecule-based magnets remain rather rare
This is surprising given the enormous potential benefits on offer. This may be partly due to technology—the design and manufacture of pressure cells for other types of measurements lag behind that of diamond study of cells area (DACs) for single-crystal XRD, and these pressure cells often do not reach the same high pressures
Summary
Over the there hashas been a significant increase in theinnumber of high-pressure singleOver thelast last5050years, years, there been a significant increase the number of high-pressure crystal and powder diffraction studies performed on molecular systems, with several comprehensive single-crystal and powder diffraction studies performed on molecular systems, with several reviews on the subject, the effect of pressure on amino acids [1], complexes and comprehensive reviewsincluding on the subject, including the effect of pressure onmetal amino acids [1],[2]. Examples include measuring changes in conductivity and band structure in conductivity [12], Mössbauer [13], and magnetic measurements These combined studies have molecular conductors [12], monitoring framework ‘breathing effects’ on theenforced uptake ofchanges guest species revealed how structural distortions, caused by increasing pressure, have in the within porous metal–organic framework materials [14], and how changes structural in encapsulated physical properties of materials. We highlightmaterials our efforts in developing magneto-structural guest species within porous metal–organic framework [14], and how structural changes in correlations transition metal molecule-based magnets; doarea not where attempt to approach cover examples froma encapsulatedinfluorophores affect emission properties [11].we One this has made other researchers, which are many and varied [15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31].
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