Gold(III) Alkyne Complexes: Bonding and Reaction Pathways

The synthesis and characterization of hitherto hypothetical Au^III π‐alkyne complexes is reported. Bonding and stability depend strongly on the trans effect and steric factors. Bonding characteristics shed light on the reasons for the very different stabilities between the classical alkyne complexes of Pt^II and their drastically more reactive Au^III congeners. Lack of back‐bonding facilitates alkyne slippage, which is energetically less costly for gold than for platinum and explains the propensity of gold to facilitate C−C bond formation. Cycloaddition followed by aryl migration and reductive deprotonation is presented as a new reaction sequence in gold chemistry.     

Isolation of the Copper Redox Steps in the Standard Selective Catalytic Reduction on Cu‐SSZ‐13

Operando X‐ray absorption experiments and density functional theory (DFT) calculations are reported that elucidate the role of copper redox chemistry in the selective catalytic reduction (SCR) of NO over Cu‐exchanged SSZ‐13. Catalysts prepared to contain only isolated, exchanged Cu^II ions evidence both Cu^II and Cu^I ions under standard SCR conditions at 473 K. Reactant cutoff experiments show that NO and NH₃ together are necessary for Cu^II reduction to Cu^I. DFT calculations show that NO‐assisted NH₃ dissociation is both energetically favorable and accounts for the observed Cu^II reduction. The calculations predict in situ generation of Brønsted sites proximal to Cu^I upon reduction, which we quantify in separate titration experiments. Both NO and O₂ are necessary for oxidation of Cu^I to Cu^II, which DFT suggests to occur by a NO₂ intermediate. Reaction of Cu‐bound NO₂ with proximal NH₄⁺ completes the catalytic cycle. N₂ is produced in both reduction and oxidation half‐cycles.     

NHC-Stabilized Iridium Nanoparticles as Catalysts in Hydrogen Isotope Exchange Reactions of Anilines

The preparation of N-heterocyclic carbene-stabilized iridium nanoparticles and their application in hydrogen isotope exchange reactions is reported. These air-stable and easy-to-handle iridium nanoparticles showed a unique catalytic activity, allowing selective and efficient hydrogen isotope incorporation on anilines using D₂ or T₂ as isotopic source. The usefulness of this transformation has been demonstrated by the deuterium and tritium labeling of diverse complex pharmaceuticals.     

Atomically Tailored Gold Nanoclusters for Catalytic Application

Recent advances in the synthetic chemistry of atomically precise metal nanoclusters (NCs) have significantly broadened the accessible sizes and structures. Such particles are well defined and have intriguing properties, thus, they are attractive for catalysis. Especially, those NCs with identical size but different core (or surface) structure provide unique opportunities that allow the specific role of the core and the surface to be mapped out without complication by the size effect. Herein, we summarize recent work with isomeric Au_n NCs protected by ligands and isostructural NCs but with different surface ligands. The highlighted work includes catalysis by spherical and rod‐shaped Au₂₅ (with different ligands), quasi‐isomeric Au₂₈(SR)₂₀ with different R groups, structural isomers of Au₃₈(SR)₂₄ (with identical R) and Au₃₈S₂(SR)₂₀ with body‐centred cubic (bcc) structure, and isostructural [Au₃₈L₂₀(PPh₃)₄]²⁺ (different L). These isomeric and/or isostructural NCs have provided valuable insights into the respective roles of the kernel, surface staples, and the type of ligands on catalysis. Future studies will lead to fundamental advances and development of tailor‐made catalysts.