Protonation Dynamics of a Doubly Reduced Biomimetic [FeFe]-Hydrogenase Catalyst

Saksida, Črt (author), Lavrenčič Štangar, Urška (mentor), Lomoth, Reiner (comentor)

The transition to a carbon-free society has driven interest in biomimetic [FeFe]-hydrogenase catalysts for hydrogen evolution. In the active site of the natural enzyme, both the metal center and the nitrogen in the azadithiolate (ADT) bridge are protonated, followed by proton shuttling from the ADT to the terminal hydride, ultimately forming an H–H bond. It has been suggested that under strongly reducing conditions the Fe$_2$(μ-ADT)(CO)$_6$ (1) turnover mechanism involves a doubly reduced species (1$^{2-}$), and this study investigates its protonation dynamics. Additionally, an analogous catalyst with a benzenedithiolate (BDT) bridge (2) was studied to exclude the possibility of protonating the second basic site. Using stopped-flow Fourier transform infrared (FTIR) spectroscopy, we characterized the metal-protonated doubly reduced species, identified by ~80 cm$^{-1}$ shift of the three most intense characteristic absorption bands, by introducing 1$^{2-}$ to phenol and 2$^{2-}$ to 3,5-dichlorophenol. Using a stronger acid (pentachlorophenol), a second protonation event occurred, in case of 1, leading to H$_2$ evolution and catalyst turnover. Upon reductant depletion, only the first reduction is reached, producing 1⁻, which immediately disproportionates, dimerizes, and forms degradation products in the presence of a proton source. When 2$^{2-}$ is mixed with pentachlorophenol, a degradation product also forms, effectively deactivating the catalyst. Understanding the proton reduction mechanism provides valuable insights for future research in optimizing the catalyst design to influence the reactivity, basicity and stability of the key intermediates.

• Language: Slovenian
• Keywords: biomimetični katalizator, [FeFe]-hidrogenaze, nastanek vodika, stopped-flow, časovno razločena FTIR
• Work type: Bachelor thesis/paper
• Typology: 2.11 - Undergraduate Thesis
• Organization: FKKT - Faculty of Chemistry and Chemical Technology
• Year: 2026
• PID: 20.500.12556/RUL-184033
• COBISS.SI-ID: 289119747
• Publication date in RUL: 24.06.2026

Secondary language

• Language: English
• Title: Dinamika protonacije dvojno reducirane oblike biomimetičnega [FeFe]-hidrogenaznega katalizatorja

Abstract

Prehod v brezogljično družbo je povečal zanimanje za biomimetične katalizatorje [FeFe]-hidrogenaz za proizvodnjo vodika. V aktivnem mestu naravnega encima sta protonirana tako kovinski center kot dušik v azaditiolatnem (ADT) mostičku, čemur sledi prenos protona z ADT na terminalni hidrid in končna tvorba vezi H–H. Predlagano je bilo, da mehanizem obrata kompleksa Fe$_2$(μ-ADT)(CO)$_6$ (1) v močno reducirajočih pogojih vključuje dvojno reducirano zvrst (1$^{2-}$), zato smo v tej raziskavi proučevali njeno dinamiko protonacije. Poleg tega smo preučevali tudi analogni katalizator z benzen-1,2-ditiolatnim (BDT) mostičkom (2), da bi izključili možnost protonacije drugega bazičnega mesta. Z uporabo stopped-flow infrardeče spektroskopije s Fourierjevo transformacijo (FTIR) smo karakterizirali protonirano dvojno reducirano zvrst na kovinskem centru, ki jo prepoznamo po približno 80 cm$^{-1}$ premiku treh najintenzivnejših karakterističnih absorpcijskih vrhov, pri reakciji 1$^{2-}$ s fenolom in 2$^{2-}$ s 3,5-diklorofenolom. Pri uporabi močnejše kisline (pentaklorofenola) je prišlo do druge protonacije, ki je v primeru kompleksa 1 vodila do izhajanja H₂ in obrata katalizatorja. Po porabi reducenta se sistem vrne le do enkrat reducirane zvrsti 1⁻, ki v prisotnosti protonskega vira hitro disproporcionira, dimerizira in tvori razpadne produkte. Pri reakciji 2$^{2-}$ s pentaklorofenolom prav tako nastane razpadni produkt, kar povzroči deaktivacijo katalizatorja. Razumevanje mehanizma redukcije protonov omogoča pomemben vpogled za nadaljnje načrtovanje katalizatorjev z izboljšano reaktivnostjo, bazičnostjo in stabilnostjo ključnih intermediatov.

• Keywords: Biomimetic, [FeFe]-Hydrogenase, Hydrogen Evolution Catalyst, Stopped-Flow, Time-Resolved FTIR