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.