Experimental and Density Functional Theory Analysis of Serial Introductions of Electron-Withdrawing Ligands into the Ligand Shell of a Thiolate-Protected Au₂₅ Nanoparticle

Abstract

The progress of ligand exchange reactions between the ligands of Au 25 (SR) 18 − nanoparticles (SR = S(CH 2 ) 2 Ph) and thiols with electron-withdrawing substituents (HSPh- p -X; X = Br, NO 2 ) was monitored using 1 H nuclear magnetic resonance. As the reactions proceed, the introduction of the electron withdrawing −SPhX ligands into the nanoparticle ligand shell causes a shift of the nanoparticle redox waves (Au 25 1+/0 and Au 25 0/1− ) to more positive potentials. Combining the NMR and electrochemical results reveals a nearly linear shift of the redox formal potentials as a function of the average number of exchanged ligands: ∼42 and 25 mV/ligand for X = −NO 2 and −Br, respectively. Using a simple model electron-withdrawing ligand (−SCH 2 Cl), density functional theory (DFT) was used to study in detail the effects on the nanoparticle electronic structure caused by exchange of this ligand for −SCH 3 . The calculations show how the electronegative −X group changes the polarization of the nanoparticle and the charge distribution among the ligands, the protecting (−SR−Au−SR−Au−SR−) semirings, and the Au 13 core. The HOMO−LUMO gap is unchanged by the ligand exchanges; both states are equally stabilized by the presence of each incoming ligand, by ∼60 mV/ligand. Charge analysis suggests no significant changes in the Au 13 core, even after complete exchange. Rather, the charge is transferred inside the ligands, mostly from nearest-neighbor atoms of the semirings.