Ordering of water molecules switches on magnetic pairing in copper complexes

Researchers at Tohoku University, working with National Taipei University of Technology (Taipei Tech), report that the orientation of water molecules can determine whether magnetic copper ions couple to one another. In two fluorinated copper(II) complexes, water molecules settling into an ordered arrangement on cooling produced antiferromagnetic pairs of ions that are not joined by strong chemical bonds. The study appears in Inorganic Chemistry.

Why hydrogen bonds are an unusual route

Molecular magnets are built from discrete molecules that each behave as a small magnetic unit. Chemists can adjust their metal centres, ligands and crystal packing, which makes them attractive for designing new magnetic behaviour. Most work has concentrated on interactions passing through direct contact or covalent bonding pathways. Hydrogen bonds are common in molecular crystals, but how far they can carry magnetic coupling between distant ions has been poorly understood.

The team tested whether hydrogen bonding alone could link copper ions that lack a strong bonded connection.

Structure changes with temperature

The researchers synthesised two pincer-type copper(II) complexes carrying fluorine substituents, labelled 2FH-CuCl₂·H₂O and 3F-CuCl₂·H₂O. They followed the crystal structures as temperature changed using single-crystal X-ray diffraction and neutron diffraction, the latter at the 4G-GPTAS instrument at the JRR-3 research reactor.

At room temperature, the water molecules bound to the copper ions point in disordered directions, and the hydrogen-bonding pattern is the same throughout the crystal. Below 260 K, the water molecules adopt an ordered arrangement. This divides the uniform hydrogen-bonding pattern into two distinct types and brings neighbouring molecules together as pairs.

Magnetic evidence for paired ions

Magnetic measurements supported the structural picture. Both compounds showed broad maxima in magnetic susceptibility at roughly 5 to 7 K, followed by a fall towards zero. This is characteristic of antiferromagnetic pairing, where the moments of the two partners cancel. The data fitted a model of isolated magnetic pairs with interaction strengths of about 10 K. Density functional theory calculations indicated that the hydrogen-bonding network is the likely pathway for this coupling.

The result initially looked inconsistent. The room-temperature structure does not allow molecules to pair, yet the magnetisation data showed pairs. Norman Lu of Taipei Tech, a co-corresponding author, said resolving this required closer study of the water molecules themselves. The low-temperature ordering explained the discrepancy.

Significance and next steps

The work suggests that hydrogen bonding, which is common and readily adjusted, could serve as a design element for magnetic coupling across relatively large distances in a crystal. Kazuhiro Nawa, associate professor at Tohoku and lead author, described it as a chemical route to tune magnetism without linking the magnetic ions through strong bonds.

Because fluorination and other substitutions alter molecular packing and hydrogen-bond formation, the authors point to a systematic way of mapping how small structural changes shift magnetic behaviour. The same approach might also be relevant to other functional molecular materials in which minor structural differences have large effects.

The findings are limited to two related copper complexes, and the pairing appears only at low temperatures, with interaction strengths of around 10 K. Testing whether the principle holds across other metal ions, ligands and hydrogen-bonding motifs will be the key step in judging how general it is.

The paper is Nawa et al., "Formation of Antiferromagnetic Spin Dimer Induced by Orientation Ordering of Water Molecule in Fluorinated Pincered Cu(II) Complexes", Inorganic Chemistry (2026).

Source: Water molecules turn out to be secret messengers for magnetism (Phys.org, 11 November 2026).

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