Gold Tripyrrindione: Redox Chemistry and Reactivity with Dichloromethane

The identification of ligands that stabilize Au­(III) centers has led to the isolation of complexes for applications in catalysis, gold-based therapeutics, and functional materials. Herein, we report the coordination of gold by tripyrrin-1,14-dione, a linear tripyrrole with the scaffold of naturally occurring metabolites of porphyrin-based protein cofactors (e.g., heme). Tripyrrindione H3TD2 binds Au­(III) as a trianionic tridentate ligand to form square planar complex [Au­(TD2)­(H2O)], which features an adventitious aqua ligand. Two reversible ligand-based oxidations of this complex allow access to the other known redox states of the tripyrrindione framework. Conversely, (spectro)­electrochemical measurements and DFT analysis indicate that the reduction of the complex is likely metal-based. The chemical reduction of [Au­(TD2)­(H2O)] leads to a reactive species that utilizes dichloromethane in the formation of a cyclometalated organo-Au­(III) complex. Both the aqua and the organometallic Au­(III) complexes were characterized in the solid state by microcrystal electron diffraction (MicroED) methods, which were critical for the analysis of the microcrystalline sample of the organo-gold species. Overall, this study illustrates the synthesis of Au­(III) tripyrrindione as well as its redox profile and reactivity leading to gold alkylation chemistry.