Surface coordination chemistry of 2,5-dihydroxythiophenol at well-defined Pd(111) electrodes: Studies by LEED, AES, HREELS and electrochemistry

The coordination chemistry and electrochemical reactivity of 2,5-dihydroxythiophenol (DHT) adsorbed from aqueous solutions onto a well-defined Pd(111) electrode surface has been studied by a combination of low-energy electron diffraction, Auger electron spectroscopy, high-resolution electron energy loss spectroscopy and electrochemistry. The results indicate that DHT is chemisorbed in two discrete non-random orientations that depend upon the concentration of the adsorbate in solution. At high concentrations, DHT is oxidatively chemisorbed through the -SH group in a vertical S-η1 orientation with the loss of a sulfhydryl hydrogen. In this mode of surface coordination, the pendant diphenol group exhibits reversible quinone/diphenol redox activity similar to that for the solution-borne (unadsorbed) species; however, the width of the voltammetric peak is much broader for the surface species due to substrate-mediated adsorbate-adsorbate interactions. At low concentrations, DHT functions as a bidentate ligand in which the diphenol and -SH groups are both oxidatively coordinated as quinonoid and S moieties at the surface. In this orientation, the organic group is no longer pendant and no quinone/diphenol redox is displayed. No LEED patterns were discernible which indicated that, regardless of orientation, the molecular adlayers did not have long-range order.