Titanium(III) chemistry: electron-transfer reactions with ground-state poly(pyridine)osmium(III) complexes, quenching reactions with excited-state poly(pyridine)ruthenium(II) complexes, and formal reduction potentials

Potentiometric titrations of aquotitanium(III) with ferric ions have been performed in hydrochloric acid solution. The potentiometric data are described by E (V vs. NHE) = 0.03 - 0.059 log ([Ti(III)]/[Ti(IV)][H+]2) at 250C and 3.0 M ionic strength and are consistent with the interpretation that Ti3+(aq) and TiO2+ are the principal titanium(III) and titanium(IV) species, respectively, present at acid concentrations of 0.1 - 3.0 M. Rate constants for the oxidation of titanium(III) by a series of osmium(III) complexes, OsL33+, where L is a bipyridine or phenanthroline derivative, have been determined at 250C and 3.0 M ionic strength. The rates are interpreted in terms of the reactions of Ti3+(aq) and TiOH2+. The rate constants are correlated with the Marcus theory, and the exchange rate constants for the Ti4+3+(aq) and TiOH3+2+ couples are estimated. The rate constants for the reaction of titanium(III) with the excited states of the ruthenium(II) complexes RuL32+ are insensitive to the reduction potentials of the ruthenium(II) complexes. Arguments are presented to show that the quenching reactions proceed predominatly by energy-transfer mechanisms. The implications of these results for the production of Ti2+(aq) in quenching reactions are discussed. 6 figures, 5 tables