Synthesis, Characterization, and Crystal Structure of Lithium Pyrrolide

Abstract

Development of clean energy is an urgent requirement because of the depletion of fossil energy sources and increasingly severe environmental pollution. However, the lack of safe and efficient hydrogen storage materials is one of the bottlenecks in the implementation of hydrogen energy. Liquid organic hydrogen carriers (LOHCs) have been recognized as potential materials for the storage and transportation of hydrogen owing to their high gravimetric and volumetric hydrogen densities, reversible hydrogen absorption and desorption ability, and ease of widespread implementation with minimal modification on the existing fueling infrastructure. While some LOHCs such as cycloalkanes and N-heterocycles have been developed for hydrogen storage, they require a high hydrogen release temperature due to the large enthalpy change of dehydrogenation. In our previous work, a metallation strategy was proposed to improve the thermodynamic properties of liquid organic hydrogen carriers for hydrogen storage, and a series of metalorganic hydrides were synthesized and investigated. Among them, sodium phenoxide-cyclohexanolate pair, lithium carbazolide-perhydrocarbazolide, and sodium anilinide-cyclohexylamide pair showed promising dehydrogenation thermodynamics and improved hydrogen storage properties. Sodium pyrrolide and sodium imidazolide were also synthesized. However, pyrrolides were not well characterized, and the structure of lithium pyrrolide was not resolved. In the present study, we synthesized sodium and lithium pyrrolides by ball milling and wet chemical methods. One equivalent of hydrogen could be released from the reaction of pyrrole and metal hydrides, indicating the replacement of H by metal. The formation of pyrrolides was confirmed by nuclear magnetic resonance (NMR), X-ray diffraction (XRD) and ultravioletvisible spectroscopy analyses. The 1 H signals attributed to C-H in the NMR spectra of the alkali metal pyrrolides shifted upfield due to the replacement of the H of N-H with a stronger electron-donating species (Li or Na), resulting in a greater shielding environment upon metallation. The absorption peaks of lithium and sodium pyrrolides showed red shifts, and the intensities became obviously stronger in the UV-Vis spectra, suggesting an enhancement of the conjugation effect, in accordance with theoretical calculations. The structure of lithium pyrrolide was determined by the combined direct space method and first-principles calculations on XRD data and Rietveld refinement. This molecule crystallizes in the monoclinic P21/c (14) space group, with lattice parameters of a = 4.4364(7) Å, b = 11.969(2) Å, c = 8.192(2) Å, β = 108.789(8)°, and V = 411.8(2) Å 3 (1 Å = 0.1 nm). Each Li + cation is surrounded by three pyrrolides via cation-N σ bonding with two pyrrolides and a cation-π interaction with the third pyrrolide, where the Li + is on the top of the π face. Our experimental findings are different from the theoretical prediction in the literature.

The Roles of Alkali/Alkaline Earth Metals in the Materials Design and Development for Hydrogen Storage

“…The sodium and lithium of pyrrolide, imidazolide, and carbazolides were synthesized through the reaction of metal hydrides and the corresponding N heterocycles. Among them, structures of the sodium of pyrrolide and imidazolide and lithium pyrrolide were successfully identified (Figure h–j). , Interestingly, besides the cation–N bondings, weak cation-π interactions between an AM cation and the adjacent aromatic ring are also observed. The existence of cation-π interactions would further stabilize the structures of H-lean metallo-N heterocycles, which will in turn help hone the DFT calculations on their thermodynamic parameters.…”