Poly(pyridine)rutenium(II)-Photoinduced Redox Reactions of Bipyridinium Cations, Poly(pyridine)rhodium Complexes, and Osmium Ammines

Article abstract of DOI:10.1021/ja00377a013

The quenching of *RuL3²⁺ (L is a 2,2'-bipyridine or 1,10-phenanthroline derivative) emission by there classes of oxidants Q has been examined The results were discussed in terms of Marcus electron-transfer model recast in the preequilibrium formalism.Substituted bipyridinium cations (methyl viologen and related compounds undergo thermodinamically favorable reduction with driving force ranging from 0.1 to 0.7 eV and rate constants in range (0.4-2.0)*10⁹ M^-1s^-1, consistent with a diffusion rate constant of 2.0*10⁹ M^-1s^-1 and an exchange rate constant c.a. 10⁶ M^-1s^-1 for Q-Q^- couples.The yields of the separated redox products RuL³⁺ and Q^- (typically 0.1 mol einstein ^-1 per quenching act) require k₃₀, the intramolecular back-reduction rate constant (to re-form ground-state RuL3²⁺ and Q), to be (2-4)*10¹⁰s^-1.Since k₃₀ increases weakly with driving force in this series, there is no evidence for inverted behavior despite the fact that ΔG⁰₃₀ is c.a. -2eV.With Q=Rh(4,4'-(CH3)2bpy)3³⁺, the quenching rate constants, k_q=(0.001-1.0)*10⁹ M^-1s^-1, exhibit a great sensitivity to the reducing power of *RuL3²⁺ and have been fitted o k₁₁ c.a. 2*10⁹ M^-1s^-1 and E⁰_Q.Q'=-0.97 V for the RhL3³⁺-RhL3²⁺ couple.This E⁰ value is strikingly similar to that (-0.9 V vs. aqueous SCE) obtained via cyclic voltammetry in acetonitrile.The cyclic voltammetry of the RhL3³⁺ complexes in water has been reexamined, and it is concluded that the irreversibility observed is due to ligand loss from rhodium(I).After correction for an energy transfer component (k_q=k_el+k_em), the rate constants for reduction of Os^III(NH3)5X (k_el c.a. 2*10⁶-2*10⁸ M^-1s^-1 for X=Cl,Br,I,H2O,or NH3) and for oxidation of Os(NH3)5N2²⁺ are in accord with theoretical expectations and osmiun (III)-osmium(II) exchange rates in the range 10²-10⁵ M^-1s^-1.Some observations, conserning Os(NH3)5Cl⁺. which was generated and characterized by radiation chemistry and flash photolysis tichniques, are also reported.