Diamidodipyrrins: Versatile Bipyrrolic Ligands with Multiple Metal Binding Modes

A straightforward, facile synthesis of diamidodipyrromethenes (diamidodipyrrins, DADP^R,R′) is presented. These tetradentate ligands readily form complexes with metal ions such as Ni²⁺ and Cu²⁺ and can adopt different binding modes with these metals. One version of the ligand (DADP^Ph,iPr) has been structurally characterized in its “free base” form, as a HBr salt, and as the Ni²⁺ and Cu²⁺ complexes. A symmetric NNOO donor set is found for the Cu²⁺ complex in the solid state, involving two carbonyl oxygen atoms and two dipyrrin nitrogen atoms, and this coordination mode has been confirmed in solution by electron paramagnetic resonance. An asymmetric NNNO binding mode found for the Ni²⁺ complex in the solid state persists in solution as revealed by ¹H NMR. The HBr salt form of the ligand shows an intriguing hydrogen-bonded head-to-head dimer arrangement. Experiments show that Cu²⁺, but not Ni²⁺, can mediate the rapid oxidation of the diamidodipyrromethane precursors to the diamidodipyrromethene ligands in the presence of dioxygen. The work here shows that diamidodipyrrins are a versatile new class of ligands in the area of nonporphyrinic pyrrole-based compounds that merit further investigation.