Self-Assembled p-Carborane Analogue of p-Mercaptobenzoic Acid on Au{111}

The p-carborane cluster analogue of p-mercaptobenzoic acid, 1-HS-12-COOH-1,12-C2B10H10, has been synthesized and characterized using nuclear magnetic resonance spectroscopy, single-crystal X-ray diffraction analysis, quantum-chemical calculations, and scanning tunneling microscopy. The single-crystal structure and selected packing aspects are discussed and presented in comparison with the two-dimensional periodic arrangements. Scanning tunneling micrographs, recorded under ambient conditions, are used to compare pure monolayers of 1-HS-1,12-C2B10H11 to coadsorbed monolayers of both the parental precursor and carboxyl-functionalized p-carboranethiolate on Au{111}. Monolayers of both constituents are further characterized by X-ray photoelectron spectroscopy, which shows good agreement between the stoichiometry of each pure monolayer and the nominal stoichiometries of the respective molecules. Results indicate that most of the molecules of both derivatives adsorb as thiolates but that a small fraction of each adsorbs as thiols, without complete SH bond scission, and consequently are labile relative to desorption. Wetting-angle measurements confirm the hydrophilic character of monolayers containing the carboxylic acid constituents. Mixed self-assembled monolayers with functionalized constituents of high axial symmetry provide a convenient basis for grafting two- and three-dimensional structures.