Mechanism of Ullmann Coupling Reaction of Chloroarene on Au/Pd Alloy Nanocluster: A DFT Study

Recently, a unique catalytic system of bimetallic Au/Pd alloy nanoclusters (NCs) for Ullmann coupling of chloroarenes (ArCl), which works at low temperature in high yield, has been developed (J. Am. Chem. Soc. 2012, 134, 20250). In this work, the full catalytic cycle of this reaction has been investigated for ArCl, producing biphenyl on the Au/Pd alloy NCs by DFT calculations with the M06-L functional. Two possible reaction pathways are proposed, namely, (i) ArCl oxidative addition followed by Cl abstraction from the NC surface occurs twice stepwise and (ii) the successive oxidative addition of two ArCl proceeds before the Cl abstraction. Both of these pathways are energetically possible, and in the latter case, the Cl atoms stay on the NC surface. The exothermic reaction pathways were obtained for this multistep reaction scheme. The rate-determining step was shown to be hydrogen transfer from dimethylformamide (DMF), which is consistent with the experimental isotope effect observation. The proton shuttle mediated by water considerably reduces the activation barrier. The side reaction, which produces benzene via hydrogenation, is prohibited by the second ArCl oxidative addition. The present study has revealed the essential mechanism of the coupling reaction on supported Au/Pd NC catalysts, suggesting that the entire reaction is controlled by several factors including the surroundings and solvents. These findings provide useful insights for the further developments of efficient NC catalysts through designing the supports, interface, and reaction conditions.