Abstract
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.