Kinetics of self-exchange of bis(1,4,7-trithiacyclononane)iron(III)/(II) in acetonitrile and in acidic aqueous solution

Abstract

HIDEO DOINE and THOMAS W. SWADDLE. Can. J. Chem. 68, 2228 (1990). The rate constant kc, of the Fe([9]anes3);+"+ self-exchange reaction cannot be measured in most common solvents because of spontaneous reductionof the Fe([9]anes3):' ion, which is also sensitive to photolysis by visible light. However, in CD3CN at -41 to -19"C, reproducible proton-decoupled 13c NMR line broadening measurements are possible, and give kc, = (5.3 ? 0.3) x lo4 kg mol-' s f ' at O°C, AH* = 10.3 ? 1.8 kJ mol-I, and AS* = -1 16 2 7 J K-' mol-I, at ionic strength I = 0.1 mol kg-'. Proton NMR line broadening experiments are marginally practicable in very acidic water (2.0 mol kg-' D2S04/D20) near O°C, and give kc, = 3.2 x lo6 kg mol-' s-I at 1°C. The relative kc, values of these and other low-spin/low-spin ~e""" self-exchange reactions follow the predictions of the Marcus-Hush theory at least qualitatively. The effect of ionic strength, however, is less than predicted, probably because of the formation of less reactive anion-cation pairs.Key words: electron transfer kinetics, crown thioether complexes HIDEO DOINE et THOMAS W. SWADDLE. Can. J . Chem. 68, 2228 (1990). Dans laplupart des solvants usuels, il n'est pas possible de mesurer laconstante de vitesse k,, de larkaction d'auto-Cchange du Fe([9]anes3);+"+; cette situation est causCe par la rkduction spontanCe de I'ion Fe([9]ane~~):+, qui est aussi sensible la photolyse par la lumikre visible. Toutefois, dans le CD3CN, entre -41 et -1g0C, il est possible d'obtenir des Clargissements reproductibles des raies des spectres de rmn du 13C dCcoupICs du proton et on peat en deduire que, a 0°C et a une force ionique T = 0,l mol kg-', kc, = (5,3 t 0,3) X lo4 kg mol-' s-', AH* = 10,3 ? 1,8 kJ mol-I, et AS* = -117 2 7 J K-' mol-I. Les expCriences d'Clargissment des raies de la rmn du proton ne sont pas tres faciles a rCaliser en milieu trks acide (2,O mol kg-' de D2S04/D20), pres de 0°C; on a pu toutefois pu etablir que, a l0C, kc, = 3.2 x lo6 kg m o l ' s-'. Les valeurs relatives de kc, pour ces rCactions d'auto-Cchange et celles d'autres rkactions semblables des Fe""" faible spin/faible spin suivent au moins qualitativement les prCdictions de la thCorie de Marcus-Hush. Toutefois, I'effet de la force ionique n'est meme pas prCdit; probablement a cause de la formation de paires d'anion-cation moins rCactives.

Electron-transfer kinetics and equilibria of copper(ii/i) complexes with 1,4,7-trithiacyclononane. A square scheme mechanism involving ligand additionBased on the presentation given at Dalton Discussion No. 4, 10–13th January 2002, Kloster Banz, Germany.

“…3 This unusual geometry has led to a large number of studies on the [9]aneS 3 complexes formed with a variety of metal ions in which the spectral properties and/or structures have been reported. 4 Takagi (Doine) and Swaddle have also studied the electron-transfer kinetics of the [9]aneS 3 complexes formed with Fe(/), 5 Co(/), 6 Pd(/), 7 Pt(/) 7 and Au(/). 7 In these latter systems it is presumed that the coordination geometry remains relatively constant during electron transfer.…”