Are Metal–Metal Interactions Involved in the Rising Enthalpies Observed in The Kubas Binding of H₂ to Hydrazine-Linked Hydrogen Storage Materials?

Models of two linked M­(III) and M­(II) (M = Ti, V, Cr) binding sites in hydrazine-linked hydrogen storage materials have been studied quantum chemically using density functional theory. The results compare favorably with previous experimental and computational results. Strong evidence is observed that the H₂ molecules bind to the metal in a Kubas manner. As seen previously in monometallic analogues,, altering the transition metal across the first row of the periodic table reduces the number of H₂ molecules that can be bound, and replacing a hydrazide ligand with a hydride increases the M-H₂ interaction energy. Evidence is presented for metal–metal interactions, which can influence the H₂ binding enthalpy and may help to explain the observed metallic properties and rising H₂ binding enthalpies with coverage of the experimental materials. An alternate explanation for the rising enthalpies is also proposed, involving a pressure-induced deformation of the structure with concomitant twisting of the bonds into conformations that allow more optimal binding of an H₂ ligand.