Effect of substituents on the water oxidation activity of [Ru(II)(terpy)(phen)Cl]+ procatalysts

A series of [Ru(II)(terpy-R)(phen-X)Cl]PF6 complexes was designed where terpy-R is the tridentate 4'-(4-methylmercaptophenyl)-2,2':6'2″-terpyridine ligand MeMPTP and phen-X is a substituted phenanthroline with hydro (1), 5-nitro (2), 5,6-dimethyl (3), and 3,4,7,8-tetramethyl (4). This series allows us to compare the reactivity of phenanthroline-containing procatalysts with that of its well-established bipyridine counterparts as well as to study the effects of electron-withdrawing and -donating substituents on water oxidation. These species were thoroughly characterized by spectroscopic and spectrometric methods, and the structures of 1, 3, and 4 were determined by single-crystal X-ray diffraction. The procatalysts 1-4 show opposite trends compared to known terpyridine/bipyridine species; the unsubstituted procatalyst 1 yields a turnover number (TON) of 410 followed by 250 and 150 for complexes 3 and 4 with electron-donating substituents. Species 2, with electron-withdrawing properties, yields the lowest TON of 60. Although the TONs decrease upon substitution, the presence of electron-donating methyl substituents enhances the rate of O2 evolution during an early stage of catalysis. Interestingly, no evidence of conversion from chlorido-containing procatalysts into expected aqua-containing catalysts was observed for 1-4 by NMR and UV-visible spectroscopy during the induction period. This observation, along with reactivity toward (NH4)2[Ce(IV)(NO3)6], suggests that water nucleophilic attack happens to a high-valent ruthenium species rather than while at the Ru(II) oxidation state. Reactivity follows a trend similar to the rate of O2 evolution in all complexes. Furthermore, the electrospray ionization mass spectrometry and (1)H NMR analyses of 1, as recovered after catalysis, indicate the presence of a chlorido ligand.