Electronic Structure of Anilinopyridinate-Supported Ru25+ Paddlewheel Compounds

The electronic structures of the diruthenium compounds Ru2(ap)4Cl (1, ap = 2-anilinopyridinate) and Ru2(ap)4OTf (2) were investigated with UV–vis, resonance Raman, and magnetic circular dichroism (MCD) spectroscopies; SQUID magnetometry; and density functional theory (DFT) calculations. Both compounds have quartet spin ground states with large axial zero-field splitting of ∼60 cm–1 that is characteristic of Ru25+ compounds having a (π*, δ*)3 electron configuration and a Ru–Ru bond order of ∼2.5. Two major visible absorption features are observed at ∼770 and 430 nm in the electronic spectra, the assignments of which have previously been ambiguous. Both bands have significant charge-transfer character with some contributions from d → d transitions. MCD spectra were measured to enable the identification of d → d transitions that are not easily observable by UV–vis spectroscopy. In this way, we are able to identify bands due to δ → δ* and δ → π* transitions at ∼16 100 and 11 200–12 300 cm–1, respectively, the...