The Kubas interaction in M(ii) (M = Ti, V, Cr) hydrazine-based hydrogen storage materials: a DFT study

Abstract

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 H(2) binding enthalpy upon alteration of the ancillary ligand from bis[(trimethylsilyl)methyl] to hydride, and the number of H(2) 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% H(2) storage may be possible for V(II) and Ti(II) respectively. Alteration of the metal does not have a large effect on the M-H(2) 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-H(2) interactions. Good evidence is found that the M-H(2) interaction is Kubas type. Orbitals showing σ-donation from H(2) 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 H(2) is similar to that of classical Kubas systems. The Kubas interaction is dominated by σ-donation from the H(2) 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.

Improving the hydrogen storage capacity of metal organic framework by chemical functionalization

“…This can be attributed to the transfer of electron density from metal ion to H 2 , which is more profound in going from Sc to Cr. Particularly in Cr-OTPe8H 2 complex; significant decrease in the charge on Cr can be observed due to donation of electron density from metal ion to ligand in accordance with previous report [50]. As a result, amount of charge transferred from H 2 to metal ion in the case of Cr-OTPe8H 2 complex is more than that of other systems.…”

Titanium hydrazide gels for Kubas-type hydrogen storage

“…The isosteric heats of hydrogen adsorption were calculated by tting the adsorption isotherms at 77 K and 87 K into the Clausius-Clapeyron equation (Fig. 23 In this mechanism there are few Kubas sites at room temperature (consistent with the isotherms of these materials at 1 bar) and increasing the pressure deforms the amorphous structure, opening up new Kubas sites as the ligands twist around the metal center. The data for all six materials are shown in Fig.…”

“…5b Other theoretical computations have shown that titanium decorated nanotubes can adsorb 8 wt% hydrogen, 4 and 12 wt% for Ti decorated polymers, 17 In both cases, each Ti atom was proposed to hold up to 5 hydrogen molecules in a Kubas type interaction. 23 For this reason we have proposed a new model, which has already been shown to be plausible by computational methods. S15 † provides the data for A100-H 2 materials).…”

“…S15 † provides the data for A100-H 2 materials). 23 In this respect, the materials operate as their own heat management system by dissipating excess energy into the very process that opens and closes the binding sites. 3.…”

Functional MXene Materials: Progress of Their Applications

“…This is because among all of the potential MXene phases, Ti 2 C has the largest superficial area for every weight. Therefore, it can be considered that Ti 2 C has a relatively high gravimetric hydrogen storage capacity . Equation can be used to calculate E b (the bond energy) between H 2 and the host material Ti 2 C: …”