Analysis of Intermediates and Products from the Dehydrogenation of Mg(BH₄)₂

The thermodynamic properties of key compounds Mg(B₃H₈)₂, MgB₂H₆, MgB₁₀H₁₀, Mg(B₁₁H₁₄)₂, Mg₃(B₃H₆)₂, and MgB₁₂H₁₂, proposed to be formed in the release of hydrogen from magnesium borohydride Mg(BH₄)₂ and the uptake of hydrogen by MgB₂, have been investigated using solid-state density functional theory (DFT) calculations. More accurate tretment of the cell-size effects with respect to the entropies was also investigated in order to improve the accuracy of the thermodynamic properties of complex borohydrides. We find that the zero-point energy corrections can lower the electronic energies of reaction by 20-30 kJ/(mol H₂) for these intermediates, while adding the thermal and entropy contibutions results in a total decrease of up to ∼50 kJ/(mol H₂). Although our treatment lowers the calculated formation energy of Mg(B₃H₈)₂, it is still too high to explain the experimental observation of B₃H₈^-. We discuss possible reasons for this disparity and propose that the formation of B₃H₈^- and H^- in a disordered amorphous phase has a large energy difference compared to the phase-separated Mg(B₃H₈)₂ and MgH₂ considered in calculations. A comparison of the experimental and NMR chemical shifts calculated within a DFT approach for known species Mg(BH₄)₂, Mg(B₃H₈)₂, Mg(B₁₁H₁₄)₂, MgB₁₀H₁₀, and MgB₁₂H₁₂ provides validation for predicting the chemical shifts of the other compounds which are yet to be confirmed experimentally. These include MgB₂H₆ and the proposed trianion species Mg₃(B₃H₆)₂ that both have favorable thermodynamics for reversible hydrogen storage in Mg(BH₄)₂ without the formation of MgH₂ as a coproduct which could phase separate and inhibit rehydrogenation.