Revealing the exchange kinetics of thiol-capped Au₂₅ nanoclusters with alkynyl ligands

Ligand exchange is an important strategy to functionalize metal nanoclusters (NCs) for enhanced properties. Thiolates and alkynyls have been widely used for NC protection; however, the possibility for alkynyl-for-thiolate exchange as well as the kinetics for the ligand exchange process have remained largely unexplored. Herein, we have reported for the first time a kinetic investigation into the alkynyl-for-thiolate exchange through the reaction of thiolated Au₂₅(SR)₁₈ with the incoming alkynyl ligands. Interestingly, our simulations revealed the electronic and steric effects of alkynyl ligands and the precursor cluster's charge state collectively govern the exchange efficiency and regioselectivity. Notably, the alkynyl-for-thiolate exchange is highly facile when reacting with the nucleophilic lithium or gold(i)-alkynyl complex (Au(C[triple bond, length as m-dash]CR) or Li(C[triple bond, length as m-dash]CR)), but fails when using HC[triple bond, length as m-dash]CR as the exchange ligand. The Au(C[triple bond, length as m-dash]CPh) complex exhibits charge-state-dependent exchange at the S₁ or S₂ position, whereas the sterically bulky Au(C[triple bond, length as m-dash]C ^t Bu) complex decelerates the exchange kinetics and universally targets the S₁ position as the exchange product. By contrast, the lithium-alkynyl complex (Li(C[triple bond, length as m-dash]CPh) or Li(C[triple bond, length as m-dash]C ^t Bu)) preferentially leads to the exchange isomer, driven by the ionic Li-C bonding that enhances the C[triple bond, length as m-dash]C π*-electron density and alkynyl nucleophilicity. Our predictions are further validated by the ligand exchange experiments between phenyl ethanethiol (PET) protected [Au₂₅(PET)₁₈]^- and Au(C[triple bond, length as m-dash]C ^t Bu). The electrospray ionization mass spectra (ESI-MS) unambiguously confirm the successful substitution of 4, 5 and 6 PET ligands by the -C[triple bond, length as m-dash]C ^t Bu ligand, and the absorption spectrum drastically changes upon alkynyl exchange. This work establishes an important atomic-level understanding of the alkynyl-for-thiolate exchange mechanism, offering a convenient strategy for realizing alkynyl and thiolate co-protected gold clusters under mild conditions.