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The synthesis of pro-ligands containing pairs of N-heterocyclic carbene (NHC) substituents on organic scaffolds was explored. These ligands were used to develop homobimetallic Ir(I) complexes containing 1,8-cyclooctadiene (COD) and carbonyl co-ligands. The catalytic activity of a bimetallic complex was investigated and compared against related monometallic complexes to determine the extent of bimetallic cooperativity present.A novel ligand, 1,1'-(2,7-di-tert-butyl-9,9-dimethyl-9H-xanthene-4,5-diyl)bis(3-methyl-1H-imidazol-3-ium) iodide, was synthesised by an Ullman coupling reaction of imidazole onto a4,5-dibromo-2,7-bis(1,1-dimethylethyl)-9,9-dimethylxanthene scaffold. Asubsequent methylation with iodomethanegeneratedthe bis-imidazolium salt. A novel bimetallic Ir(I) catalyst containing this ligand was synthesised via deprotonation using potassium carbonateand complexation with [Ir(COD)Cl]2, followed by a carbonylation of both metal centresusing carbon monoxide.The catalytic activity of these bimetallic systems was explored in the following multi-step alkyne activation reactions and compared to the reactivity of twoanalogous monometallic Ir(I) catalysts: (i) the hydroamination of alkynyl amines; and (ii) the tandem dihydroalkoxylation of alkyne diols to form spiroketals.For all catalytic transformations, the bimetallic complex was found to be an efficient catalyst with excellent turnover frequencies. Bimetallic cooperative enhancement was observed in the catalytic activity of the bimetallic Ir (I) catalyst in comparison to the activity of one of the analogous monometallic Ir(I) catalysts. |
