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

## TL;DR

This paper explores how specific ligands can stabilize high-oxidation-state praseodymium complexes through a balance of electronic and steric factors.

## Contribution

A new imidophosphorane ligand is synthesized and shown to stabilize Pr4+ and Pr5+ complexes through enhanced steric protection.

## Key findings

- The NPC2 ligand supports homoleptic Ce3+ and Pr3+ complexes that afford access to higher oxidation states.
- NPC3 ligand provides increased stabilization of Pr4+ and Pr5+ due to enhanced steric protection.
- Computational analyses confirm the role of steric encumbrance and electron-donating ability in stabilizing high-oxidation-state complexes.

## 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 Ce3+ and Pr3+ complexes that afford access to Ce4+ and electrochemically observable Pr4+ and Pr5+. The structures, electrochemistry, and computational analyses
of tetrahomoleptic NPC

2
complexes
of Ce3+, Ce4+, and Pr3+ are compared
with previously reported analogues supported by NP

*
([NP­(1,2-bis-
t
Bu-diamidoethane)­(NEt2)]−), NPC

1
([NP­(
t
Bu)­(pyrr)2]−), and NPC

3
([NP
t
Bu3]−;
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 Pr4+ and Pr5+ 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 Pr4+/3+ and Pr5+/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.