New insights into Fe-H2 and Fe-H- bonding of a [NiFe] hydrogenase mimic: a local vibrational mode study

In this work, we investigated the strength of the H- and H2 interaction with the Fe atom of a [NiFe] hydrogenase mimic, and how this interaction can be modulated by changing the Fe ligand in trans-position relative to H- and H2. We used as a quantitative measure of bond strength local vibrational force constants derived from the Konkoli-Cremer local mode analysis, complemented by the topological analysis of the electronic density and the natural bond orbital analysis. Seventeen different ligands were investigated utilizing density functional theory calculations, including sigma-donor ligands such as CH3-C2H5NH3, and H2O, -donor ligands such as Cl-, F-, and OH-, and sigma-donor/-acceptor ligands such as CN- and CO. According to the local mode analysis, Fe-H interactions are strengthened by sigma-donor or -donor ligands and weakened by sigma-donor/-acceptor ligands. In contrast, the H-H bond of H2 is weakened by sigma-donor or -donor ligands and strengthened by sigma-donor/-acceptor ligands. We also present a new metal-ligand electronic parameter (MLEP) for Fe-H ligands which can be generally applied to evaluate the Fe-H bond strength in iron complexes and iron hydrides. These results form a valuable basis for future [NiFe] hydrogenase-based catalyst design and fine tuning, as well as for the development of efficient biomimetic catalysts for H2 generation.