Coordination chemistry of a calix[4]arene-based NHC ligand: dinuclear complexes and comparison to IiPr2Me2

Abstract

The preparation and coordination chemistry of 5,17-bis(3-methyl-1-imidazol-2-ylidene)-25,26,27,28-tetrapropoxycalix[4]arene (1) is described. Starting from the bis(imidazolium) pro-ligand 1·2HI, the free carbene 1 was readily generated in solution through deprotonation using K[O^tBu] and its reactivity with rhodium(I) dimers [Rh(COD)Cl]₂ (COD = 1,5-cyclooctadiene) and [Rh(CO)₂Cl]₂ investigated. Dinuclear complexes were isolated in both cases, where the calix[4]arene-based NHC ligand adopts a bridging μ²-coordination mode, and in one case characterised in the solid-state by X-ray diffraction. Using instead an isolated and well-defined (mononuclear) silver transfer agent, generated by reaction of 1·2HI with Ag₂O in the presence of a halide extractor, reactions with [Rh(COD)Cl]₂ and [Rh(CO)₂Cl]₂ produced cationic dinuclear complexes bearing μ²-1 and μ²-Cl bridging ligands. The structural formulation of the novel dinuclear adducts of 1 was aided through spectroscopic congruence with model complexes, containing monodentate 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IⁱPr₂Me₂).