Counteranions reshape molecular packing to tune magnetism in copper complexes

Magnetic properties of molecular materials are influenced not only by their molecular components but also by their solid-state organization. In charged π-electronic systems, electrostatic and dispersion forces can arrange molecules into distinct ion-pairing structures. Oppositely charged species can form charge-by-charge assemblies, while like-charged units can stack together under favorable interactions.

The proximity and relative orientation of paramagnetic units in these assemblies affect intermolecular spin–spin interactions, allowing control over magnetic behavior through assembly pattern management. However, solid-state intermolecular spin–spin interactions in CuII complexes of π-electronic macrocycles have been observed only in limited cases.

Professor Hiromitsu Maeda's research team at Ritsumeikan University explored whether counteranions could control the assembly and magnetic behavior of thiaporphyrin CuII complexes. Their study, published in Chemical Science, focused on the CuII complexation of thiaporphyrins, which produces paramagnetic π-electronic cations. By synthesizing two such cations and exchanging chloride ions with various counteranions, the researchers observed different assembly modes and magnetic properties.

Thiaporphyrins, containing a thiophene unit within the porphyrin macrocycle, act as monoanionic ligands and complex with divalent metals to create positively charged π-electronic complexes. The incorporation of CuII introduces electron spin and charge, making these complexes valuable for investigating how ion-pairing structure influences magnetism.

The researchers obtained chloride ion pairs and then exchanged them for counteranions, including BF4-, PF6-, B(C6F5)4- (FABA-), and pentacyanocyclopentadienide (PCCp-). Single-crystal X-ray analysis, solid-state electron spin resonance (ESR), magnetic susceptibility measurements, UV/visible spectroscopy, and theoretical calculations were employed to characterize the resulting ion pairs and examine their solid-state structures and magnetic properties.

A clear structural contrast emerged between the assemblies. The PCCp- counteranion led to a charge-by-charge assembly, where one CuII complex formed a π-stacked ion pair with negligible delocalization of spin density onto the counteranion, resulting in negligible intermolecular spin–spin interactions. In contrast, ion pairs containing BF4-, PF6-, or FABA- counteranions formed π-stacked cation dimers, which assembled in a two-by-two packing mode.

ESR and magnetic susceptibility measurements revealed antiferromagnetic interactions in these dimer-based structures, with theoretical spin-density calculations supporting the experimental observations. The distance and orientation of the CuN3S units, as well as chalcogen-bonding and dipole-dipole interactions, played crucial roles in stabilizing the stacked dimers and governing the strength of the antiferromagnetic interaction.