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

[Ni(PR 2NR' 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), ( t Bu, Me), (CF3, H), and (NH2, H). Based on the structural features, relative stabilities of the NiII 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 t Bu and CF3 groups introduces steric hindrance, forcing the Ni complexes into a tetrahedral conformation. This geometric constraint destabilizes the product state in the NiII 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.