A tridentate CNO‐donor palladium(II) complex as efficient catalyst for direct C―H arylation: Application in preparation of imidazole‐based push–pull chromophores

Abstract

A series of imidazolium chlorides for the formation of tridentate CNO-donor palladium(II) complexes featuring N-heterocyclic carbene moieties have been developed from cheap and readily available starting materials with high yields.Their palladium complexes were prepared by reactions between the ligand precursors and PdCl 2 using K 2 CO 3 as base in pyridine with reasonable yields.These air-stable metal complexes were characterized using 1 H NMR and 13 C { 1 H} NMR spectroscopy and elemental analyses. Heteronuclear multiple bond correlation experiments were performed to identify key NMR signals of these compounds. The structures of two of the complexes were also established by single-crystal X-ray diffraction analysis. One of these complexes was successfully applied in the direct C-H functionalization reactions between heterocyclic compounds and aryl bromides, producing excellent yields of coupled products. The coupling reactions were scalable, allowing grams of coupled products to be obtained with a mere 2 mol% of Pd loading. The catalyst system developed allowed the large-scale preparation of several push-pull chromophores straightforwardly. Photophysical properties based on UV-visible and fluorescence spectroscopy for these chromophores were investigated. Deep blue photoluminescence with moderate quantum efficiency and twisted intramolecular charge transfer excited state were observed for these chromophores. Density functional theory (DFT) and time-dependent DFT calculations were performed to support the experimental results.

Multidentate N‐Heterocyclic Carbene Complexes of Nickel and Palladium: Structural Analysis and Catalytic Application in Mizoroki–Heck Coupling Reaction

“…The carbene carbon resonance in the tridentate complex 2a was observed at 159.7 ppm, similar to that in tridentate complex 2b (157.8 ppm) . However, these signals in the tridentate complex 2a' and 2b' bearing benzimidazole moieties were downfield shifted to ca. 169.9 ppm.…”

“…The syntheses of complexes 2b , 2b' , and 2d were reported by us previously . Following similar procedures, the new tridentate palladium complexes 2a and 2a' were prepared by heating a mixture of the corresponding ligand precursor and PdCl 2 with K 2 CO 3 as a base in neat pyridine at 50 °C, affording the air‐stable, light‐yellow solids in good yields of 74 and 70 %, respectively.…”

“…Notably, the Pd–C bond lengths in the tetradentate complexes [ 2c' : 1.909(3) Å; 2c : 1.911(4) Å] were very short compared to those in the tridentate complexes [ 2a : 1.963(9) Å; 2b : 1.962(7) Å; 2d : 1.973(7) Å] (Figure ). These short bond lengths indicate the formation of strong Pd–carbene bonds due to the tetradentate chelation in which the NHC moiety was flanked by two donor groups and thus being part of two palladacyclic rings.…”

“…Their coordination with electron‐rich metal centers facilitates the effective oxidative addition of C–Cl bonds, leading to very effective palladium complexes for catalyzing the Mizoroki–Heck coupling reaction with aryl chlorides . Recently, the skeleton was further functionalized to form palladium complexes with tridentate amidate‐ and phenoxy‐functionalized NHC ligands, which were effective in catalyzing the C–H arylation reaction between heterocyclic compounds and aryl halides . However, their catalytic performances for the Mizoroki–Heck coupling reaction has not yet been tested.…”

Palladium Complexes of Thio/Seleno‐Ether Containing N‐Heterocyclic Carbenes: Efficient and Reusable Catalyst for Regioselective C‐H Bond Arylation

“…A tridentate C,N,O‐donor palladium(II)‐NHC was also reported to catalyze the direct C‐H functionalization of heterocycles with aryl bromides. This protocol also required longer time, harsher conditions and a higher catalyst loading (2 mol‐%, 18 h, 140 °C) . Recently, palladium (II) complexes of organochalcogen bearing NHC ligands were used as efficient catalyst for the direct arylation of imidazoles.…”