Tuning Electron Delocalization and Transfer Rates in Mixed-Valent Ru3O Complexes through "Push-Pull" Effects

Electron transfer rates in a series of Oxo-centered triruthenium clusters featuring an extended aromatic ancillary ligand of the type [Ru-3(OAc)(6)(mu(3)-O)(CO)(L)(pep)], where L = 4-cyanopyridine (cpy), pyridine (py), or 4-(dimethylamino)pyridine (dmap) and pep = 4-(phenylethynyl)pyridine were investigated. The electron self-exchange rate constants for the 0/- couple were determined by 1H NMR line broadening experiments and found to range from 4.3 to 9.2 (x 10(7) M-1 s(-1)) in deuterated acetonitrile (ACN-d(3)). Relative rates of self-exchange can be rationalized on the basis of increased contact area between self-exchanging pairs, and a push-pull modulation of electron density between the pep vs ancillary pyridine ligands. Faster self-exchange was observed with increasing electron-donating character of the ancillary pyridine ligand substituent (dmap > py > cpy), and this was attributed to increased orbital Overlap between self-exchanging pairs. These results are supported by trends observed in H-1 NMR contact shifts of the pep ligand that were found to depend on the electron-donating or -withdrawing nature of the ancillary pyridine ligand.