Fully Relativistic, Comparative Investigation of Gold and Platinum Alkyne Complexes of Relevance for the Catalysis of Nucleophilic Additions to Alkynes.

For a range of additions to alkynes gold is known to exhibit a much higher catalytic activity than a corresponding platinum compound. In order to approach the origin of this behavior we first investigate the propyne activation by the gold and platinum catalysts AuCl3 and PtCl2(H2O) where both metals possess a d(8) electron configuration and where the catalysts exhibit similar steric effects. Propyne serves as a representative for alkynes. Fully relativistic ab initio calculations of these alkyne-catalyst complexes are presented at the Dirac-Hartree-Fock self-consistent field (DHF-SCF), density functional theory (DFT/B3LYP), and Green's function (GF) level in order to properly account for the large relativistic effects of gold and platinum. For the alkyne/catalyst complexes both the perpendicular and in-plane conformations were studied as these possess very similar ground state energies and may easily transform into each other. Strongly varying orbital populations together with sizable energetic and structural differences of the frontier orbitals are found and can be considered as a major source of the differing catalytic activity. These mainly comprise vanishing LUMO densities at the carbon centers in the platinum complex together with increased LUMO energies making a nucleophilic attack harder than in the gold compound. As Green's function calculations show, DFT/B3LYP seems to overestimate correlation contributions leading to an unphysical energetic lowering of many unoccupied orbitals.