Metal-Ion-Mediated Spin Coupling in Nitronyl Nitroxide Systems: From Superexchange to 4f Electron-Radical Interactions

To construct tunable spin-coupling systems and explain the role of metal ions in modulating radical-based exchange interactions, two phenanthroline-based nitronyl nitroxide (NN) radicals, Br-PhenPhNN and PhendiPhNN, were synthesized and then used as ligands; Cu+ and Gd3+ coordination complexes were prepared. Magnetic studies revealed distinct metal-dependent coupling mechanisms. Superconducting Quantum Interference Device (SQUID) magnetic susceptibility measurements revealed antiferromagnetic interactions in the diamagnetic Cu+ complexes, mediated by metal-centered superexchange between the radical ligands, with J = -5.069 cm-1 for Cu+(Br-PhenPhNN)2 and J = -1.394 cm-1 for Cu+(PhendiPhNN)2. In contrast, the paramagnetic Gd3+ complexes exhibit ferromagnetic 4f electron-radical coupling, where JGd-rad = 2.429 cm-1 for Gd3+(Br-PhenPhNN)2 and 1.494 cm-1 for Gd3+(PhendiPhNN)2, accompanied by antiferromagnetic radical-radical interactions with Jrad-rad = -8.677 and -6.694 cm-1, respectively. These results show that Cu+ complexes are dominated by superexchange interactions, whereas Gd3+ complexes feature coexisting multiple competing spin interactions. This distinction highlights the crucial role of the metal center in controlling both the type and strength of radical spin coupling and provides guidance for the rational design of molecular magnetic materials.