Favorable protonation of the (μ-edt)[Fe₂(PMe ₃)₄(CO)₂(H-terminal)]⁺ hydrogenase model complex over its bridging μ-H counterpart: A spectroscopic and DFT study

The mechanism of hydrogen production in [FeFe] hydrogenase remains elusive. However, a species featuring a terminal hydride bound to the distal Fe is thought to be the key intermediate leading to hydrogen production. In this study, density functional theory (DFT) calculations on the terminal (H-term) and bridging (μ-H) hydride isomers of (μ-edt)-[Fe₂(PMe ₃)₄(CO)₂H]⁺ are presented in order to understand the factors affecting their propensity for protonation. Relative to H-term, μ-H is 12.7 kcal/mol more stable, which contributes to its decreased reactivity towards an acid. Potential energy surface (PES) calculations for the reaction of the H-term isomer with 4-nitropyridinium, a proton source, further reveal a lower activation energy barrier (14.5 kcal/mol) for H-term than for μ-H (29 kcal/mol). Besides these energetic considerations, the H-term isomer displays a key molecular orbital (MO <139>) that has a relatively strong hydride (1s) contribution (23%), which is not present in the μ-H isomer. This indicates a potential orbital control of the reaction of the hydride complexes with acid. The lower activation energy barrier and this key MO together control the overall catalytic activity of (μ-edt)[Fe₂(PMe₃)₄(CO)₂(H-term)] ⁺. Lastly, Raman and IR spectroscopy were performed in order to probe the ν(Fe-H) stretching mode of the two isomers and their deuterated counterparts. A ν(Fe-H) stretching mode was observed for the μ-H complex at 1220 cm^-1. However, the corresponding mode is not observed for the less stable H-term isomer.