Electronic Structure of Anilinopyridinate-Supported Ru25+ Paddlewheel Compounds

The electronic structures of the diruthenium compounds Ru-2(ap)(4)Cl (1, ap = 2-anilinopyridinate) and Ru-2(ap)(4)OTf (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 similar to 60 cm(-1) that is characteristic of Ru-2(5+) compounds having a (pi*, delta*)(3) electron configuration and a Ru Ru bond order of similar to 2.5. Two major visible absorption features are observed at similar to 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 delta > delta* and delta > pi* transitions at similar to 16 100 and 11 200-12 300 cm(-1), respectively, the latter band being sensitive to the pi-donating character of the axial ligand. The Ru Ru stretches are coupled with pyridine rocking motions and give rise to observed resonance Raman peaks at 350 and 420 cm(-1), respectively.