Probing the Electronic Structure of a Ru(II) Iminonitroxide Biradical Complex by UV-Vis-NIR Spectroscopy and TD-DFT

Coordination of organic biradicals to transition metals is a promising route to multifunctional materials, but controlling the metal oxidation state during complexation remains challenging. Here we attempted to prepare a ruthenium(III) complex with the 2,6-bis(1-oxyl-4,4,5,5-tetramethyl-4,5-dihydro-1-H-imidazol-2-yl)pyridine (bisimpy), tridentate imino-nitroxide, from [RuCl3(PhCN)3]. Instead, redox processes between the ligand and the ruthenium precursor in solution yielded the ruthenium(II) complex [Ru(bisimpy)(PhCN)Cl2]. The product was characterized by single-crystal X-ray diffraction and UV-vis absorption in solution supported by TD-DFT and BS-DFT calculations. Structural analysis confirms the tridentate coordination of bisimpy to a centrosymmetric Ru(II) ion, rendering the two iminonitroxide fragments equivalent. UV-vis spectra show intense ligand-centered bands in the UV region and MLCT transitions in the near-IR region absent in the free biradical. TD-DFT computation reproduces the experimental spectra assigning the features to metal-to-ligand, and mixed ligand–ligand/metal–ligand transitions. Notably, BS-DFT computation predicts a dramatic change in the intramolecular exchange between the two radical centers: whereas the free biradical is weakly ferromagnetic, coordination to the diamagnetic Ru(II) center switches the coupling to strongly antiferromagnetic through a metal-mediated superexchange pathway. These results show that the imino-nitroxide biradical directs the reaction toward Ru(II) rather than the targeted Ru(III) species and demonstrate that a formally diamagnetic d⁶ metal can actively modulate radical–radical magnetic communication. In the Ru(II) complex, the diamagnetic low-spin 4d⁶ center (S = 0) does not participate in direct magnetic exchange with the radicals, but it mediates a strong antiferromagnetic coupling between the two radical centers through superexchange involving the 4d metal orbitals.