Abstraction of the I Atom from CH3I by Gas-Phase Aun- (n = 1-4) via Reductive Activation of the C-I Bond

Gas-phase reactions of atomic gold anions and small gold cluster anions, Au-n(-) (n = 1-4), with CH3I were investigated to clarify the effect of the cluster size on C-I bond activation and to elucidate the key properties of Au clusters that govern the reactivity. AunI- identified by mass spectrometry was observed as a common reaction product. Photoelectron spectroscopy and density functional theory calculations revealed that Au2I- has a linear structure in which the I atom is bonded to Au-2, and Au3I- and Au4I- take a two-dimensional structure in which the I atom is bonded to triangular Au-3 moieties. Pseudo-first-order kinetic analyses of the reaction revealed the inverse correlation of the reactivity of Au-n(-) toward CH3I and the electron affinity of Au-n, indicating the reductive activation of the C-I bond. Especially, Au-2(-) showed the highest reactivity to form Au2I- as the main product, whereas the adduct compound Au2CH3I- was hardly formed, in sharp contrast to the reaction of Au- reported previously. On the basis of theoretical calculations, we propose that the reaction proceeded dominantly via the I atom abstraction pathway (attack of Au-2(-) from the I atom side), which is highly preferential from the viewpoint of both the energetics and a steric factor. This study demonstrates that not only the reactivity but also the reaction mechanisms and products are governed by the cluster size in C-I bond activation by Au clusters at the smallest size region.