Exploring Molybdenum(I) pincer complexes as Catalysts for Pyridine Hydrogenation

Abstract

As the transition to cleaner energy intensifies, N-heterocycles as liquid organic hydrogen carriers (LOHCs) offer a promising approach. However, their reliance on noble metals such as ruthenium, iridium, and platinum poses sustainability challenges.
In this study, 60 Mo(I) pincer complexes were screened using a combination of MACE and DFT. MACE was used to generate initial 3D molecular structures via force field optimization. These were further analysed using DFT calculations with the PBE0-D3BJ functional and def2-SVP basis set under standard conditions. Gibbs free energies were computed and used to evaluate pyridine binding energies across the ligand set.
The results indicate that pyridine binding to Mo(I) complexes is unfavourable, despite several complexes exhibiting sufficiently hydridic Mo-H bonds to suggest potential catalytic activity. The hydride charge appears to be conformation dependent, with certain configurations decreasing or increasing the hydridic charge. Oxidation of Mo(I) was observed in some systems, leading to pincer ligand decomposition. Additionally, nitrogen donor arms often dissociate from the metal centre, resulting in 5-coordinated geometries.
The unfavourable binding energy suggest that inner sphere mechanisms are unlikely. Instead, the findings support the plausibility of outer sphere pathways, especially given the lack of correlation between pyridine binding energy and the hydridic charge. Furthermore, the observed dissociation of nitrogen donor arms hint at potential ligand hemilability, which may influence catalytic dynamics and warrants further investigation.