Diamidodipyrrins: Versatile Bipyrrolic Ligands with Multiple Metal Binding Modes

A straightforward, facile synthesis of diamidocipyrromethenes (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,tPr)) 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 Cull 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 H-1 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.