Abstraction of the I Atom from CH3I by Gas-Phase Au n - (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. Au _n I^- 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 Au₂, and Au₃I^- and Au₄I^- take a two-dimensional structure in which the I atom is bonded to triangular Au₃ 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, Au₂ ^- 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 Au₂ ^- 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.

An ab initio study on coinage atom-inserted cyanide/isocyanide: XMCN/XMNC (M = coinage atoms; X = halogen)

The coinage atom-inserted cyanide/isocyanide compounds, XMCN and XMNC (X = halogens) formed by the insertion of a coinage atom into the X-C(N) bonds of XCN (or XNC), were investigated by ab initio methods. XMCN was predicted to be more stable than XMNC, which is different from the case of XUCN/XUNC reported previously. Based on the analyses on the ionization dissociation pathways, the M-C (or M-N) bond is more easily broken than the X-M bond. Moreover, the order of the M-C (or M-N) bond energy in XMCN (or XMNC) is XAuCN (XAuNC) > XCuCN (XCuNC) > XAgCN (XAgNC). The shift characters of v C-N in XMCN (or XMNC) with respect to the concerning precursor can be used to identified XMCN and XMNC experimentally. The results of charge decomposition analysis (CDA) and atoms-in-molecule (AIM) illustrate that the X-M and M-C(N) bond behaves as a coordination bond, while the C-N bond is a typical polar covalent bond. The higher thermodynamic stability of XMCN is the result of the -CN group having better coordination ability than the -NC group.

Photoassisted Homocoupling of Methyl Iodide Mediated by Atomic Gold in Low-Temperature Neon Matrix

Infrared spectroscopy and density functional theory calculations showed that the gold complexes [CH₃-Au-I] and [(CH₃)₂-Au-I₂], in which one and two CH₃I molecule(s), respectively, are oxidatively adsorbed on the Au atoms, are formed in a solid neon matrix via reactions between laser-ablated gold atoms and CH₃I. Global reaction route mapping calculations revealed that the heights of the activation barriers for the sequential oxidative additions to produce [CH₃-Au-I] and [(CH₃)₂-Au-I₂] are 0.53 and 1.00 eV, respectively, suggesting that the reactions proceed via electronically excited states. The reductive elimination of ethane (C₂H₆) from [(CH₃)₂-Au-I₂] leaving AuI₂ was hindered by an activation barrier as high as 1.22 eV but was induced by visible-light irradiation on [(CH₃)₂-Au-I₂]. These results demonstrate that photoassisted homocoupling of CH₃I is mediated by Au atoms via [(CH₃)₂-Au-I₂] as an intermediate.