Synthesis of a Sterically Encumbered Pincer Au(III)−OH Complex

We report the synthesis and crystallographic characterization of a novel Au(III)−OH complex featuring a N^N^N-pincer ligand. Reactivity studies towards oxygen atom transfer (OAT), a type of reactivity previously reported for a Au(III)−OH complex, indicate that this complex provides both a sterically encumbered Au atom and a sterically poorly accessible OH group leading to no reactivity with a series of phosphines. The steric encumbrance sets this example apart from the known examples of Au(III)−OH (pincer) complexes, which commonly feature planar ligands that provide little control over steric accessibility of the Au and O atoms in these complexes. Implications for the mechanism of OAT from Au−OH complexes are briefly discussed

Spectroscopic Manifestations and Implications for Catalysis of Quasi-d¹⁰ Configurations in Formal Gold(III) Complexes.

Several gold +I and +III complexes are investigated computationally and spectroscopically, focusing on the d-configuration and physical oxidation state of the metal center. Density functional theory calculations reveal the non-negligible electron-sharing covalent character of the metal-to-ligand σ-bonding framework. The bonding of gold(III) is shown to be isoelectronic to the formal CuIII complex [Cu(CF3)4]1- , in which the metal center tries to populate its formally unoccupied 3dx2-y2 orbital via σ-bonding, leading to a reduced d10 CuI description. However, Au L3-edge X-ray absorption spectroscopy reveals excitation into the d-orbital of the AuIII species is still possible, showing that a genuine d10 configuration is not achieved. We also find an increased electron-sharing nature of the σ-bonds in the AuI species, relative to their AgI and CuI analogues, due to the low-lying 6s orbital. We propose that gold +I and +III complexes form similar bonds with substrates, owing primarily to participation of the 5dx2-y2 or 6s orbital, respectively, in bonding, indicating why AuI and AuIII complexes often have similar reactivity

Gold(III) Catalyzed Overman Rearrangements: Controlling Steric Interactions using Pincer-Type Ligands

Spatial control of reactivity is intrinsically difficult in gold catalysis due to the linear coordination mode of Au(I) and the commonly flat ligands employed for Au(III) complexes. Our recent report of a novel and sterically encumbered (NNN)diiPrAu−OH complex (Eur. J. Inorg. Chem. 2021, 3561–3564.) suggested that the (NNN) ligand framework is capable of sterically interacting with substrates through its conveniently oriented aryl groups. We have now examined these steric properties in more detail by varying the ortho-substituent of the aryl group in (NNN)xAu−Cl complexes. With just small modifications we were able to vary the buried volume around the Cl atom and correlate this to yields obtained in a Au-catalyzed Overman rearrangement. Computationally we further elucidate that the stark difference in yields obtained originates from a shift in binding mode of the substrate to the Au catalyst in the rate limiting step of the reaction. We thus conclude that delicate spatial control can be exercised in gold catalysis and propose the (NNN) ligand framework to be an attractive platform for the efficient design of Au(III) complexes for stereoselective catalysis

Cyclobutadiene Arene Complexes of Rhodium and Iridium

Reactions of [(C₂H₄)₂RhCl]₂ or [(coe)₂RhCl]₂ (coe = cyclooctene) with AgPF₆ and arenes, followed by addition of 3-hexyne, give the cyclobutadiene complexes [(C₄Et₄)­Rh­(arene)]⁺ in 40–65% yield (arene = <i>tert</i>-butylbenzene, <i>p</i>-xylene, mesitylene, 4-mesitylbutanoic acid). In the absence of arenes, the hexaethylbenzene complex [(C₄Et₄)­Rh­(C₆Et₆)]⁺ is formed in 70% yield as a result of cyclotrimerization of 3-hexyne in the coordination sphere of rhodium. Similar reaction of [(coe)₂IrCl]₂ with AgPF₆ and 3-hexyne leads to [(C₄Et₄)­Ir­(C₆Et₆)]⁺, which is apparently the first reported cyclobutadiene iridium complex. DFT calculations suggest that formation of the model cyclobutadiene complex [(C₄Me₄)­Rh­(C₆H₆)]⁺ from bis­(alkyne) intermediate [(C₂Me₂)₂Rh­(C₆H₆)]⁺ can proceed via a metallacycle transition state with a low energy barrier of 14.5 kcal mol^–1