On the ligand exchange, redox, and disproportionation processes of tetrachloroaurate in the presence of a pyridine derivative.

The potential applications of AuI precursors and pyridine derivatives for shape-directed nanoparticle growth has provided an impetus for the present study of the chemistry of tetrachloroaurate (AuCl 4 −) in the presence of 4-methoxypyridine (Py/). Electrochemical experiments show that the addition of Py/ to AuCl 4 − leads to the co-existence of both AuIII and AuI species as evidenced by the observation of two reduction events in the cyclic voltammetry. Kinetic analyses have been used to show that multiple ligand exchange processes occur over the course of several minutes and result in the formation of a AuIII species with two Py/ and a variable number of chloro and hydroxo ligands in the inner sphere. The kinetics of the initial Py/ substitution reaction are shown to follow a modified form of the two-term rate law typical of square planar complexes. The observation of a slow, but spontaneous, conversion of AuIII species to AuI species has been explained by estimating the standard reduction potential for the pyridine/pyridine-N-oxide redox couple and demonstrating that it can drive a galvanic reaction with the AuPy 2 / C l q (OH) 2 − q + / AuPy 2 / + redox couple. The previously reported tendency of this system to form highly anisotropic gold nanoparticles on conductive metal oxide electrodes is explained by a thermodynamic analysis of the disproportionation reaction of AuPy 2 / +. Image, graphical abstract [ABSTRACT FROM AUTHOR]