RESUMO
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.
RESUMO
Molecular models of the M-H(2) binding sites of experimentally characterised amorphous vanadium hydrazide gels are studied computationally using gradient corrected density functional theory, to probe the coordination number of the vanadium in the material and the nature of the interaction between the metal and the H(2) molecules. The H(2) is found to bind to the vanadium through the Kubas interaction, and the first quantum theory of atoms-in-molecules analysis of this type of interaction is reported. Strong correlation is observed between the electron density at the H-H bond critical point and the M-H(2) interaction energy. Four coordinate models give the best reproduction of the experimental data, suggesting that the experimental sites are four coordinate. The V-H(2) interaction is shown to be greater when the non-hydrazine based ligand, THF, of the experimental system is altered to a poorer π-acceptor ligand. Upon altering the metal to Ti or Cr the M-H(2) interaction energy changes little but the number of H(2) which may be bound decreases from four (Ti) to two (Cr). It is proposed that changing the metal from V to Ti may increase the hydrogen storage capacity of the experimental system. A 9.9 wt% maximum storage capacity at the ideal binding enthalpy for room temperature performance is predicted when the Ti metal is combined with a coordination sphere containing 2 hydride ligands.
RESUMO
To verify the role of the Kubas interaction in transition metal grafted mesoporous silicas, and to rationalize unusual rising enthalpy trends with surface coverage by hydrogen in these systems, computational studies have been performed. Thus, the interaction of H2 with the titanium centers in molecular models for experimentally characterized mesoporous silica-based H2 absorption materials has been studied quantum chemically using gradient corrected density functional theory. The interaction between the titanium and the H2 molecules is found to be of a synergic, Kubas type, and a maximum of four H2 molecules can be bound to each titanium, in good agreement with previous experiments. The average Ti-H2 interaction energies in molecules incorporating benzyl ancillary ligands (models of the experimental systems) increase as the number of bound H2 units increases from two to four, in agreement with the experimental observation that the H2 adsorption enthalpy increases as the number of adsorbed H2 molecules increases. The Ti-H2 interaction is shown to be greater when the titanium is bound to ancillary ligands, which are poor π-acceptors, and when the ancillary ligand causes the least steric hindrance to the metal. Extension of the target systems to vanadium and chromium shows that, for molecules containing hydride ancillary ligands, a good relationship is found between the energies of the frontier molecular orbitals of the molecular fragments, which interact with incoming H2 molecules, and the strength of the M-H2 interaction. For the benzyl systems, both the differences in M-H2 interaction energies and the energy differences in frontier orbital energies are smaller than those in the hydrides, such that conclusions based on frontier orbital energies are less robust than for the hydride systems. Because of the high enthalpies predicted for organometallic fragments containing hydride ligands, and the low affinity of Cr(III) for hydrogen in this study, these features may not be ideal for a practical hydrogen storage system.
