Substituent Effects on the Exchange Dynamics of Ligands on 1.6 nm Diameter Gold Nanoparticles

Abstract

The kinetics of exchange ofphenylethanethiolate ligands (PhC2S) of monolayer-protected clusters (MPCs, average formula Au140(PhC2S)53) by para-substituted arylthiols (p-X-ArSH) are described. 1H NMR measurements of thiol concentrations show that the exchange reaction is initially rapid and gradually slows almost to a standstill. The most labile ligands, exchanging at the shortest reaction times, are thought to be those at defect sites (edges, vertexes) on the nanoparticle core surface. The pseudo-first-order rate constants derived from the first 10% of the exchange reaction profile vary linearly with in-coming arylthiol concentration, meaning that the labile ligands exchange in a second-order process, which is consistent with ligand exchange being an associative process. A linear Hammett relationship with slope p = 0.44 demonstrates a substituent effect in the ligand place exchange reaction, in which the bimolecular rate constants increase for ligands with electron-withdrawing substituents (1.4 x 10-2 and 3.8 x 10(-3) M(-1) s(-1) for X = NO2 and 4-OH, respectively). This is interpreted as the more polar Au-S bonds at the defect sites favoring bonding with more electron deficient sulfur moieties. At longer reaction times, where ligands exchange on nondefect (terrace) as well as defect sites, the extent of ligand exchange is higher for thiols with more electron-donating substituents. The difference between short-time kinetics and longer-time pseudoequilibria is rationalized based on differences in Au-S bonding at defect vs nondefect MPC core sites. The study adds substance to the mechanisms of exchange of protecting ligands on nanoparticles. The scope and limitations of 1H NMR spectroscopy for determining rate data are also discussed.

Divalent Metal Nanoparticles

“…52 The chemistry of the incoming ligands matters as well: for para- substituted arylthiols, the reaction rate is increased for ligands with electron withdrawing substituents, a functionality that favors the polar Au-S bond. 48 Characteristics of the NP core itself also affect the place-exchange kinetics. Faster exchange is observed on more positively charged cores, and the equilibrium exchange percentage on these NPs is also higher.…”

The SERS effect in coordination chemistry

“…Kinetic and thermodynamic factors are involved in the balance of the covalent interaction between the surface and the ligand, as well as in the non-covalent ligand interactions, including the electrostatic and steric effects at the surface [139,143]. In general, the kinetics seem to proceed according to pseudo-first order and second order processes [142,144,145], but they have also been described by means of Langmuir models [139] and exponential approximations [146,147].…”

Stability, Oxidation, and Shape Evolution of PVP-Capped Pd Nanocrystals

“…Surface defects can also influence the co-ordination of capping ligands 38 and a more defective surface may disrupt the packing or lower the coverage of the PVP capping layer, facilitating easier oxidation of the Pd NCs.…”