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) Au-38(SC2Ph)(24) and Au-140(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 Au-38(SC2Ph)(24) MPC, measured using H-1 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 Au-38(SC2Ph)(24) are very close to those of corresponding exchange reactions on the larger nanoparticle Au-140(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 Au-38 and Au-140 cores, the latter being much slower presumably due to its larger terrace-like surface atom content. The reverse exchange reaction was studied for Au-38(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,K-PE = 4.0 that is independent of X.