Effect of boron halogenation on dihydrogen bonds: A quantum mechanical approach

Dihydrogen bond (DHB) interaction that exists in ammoniated metal borohydride systems is recognized as an effective intermediate step involved in the evolution of H₂ molecules. Mechanism of DHB formation and its electronic properties upon halogenations were explored for Mg(BH₄)₂·2NH₃⋯M(BH₃X) (where M = Li, Na, K, and X = H, F, Cl, and Br) systems using ab initio (MP2) and DFT (ωB97XD) calculations. The influence of halogens in varying the nature of the DHB that forms in Mg(BH₄)₂·2NH₃⋯M(BH₃X) complexes was explored with the Quantum Theory of Atoms In Molecule (QTAIM) analysis. Further, Energy decomposition analysis (EDA) was made to understand the strength of DHB interaction through the calculation of the E_disp term in interaction energy. The results obtained from EDA and QTAIM were found to correlate well with the structural parameter and the interaction energy values. This study reveals the influence of halogenations on tuning the electronic properties of DHB interaction in all the complexes. Our results suggest that the effective substitution of halogens in BH₄ molecule enhances DHB interaction, and the impact of halogenations on DHB has been revealed through QTAIM, EDA, Natural Bond Order (NBO), Non-covalent Interaction (NCI), and Bader charge analyses.

Dihydrogen bonds in ammine metal borohydride systems are responsible for its large hydrogen storage capacity.