Forming Chemisorbed Single-Molecule Junctions through Loss of Stable Carbocations

Recent studies have found that "chemically inert" gold surfaces may drive S-C(sp³) bond cleavage reactions in thioether (-SR) linker groups, providing access to single-molecule junctions with chemisorbed Au-S contacts following the elimination of R⁺. Here, we demonstrate that such transformations occur more readily at elevated temperatures, rough surfaces, and in nonpolar solvents. We further show that a greater proportion of chemisorbed bonds are formed when R = -CPh₃ or -C₇H₇ than when R = -^tBu, consistent with the relative stability of [^tBu]⁺ < [CPh₃]⁺ ∼ [C₇H₇]⁺ carbocations. Our contact chemistry assignments are supported by first-principles transmission calculations, and we apply potential energy calculations to expose the relatively small influence of applied external electric fields on this bond breaking process. Together, this work provides a deeper understanding of reactivity at metal surfaces, of broad relevance to heterogeneous catalysis and critical to the stability and function of molecular junctions and monolayers.