Theoretical Prediction of Metastable Intermediates in the Decomposition of Mg(BH₄)₂

We have studied the decomposition pathway and products of Mg-borohydride using density functional theory (DFT) calculations of free energy (including vibrational contributions) in conjunction with a Monte Carlo-based crystal structure prediction method, the prototype electrostatic ground state (PEGS) search method. We find that a recently proposed Mg(B 3 H 8 ) 2 intermediate (Chong et al. Chem. Commun. 2011, 47, 1330) is energetically highly unfavorable with respect to decomposition into MgB 12 H 12 and hence is not a thermodynamic reaction product. We systematically search for low-energy structures of Mg-triboranes [Mg(B 3 H 8 ) 2, MgB 3 H 7, and Mg 3 (B 3 H 6 ) 2 ], closo-borane MgB n H n ( n = 6, 7, 8, 9, 10, 11), and Mg(B 11 H 14 ) 2 compounds using PEGS simulations, refining the resulting structures with accurate DFT calculations. We find that none of these compounds break the previously determined thermodynamically stable decomposition path: Mg(BH 4 ) 2 → 1 / 6 MgB 12 H 12 + 5 / 6 MgH 2 + 13 / 6 H 2 → MgB 2 + 4H 2 . However, the reaction [Mg(BH 4 ) 2 → 1 / 3 Mg 3 (B 3 H 6 ) 2 + 2H 2 ] involving a Mg 3 (B 3 H 6 ) 2 product has an enthalpy close to that of the MgB 12 H 12 pathway and falls within the desired enthalpy window for near-ambient reversibility [20–50 kJ/(mol H 2 )]. This indicates that (1) if MgB 12 H 12 is kinetically hindered in the decomposition of Mg(BH 4 ) 2 such as in the aforementioned reference (Chong et al. Chem. Commun. 2011, 47, 1330), Mg 3 (B 3 H 6 ) 2 might be yielded as a metastable intermediate, and (2) Mg 3 (B 3 H 6 ) 2 could possibly be rehydrided back to Mg(BH 4 ) 2 under modest H 2 ( T, p ) conditions. We suggest that the observed intermediate is not [B 3 H 8 ] but could be another triborane such as [B 3 H 6 ].