The role of the radical tetrazine bridging ligand in spin-only magnetic coupling in complex dimers

Abstract

This study explores unusual magnetic coupling between two metal centers in dimers bridged by a tetrazine ring. Computational analysis was performed to understand how a radical perturbation on the bridging ligand influences the magnetic properties of previously synthesized cobalt (Co) and nickel (Ni) dimers. We applied broken-symmetry density functional theory (BS-DFT) functionals to gain deeper insights into the magnetic interactions. This approach provided a comprehensive quantitative picture of the coupling nature of the metal centers. To study the effect of the radical, the two dimers are studied in two oxidation states of bridging tetrazine: the neutral state and the radical state. The radical state was achieved by reducing the tetrazine ring through one-electron transfer. This approach allowed us to monitor the alterations in magnetic properties due to monoradical perturbation. For the dimers in their oxidized form, we observed ferromagnetic coupling between the Ni(II) centers, in contrast to the corresponding neutral Co(II) dimers, which exhibited strong antiferromagnetic coupling. Concerning the radical species, in both Co and Ni dimers, the magnetic calculations aligned with experimental measurements. This alignment demonstrated that the magnetic coupling between the metal and the radical led to robust ferromagnetic coupling. These results were further validated through orbital Mulliken spin density analysis. To support our findings, we developed an in-house code that generated temperature-dependent magnetic moment diagrams, coupled with energy-dependent spin-coupled states. These diagrams provided evidence of the ferromagnetic nature of the two radical dimers.