REDUCTIVE QUENCHING OF THE EMISSION OF ETHYL-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 mus) has b een characterized in water at 25-degrees-C (0.5 M ionic strength). The excited state is a very strong oxidant and is very rapidly quenched b y a number of moderately reducing anions (anion, 10(-9)k(q), M-1 s-1): NO2-, 2.0; N3-, 4.0; I-, 6.5. The rate constants for quenching by aqu a ions are in fair agreement with values calculated from the Marcus cr oss-relation and self-exchange rates for the Os(VI)(tmc)O2(2+)/Os(V)( tmc)O2+ couple (k(ex) = 1 X 10(5) M-1 s-1) and the aqua ion couple (aq ua ion, k(q)): Fe(H2O)62+, 1.0 X 10(9) M-1 s-1; Co(H2O)62+, 1.0 x 10(6 ) M-1 s-1; Ce(III)(aq), less-than-or-equal-to 1.0 X 10(5) M-1 s-1. The k(q) (M-1 s-1) values for quencher (Q) = Cl- (4 X 10(5)), HCO2- (1.5 X 10(6)), Br-(1.7 X 19(8)), SCN- (5.6 X 10(9)), CO32- (1.6 X 10(8)), a nd OH- (1.0 X 10(7)) are used to estimate self-exchange rate constants for the Q+/Q couples. The first estimates are presented for the CO32- /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, calculat ed assuming that only the outer-shell barrier contributes to the intri nsic barrier to outer-sphere electron transfer and that a two-sphere d ielectric continuum model is applicable, are orders of magnitude small er than the self-exchange rates inferred from the quenching data and l iterature data.