Temperature Dependence of Solid-State Electron Exchanges of Mixed-Valent Ferrocenated Au Monolayer-Protected Clusters

Abstract

Electron transfers (ETs) in mixed-valent ferrocene/ferrocenium materials are ordinarily facile. In contrast, the presence of ~1:1 mixed-valent ferrocenated thiolates in the organothiolate ligand shells of <2 nm diameter Au225, Au144, and Au25 monolayer-protected clusters (MPCs) exerts a retarding effect on ET between them at and below room temperature. Near room temperature, in dry samples, bimolecular rate constants for ET between organothiolate-ligated MPCs are diminished by the addition of ferrocenated ligands to their ligand shells. At lower temperatures (down to ~77 K), the thermally activated (Arrhenius) ET process dissipates, and the ET rates become temperature-independent. Among the Au225, Au144, and Au25 MPCs, the temperature-independent ET rates fall in the same order as at ambient temperatures: Au225 > Au144 > Au25. The MPC ET activation energy barriers are little changed by the presence of ferrocenated ligands and are primarily determined by the Au nanoparticle core size.

Charge-Transfer Effects in Ligand Exchange Reactions of Au25 Monolayer-Protected Clusters

“…Together with the present data, it is evident that a rate-determining step of the ligand exchange reaction involves at least partial transfer of charge from the incoming ligand to the semirings of the Au 25 molecule. Table 2 shows the effect of temperature on the rate constants for ligand exchanges of incoming benzene-thiol and benzeneselenol with Au 25 (SC 2 Ph) 18 MPCs having a −1 charge state. The −10 and 25°C temperature range was chosen to aim at rate constants on the order of 10 −4 M −1 s −1 , which allow better NMR quantitation.…”

“…2 The crystal structures of the molecule-like cluster 3 Au 25 (SC 2 Ph) 18 and others 4 have been elucidated and several catalytic and sensing roles 5 identified, providing impetus to seek complete understanding of their structure−property relationships. In previous studies, 6−8 ligand exchanges of Au 25 (SC 2 Ph) 18 were found to be rapid, occur primarily at solvent-accessible sites, and follow an associative mechanism, being first-order in the nanoparticle (NP) and in the incoming ligand, as in Scheme 1. Introduction of electron-withdrawing ligands such as 4-bromobenzenethiol (BrPhSH) into the NP ligand shell produces a shift of the Au 25 +1/0 and Au 25 0/−1 NP redox waves to more positive potentials.…”

“…Highly monodisperse Au 25 (SC 2 Ph) 18 was synthesized, and known charge states (i.e., 50% Au 25 +1 , 50% Au 25 0 ) were prepared as reported previously. 17,18 All 1 H NMR measurements were made using a Bruker 500 mHz spectrometer.…”

“…17,18 All 1 H NMR measurements were made using a Bruker 500 mHz spectrometer. About 20 mg of Au 25 NPs was dissolved in ∼1 mL of CD 2 Cl 2 containing a known amount of acetonitrile (serving as an internal standard), and the solution was transferred into an NMR tube.…”

“…Plot of the natural log of fraction of ligands of Au 25 (SC 2 Ph)18 −1 not replaced by molecules of incoming ligand, either thiolate or selenolate, during the ligand exchange reaction as monitored by 1 H NMR. The first 10 data points, collected within the first 10 min of exchange, were linearized to obtain the initial ligand exchange rate constant.…”