Abstract
Hydrogen bonding plays a foundational role in the life, earth, and chemical sciences, with its richness and strength depending on the situation. In molecular materials, these interactions determine assembly mechanisms, control superconductivity, and even permit magnetic exchange. In spite of its long-standing importance, exquisite control of hydrogen bonding in molecule-based magnets has only been realized in limited form and remains as one of the major challenges. Here, we report the discovery that pressure can tune the dimensionality of hydrogen bonding networks in CuF2(H2O)2(3-chloropyridine) to induce magnetic switching. Specifically, we reveal how the development of exchange pathways under compression combined with an enhanced ab-plane hydrogen bonding network yields a three dimensional superexchange web between copper centers that triggers a reversible magnetic crossover. Similar pressure- and strain-driven crossover mechanisms involving coordinated motion of hydrogen bond networks may play out in other quantum magnets.
Highlights
Hydrogen bonding plays a foundational role in the life, earth, and chemical sciences, with its richness and strength depending on the situation
This system differs from the prior examples in that the structure is fully molecular rather than being covalently bound (Fig. 1(a)). It displays a buckled network of intermolecular hydrogen bonds between the H2O ligands and fluoride centers that act as superexchange linkages between the copper centers within the ab plane
We propose that intermolecular hydrogen bonding between the water ligand and chlorine acts as an additional superexchange pathway between copper centers along the c axis, adding a third dimension to the hydrogen bonding network in CuF2(H2O)2(3chloropyridine) above 0.8 GPa (Fig. 1 (b))
Summary
Hydrogen bonding plays a foundational role in the life, earth, and chemical sciences, with its richness and strength depending on the situation. This system differs from the prior examples in that the structure is fully molecular rather than being covalently bound (Fig. 1(a)) It displays a buckled network of intermolecular hydrogen bonds between the H2O ligands and fluoride centers that act as superexchange linkages between the copper centers within the ab plane. The latter pathway forms because compression reduces interatomic distances, aligns the Cl-containing ring, and widens the H2O ligands, leading to a three dimensional hydrogen bonding network between copper centers This increased superexchange network dimensionality drives the 0.8 GPa magnetic crossover. We conclude that magnetic tunability in CuF2(H2O)2(3-chloropyridine) derives from and depends upon the presence of flexible intermolecular hydrogen bonding networks Further compression reveals another distortion between 4 and 5.5 GPa involving the bipyramidal copper environment at this time, it is not known whether there is a magnetic component. In addition to establishing how pressure-induced changes in bond lengths and angles control magnetism in hydrogen bonded quantum magnets like CuF2(H2O)2(3-chloropyridine), these findings are important for unraveling spin crossover processes and energy transfer mechanisms in other functional materials like multiferroics
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