Water is not usually thought of as a magnetic material. But a new study led by researchers at Tohoku University's Institute of Multidisciplinary Research for Advanced Materials, in collaboration with National Taipei University of Technology (Taipei Tech), has found that tiny shifts in the orientation of water molecules can flip on magnetic partnerships between metal ions that are not even directly bonded to each other. The discovery offers chemists a surprising new tool: instead of wiring magnetic ions together with strong chemical bonds, they may be able to use something as simple and common as hydrogen bonding to link them up.

"We found that distant magnetic moments can interact via hydrogen bonding, and these interactions can be controlled by the orientation of water molecules," said Kazuhiro Nawa, associate professor at Tohoku University's Institute of Multidisciplinary Research for Advanced Materials. "This gives us a chemical route to tune magnetism without directly connecting the magnetic ions through strong bonds."

Molecular magnets are materials built from individual molecules that each act as a tiny magnetic unit. Because chemists can tune the elements, ligands, and crystal arrangement of these molecules, molecular magnets offer a flexible playground for designing new magnetic behavior. Traditionally, though, scientists have focused on magnetic interactions that travel through direct contact between magnetic ions or through pathways of strong covalent bonds. Weaker connections, like hydrogen bonds, are common in molecular materials, yet how they might carry magnetic interactions between distant ions has remained poorly understood.

The team set out to test a specific idea: that hydrogen bonding itself could serve as a bridge, carrying magnetic interactions between copper ions even when those ions are not directly linked by strong bonds.

To investigate, the researchers synthesized two fluorinated pincer-type copper(II) complexes, named 2FH-CuCl₂·H₂O and 3F-CuCl₂·H₂O. Using single crystal X-ray diffraction and neutron diffraction experiments at the 4G-GPTAS instrument at the JRR-3 research reactor, they examined how the crystal structures of these compounds changed with temperature.

At room temperature, water molecules attached to the copper ions point in a mix of disordered directions, and the compounds' hydrogen bonding pattern is uniform throughout the crystal. But when cooled below 260 K, the water molecules snap into an ordered arrangement. This reshuffling splits the once uniform hydrogen bonding pattern into two distinct types, and in the process it pairs up neighboring molecules.

Magnetic measurements backed up the structural story. The compounds showed broad peaks in their magnetic susceptibility around 5 to 7 K, followed by a drop toward zero, a signature of antiferromagnetic pairing, in which the magnetic moments of paired molecules cancel each other out. The data fit a model of magnetic pairs with interaction strengths of roughly 10 K, and density functional theory calculations pointed to the hydrogen bonding network as the likely pathway carrying that interaction.

"A notable result was the clear connection between water molecule ordering and the formation of magnetic pairs," said Norman Lu, co-corresponding author of the study and professor at Taipei Tech. "Since the room temperature structure prohibits the pairing of molecules, it was genuinely surprising to see antiferromagnetic pairs show up in the magnetization data. It took closer study of the water molecules themselves to resolve that inconsistency."

The finding matters because it hands chemists a new dial to turn when designing magnetic materials. Rather than relying only on strong, direct chemical bonds to link magnetic ions, researchers may be able to use hydrogen bonding, one of the most common and adjustable interactions in chemistry, to carry magnetic coupling across relatively large distances within a crystal.

Because features like fluorination and other chemical substitutions can reshape how molecules pack together and how their hydrogen bonds form, the approach opens the door to systematically mapping how small structural tweaks produce large shifts in magnetic behavior. Beyond magnetism, the same strategy could help researchers studying other functional molecular materials where subtle structural changes drive outsized effects.