Charge-transfer effects in ligand exchange reactions of Au₂₅ monolayer-protected clusters

Reported here are second-order rate constants of associative ligand exchanges of Au₂₅L₁₈ nanoparticles (L = phenylethanethiolate) of various charge states, measured by proton nuclear magnetic resonance at room temperature and below. Differences in second-order rate constants (M^-1 s^-1) of ligand exchange (positive clusters ∼1.9 × 10^-5 versus negative ones ∼1.2 × 10^-4) show that electron depletion retards ligand exchange. The ordering of rate constants between the ligands benzeneselenol > 4-bromobenzene thiol > benzenethiol reveals that exchange is accelerated by higher acidity and/or electron donation capability of the incoming ligand. Together, these observations indicate that partial charge transfer occurs between the nanoparticle and ligand during the exchange and that this is a rate-determining effect in the process.