BX(H): exploring the limits of isotopologue selectivity of hydrogen adsorption.

We study the isotopologue-selective binding of dihydrogen at the undercoordinated boron site of BX (X = H, F, Cl, Br, I, CN) using quantum chemistry. With a Gibbs free energy of H attachment reaching up to 80 kJ mol (Δ at 300 K for X = CN), these sites are even more attractive than most undercoordinated metal centers studied so far. We thus believe that they can serve as an edge case close to the upper limit of isotopologue-selective H adsorption sites. Differences of the zero-point energy of attachment average 5.0 kJ mol between D and H and 2.7 kJ mol between HD and H, resulting in hypothetical isotopologue selectivities as high as 2.0 and 1.5, respectively, even at 300 K. Interestingly, even though attachment energies vary substantially according to the chemical nature of X, isotopologue selectivities remain very similar. We find that the H-H activation is so strong that it likely results in the instantaneous heterolytic dissociation of H in all cases (except, possibly, for X = H), highlighting the extremely electrophilic nature of BX despite its negative charge. Unfortunately, this high reactivity also makes BX unsuitable for practical application in the field of dihydrogen isotopologue separation. Thus, this example stresses the two-edged nature of strong H affinity, yielding a higher isotopologue selectivity on the one hand but risking dissociation on the other, and helps define a window of adsorption energies into which a material for selective adsorption near room temperature should ideally fall.