(CF₃)₃Au as a Highly Acidic Organogold(<scp>III</scp>) Fragment

Abstract

The Lewis acidity of perfluorinated trimethylgold (CF₃ )₃ Au was assessed by theoretical and experimental methods. It was found that the (CF₃ )₃ Au unit is much more acidic than its nonfluorinated analogue (CH₃ )₃ Au, and probably sets the upper limit of the acidity scale for neutral organogold(III) species R₃ Au. The significant acidity increase on fluorination is in line with the CF₃ group being more electron-withdrawing than CH₃ . The solvate (CF₃ )₃ Au⋅OEt₂ (1) is presented as a convenient synthon of the unsaturated, 14-electron species (CF₃ )₃ Au. Thus, the weakly coordinated ether molecule in 1 is readily replaced by a variety of neutral ligands (L) to afford a wide range of (CF₃ )₃ Au⋅L compounds, which were isolated and characterized. Most of these mononuclear compounds exhibit marked thermal stability. This enhanced stabilization can be rationalized in terms of substantially stronger [Au]-L interactions with the (CF₃ )₃ Au unit. An affinity scale of this single-site, highly acidic organogold(III) fragment was calculated by DFT methods and experimentally mapped for various neutral monodentate ligands. The high-energy profile calculated for the fluorotropic [Au]-CF₃ ⇌F-[Au]←CF₂ process makes this potential decomposition path unfavorable and adds to the general stabilization of the fragment.