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, <b>NPC</b>^<b>2</b> ([NP(^<i>t</i>Bu)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 <b>NPC</b>^<b>2</b> complexes of Ce³⁺, Ce⁴⁺, and Pr³⁺ are compared with previously reported analogues supported by <b>NP</b>^<b>*</b> ([NP(1,2-bis-^<i>t</i>Bu-diamidoethane)(NEt₂)]^-), <b>NPC</b>^<b>1</b> ([NP(^<i>t</i>Bu)(pyrr)₂]^-), and <b>NPC</b>^<b>3</b> ([NP^<i>t</i>Bu₃]^-; ^<i>t</i>Bu = <i>tert</i>-butyl) ligands. Across the <b>NPC</b>^{<b><i>x</i></b>} (<i>x</i> = 1-3) series, ligand substitution results in modest changes in redox potentials, consistent with minimal perturbation of the <i>f</i>-orbital manifold. Despite similar electronic donor properties, <b>NPC</b>^<b>3</b> 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 <b>NPC</b>^<b>1</b> and <b>NPC</b>^<b>2</b>. 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 <b>NPC</b>^{<b><i>x</i></b>} 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.