Structural Diversity in Cyclometalated Diiridium(III) Complexes with Bridging syn and anti ₂-Oxamidato and ₂-Dithioxamidato Ligands

Six new diiridium complexes containing 2-methyl-6-phenylpyridyl as the cyclometalating ligand with a mu(2)-oxamidato or a mu(2)-dithioxamidato ligand as the bridge have been synthesized in 60-73 % yields. These complexes were revealed by multinuclear NMR spectroscopy to contain inseparable mixtures of diastereomers (rac, Delta Delta/Lambda Lambda and meso, Delta Lambda) with bridges in anti and syn configurations. The remarkable variety of isomers present was confirmed by X-ray crystallography on single crystals grown from mixtures of each complex. In one complex with a N,N'-bis(4-trifluoromethylphenyl)-mu(2)-oxamidato bridge, two single crystals of anti and syn isomers were structurally determined. Two single crystals of the mu(2)-dithioxamidato bridge complex were found to contain rac and meso forms of the syn isomer. Hybrid DFT computations on the four isomers of each diiridium complex revealed negligible energetic preferences for one isomer despite the methyl groups in the 2-methyl-6-phenylpyridyl cyclometalating ligands being close to the neighboring methyl groups and the bridge, thus supporting the experimental findings of isomer mixtures. Two distinct broad emissions with maxima at 522-529 nm and at 689-701 nm observed in these complexes in dichloromethane are attributed to mixed metal-ligand to ligand charge transfer (MLLCT) excited states involving the pyridyl and bridge moieties respectively with the aid of electronic structure computations.