Correlation between the electrochemical response and main components structure in solutions for rechargeable Mg batteries based on THF and the reaction products of tBuMgCl and AlCl₃

The electrochemical response of ethereal solutions containing magnesium organohaloaluminate complexes has drawn great interest in recent decades owing to their relevance to rechargeable magnesium batteries, as demonstrated with solutions containing complexes formed by reacting R₂Mg and AlCl₂R moieties in ethers like tetrahydrofuran (THF). However, most of previous reports focused on battery related performances, and less on the structure of the active species. Herein, we focus on (1) identifying electroactive species and (2) correlating the electrochemical properties of their solutions to the preparation modes: either through reactions of their precursors in THF, or by dissolving isolated crystallized products in the ether solvent. Specifically, we explore the products of the reaction of the Grignard reagent t-BuMgCl with AlCl₃ (1:1) in THF, and how their presence in solutions affect their electrochemical behavior. We isolated two complex products, one non-soluble and one soluble in THF, which exhibit two distinct single-crystal structures. The molecular structure of the two complexes obtained from the single-crystal X-ray diffraction (SCXRD) and nuclear magnetic resonance (NMR) measurements was correlated with the electrochemical behavior of their solutions in THF, as reflected by their cyclic voltammetry (CV) response. We found that one product (compound I) has the molecular structure [MgCl·THF₅]⁺ [AlCl₄]^– and it is electrochemically inactive in terms of reversible Mg dissolution/deposition. The molecular structure of the second compound (II), [Mg₂(μ-Cl)₃·THF₆]⁺ [t-BuAlCl₃]^–, has not been reported before, and its solutions in THF exhibit reversible Mg dissolution/deposition behavior. Our study clearly demonstrates the possibility of tuning judiciously the electrochemical response of ethereal solutions of magnesium organohaloaluminate complexes through their structural modifications.