| Abstract
| - Dense monolayers of[Os(bpy)2(p3p)2]2+,where bpy is 2,2‘-bipyridyl and p3p is4,4‘-trimethylenedipyridine,have been formed by spontaneous adsorption onto clean platinummicroelectrodes. Cyclic voltammetry ofthese monolayers is nearly ideal, and the area occupied per moleculesuggests that only one of the p3p ligandsbinds to the electrode surface, the other being available forprotonation. Chronoamperometry conducted ona microsecond time scale has been used to measure the heterogeneouselectron transfer rate constant k for theOs2+/3+ redox reaction. For electrolyteconcentrations above 0.1 M, heterogeneous electron transferischaracterized by a single unimolecular rate constant(k/s-1). Tafel plots of thedependence of ln k on theoverpotential η show curvature, and larger cathodic than anodic rateconstants are observed for a given absolutevalue of η. This response is consistent with electron transferoccurring via a through-space tunnelingmechanism. Plots of k vs pH are sigmoidal, and thestandard heterogeneous rate constant k° decreasesfrom(6.1 ± 0.2) × 104 to (1.6 ± 0.1) × 104s-1 as the pH of the contacting solution isdecreased from 5.05 to1.07. When in contact with pH 5.05 electrolyte, theelectrochemical enthalpy ΔH⧧ is 37.5 ± 2.1kJ mol-1,which decreases to 24.6 ± 1.5 kJ mol-1 at apH of 1.07. The reaction entropy ΔSrc°is independent of thepH over this range, maintaining a value of 82 ± 7 Jmol-1 K-1. Incontrast to the behavior expected fromthe decrease of k with decreasing pH, the free energy ofactivation ΔG⧧ decreases with decreasing pH.Theelectronic transmission coefficient κel, describing theprobability of electron transfer once the nucleartransitionstate has been reached, is considerably less than unity for all pH'sinvestigated. κel decreases withdecreasingsolution pH, suggesting an increasingly weaker electronic interactionbetween the metallic states of the electrodeand the orbitals of the redox center as the monolayer becomesprotonated. These results suggest that monolayerprotonation modulates the heterogeneous electron transfer rate bychanging the through-space electron transferdistance. This may be caused either by a change in the tilt anglebetween the adsorbate and the electrode orby the methylene spacer units within the bridging ligand becomingextended, when the monolayer is protonated.
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