Unraveling Meso-Substituent Steric Effects on the Mechanism of Hydrogen Evolution Reaction in Ni^II Porphyrin Hydrides Using DFT Method

Substituents at the meso-site of metalloporphyrins profoundly influence the hydrogen evolution reaction (HER) mechanism. This study employs density functional theory (DFT) to computationally analyze Ni-II-porphyrin and its hydrides derived from tetrakis(pentafluorophenyl)porphyrin molecules, presenting stereoisomers in ortho- or para-positions. The results reveal that the spatial resistance effect of meso-substituted groups at the ortho- and para-positions induces significant changes in Ni-N bond lengths, angles, and reaction dynamics. For ortho-position substituents forming complex I, a favorable 88.88 angstrom(3) spherical space was created, facilitating proton coordination and the formation of H-2 molecules; conversely, para-position substituents forming complex II impeded H-2 formation until bimolecular complexes arose. Molecular dynamics (MD) analysis and comparison were conducted on the intermediation products of I-H-2 and (II-H)(2), focusing on the configuration and energy changes. In the I-H-2 products, H-2 molecules underwent separation after 150 fs and overcame the 2.2 eV energy barrier. Subsequently, significant alterations in the spatial structure were observed as complex I deformed. In the case of (II-H)(2), it was influenced by the distinctive "sandwich" configuration; the spatial structure necessitated overcoming a 6.7 eV energy barrier for H-2 detachment and a process observed after 2400 fs.