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

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 Ag2O. The structure of 5a could be verified by an X-ray single-crystal structure determination. 1H 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.