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(B3H8)(2) intermediate (Chong et al. Chem. Commun. 2011, 47, 1330) is energetically highly unfavorable with respect to decomposition into MgB12H12 and hence is not a thermodynamic reaction product. We systematically search for low-energy structures of Mg-triboranes [Mg-(B3H8)(2), MgB3H7, and Mg-3(B3H6)(2)], closo-borane MgBnHn (n = 6, 7, 8, 9, 10, 11), and Mg(B11H14)(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(BH4)(2) -> 1/6MgB(12)H(12) + 5/6MgH(2) + 13/6H(2) -> MgB2 + 4H(2). However, the reaction [Mg(BH4)(2) -> 1/3Mg(3)(B3H6)(2) + 2H(2)] involving a Mg-3(B3H6) product has an enthalpy close to that of the MgB12H12 pathway and falls within the desired enthalpy window for near-ambient reversibility [20-50 kJ/ (mol H-2)]. This indicates that (1) if MgB12H12 is kinetically hindered in the decomposition of Mg(BH4)(2) such as in the aforementioned reference (Chong et al. Chem. Commun. 2011, 47, 1330), Mg-3(B3H6)(2) might be yielded as a metastable intermediate, and (2) Mg-3(B3H6)(2) could possibly be rehydrided back to Mg(BH4)(2) under modest H-2 (T,p) conditions. We suggest that the observed intermediate is not [B3H8] but could be another triborane such as [B3H6].