Changing the counteranion decides whether copper thiaporphyrin cations pair up magnetically

The experiment's strength is its control. Each copper cation stayed fixed and only its partner anion changed ^[4]. Any difference in magnetism therefore comes from the anion and what it does to the packing. The team examined each salt several ways: single-crystal X-ray structures, solid-state ESR, magnetic susceptibility, and calculations of interaction energies and spin density ^[5].

The spin-density calculations rule out the most obvious alternative. In the PCCp salt with charge-by-charge stacks, the spin stayed on the CuN3S core, with negligible spread onto the anion ^[6]. In that salt the anion holds the copper centers apart and does not carry spin between them. In the dimer salts, the calculations put opposite spins on the two stacked cations ^[9]. The structures pointed to the distance and orientation of the CuN3S units as the factors that set the coupling ^[11].

The dimers are the chemically interesting part. Each one stacks two cations, and like charges repel. Like-charged pi-electronic units can overcome that repulsion and stack when other intermolecular interactions favor it ^[14]. According to the report, chalcogen bonding and dipole-dipole interactions held these dimers together ^[11]. Spin-spin interactions depend on how close paramagnetic units sit and how they are oriented ^[15], so bringing two copper cores face to face is what lets them couple.

I think the evidence supports calling packing a design lever for one decision: whether neighboring copper spins stay isolated or pair up. The anion decided whether dimers formed at all, and also how they were arranged in the crystal ^[12]. Finer tuning is less firmly established. Among the dimer salts, differences in local sulfur-nitrogen contacts and in how the dimers packed were associated with differences in antiferromagnetic strength, according to the report ^[10].

The sample is small. Two cations and four named anions allow at most eight pairings ^[1]. The published account does not report coupling constants, measurement temperatures or how many of those pairings were crystallized, so it does not show how wide a range of coupling strengths the anion can reach.

Before this work, solid-state spin coupling between copper(II) complexes of pi-electronic macrocycles had been reported in only a few cases ^[13]. Maeda's stated aim goes beyond this series. "The design of π-electronic systems with charge and spin would provide fascinating strategies for the construction of supramolecular spintronic materials," he said ^[16].