Nitrogen fixation by a molybdenum catalyst mimicking the function of the nitrogenase enzyme: A critical evaluation of DFT and solvent effects

Compounds mimicking the enzyme nitrogenase represent promising alternative routes to the current Haber-Bosch industrial synthesis of ammonia from molecular hydrogen and nitrogen. In this work, we investigated the full catalytic cycle of one of such compounds, Mo(HIPTN3N) (with HIPT = hexaisopropylterphenyl), by means of DFT calculations. Our results suggest these large ligands to exert mainly a steric influence on the structural properties of the catalyst. In addition, we provided a structural and electronic characterization of the putative reaction intermediates along with a picture of the electronic mechanism of molecular nitrogen N-N bond breaking. A large discrepancy was observed between calculated and experimental reaction free energies, suggesting that in the present case the predictability of DFT reaction energies is limited. Investigation of explicit solvation of specific catalytic intermediates as well as of the protonation and reducing agents reveal the crucial role played by the solvent molecules (benzene and heptane) particularly for protonation steps. Furthermore, the analysis of several DFT functionals indicates that these have to be carefully chosen in order to reproduce the energetic profile of reduction steps. This study shows how DFT calculations may be a powerful tool to describe structural and electronic properties of the intermediates of the catalytic cycle, yet, due to the complexity of the system, reaction energies cannot be easily reproduced without a careful choice of the solvation model and the exchange-correlation functional.