Initial Growth Mechanisms of Gold-Phosphine Clusters

Abstract

Gold-phosphine cluster growth was studied at the BP86/TZV level of theory, starting from the precursor AuClPR3 (R = H, Me, and Ph), up to clusters containing four gold atoms. For all three substituents, AuClPR3 is linear. For R = H and R = Me, the anionic species AuClPR3− are bent, whereas for R = Ph, it is linear because of the addition of the electron in a π* orbital mainly localized on the benzyl rings. Chloride ion dissociation, especially from negatively charged clusters, is very common during cluster growth. Phosphine groups can also dissociate, although this becomes less favorable as the R group becomes larger. The growth products obtained strongly depend on the relative orientation of the reactants, leading to a wide variety of clusters. The inclusion of a classical dispersion term gives similar monomers and dimers to regular BP86 but may lead to different trimers. Most reactions involved in cluster growth are exothermic. Dispersion forces tend to increase the reaction energies involving a low charge and decrease the reaction energies involving a high negative charge. Trimers tend to have a triangular core and be negatively charged. Tetramers have a diamond, tetrahedral, or Y-shaped core. Diamonds and tetrahedra mostly have a positively charged or neutral core, as opposed to Y shapes, which have a negatively charged or neutral core. The optimized tetrahedral structure Au4Cl2(PH3)4 was found to be similar to the experimental cluster [Au4(μ-I)2(PPh3)4] and to be one of the lowest energy structures.