Computational Elucidation of Electronic Structure and Noncovalent Interactions in NHC-Supported Palladium-PEPPSI Complexes for Acceptorless Alcohol Dehydrogenation

A systematic density functional theory (DFT) investigation was conducted to elucidate the structural stability, electronic properties, and noncovalent interactions of NHC-supported Pd-PEPPSI complexes (cata-1 to cata-5) relevant to acceptorless alcohol dehydrogenation. All geometries were optimized at the M06-2X-D3/def2-TZVPP level, and frequency analyses confirmed true minima without imaginary modes. Thermodynamic evaluation revealed subtle stability differences among the complexes, with cata-2 exhibiting the most favorable Gibbs free energy. Frontier molecular orbital analysis showed closely comparable HOMO energies (-7.09 to -7.01 eV) and HOMO–LUMO gaps (6.29 to 6.35 eV), indicating high kinetic stability with slight variations in electronic softness and reactivity. Differences in dipole moment and electrophilicity index demonstrate that ligand substitution effectively modulates polarization and charge distribution around the Pd center. Charge analyses using Hirshfeld, electrostatic potential (ESP), and natural population analysis (NPA) consistently confirm the electrophilic character of palladium and highlight substituent-dependent electronic redistribution within the coordination sphere. Atoms-in-molecules (AIM) topology reveals weak closed-shell N···H and I···H interactions (ρ ^BCP ≈ 0.003-0.011 a.u.) that contribute to structural stabilization without covalent character. Vibrational analysis further confirms the integrity of the square-planar Pd coordination environment, with characteristic Pd-I stretching modes near ~120 cm ^-1 . Overall, the results establish clear structure–property relationships, demonstrating that ligand architecture fine-tunes electronic distribution, polarization, and intermolecular stabilization while preserving structural robustness, thereby providing fundamental computational insights for the rational design of palladium catalysts for sustainable dehydrogenation processes.