'Nindigo': A New Ligand Architecture

By applying standard carbonyl-to-imine conversion chemistry to the famous dye molecule indigo, a team of chemists led by Robin G. Hicks of the University of Victoria, in British Columbia, has created a ligand with two β-diketimine metal-binding sites fused together in a bridging architecture (Chem. Commun. 2010, 46, 6753). The diketimine structural motif in which a pair of nitrogen atoms binds a metal is popular for its easy construction and for its tunability by using different alkyl or aryl substituent groups. But incorporating two diketimines in one ligand appears to be a chemical first, the researchers report. They treated indigo with various substituted anilines, titanium tetrachloride, and 1,4-diazabicyclo[2.2.2]octane in refluxing bromobenzene solvent to make indigo diketimine derivatives. The indigo-aniline reaction was actually described a century ago, Hicks notes, but until now a reproducible high-yield synthesis had not been reported. The researchers used the new class of ligands, which they have dubbed “Nindigo,” to make bispalladium complexes that are redox active and exhibit an intense near-infrared absorption.