Ligand Heterogeneity on Monolayer-Protected Gold\nClusters

Abstract

This paper describes the effects of oxidative electronic charging of the Au cores of the monolayer-protected clusters (MPCs), Au₁₄₀(S(CH₂)₅CH₃)₅₃ and Au₃₈(SCH₂CH₂Ph)₂₄, on nuclear magnetic resonance\n(NMR) spectra of their monolayer ligand shells. Previously unresolved fine structure in the ¹³C NMR\nhexanethiolate methyl and C5 methylene resonances is seen in spectra of solutions of monodisperse\nAu₁₄₀(S(CH₂)₅CH₃)₅₃ MPCs, reflecting magnetically inequivalent ligand sites. Incremented increases in\npositive cluster core charge, effected by electrochemical charging, cause the spectral fine structure of the\nmethyl resonance to coalesce, becoming a single peak at the Au₁₄₀³⁺ charge state. The spectral changes\nare reversible; charging back to the original core charge state regenerates the methyl ¹³C resonance fine\nstructure. Adding an equimolar quantity of a Au(I) thiolate complex, Au^I[SCH₂(C₆H₄)C(CH₃)₃], to an\nuncharged Au₁₄₀(S(CH₂)₅CH₃)₅₃ MPC solution in <i>d</i><i>₂</i>-methylene chloride causes partial spectral coalescence.\n¹³C NMR spectra of Au₃₈(SCH₂CH₂Ph)₂₄ MPCs exhibit roughly comparable spectral changes upon positive\ncore charging to the ‘0', ‘+1', and ‘+2' states. The NMR results indicate that exchange between magnetically\ninequivalent sites occurs at rates of 100 to 400 s^-1, a rate believed to be too fast to be accountable by actual\nexchanges of ligands between different sites on the Au core. We also describe changes in core electronic\nspectra of Au₁₄₀(S(CH₂)₅CH₃)₅₃ induced by positive charging, measured using spectroelectrochemistry.