title: Favorable Protonation of the (μ-edt)[Fe(2)(PMe(3))(4)(CO)(2)(H-terminal)](+) Hydrogenase Model Complex Over Its Bridging μ-H Counterpart: A Spectroscopic and DFT Study.
abstract: 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)-[Fe(2)(PMe(3))(4)(CO)(2)H](+) 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)[Fe(2)(PMe(3))(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.
authors.affiliation: Department of Chemistry, University of Michigan, Ann Arbor, MI 48109, USA.
authors.forename: Mary Grace I
authors.initials: MG
authors.lastname: Galinato
authors.forename: C Matthew
authors.initials: CM
authors.lastname: Whaley
authors.forename: Dean
authors.initials: D
authors.lastname: Roberts
authors.forename: Peng
authors.initials: P
authors.lastname: Wang
authors.forename: Nicolai
authors.initials: N
authors.lastname: Lehnert