Reductive Quenching of the Emission of trans-Dioxo(1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane)osmium(VI) in Water

Siegfried Schindler, Edward W. Castner, Carol Creutz, Norman Sutin

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Abstract

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, 109kq, 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 *OsVI(tmc)O22+/Osv(tmc)O2+ couple (kex = 1 × 105 M−1 s−1) and the aqua ion couple (aqua ion, kq): Fe(H2O)62+, 1.0 × 109 M−1 s−1; Co(H2O)62+, 1.0 × 106 M−1 s−1; CeIII(aq), ≤1.0 × 105 M−1 s−1. The kq (M−1 s−1) values for quencher (Q) = Cl (4 × 105), HCO2 (1.5 × 106), Br(1.7 × 108), SCN(5.6 × 109), CO32− (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 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, 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.

Original languageEnglish (US)
Pages (from-to)4200-4208
Number of pages9
JournalInorganic Chemistry
Volume32
Issue number20
DOIs
Publication statusPublished - Jan 1 1993
Externally publishedYes

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All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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