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

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, NPC² ([NP(^tBu)₂(pyrr)]^-; pyrr = pyrrolidinyl), and its ability to support homoleptic Ce³⁺ and Pr³⁺ complexes that afford access to Ce⁴⁺ and electrochemically observable Pr⁴⁺ and Pr⁵⁺. The structures, electrochemistry, and computational analyses of tetrahomoleptic NPC² complexes of Ce³⁺, Ce⁴⁺, and Pr³⁺ are compared with previously reported analogues supported by NP^* ([NP(1,2-bis-^tBu-diamidoethane)(NEt₂)]^-), NPC¹ ([NP(^tBu)(pyrr)₂]^-), and NPC³ ([NP^tBu₃]^-; ^tBu = 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, NPC³ provides increased stabilization of Pr⁴⁺ and Pr⁵⁺ complexes due to enhanced steric protection, leading to improved electrochemical reversibility and chemical stability relative to complexes of NPC¹ and NPC². 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.