Does Core Size Matter in the Kinetics of Ligand Exchanges of Monolayer-Protected Au Clusters?

This paper compares the kinetics of exchanges of phenylethanethiolate ligands (PhC2S−) of the monolayer-protected clusters (MPCs) Au₃₈(SC2Ph)₂₄ and Au₁₄₀(SC2Ph)₅₃ with p-substituted arylthiols (p-X−PhSH), where X = NO₂, Br, CH₃, OCH₃, and OH. First-order rate constants at 293 K for exchange of the first ca. 25% of the ligands on the molecule-like Au₃₈(SC2Ph)₂₄ MPC, measured using ¹H NMR, vary linearly with the in-coming arythiol concentration; ligand exchange is an overall second-order reaction. Remarkably, the second-order rate constants for ligand exchange on Au₃₈(SC2Ph)₂₄ are very close to those of corresponding exchange reactions on the larger nanoparticle Au₁₄₀(SC2Ph)₅₃ MPCs. These are the first results that quantitatively show that the chemical reactivity of different sized nanocrystals is almost independent of size; presumably, this is because the locus of the initial ligand exchanges is a common kind of site, thought to be the nanocrystal vertexes. The rates of later stages of exchange (beyond ca. 25%) differ for Au₃₈ and Au₁₄₀ cores, the latter being much slower presumably due to its larger terrace-like surface atom content. The reverse exchange reaction was studied for Au₃₈(p-X−arylthiolate)₂₄ MPCs (X = NO₂, Br, and CH₃), where the in-coming ligand is now phenylethanethiol. Remarkably, the rate constants of both forward and reverse exchanges display identical substituent effects, which implies a concurrent bonding of both in-coming and leaving ligands to the Au core in the rate-determining step, as in an associative mechanism. X = NO₂ gives the fastest rates, and the ratio of forward and reverse rate constants gives an equilibrium constant of K_EQ,PE = 4.0 that is independent of X.