Changing counteranions switches thiaporphyrin Cu^II complexes between charge-by-charge packing and antiferromagnetic π-stacked dimers in the solid state

Researchers at Ritsumeikan University and collaborating institutions have shown that counteranions control the solid-state assembly of paramagnetic thiaporphyrin Cu^II complex cations and thereby tune their magnetic behavior. PCCp⁻ formed a charge-by-charge assembly without significant spin–spin interaction, whereas nonplanar counteranions promoted π-stacked cation dimers exhibiting antiferromagnetic interactions. These findings demonstrate that ion-pairing assembly can control molecular spin arrangements, providing a strategy for designing supramolecular materials with tunable magnetic properties for spintronic applications.

Magnetic properties in molecular materials depend not only on the molecular components themselves but also on their solid-state organization. In charged π-electronic systems, electrostatic and dispersion forces can organize molecules into distinct ion-pairing structures. Oppositely charged species may form charge-by-charge assemblies, whereas like-charged π-electronic units can, under favorable intermolecular interactions, overcome electrostatic repulsion and form stacked dimers. Because intermolecular spin–spin interactions are sensitive to the proximity and relative orientation of paramagnetic units, controlling the assembly pattern provides a route to modulating magnetic behavior. Yet solid-state intermolecular spin–spin interactions in Cu^II complexes of π-electronic macrocycles have been reported only in limited cases.

A research team led by Professor Hiromitsu Maeda at Ritsumeikan University investigated whether counteranions could be used to control the assembly and magnetic behavior of thiaporphyrin Cu^II complexes. Their findings were published online in the journal Chemical Science on August 24, 2026. Thiaporphyrins contain a thiophene unit within the porphyrin macrocycle and can act as monoanionic ligands. Complexation with divalent metals produces positively charged π-electronic complexes through partial charge compensation. Incorporating Cu^II introduces electron spin as well as charge, making these complexes useful building blocks for studying how ion-pairing structure influences magnetism. “In this study, the Cu^II complexation of thiaporphyrins was investigated to afford paramagnetic π-electronic cations that modulate ion-pairing assembly modes in combination with coexisting anions,” said Professor Maeda.

The researchers synthesized two thiaporphyrin Cu^II complex cations and first obtained them as chloride ion pairs. The chloride ions were then exchanged for several counteranions, including BF₄⁻, PF₆⁻, B(C₆F₅)₄⁻ (FABA⁻), and pentacyanocyclopentadienide (PCCp⁻). The resulting ion pairs were characterized by single-crystal X-ray analysis, solid-state electron spin resonance (ESR), magnetic susceptibility measurements, UV/visible spectroscopy, and theoretical calculations to examine their solid-state structures and magnetic properties. Computational approaches were also used to evaluate interaction energies and spin-density distributions, enabling the team to connect molecular packing directly with magnetic behavior.

A clear structural contrast emerged. With the π-electronic PCCp⁻ counteranion, one Cu^II complex formed a charge-by-charge assembly based on π-stacked ion pairs. Another PCCp⁻ ion pair exhibited axial Cu–N coordination. In the charge-by-charge arrangement, spin density remained largely localized on the CuN₃S core, with negligible delocalization onto PCCp⁻. This was consistent with the absence of significant intermolecular spin–spin interactions.

Nonplanar counteranions produced a different outcome. Ion pairs containing BF₄⁻, PF₆⁻, or FABA⁻ formed π-stacked cation dimers that assembled in a two-by-two packing mode. ESR and magnetic susceptibility measurements indicated antiferromagnetic interactions in these dimer-based structures. Theoretical spin-density calculations supported the experimental observations, showing opposite spins localized on the respective stacked Cu^II-containing cations. Among these assemblies, differences in local S/N contacts and interdimer packing were associated with differences in the strength of the antiferromagnetic interaction.

The results further indicated that magnetic behavior was governed by specific structural factors. The distance and orientation of the CuN₃S units were crucial for spin–spin coupling, while chalcogen-bonding and dipole–dipole interactions contributed to stabilizing the stacked dimers. The counteranion therefore influenced not only whether dimerization occurred but also how those dimers were arranged in the crystal. “The design of π-electronic systems with charge and spin would provide fascinating strategies for the construction of supramolecular spintronic materials,” concluded Prof. Maeda.

Together, the findings demonstrate that counteranions can direct the assembly of paramagnetic molecular cations and, in turn, modulate their collective magnetic properties. By using ion pairing to control dimerization and spin arrangements, the study provides a molecular design strategy for constructing supramolecular spintronic materials in which magnetic behavior can be tuned through assembly. The work highlights the potential of π-electronic systems that combine charge and spin as building blocks for functional supramolecular materials.

Title of original paper: Ion-pairing-modulated magnetic properties of thiaporphyrin Cu^II complex cations
Journal: Chemical Science
DOI: 10.1039/d6sc04368b

About Professor Hiromitsu Maeda from Ritsumeikan University, Japan

Prof. Hiromitsu Maeda is a Professor in the Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University, Japan, and a Fellow of the Ritsumeikan Advanced Research Academy (RARA). He earned his Ph.D. from Kyoto University in 2004, following a three-month research stay at the University of Texas at Austin in 2001. He joined Ritsumeikan University's College of Science and Engineering in 2004 and transferred to the College of Pharmaceutical Sciences in 2008, becoming a Professor in 2014. He moved to the College of Life Sciences in 2016 and has held several additional research appointments in Japan and abroad.

Funding information

This work was supported by JSPS KAKENHI Grant Numbers JP22H02067 and JP23K23335 for Scientific Research (B), JP24KJ2151 for JSPS Fellows, JP23K17951 for Challenging Research (Exploratory), and JP20H05863 for Transformative Research Areas (A) “Condensed Conjugation,” and Ritsumeikan Global Innovation Research Organization (R-GIRO) project (2022-27). Theoretical calculations were partially performed at the Research Center for Computational Science, Okazaki, Japan (22-IMS-C077, 23-IMS-C069, 24-IMS-C067, 25-IMS-C069, and 26- IMS-C067). A part of this work was conducted in Institute for Molecular Science, supported by Advanced Research Infrastructure for Materials and Nanotechnology in Japan (JPMXP1225MS1061, JPMXP1225MS1075) of the Ministry of Education, Culture, Sport, Science and Technology (MEXT), Japan. We thank Dr. Hiroki Tanaka, Mr. Keita Ono, and Mr. Kohei Kawami, Ritsumeikan University, for the preparation of NaPCCp, Dr. Nobuhiro Yasuda, Dr. Yuiga Nakamura, Dr. Toshiyuki Sasaki, Dr. Shintaro Kobayashi, and Dr. Kouhei Ichiyanagi, JASRI/SPring-8 (2023A1645, 2023B1755, 2024A1465, 2024B1258, and 2025B1348) for synchrotron radiation single-crystal X-ray analysis, Dr. Shigeo Kuwamoto and Dr. Noboru Ohta, JASRI/ SPring-8 for synchrotron-radiation XRD measurements (BL19B2 and BL40B2 at SPring-8: 2025A1951, 2025B1688, and 2025B1870), and Prof. Hitoshi Tamiaki, Ritsumeikan University, for various measurements.