Reductive quenching of the emission of trans-dioxo(1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane)osmium(VI) in water

Reductive quenching of the red emission of the metal-centered excited state of the title osmium(VI) complex (0.3 mM, lifetime 1.0 μs) has been characterized in water at 25°C (0.5 M ionic strength). The excited state is a very strong oxidant and is very rapidly quenched by a number of moderately reducing anions (anion, 10^-9k_q, M^-1 s^-1): NO₂ ^-, 2.0; N₃ ^-, 4.0; I^-, 6.5. The rate constants for quenching by aqua ions are in fair agreement with values calculated from the Marcus cross-relation and self-exchange rates for the *Os^VI(tmc)O₂ ²⁺/Os^V(tmc)O ₂ ⁺ couple (k_ex = 1 × 10⁵ M^-1 s^-1) and the aqua ion couple (aqua ion, k_q): Fe(H₂O)₆ ²⁺, 1.0 × 10⁹ M^-1 s^-1; Co(H₂O)₆ ²⁺, 1.0 × 10⁶ M^-1 s^-1; Ce^III(aq), ≤1.0 × 10⁵ M^-1 s^-1. The k_q (M^-1 s^-1) values for quencher (Q) = Cl^- (4 × 10⁵), HCO₂ ^- (1.5 × 10⁶), Br^- (1.7 × 10⁸), SCN- (5.6 × 10⁹), CO₃ ^2- (1.6 × 10⁸), and OH- (1.0 × 10⁷) are used to estimate self-exchange rate constants for the Q⁺/Q couples. The first estimates are presented for the CO₃ ^2-/CO₃ ^- (0.4 M^-1 s^-1), OH^•/OH-(300 M^-1 s^-1), and HCO₂/HCO₂ ^- (300 M^-1 s^-1) couples. Self-exchange rates for the X^•/X^- halogen couples, calculated assuming that only the outer-shell barrier contributes to the intrinsic barrier to outer-sphere electron transfer and that a two-sphere dielectric continuum model is applicable, are orders of magnitude smaller than the self-exchange rates inferred from the quenching data and literature data.