The Kubas interaction in M(II) (M = Ti, V, Cr) hydrazine-based hydrogen storage materials: a DFT study
The Cr(II) binding sites of an experimentally realised hydrazine linked hydrogen storage material have been studied computationally using density functional theory. Both the experimentally determined rise in H2 binding enthalpy upon alteration of the ancillary ligand from bis[(trimethylsilyl)methyl] to hydride, and the number of H2 molecules per Cr centre, are reproduced reasonably well. Comparison with analogous Ti(II), V(II) and Mn(II) systems suggests that future experiments should focus on the earliest 3d metals, and also suggests that 5 and 7 wt% H2 storage may be possible for V(II) and Ti(II) respectively. Alteration of the metal does not have a large effect on the M–H2 interaction energy, while alteration of the ancillary ligand bound to the metal centre, from bis[(trimethylsilyl)methyl] or hydride to two hydride ligands, THF and only hydrazine based ligands, indicates that ancillary ligands that are poor π-acceptors give stronger M–H2 interactions. Good evidence is found that the M–H2 interaction is Kubas type. Orbitals showing σ-donation from H2 to the metal and π-back-donation from the metal to the dihydrogen are identified, and atoms-in-molecules analysis indicates that the electron density at the bond critical points of the bound H2 is similar to that of classical Kubas systems. The Kubas interaction is dominated by σ-donation from the H2 to the metal for Cr(II), but is more balanced between σ-donation and π-back-donation for the Ti(II) and V(II) analogues. This difference in behaviour is traced to a lowering in energy of the metal 3d orbitals across the transition series.