Catalyst Design for Small Molecules Activation and Reduction by Electrocatalysis – CaDeSMARE

The project involved several aspects:
- Synthetic work: (i) Preparation of a family of polydentate N ligands and complexes of different transition metals from group 6 (Mo) to group 8 (Fe, Ru); (ii) Reactivity studies toward the reduced complexes (H2 and N2 complexes) (iii) Subsequent reactivity toward protons and electrons; Observation/Isolation and characterization of relevant reaction intermediates.
- Analytical characterizations of the complexes with different techniques: (i) Multinuclear NMR; (ii) X-ray diffraction studies when single-crystals can be obtained; (iii) electrochemistry.

A first task involved ligand synthesis and their coordination to various metal centers, in their “high oxidation state”. Three different ligands, N3-iPr, N3-Allyl and N4-Pyridine, based on the same “N3” core were identified. The syntheses are based on the chemistry developed by the LCBPT team. N3 and N4 ligands were synthesized and delivered them to the Toulouse partners for complex synthesis.
8.0 grams of the ligand N3-iPr were produced. 3.0 grams of N3-allyl and 3.0 g of N4-Pyridine ligands were produced.
In a second part, the coordination of these ligands was evaluated by the two partners in Toulouse (LCC and LHFA).
The three ligands were tested in the synthesis of group 8 metal centers in a first stage. The synthesis focused on ruthenium hydride complexes of the type (N3-iPr)RuH3(PR3)+. The variation in R groups was done to probe the possible interactions among Hydride ligands. Notably, the different phosphines tested lead to the formation of complexes bearing the H(H2) form for PPh3 while the trihydrides were observed with the trialkyl phosphines. DFT studies were performed and rationalized that the energy differences between the two forms are small, which induces a fast dynamic exchange of the H atoms in the complexes. This process was evidenced by NMR studies.
The coordination of N3-iPr to a classical Ru(II) precursor allowed the isolation of the desired (N3-iPr)RuCl2(PPh3).
These complexes being diamagnetic, they could be characterized by classical NMR techniques, but also crystallized, definitely proving their 3D structures.

In parallel, Fe(II) complexes were synthesized with N3-iPr. Two paramagnetic complexes, (N3-iPr)Fe(OTf)2 and [(N3-iPr)FeCl2]2 could be characterized by 1H NMR as well as by X-ray diffraction.

Finally, we studied several entries into Mo complexes. The classical approach, using Mo(III) precursors only lead to insoluble complexes, with the three ligands. Nevertheless, appropriate elemental analyses were obtained which supported the formation of the [(N3)MoCl3] and [(N4-pyridine)MoCl3] complexes.

Having obtained several complexes featuring the N3 and N4-pyridine ligands, their potential in N2 fixation was evaluated. A first step involved the reduction of the precursors under N2 to favor N2 coordination at the metal center, a prerequisite to any functionalization.
Most unfortunately, none of the numerous attempts were successful. In particular, the use of IR did not evidence formation of Ru-N2 or Mo-N2 reduced complexes.
The most positive result was obtained with the Fe complex [(N3-iPr)FeCl2]2. Its reduction by stoichiometric amounts of KC8 resulted in the formation of paramagnetic species. Hydrolysis of the mixture with HCl and characterization of the products by 1H NMR demonstrated the formation of minor amounts of NH4+, a direct evidence of the formation of a “Fe-N2” intermediate. Efforts were then paid to isolate and characterize this complex, but were unsuccessful.

The polydentate N based ligands central to the proposal have been successfully synthesized in large scale, which allowed the subsequent studies of their coordination to various metal complexes. Several complexes of Mn, Mo, Fe and Ru could be obtained, although the paramagnetism of some complexes hampered their full characterization. Subsequent reduction was attempted to favor N2 coordination, a prerequisite for subsequent studies of functionalization. Unfortunately, we were not able to isolate any M-N2 complexes with the chosen ligand systems. Similarly, proton reduction studies to form H2 were unsuccessful.
Because of these dead ends, the LHFA and LPEM groups studied the mechanism of N2 splitting at a (triphosphine)Mo complex developed by the LHFA group. This study was the basis of the ANR project “REDNEC”.

1) Espada, M. F.; Bennaamane, S. ; Liao, Q. ; Saffon-Merceron, N.; Massou, S. ; Clot, E. ; Nebra, N.; Fustier-Boutignon M.; Mézailles, N. Angew. Chem., Int. Ed., 2018, 57, 12865-12868.
2) Bennaamane, S. ; Espada, M. F.; Yagoub, I. ; Saffon-Merceron, N.; Fustier-Boutignon M.; Nebra, N.; Clot, E. ; Mézailles, N. Eur. J. Inorg. Chem. 2020, 1499. doi.org/10.1002/ejic.201901295. Special issue : Nitrogen Fixation.
3) Bennaamane, S. ; Espada, M. F.; Mulas, A. ; Personeni, T. ; Saffon-Merceron, N.; Fustier-Boutignon M.; Bucher, C. ; Mézailles, N. Angew. Chem., Int. Ed., 2021, 60, 20210 –20214.
4) L. Merakeb, M. Robert, Curr. Opin. Electrochem. 2021, 29:100834.
5) L. Merakeb, S. Bennaamane, J. De Freitas, E. Clot, N. Mézailles, M. Robert, Angew. Chem. Int. Ed., 2022, 61, doi.org/10.1002/anie.202209899.

We will address the fundamental and major societal issues related to cost-effective and energetically efficient production of NH3 from N2 via homogeneous catalysis and the catalytic reversible transformation of the couple H+/H2 for fuel cell applications. Our goal is to tackle these problems together. Indeed, the reduction of N2 into ammonia can be achieved either via heterogeneous catalysis using H2 (Haber-Bosch process) or via homogeneous catalysis using H+/electrons (Nitrogenase enzymes). Our ultimate goal is therefore to use the catalysts developed for H2/H+ + electrons equilibrium to allow the development of the yet unknown N2 reduction to NH3 using H2 in a homogeneous fashion. New catalysts based on a fine tuning of the ligand/metal combination are targeted. The key feature of these complexes is the use of novel tetra-amine type ligands [N4], designed to be tris-coordinated to the metal center and possessing one pendant amine moiety able to act as a proton relay. Several metal centers will be studied: Mo, W, Fe, Ru and Co. With these new complexes, we will investigate: (i) the coordination and the activation of N2 and H2; (ii) the reduction of N2 into ammonia using either the H+/electron couple or H2; and the reversible electro-catalyzed H+/H2 transformation; (iii) the role of the new class of ligand in such reactions. The success is based on a consortium with complementary competences: polydentate N ligands, coordination chemistry, electrochemically induced catalysis and mechanistic studies.