New insights into Fe–H$$_{2}$$ and Fe–H$$^{-}$$ bonding of a [NiFe] hydrogenase mimic: a local vibrational mode study

Abstract

In this work, we investigated the strength of the $${\hbox {H}}^{-}$$ and $${\hbox {H}}_{2}$$ 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 $${\hbox {H}}^{-}$$ and $${\hbox {H}}_{2}$$ . 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 $${\hbox {CH}}_{3}^{-}$$ , $${\hbox {C}}_{2}{\hbox {H}}_{5}^{-}$$ , $${\hbox {NH}}_{3}$$ , and $${\hbox {H}}_{2}\hbox {O}$$ , $${\pi }$$ -donor ligands such as $${\hbox {Cl}}^{-}$$ , $${\hbox {F}}^{-}$$ , and $${\hbox {OH}}^{-}$$ , and $${\sigma }$$ -donor/ $${\pi }$$ -acceptor ligands such as $${\hbox {CN}}^{-}$$ and CO. According to the local mode analysis, Fe–H interactions are strengthened by $${\sigma }$$ -donor or $${\pi }$$ -donor ligands and weakened by $${\sigma }$$ -donor/ $${\pi }$$ -acceptor ligands. In contrast, the H–H bond of $${\hbox {H}}_{2}$$ is weakened by $${\sigma }$$ -donor or $${\pi }$$ -donor ligands and strengthened by $${\sigma }$$ -donor/ $${\pi }$$ -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 $${\hbox {H}}_{2}$$ generation.