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 CuII 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 CuII 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 CuII 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 CuII 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 CuII 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 CuII 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 CuII-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.
