Synthesis, Characterization, Crystal Structures, and Catalytic C–C Coupling and Hydrosilylation Reactions of Palladium(II) Complexes Derived from CNC Pincer‐Type N‐Heterocyclic Carbenes

Abstract

A series of pyridine linker containing bis(benz)imidazolium salts serving as precursors to CNC pincer-type N-heterocyclic carbene (NHC) ligands were prepared by successive Nalkylation of the azole core. Binuclear NHC complexes of silver(I) were synthesized by in situ deprotonation of the corresponding (benz)imidazolium salt with silver(I) oxide in acetonitrile at elevated temperature in the dark. Subsequently, pincer-type palladium(II)-NHC complexes were prepared by transmetalation using silver(I)-NHC complexes in acetonitrile at elevated temperatures. All compounds were fully characterized by elemental analysis, FTIR spectroscopy, and 1 H and 13 C NMR spectroscopy. X-ray diffraction studies on single crystals of three compounds confirmed the distorted[a] The

Chelating Bis(N‐Heterocyclic Carbene) Palladium‐Catalyzed Reactions

“…Interestingly, pincer‐type palladium complexes 89 – 93 with hydrophobic wingtip groups were employed as precatalysts in the aqueous‐phase Suzuki–Miyaura reactions, according by Haque and co‐workers . Among these, complexes 89 and 91 with allyl wingtips exhibited superior activity.…”

“…As shown in Scheme , Haque and co‐workers performed the hydrosilylation of styrene by employing complexes 89 – 93 as catalysts (Figure ) . In the presence of catalyst 89 , the C−Si bonds were coupled more efficiently, owing to reduced steric hindrance.…”

“…Interestingly,p incer-typep alladium complexes 89-93 with hydrophobic wingtip groups were employeda sp recatalysts in the aqueous-phase Suzuki-Miyaura reactions, according by Haque and co-workers. [64] Amongt hese, complexes 89 and 91 with allyl wingtips exhibited superior activity.T he increasing steric bulk of the wingtipg roups slightly disfavored the reaction. The complex with ether substituents (90)l ed to slower catalytic reactions, owing to the probablec oordination of the ether groups to the palladium center.C omplex 89 displayed consistently high activity during substrate screening.…”

“…As shown in Scheme 6, Haque andc o-workers performed the hydrosilylation of styrene by employing complexes 89-93 as catalysts ( Figure 15). [64] In the presence of catalyst 89,t he CÀSi bonds were coupled more efficiently, owing to reduced steric hindrance. The electronic effect of the nitrogen substituent in complex 90 was thought to be the reason for its lower catalytic activity.N otably,t he regioselectivity of the reactionw as not altered when different catalysts were employed.…”

Recent Advances in the Schiff Bases and N‐Heterocyclic Carbenes as Ligands in the Cross‐Coupling Reactions: A Comprehensive Review

“…Haque et al . reported a series of pyridine linker comprising bis(benz)imidazolium salts serving as precursors to Carbon‐Nitrogen‐Carbon (CNC) pincer‐type NHC ligands synthesized by N ‐alkylation of the azole core. The pincer‐type Pd(II)–NHC complexes ( C208 and C209 ) were synthesized by transmetalation using Ag(I)–NHC complexes in acetonitrile at high temperature providing 79–80% yields as shown in Scheme .…”