Decaborane Thiols as Building Blocks for Self-Assembled Monolayers on Metal Surfaces

Three nido-decaborane thiol cluster compounds, [1-(HS)-nido-B₁₀H₁₃] 1, [2-(HS)-nido-B₁₀H₁₃] 2, and [1,2-(HS)₂-nido-B₁₀H₁₂] 3 have been characterized using NMR spectroscopy, single-crystal X-ray diffraction analysis, and quantum-chemical calculations. In the solid state, 1, 2, and 3 feature weak intermolecular hydrogen bonding between the sulfur atom and the relatively positive bridging hydrogen atoms on the open face of an adjacent cluster. Density functional theory (DFT) calculations show that the value of the interaction energy is approximately proportional to the number of hydrogen atoms involved in the interaction and that these values are consistent with a related bridging-hydrogen atom interaction calculated for a B₁₈H₂₂·C₆H₆ solvate. Self-assembled monolayers (SAMs) of 1, 2, and 3 on gold and silver surfaces have been prepared and characterized using X-ray photoelectron spectroscopy. The variations in the measured sulfur binding energies, as thiolates on the surface, correlate with the (CC2) calculated atomic charge for the relevant boron vertices and for the associated sulfur substituents for the parent B₁₀H₁₃(SH) compounds. The calculated charges also correlate with the measured and DFT-calculated thiol ¹H chemical shifts. Wetting-angle measurements indicate that the hydrophilic open face of the cluster is directed upward from the substrate surface, allowing the bridging hydrogen atoms to exhibit a similar reactivity to that of the bulk compound. Thus, [PtMe₂(PMe₂Ph)₂] reacts with the exposed and acidic B–H–B bridging hydrogen atoms of a SAM of 1 on a gold substrate, affording the addition of the metal moiety to the cluster. The XPS-derived stoichiometry is very similar to that for a SAM produced directly from the adsorption of [1-(HS)-7,7-(PMe₂Ph)₂-nido-7-PtB₁₀H₁₁] 4. The use of reactive boron hydride SAMs as templates on which further chemistry may be carried out is unprecedented, and the principle may be extended to other binary boron hydride clusters.