N‑Heterocyclic Carbenes with a <i>N</i>‑2,4-Dinitrophenyl Substituent: Comparison with PPh₃ and IPr

Synthesis and characterization of N-heterocyclic carbenes (NHCs) bearing a 2,4-dinitrophenyl (DNP) substituent on an NHC framework were performed. The treatment of 1-(2,4-dinitrophenyl)-1H-imidazole (1) with 1-bromo-2,4-dinitrobenzene or methyl triflate afforded imidazolium salts 2a·HBr or 2b·HOTf, respectively, which were corresponding precursors of NHC ligands 1,3-bis­(2,4-dinitrophenyl)-1H-imidazol-2-ylidene (2a) and 1-(2,4-dinitrophenyl)-3-methyl-1H-imidazol-2-ylidene (2b). Rh and Au complexesRhCl­(2a)­(cod) [3a (cod = 1,5-cyclooctadiene)], RhCl­(2b)­(cod) (3b), RhCl­(2a)­(CO)2 (4a), RhCl­(2b)­(CO)2 (4b), and AuCl­(2a) (5a)were synthesized using 2a·HBr and 2b·HOTf. IR, NMR, and crystallographic analysis of the Rh complexes demonstrated that the DNP substituent remarkably decreased the σ-donating ability of the carbenic carbon and increased the π-accepting ability. In addition, IR spectroscopy of Rh dicarbonyl complexes revealed that the average CO stretching frequency of 2a was equal to that of PPh3. Au complex 5a exhibited a significantly higher catalytic activity than AuCl­(PPh3) 5d in the Au­(I)-catalyzed hydroalkoxylation of cyclohexene. The computational analysis of the Au­(I) complexes supported the experimental data, and the results suggested that the Au–Ccarbene π-back-bonding interaction energy of 5a was 17–20% larger than that of 5d.