All-Metal Aromatic Sandwich Binuclear Complexes: Electronic Structures, Aromaticity and Interactions with Hydrogen via Multicenter Bonds

According to the first-principle calculations, the aromatic Al42− unit can sandwich the two transition metal (TM) atoms forming a stable all-metal binuclear compound [Al4TM2Al4]q- (q=-2,0,2; TM=Sc, Ti, V, Y, Zr, Nb, La, Hf, Ta). The stability analysis shows that these all-metal binuclear compounds Al4TM2Al4 exist as stable complexes forming aluminum rings which can maintain their two-fold aromaticity of the molecule, which is either the σ or the π aromaticity. Moreover the interaction between Al4Sc2Al4 and hydrogen atom has been also studied using density functional theory (DFT). It has been found that one Al4Sc2Al4 molecule can adsorb 12 hydrogen atoms, whose hydrogen storage mass density is 3.77 wt%. Furthermore, the average binding energy which is in the range between 0.2-0.7 eV/H2, can be either adsorbed or desorbed reversibly under near standard conditions. We have also found that a multicenter bond is formed between the H atoms and the Al or Sc atoms during the process of hydrogen adsorption, which is characterized by specific vibration frequency. Therefore, in order to store hydrogen reversibly and effectively, we can select the specific mode in the infrared band which is able to induce the hydrogen desorption from that which is adsorbed by Al8Sc2.