Conformational constraints in [Ni(PR2NR′2)2] complexes for tuning H2 production and oxidation: a DFT-based ligand design study

Abstract

[Ni(P^R₂N^R′₂)₂] complexes catalyze H₂ production or oxidation depending on the nature of the two bidentate P₂N₂ ligands. Ligand design plays a crucial role in determining the reaction direction and catalytic properties. In this study, density functional theory (DFT) calculations were performed to analyze the structural and energetic properties of Ni complexes with (R, R′) = (H, H), (Me, Me), (Cy, Me), (^tBu, Me), (CF₃, H), and (NH₂, H). Based on the structural features, relative stabilities of the Ni^II and Ni⁰ complexes, and their energy profiles, these Ni complexes were classified into three groups, I–III. In group I, the reaction is slightly exothermic toward H₂ production. In group II, the presence of ^tBu and CF₃ groups introduces steric hindrance, forcing the Ni complexes into a tetrahedral conformation. This geometric constraint destabilizes the product state in the Ni^II oxidation state, shifting the thermodynamics toward H₂ oxidation. Conversely, destabilizing the reactant state in the Ni⁰ oxidation state can be achieved using a tetradentate ligand in which the two bidentate P₂N₂ ligands are connected by trimethylene, –(CH₂)₃–, units at the P atoms. This ligand, classified as group III, maintains a square planar conformation, rendering H₂ production highly exothermic. These findings align with experimental observations of similar complexes and underscore the importance of ligand geometry and substituent effects.