POLY(DIHYDROGEN) COMPLEXES AND HYDROGEN TRANSFER – 3H2

We used the full arsenal of usual techniques for the characterization of the organometallic complexes and of the new composites (Multinuclear Magnetic Resonance both in solution and in the solid state, Infra-Red and UV, X-ray diffraction on crystals and powders, Photoelectron spectrometry, Scanning and Transmission Electron Microscopy…). Moreover, neutron diffraction proved to be crucial to locate the hydrogen atoms bonded to the metal center either as a terminal or bridging hydride or as a dihydrogen ligand. DFT (Density Functional Theory) calculations provided key information for the characterization of the new species and to determine the mechanisms taking place.
To study the hydrogen storage properties of our devices, we used extensively thermal and calorimetric techniques: thermogravimetric, temperature programmed desorption and calorimetric measurements…The hydrogen absorption kinetics were investigated under hydrogen pressures using a Sievert apparatus and desorption properties were studied by Temperature Programmed Desorption analysis.

We have demonstrated the interest of using well-defined organometallic complexes featuring hydride ligands to dope MgH2. This has a tremendous impact on desorption/absorption kinetics, on the storage capacity and importantly on the reversibility, which are key issues in the domain of energy. Advanced thermal and calorimetric studies have been performed to evaluate the storage properties of our new devices and to understand the fate of the complexes before and after hydrogen absorption/desorption cycles as well as to study the hydrogen transfer mechanisms using a large variety of techniques (NMR, Neutron diffraction, DFT calculations…). Destabilization of MgH2, a major drawback for a use at moderate temperatures, was achieved thanks to doping by only 1.8%wt of an organometallic complex.

The initial objectives made it possible to validate the interest of doping MgH2 by well-defined transition metal complexes and in particular hydrides. Depending on the results obtained, we have been able to orient our research to move towards composites that increasingly have storage properties that are closer to the specifications. In particular, we lowered the desorption temperature below 200 ° C and validated the reversibility and rapid absorption of H2 at low temperature (<100 ° C). This makes it possible to envisage an extremely promising extension to our studies by working both on the nature of the doping agent and on the doping conditions.
Our mechanistic studies on transfer processes also open up very interesting prospects for targeting better catalysts.
The impact of our work allows us to envision new collaborations both nationally and internationally.

3 papers have been so far published. The first one on the impact of polyhydride ruthenium complexes as doping agent for MgH2. We could improve the desorption/absorption kinetics by switching to nickel catalysts as shown in our second paper. As reported in our third paper, we achieved remarkable properties by the use of the nickel hydride NiHCl(PCy3)2 with desorption temperatures lower than 200°C and 6wt% absorption at 100°C in less than 30 min.
1. Impact of the addition of poly-dihydrogen ruthenium complexes on the hydrogen storage properties of the Mg/MgH2 system.
B. Galey, S. Sabo-Etienne, A. Auroux, M. Grellier, S. Dhaher, G. Postole, Sustainable Energy & Fuels 2018, 2, 2335-2344.
2. Enhancing hydrogen storage properties of the Mg/MgH2 system by the addition of bis(tricyclohexylphosphine)nickel(II) dichloride
B. Galey, A. Auroux, S. Sabo-Etienne, M. Grellier, G. Postole, Int. J. Hydrog. Energy, 2019, 44, 11939-11952.
3. Improved hydrogen storage properties of Mg/MgH2 thanks to the addition of nickel hydride complex precursors.
B. Galey, A. Auroux, S. Sabo-Etienne, S. Dhaher, M. Grellier, G. Postole
Int. J. Hydrog. Energy, 2019, 44, 28848-28862.

We want to address fundamental issues involving a unique class of complexes, the dihydrogen complexes. Dihydrogen complexes in which dihydrogen is coordinated to a transition metal centre without H-H bond breaking represent an ideal class in terms of binding strength midway between metal hydrides and physisorption. We propose to investigate the following challenging targets: 1) Establish, in the solid state, the properties of complexes bearing one or more H2 ligands, to understand the synergy between H2 and the transition metal for a better control of the hydrogen transfer processes that ideally should be reversible. 2) Explore synthetic strategies to obtain the first complex bearing three dihydrogen ligands, and evaluate the potential of such a high number of dihydrogen coordinated to a metal center. The concept that increasing the number of dihydrogen ligands for a better hydrogen release has to be demonstrated. 3) Evaluate the properties of dihydrogen and poly-dihydrogen complexes as doping agents for attractive metal hydrides. Mechanistic investigations aimed at determining the key interactions for a reversible hydrogen release will be conducted. Thermodynamics and kinetics will be studied by a variety of techniques including calorimetric and thermal analysis, neutron diffraction techniques and DFT calculations. Major breakthroughs are expected from our strategy based on a consortium bringing together teams with complementary and unique expertise. Future applications could be expected in the areas of energy, hydrogen storage, and catalysis.