Tandem reactivity of a redox-active Frustrated Lewis Pair for H2 oxidation on [FeFe]-platform – OxySplit-H2
Redox-active frustrated Lewis pair reactivity in an [FeFe] model for H2 oxidation
This project aims to develop a new strategy to reduce the use of noble, rare, and expensive metals for hydrogen activation. It is based on a Frustrated Lewis Pair (FLP) formed by two molecules—one iron-based and one boron-based—that enables the cleavage of the H-H bond.
Design of FLPs capable of activating H2
H2 is considered a green energy vector. However, the H-H bond is chemically inert and is typically activated by noble, rare, and expensive metals. Since their discovery in 2006, Frustrated Lewis Pairs (FLPs) have demonstrated their ability to cleave H2 into H+/H- ions. This reaction results from the simultaneous interaction of a Lewis acid (LA) and a Lewis base (LB). Despite the potential use of transition metals as Lewis acids or bases in FLPs, no FLP has yet achieved the oxidation of H2. In nature, hydrogenases are capable of reversibly converting H2 to protons and electrons. To oxidize H2, the bimetallic center of [FeFe]-hydrogenases acts as the LA, while an amine in the second coordination sphere serves as the LB, together functioning as an FLP. However, only a limited number of organometallic models can perform H2 oxidation, and the catalytic process is more efficient in the natural enzyme than in organometallic models. Thus, the project proposes to study this FLP-type reactivity—reverse to that found in nature—between a dinuclear iron derivative (LB) and a borane-type LA to activate and then oxidize H2.
The project has been divided into two distinct parts.
The first part involves the synthesis of dinuclear iron complexes, inspired by the active site of [FeFe]-hydrogenases, which will be combined with boranes of the type BR3 (R: Ph, C6F5, etc.) to form intermolecular FLPs. These will be studied using spectroscopy (NMR, IR). Subsequently, stable Fe(I)Fe(I) derivatives functionalized with an organic arm bearing a boryl group in the second coordination sphere will be developed via a hydroboration reaction, in order to form intramolecular FLPs.
The second part will focus on the study of the synthesized FLPs and their reactivity toward H2 (spectroscopic monitoring, theoretical rationalization). If stoichiometric activation of H2 is observed, experimental conditions will be optimized to achieve catalytic conversion.
The project enabled the development of several FLPs based on diiron complexes and borane derivatives capable of activating H2.
Two intermolecular FLPs were studied: [Fe2(µ-pdt)L(CO)4] (L: (PMe3)2, dmpe (1,2-bis(dimethylphosphino)ethane)) / B(C6F5)3. These FLPs cleave the H2 molecule, forming a protonated diiron cation and a borohydride anion. Theoretical studies were carried out, rationalizing the observed differences in yield during this reactivity: both electronic and steric effects are responsible for the observed differences. This work has been published: DOI 10.1039/D1SC06975F.
By synthesizing a more basic diiron complex, substituted with two dmpe ligands, no FLP-type reactivity was observed in the presence of B(C6F5)3 and H2. However, this complex reacts regioselectively with organochlorine solvents via chloronium transfers (publication DOI 10.1021/acs.inorgchem.3c03481). This reactivity had never been reported for a dinuclear iron complex. Furthermore, it reacts with Brønsted acids to form bridging or terminal hydride complexes, where regioselectivity depends on both the steric bulk of the associated base and the strength of the acid (publication DOI 10.1002/chem.202404353).
Finally, diiron complexes functionalized with a Lewis acid moiety (borane-type) were developed using a hydroboration reaction. The Lewis acid function was successfully placed on the dithiolate bridge via dithiolene derivatives (publication on the synthesis of precursors: DOI 10.1002/ejic.202400061) and also on a ligand derived from a phosphine (work in preparation for publication). The reactivity of this type of intramolecular Fe2/B assembly with H2 is still under investigation.
The intramolecular diiron/borane complexes synthesized in this project could also be used for the activation of other small resource molecules (CO2, N2, etc.). Furthermore, only iron-containing compounds have been synthesized so far. It would be relevant to extend this study to heterobimetallic complexes in order to compare their reactivity with that of diiron complexes.
Development of efficient, eco- and environment friendly substrates for the oxidation of H2, a green energy carrier, is required to substitute rare and expensive noble metals. Frustrated Lewis Pairs (FLP) allow the heterolysis splitting of H2 but no electron is released from H2. Inspired by [FeFe]-hydrogenases in nature, organometallic models are developed, but very few are able to oxidise H2. Aiming H2 oxidation, this project propose to study the intermolecular or intramolecular FLP formed by a borane-based Lewis acid associated to a cooperative bimetallic assembly Fe(I)Fe(I) acting as a Lewis base, able to transfer both electrons and protons. Novel FLP combining a [FeFe] bimetallic complex and a borane will be characterized by spectroscopic and electrochemical methods, reactivity with H2 will then be studied.
Project coordination
Lucile Chatelain (CHIMIE, ELECTROCHIMIE MOLECULAIRES ET CHIMIE ANALYTIQUE)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
Partnership
CEMCA CHIMIE, ELECTROCHIMIE MOLECULAIRES ET CHIMIE ANALYTIQUE
Help of the ANR 199,908 euros
Beginning and duration of the scientific project:
March 2021
- 48 Months
