# Tohoku University: water molecule orientation switches on magnetic pairing between copper ions in two Cu(II) complexes

- Published: 2026-11-10 13:58 UTC
- Section: Science › Chemistry
- Tags: molecular magnets, hydrogen bond magnetic coupling, water molecule ordering, copper(II) complexes, antiferromagnetic dimer, neutron diffraction JRR-3, magnetic susceptibility, fluorinated pincer complexes
- URL: https://realtech.media/news/rt-7db77337cdb3a8ee36cc4368_tohoku_university_water/

**In two fluorinated copper(II) complexes, ordering of water molecules below 260 K creates hydrogen-bond pathways that carry antiferromagnetic coupling between ions not directly bonded.**

![Tohoku University: water molecule orientation switches on magnetic pairing between copper ions in two Cu(II) complexes](https://realtech.media/media/2026/11/0281c9240d43650e.png)
*tohoku.ac.jp*

Researchers at Tohoku University's Institute of Multidisciplinary Research for Advanced Materials, together with National Taipei University of Technology (Taipei Tech), report that the orientation of water molecules can switch on magnetic interaction between copper ions that are not directly bonded, according to tohoku.ac.jp [1]. The work was published in Inorganic Chemistry. The team studied two synthesized compounds and found antiferromagnetic pairs forming at low temperature [1].

Molecular magnets are built from individual molecules, each acting as a small magnetic unit, and chemists can tune their elements, ligands and crystal packing. Earlier work mostly examined magnetic interactions through direct contact between ions or through strong covalent bonds. How weaker hydrogen bonds might carry magnetic interactions between distant ions has remained poorly understood [1].

## Structure: water ordering below 260 K

The researchers synthesized two fluorinated pincer-type copper(II) complexes, 2FH-CuCl₂·H₂O and 3F-CuCl₂·H₂O. They tracked temperature-dependent changes in the crystal structures with single-crystal X-ray diffraction and with neutron diffraction on the 4G-GPTAS instrument at the JRR-3 research reactor [1].

At room temperature, water molecules bound to the copper ions point in disordered directions, and the hydrogen-bonding pattern is uniform across the crystal. Below 260 K the water molecules adopt an ordered arrangement. The uniform pattern then splits into two distinct types of hydrogen bonds, and neighboring molecules become paired [1].

## Magnetism: antiferromagnetic pairs

Magnetic susceptibility of the compounds showed broad peaks around 5 to 7 K, followed by a drop toward zero. This is a signature of antiferromagnetic pairing, in which the magnetic moments of paired molecules cancel. The data fit a model of magnetic pairs with interaction strengths of roughly 10 K. Density functional theory calculations indicated that the hydrogen-bond network is the likely pathway for this interaction [1].

Norman Lu of Taipei Tech, a co-corresponding author, noted that the room-temperature structure does not allow pairing of molecules, so the antiferromagnetic pairs in the magnetization data were unexpected. Resolving the inconsistency required a closer look at the water molecules themselves [1].

"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 [1]. According to the authors, this offers a chemical route to tune magnetism without linking magnetic ions through strong bonds, and fluorination and other substitutions can change how molecules pack and how hydrogen bonds form [1].

## Sources

1. [tohoku.ac.jp](https://www.tohoku.ac.jp/en/press/water_molecules_turn_out_to_be_secret_messengers_for_magnetism.html)
2. [ResearchSEA](https://www.asiaresearchnews.com/node/23671)

---

Source: RealTech, realtech.media — https://realtech.media/news/rt-7db77337cdb3a8ee36cc4368_tohoku_university_water/