Synthesis of pincer-type N-heterocyclic carbene palladium complexes with a hemilabile ligand and their application in cross-coupling catalysis

Abstract

Benzimidazolium salts containing both a neutral imine and a masked carboxylate functional group for potential metal chelation were prepared. Palladation of the ester-protected ligand afforded a N,C-bidentate carbene complex 4. Subsequent ester hydrolysis preserved the bidentate coordination mode and yielded complex 5 with a pending COOH group exclusively.However, when ester deprotection was carried out prior to metalation, the N,C,O-tridentate pincer-type coordinated palladium complex 7 was obtained. Proton-abstraction of the dangling COOH group in the bidentate ligand of complex 5 by treatment with a base led to the formation of the N,C,O-tridentate coordinated Pd system 7, and inversely, exposure of the tridentate bound Pd complex 7 with acid afforded the N,C-bidentate ligand coordination mode in complex 5, demonstrating hemilability of the oxygen donor site in the pincer ligand. All three palladium(II) complexes 4, 5, and 7 were evaluated in cross-coupling catalysis and revealed distinct activity differences that are dependent on the type of coupling (Suzuki vs.Heck) and the substrate (Ar-Br vs. Ar-Cl). These differences suggest that judicious choice of donor groups in pincer-type complexes is a viable strategy for catalyst optimization.

Triple the fun: tris(ferrocenyl)arene-based gold(i) complexes for redox-switchable catalysis

“…Among them, triferrocene SP1 (Scheme 1, bottom) had not been crystallographically characterised until now 69 and is most likely formed from hindered reductive elimination followed by a second transmetallation step of 2 onto the Pd catalyst. 70 The formation of pentanuclear SP2 during the preparation of 4e can be similarly rationalised. Together with the isolation of only diferrocenylated bromotriuorobenzene SP3, these ndings point towards the use of our synthetic protocol to access ever more complex and functionalisable redox-active structures.…”

Smart N-Heterocyclic Carbene Ligands in Catalysis

“…Since the publication of these two early examples of pyridine-NHC hemilabile ligands in 2000, , the number of complexes containing NHC ligands with nitrogen donor groups has increased enormously, and these include examples of coordination to palladium, − platinum, rhodium, ,− ,− iridium, ,− ,,,− ruthenium, ,,,− nickel, ,,− silver, − gold, copper, ,, and iron. , A s...…”

Recent Advances in Catalysis with Transition‐Metal Pincer Compounds

“…In 2014, Albrecht and co‐workers produced Pd complex 66 having a N(NCH) bidentate ligand functionalized with a carboxylic acid; in basic media, this complex turns into Pd–N(NHC)O pincer complex 67 (Scheme ) . Pincer complex 67 is active in the Mizoroki–Heck coupling of disubstituted bromobenzenes with styrene under mild conditions (NaOAc, 50 °C, 2 h), whereas the complex with the bidentate ligand exhibits poor activity under similar catalytic conditions.…”