Coordination chemistry of a calix[4]arene-based NHC ligand: dinuclear complexes and comparison to I super(i)Pr sub(2)Me sub(2)

The preparation and coordination chemistry of 5,17-bis(3-methyl-1-imidazol-2-ylidene)-25,26,27,28-tetrapropoxyca l ix[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 super()tu] and its reactivity with rhodium(i) dimers [Rh(COD)Cl] sub(2) (COD = 1,5-cyclooctadiene) and [Rh(CO) sub(2)Cl] sub(2) investigated. Dinuclear complexes were isolated in both cases, where the calix[4]arene-based NHC ligand adopts a bridging mu super(2)-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 Ag sub(2)O in the presence of a halide extractor, reactions with [Rh(COD)Cl] sub(2) and [Rh(CO) sub(2)Cl] sub(2) produced cationic dinuclear complexes bearing mu super(2)-1 and mu super(2)-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 (I super(i)Pr sub(2)Me sub(2)).