N-Heterocyclic Carbene Complexes of Mercury, Silver, Iridium, Platinum, Ruthenium, and Palladium Based on the Calix[4]arene Skeleton

Abstract

Two independent pathways to Ag(I)-N-heterocyclic-calix [4]arene complexes (5a,b and 6-8) starting from three calix [4]arenes bearing one or two (distal or proximal) imidazolium substituents attached at the upper rim via a methylene linker are described. Complexes 5 were synthesized using the free carbene route, and complexes 6-8 were obtained via in situ formation of the N-heterocyclic carbene by deprotonation and reaction with Ag 2 O. The structure of 5a could be verified by an X-ray single-crystal structure determination. 1 H NMR studies of the compounds 5a,b showed dynamic behavior in solution, similar to a conformational change observed for known phosphine-based calixarene ligands. Using the imidazolium-calixarenes as precursors, NHC complexes of mercury, iridium, platinum, and ruthenium have been prepared and characterized. Additionally, cis and trans Pd(II)-N-heterocyclic-calix[4]arene complexes (3 and 13) were synthesized independently by in situ deprotonation of 2 with Pd(OAc) 2 and by transmetalation using Ag(I) complex 6 as a supramolecular carbene transfer reagent, respectively. Both isomeric complexes 3 and 13 were tested as catalysts in the Suzuki-Miyaura reaction and compared to previously reported in situ systems. Both isolated complexes exhibit, independently of their geometry, the same catalytic activity, which is superior to the in situ system used for comparison.

New Amphiphilic Imidazolium/Benzimidazolium Calix[4]arene Derivatives: Synthesis, Aggregation Behavior and Decoration of DPPC Vesicles for Suzuki Coupling in Aqueous Media

“…As it was previously shown by Schatz group, tetra-O-alkyl calixarene derivatives bearing at the distal positions of upper rim two [(3- N -substituted imidazolium-1-yl)methyl] fragments formed stable N -heterocyclic carbenes and corresponding cis/trans Pd(II) complexes which effectively catalyzed Suzuki–Miyaura coupling [ 25 , 26 ]. This structural motive of NHC precursors with two imidazolium fragments on the upper rim seems to be very attractive for designing self-organizing systems for different catalytic transformations in water.…”

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

“…It was assumed that the strain in the metallocycle prevented rearrangement of the trans ‐chelating complex to the active species, which consequently accounted for its low catalytic efficiency. According to the description of Schatz and co‐workers, a calix[4]arene‐based Pd–bis(NHC) complex with cis configuration ( 70 ) was generated through direct metalation of its corresponding bis‐imidazolium salt by using Pd(OAc) 2 in DMF at 90 °C, whereas trans ‐ 69 was obtained by transmetalation of the corresponding disilver complex employing [PdCl 2 (CH 3 CN) 2 ] in CH 2 Cl 2 at 55 °C . The Suzuki–Miyaura reaction between 4‐chlorotoluene and phenylboronic acid was employed as a model reaction.…”

1,3-Bis(2-cyanobenzyl)imidazolium bromide

“…For details and applications of N-heterocylic carbene, see: Wanzlick & Kleiner (1961); Fahlbusch et al (2009); Demir et al (2009); Grasa et al (2002); Buchowicz et al (2006); Marko et al (2002).…”

“…Since the investigation of N-heterocyclic carbene (NHC) chemistry by Wanzlick and Kleiner (1961), NHCs have played a major role as ligands in coordination and organometallic chemistry (Fahlbusch et al, 2009). During the past decades it has been proven as an alternative to tertiary phosphines in homogeneous catalysis.…”