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Correlation of heterogeneous electron transfer rates with structural change in cobalt tetraaza macrocyclic complexes

Journal of Electroanalytical Chemistry
Publication Date
DOI: 10.1016/0022-0728(88)80196-7


Abstract Heterogeneous electron transfer rates are reported for reduction of four [Co[N 4](H 2O) 2] 3+ complexes at Pt electrodes in perchloric acid solution and correlated with changes in molecular structure. N 4 is one of the four tetraaza ligands 1,4,8,11-tetraazacyclotetradecane (cyclam); 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene (TIM); 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene (Me 6N 4) and 12,14-dimethyl-1,4,8,11-tetraazacyclotetradeca-1,11-diene-13-one (Me 2N 4-one). The structural change accompanying reduction of Co(III) to Co(II) is lengthening of the axial Co-OH 2 bonds, which occurs to an extent (30 to 50 ppm) that is dependent on the nature of the N 4 ligand. The rate of electron transfer decreases (TIM > Me 4N 4-one > cyclam > Me 6N 4) as the extent of this displacement increases in accordance with the Marcus theory. The double layer corrected rate constants ( k corr s = 8 × 10 −4 to 2.9 × 10 −5 cm s −1) are in good agreement with “absolute” values calculated from the Marcus theory and also with values derived from homogeneous solution self-exchange rates. The generally ideal behavior of the [Co[N 4](H 2O) 2] 3+/2+ couple is contrasted with other systems which experience large changes in nuclear coordinates during electron transfer but do not follow the Marcus theory as closely.

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