Synthesis and Structures of Benzyl‐ and Amido‐Substituted N‐Heterocyclic Palladium Carbene Complexes

• Published in: ChemistrySelect (Weinheim) 2022-06, Vol.7 (22), p.n/a
• Main Authors: Wang, Xin‐Ya, Yang, Hsueh‐Hui, Chang, Li‐Kuang, Chiu, Tzu‐Hao, Liu, Fu‐Chen, Lin, Ivan J. B.
• Format: Article
• Language: English
• Subjects: catalytic reaction; C−C coupling; NHC carbenes; palladium
• ISSN: 2365-6549; 2365-6549
• DOI: 10.1002/slct.202201700

The benzyl‐ and amido‐substituted benzimidazolium ligand precursor [Bz‐BimH‐CH2CONH2]Br (1) was synthesized. Under different conditions, the reactions of this ligand precursor with PdBr2 produced palladium carbene complexes: Pd[(Bz‐Bim‐CH2CONH2)2Br2] (2), Pd(Bz‐Bim‐CH2CONH)2 (3), [Pd(Bz‐Bim‐CH2CONH2)(Py)Br2] (4), or [Pd(Bz‐Bim‐CH2CONH2)(CH3CN)Br2] (5). These complexes were characterized by NMR, IR, and single‐crystal X‐ray diffraction analyses. Complex 2 contains two trans rotamers in both the solution and solid state, and a crystal that possesses a trans‐anti conformation is confirmed through single‐crystal X‐ray diffraction analysis. Complex 3 contains two cis‐orientated bidentate carbene ligands, and complexes 4 and 5 are mono‐carbene complexes. The chelate rings in complex 3 rendered the methylene protons of the amido‐substituted diastereotopic, and a long range (six bond) through bond interaction between the carbonyl carbon and the methylene protons of the benzyl‐substituent was observed through the HMBC experiment. The catalytic activity studies indicated 4 and 5 are better catalysts in Suzuki‐Miyaura coupling reaction. Under different conditions, three different types of benzyl‐ and amido‐substituted N‐heterocyclic palladium carbene complexes, including the trans bi‐carbene complex 2, cis bis‐bidentate carbene complex 3, and the mono‐carbene complexes 4 and 5, were prepared from the reactions of the benzyl‐ and amido‐substituted benzimidazolium ligand precursor with palladium bromide. The catalytic activity studies indicated complexes 4 and 5 are better catalysts in Suzuki‐Miyaura coupling reaction.