Protonation effects on superexchange across gold/osmium bis(bipyridyl) tetrazine chloride monolayer interfaces

Monolayers of [Os(bpy)(2)(4-tet)Cl](+), where bpy is 2,2'-bipyridyl and 4-t et is 3.6-bis(4-pyridyl)-1,2,4,5-tetrazine, nave been formed by spontaneous adsorption onto clean gold microelectrodes. These monolayers are stable an d exhibit well-defined voltammetric responses for the Os2+/3+ redox reactio n across a wide range of solution pH values. The shift in the formal potent ial with increasing perchlorate concentration indicates that the oxidized f orm is ion-paired to a single additional perchlorate anion. The tetrazine b ridge between the [Os(bpy)(2)Cl](+) moiety and the electrode surface underg oes a reversible protonation/deprotonation reaction. The pK(a) of the tetra zine within the monolayer has been determined as 3.7 +/- 0.3 from the pH de pendence of the interfacial capacitance. Significantly, this value is indis tinguishable from that found for the complex dissolved in essentially aqueo ns solution suggesting that these monolayers are highly solvated. High-spee d cyclic voltammetry reveals that the redox switching mechanism is best des cribed as a nonadiabatic, through-bond tunneling process. Significantly, wh ile protonating the bridging ligand does not influence the free energy of a ctivation, 10.3 +/- 1.1 kJ mol(-1), k degrees decreases by 1 order of magni tude from 1.1 x 10(4) to 1.2 x 10(3) s(-1) on going from a deprotonated to protonated bridge. These observations are interpreted in terms of a through -bond hole tunneling mechanism in which protonation decreases the electron density on the bridge and reduces the strength of the electronic coupling b etween the metal center and the electrode.