Pyrrophens: Pyrrole-Based Hexadentate Ligands Tailor-Made for Uranyl (UO₂²⁺) Coordination and Molecular Recognition

Derivatives of a novel pyrrole-containing Schiff base ligand system (called "pyrrophen") are presented which feature substituted phenylene linkers (R¹ = R² = H (HL); R¹ = R² = CH₃ (HL)) and a binding pocket modeled after macrocyclic species. These ligands bind neutral CH₃OH in the solid state through pyrrolic hydrogen-bonding. The interaction of the uranyl cation (UO₂²⁺) and HL yields planar hexagonal bipyramdial uranyl complexes, while the Cu²⁺ and Zn²⁺ complexes were found to self-assemble as dinuclear helicate complexes (M₂L₂) with HL under identical conditions. The favorable binding of UO₂²⁺ over Zn²⁺ provides insight into the molecular recognition of uranyl over other metal species. Structural features of these complexes are examined with special attention to features of the UO₂²⁺ coordination environment which distinguish them from other related salophen and porphyrinoid complexes.