Abstract
In this study, a series of unsymmetrical 1,3-disubstituted benzimidazolium chlorides 2a–g with two nitrogen atoms substituted by different alkyl groups were synthesized in high yields as N-heterocyclic carbene (NHC) precursors. These benzimidazolium salts were then converted into the corresponding Pd–NHC complexes of the PEPPSI family (PEPPSI = pyridine-enhanced precatalyst preparation, stabilization, and initiation) 3a–g. The structures of all compounds were characterized by
1
H nuclear magnetic resonance (NMR) spectroscopy,
13
C NMR spectroscopy, infrared spectroscopy, and elemental analysis, which support the proposed structures. The structure of the 3c complex was determined by X-ray crystallography. More detailed structural characterization of the 3c complex was performed through single-crystal X-ray diffraction, which supports the proposed structures. The Pd–NHC–PEPPSI complexes were used as catalysts in the direct C
5
-arylation of 2-acetyl furan, 2-acetylthiophene, and 2
n
-propylthiazole with different aryl bromides. These complexes exhibited moderate-to-high catalytic activities and selectively at the C
5
position. Furthermore, the Pd–NHC–PEPPSI complexes were evaluated for their potential antibacterial properties against a panel of bacterial strains, such as
Micrococcus luteus
,
Listeria monocytogenes
,
Salmonella typhimurium
,
Staphylococcus aureus
,
Candida albicans
, and
Pseudomonas aeruginosa
. The Pd–NHC–PEPPSI complex 3f showed better activity than
ampicillin
against
Micrococcus luteus
, with an MIC of 0.035 mg mL
−1
. In addition, the antioxidant activities of the complexes 3d and 3f showed considerable free radical scavenging activity.