Coordination-Triggered Coupling-Pathway Control in Organic Diradical Cations

Organic diradicals with stimuli-responsive spin interaction tunability have attracted considerable interest due to their unique controllable magnetic behaviors. However, due to the challenges of introducing tuning centers into the highly reactive diradical systems, only limited cases of tunable diradicals have been reported. Herein, we demonstrated that a boron center placed directly between two spin sources can serve as a coordination-responsive antiferromagnetic coupling unit to modulate magnetic interactions in an organic diradical system. We developed a boron-based diradical cation which can be readily coordinated by acetonitrile. Both the non-coordinated diradical cation and the adduct were comprehensively characterized by single-crystal X-ray diffraction, SQUID magnetometry, EPR spectroscopy, and further analyzed by DFT calculations. After coordination, the spin interaction through boron is weakened, resulting in a significant reduction in the antiferromagnetic coupling strength while shortening the average spin-spin distance. The non-coordinated form possesses a singlet ground state, while the adduct has nearly degenerate singlet and triplet states. To the best of our knowledge, this work represents the first experimental demonstration of coordination-triggered coupling pathway control in organic diradical cations, thereby providing a new approach for controlling spin interactions through chemical coordination.