ASSESSING THE ACTIVATING EFFECT OF THE (ETA(5)-C5ME5)RU- A KINETIC AND NMR-STUDY OF THE IONIZATION OF THE ANE)-(ETA(5)-PENTAMETHYLCYCLOPENTADIENYL)RUTHENIUM CATION IN H2O-ME2SO MIXTURES( GROUP )

Rates of proton abstraction from the thane)(eta(5)-pentamethylcylopent adienyl)ruthenium cation (6) by a variety of bases B (OH-, carboxylate ions, primary and secondary amines) and of reprotonation of the resul ting carbon base C-6 by the conjugate acids BH have been measured in a 50:50 (v/v) H2O-Me2SO mixture at 25 degrees C. The intrinsic reactivi ty of 6, as determined from the Bronsted plots for carboxylate ion or primary amine reactions is rather low: log k(0)(RCOO-) = 0.6; log k(0) (RNH2) = 0.1. These values together with the pK(a) value for the ioniz ation of 6 (pK(a) = 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 absorbin g the negative charge generated by the ionization of 6. Additional evi dence that the cation 6 ionizes to give a zwitterionic carbon base wit h an exocyclic nitronate functionality comes from the observation that C-6 undergoes instantaneous protonation at low pH (pH < 3) to give a nitronic acid. H-1 and C-13 NMR data collected for the ionization of 6 in 50:50 (v/v) H2O-Me2SO and pure Me2SO agree with the above conclusi ons. From a comparison of the behavior of 6 with that of (4-nitropheny l)nitromethane, the inductive effect of a RuCp(+)-coordinated phenyl ring is shown to be identical to that of a 4-nitrophenyl ring.