Theoretical study of H-2 splitting and storage by boron-nitrogen-based systems: a bimolecular case and some qualitative aspects

The experimentally observed reversible hydrogen activation reaction by trans-2,6-dimethyl-2,6-diphenylpiperidine and B(C6F5)(3) is modeled at RI-SCS-MP2/aug-cc-pVTZ//B3LYP/6-31G* level. A Morokuma analysis is performed for the transition state and for the product to study the energy contributions. The role of solvent effects, and substitution of ligands is discussed. Some more general points are made on `frustration' (meaning the prevention of B-N bond formation by bulky ligands) and `Coulomb pays for Heitler-London' (meaning that the attraction between the product counterions is comparable with the homolytic splitting energy of H-2). Both ideas refer to hypothetical deviations from the experimental route. A simple NH3 + H-2 + BH3 <-> (NH4+)(BH4-) model reaction is used to estimate the frustration energy and to locate the ionic/covalent crossings of this `bimolecular' model. The ionic structures are stabilised further in oligomers (and crystals), a phenomenon termed as `collective Madelung ionisation' (CMI) and illustrated by a [(NH4+)(BH4-)](4) tetramer.