A DFT-Based Protocol for Modeling the Structure and Reactivity of Gold(III) Complexes

In this study, distinct computational protocols were employed to investigate the structure and kinetic properties of the aquation reaction of the [Au(dien-H)Cl]⁺ Au(III) complex. A total of 154 protocols with nonrelativistic Hamiltonians were initially assessed, comprising 31 basis sets for Au, 52 basis sets for ligand atoms, and 71 levels of theory (including HF, MP2, and 69 DFT-functionals). Additionally, seven protocols with relativistic Hamiltonians, using all-electron basis sets for Au, were evaluated. The results indicate that the structure is relatively insensitive to the computational protocol. In contrast, the activation Gibbs free energy (${\Delta G}_{\mathrm{aq}}^{‡}$) are highly sensitive to both the level of theory and basis sets choice. Notably, the basis set used for ligand atoms plays a key role in accurately predicting kinetic parameters. Among the tested 397 combinations, the B3LYP/def2-SVP/6-31G(d,p) protocol yielded the overall best agreement with experimental data for the reference complex. However, for bulkier [Au(R-dien-H)Cl]⁺ derivatives, diffuse functions on ligand atoms are essential, making 6-31+G(d) the recommended basis set. When all five Au(III) complexes are considered, the optimal performance is achieved using B3LYP with the Stuttgart-RSC ECP for Au and 6-31+G(d) for ligand atoms. This combination offers a good balance between accuracy and computational cost, making it a practical choice even for larger Au(III) complexes.