Assessing the Activating Effect of the (η5-C5Me5)Ru+ Group: A Kinetic and NMR Study of the Ionization of the (η6-Phenylnitromethane)- (η5-pentamethylcyclopentadienyl)ruthenium Cation in H2O−Me2SO Mixtures

Abstract

Rates of proton abstraction from the (η6-phenylnitromethane)(η5-pentamethylcylopentadienyl)ruthenium cation (6) by a variety of bases B (OH-, carboxylate ions, primary and secondary amines) and of reprotonation of the resulting carbon base C-6 by the conjugate acids BH have been measured in a 50:50 (v/v) H2O−Me2SO mixture at 25 °C. The intrinsic reactivity of 6, as determined from the Brönsted plots for carboxylate ion or primary amine reactions is rather low: log = 0.6; log = 0.1. These values together with the pKa value for the ionization of 6 (pKa = 5.90) are typical for the formation of a nitronate-type carbon base, indicating that the RuCp*+-complexed phenyl ring is not capable of competing with the exocyclic nitro group in absorbing the negative charge generated by the ionization of 6. Additional evidence that the cation 6 ionizes to give a zwitterionic carbon base with an exocyclic nitronate functionality comes from the observation that C-6 undergoes instantaneous protonation at low pH (pH < 3) to give a nitronic acid. 1H and 13C NMR data collected for the ionization of 6 in 50:50 (v/v) H2O−Me2SO and pure Me2SO agree with the above conclusions. From a comparison of the behavior of 6 with that of (4-nitrophenyl)nitromethane, the inductive effect of a RuCp*+-coordinated phenyl ring is shown to be identical to that of a 4-nitrophenyl ring.