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 [Ru₃(OAc)₆(μ₃-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 (× 10⁷ M^–1 s^–1) in deuterated acetonitrile (ACN-<i>d</i>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 ¹H NMR contact shifts of the pep ligand that were found to depend on the electron-donating or -withdrawing nature of the ancillary pyridine ligand