Sterically Demanding AgI and CuI N‐Heterocyclic Carbene Complexes: Synthesis, Structures, Steric Parameters, and Catalytic Activity

• Published in: Chemistry : a European journal 2020-04, Vol.26 (24), p.5530-5540
• Main Authors: Cervantes‐Reyes, Alejandro, Rominger, Frank, Hashmi, A. Stephen K.
• Format: Article
• Language: English
• Subjects: Aromatic compounds; buried volume; carbene ligands; Catalysts; Catalytic activity; Chemical synthesis; Chemistry; Condensates; Coordination compounds; Copper; High temperature; Ligands; Metal complexes; N-heterocyclic carbenes; Nitrogen atoms; Parameters; Phenylacetylene; Reagents; Salts; silver
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202000600

The synthesis and full characterization of new air‐stable AgI and CuI complexes bearing structurally bulky expanded‐ring N‐heterocyclic carbene (erNHC) ligands is presented. The condensation of protonated NHC salts with Ag2O afforded a collection of AgI complexes, and their first use as ligand transfer reagents led to novel isostructural CuI or AuI complexes. In situ deprotonation of the NHC salts in the presence of a copper(I) source, provides a library of new CuI complexes. The solid‐state structures feature large N‐CNHC‐N angles (118–128°) and almost identical angles between the aryl groups on the nitrogen atoms and the plane of the N‐C‐N unit of the carbene (i.e. torsion angles close to 0°). Among the steric parameters, the percent buried volume (%Vbur) values span easily in the 50–57 % range, and that one of (9‐Dipp)CuBr complex (%Vbur=57.5) overcomes to other known erNHC–metal complexes reported to date. Preliminary catalytic experiments in the copper‐catalyzed coupling between N‐tosylhydrazone and phenylacetylene, afforded 76–93 % product at the 0.5–2.5 mol % catalyst loading, proving the stability of CuI erNHC complexes at elevated temperatures (100 °C). A unique family of AgI and CuI complexes possessing sterically demanding N‐heterocyclic carbene ligands is described. Their structural and steric profiling reveals the highest buried volumes (%Vbur up to 57.5) for any expanded‐ring backbone NHC complex. For the first time, the AgI complexes have been employed as ligand‐transfer reagents while the CuI complexes proved to be efficient candidates for catalysis (see figure).