Multinuclear magnetic resonance spectroscopy and density function theory calculations for the identification of the equilibrium species in THF solutions of organometallic complexes suitable as electrolyte solutions for rechargeable Mg batteries

We present a multinuclear nuclear magnetic resonance (NMR) and density functional theory (DFT) study of electrolyte solutions based on organometallic complexes with aromatic ligands. These solutions constitute a unique electrolyte family possessing a wide electrochemical window, making them suitable for rechargeable magnesium batteries. In our previous study we identified equilibrium species in the solutions based on a combination of Raman spectroscopy and single-crystal XRD analyses,(1) and herein we extend our studies to include multinuclear NMR analyses. These solutions are comprised of the metathesis reaction products of MgCl_2-xPh_x and AlCl_3-yPh_y in various proportions, in THF. In principle, these reactions involve the exchange of ligands between the magnesium and the aluminum based compounds, forming ionic species and neutral molecules, such as Mg₂Cl₃⁺·6THF, MgCl₂· 4THF and AlCl_4-yPh_y^- (y = 0-4). The identification of the solution phase species from the spectroscopic results is supported by spectral analyses of specially synthesized reference compounds and DFT quantum-mechanical calculations. The current approach reveals new aspects about the NMR shift of the organometallic complexes and, in particular, facilitates differentiation between ionic and neutral species.