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Does Core Size Matter in the Kinetics of Ligand Exchanges of Monolayer-Protected Au Clusters?
journal contributionposted on 2005-03-02, 00:00 authored by Rui Guo, Yang Song, Gangli Wang, Royce W. Murray
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 KEQ,PE = 4.0 that is independent of X.