Ligand Design Criteria for the Stability of High Oxidation State Praseodymium Complexes

High Resolution Image Download MS PowerPoint Slide The isolation of high-oxidation-state lanthanide complexes requires a balance of electron-donating ligand environment, steric protection, and ligand redox stability. Herein, we report the synthesis of a new imidophosphorane ligand, NPC 2 ([NP( t Bu) 2 (pyrr)] −; pyrr = pyrrolidinyl), and its ability to support homoleptic Ce 3+ and Pr 3+ complexes that afford access to Ce 4+ and electrochemically observable Pr 4+ and Pr 5+ . The structures, electrochemistry, and computational analyses of tetrahomoleptic NPC 2 complexes of Ce 3+, Ce 4+, and Pr 3+ are compared with previously reported analogues supported by NP * ([NP(1,2-bis- t Bu-diamidoethane)(NEt 2 )] − ), NPC 1 ([NP( t Bu)(pyrr) 2 ] − ), and NPC 3 ([NP t Bu 3 ] −; t Bu = tert -butyl) ligands. Across the NPC x ( x = 1–3) series, ligand substitution results in modest changes in redox potentials, consistent with minimal perturbation of the f -orbital manifold. Despite similar electronic donor properties, NPC 3 provides increased stabilization of Pr 4+ and Pr 5+ complexes due to enhanced steric protection, leading to improved electrochemical reversibility and chemical stability relative to complexes of NPC 1 and NPC 2 . Systematic density functional theory calculations on both experimentally isolated and nonisolated complexes rationalize the experimentally observed insensitivity of the Pr 4+/3+ and Pr 5+/4+ redox couples to ligand substitution across the NPC x series, while identifying steric encumbrance, electron-donating ability, and counterion-retention as key factors governing the accessibility and stabilization of high-oxidation-state lanthanide complexes.