Improvement in hydrogen binding ability of closo-dicarboranes via functionalization and designing of extended frameworks.

Neutral closo-dicarboboranes are reported to have very low H2 binding ability. Herein, we report an improvement in H2 binding energy (Eb) of C2B4H6 by substituting H atoms with different functional groups like X = F, Cl, Br, and XY = BO, CN and NC via quantum-chemical density functional theory based computations. In going from B6H62- to C2B4H6, the Eb value is reduced from 14.6 kJ mol-1 to 2.7 kJ mol-1. C2B4X6 and C2B4(XY)6 systems, which can bind a total of eight H2 molecules, with one H2 molecule occupying at each B-B-C face, possess an Eb value per H2 in the range of 4.5 kJ mol-1 for X = F, 3.9 kJ mol-1 for X = Cl, 5.9 kJ mol-1 for X = Br, 6.8 kJ mol-1 for XY = BO, 5.8 kJ mol-1 for XY = CN and 5.2 kJ mol-1 for XY = NC. The improvement in Eb value is found to be the highest in case of C2B4(BO)6, which has the ability to bind 6.6 gravimetric wt% of H2. The situation can be made more favorable by applying an external electric field. Energy decomposition analysis reveals that although the dispersion interaction (ca. 55-65%) has significant role in binding H2 with such types of molecules, contribution from electrostatic and orbital interaction is also considerable. Further, we modeled an extended system by linking C2B4(BO)n through 'C ≡ C' units for H2 storage purpose. The energy difference between the highest occupied and the lowest unoccupied molecular orbitals gradually lessens with the increase in molecular length. Therefore, it can be tuned gradually by controlling the chain length, which may further open up their potency in the field of electronics. Graphical abstract C2B4X6 (X = F, Cl, Br) and C2B4(XY)6 (XY = BO, CN, NC) show enhanced H2 binding ability from C2B4H6. Further, 1D, 2D and 3-D frameworks can be built by joining C2B4(BO)n units via 'C ≡ C' linkage.