Hydrogen storage in copper prussian blue analogues: Evidence of H ₂ coordination to the copper atom

The hydrogen adsorption in porous Prussian blue analogues shows the highest value for copper, suggesting the possibility that a direct interaction between the copper atom and the hydrogen molecule is established. The bonding of copper (2+) to the CN group of cyanometallates shows a unique behavior. The trend of copper to receive electrons in its 3d hole to adopt an electronic configuration close to 3d¹⁰ is complemented by the ability of the CN group to donate electrons from its 5σ orbital, which has certain antibonding character. Because of this cooperative effect, the electronic configuration of the copper atom at the cavity surface is close to Cu(+). The resulting large availability of electron density on the copper atom favors its interaction with the antibonding σ* orbital of the hydrogen molecule. The charge removed from the metal t_2g orbitals is compensated (donated) by H₂ through a side-on a interaction. From these combined mechanisms, where H₂ behaves as an acceptor-donor ligand for the copper atom, the high ability that copper hexacyanometallates show for the hydrogen storage could be explained. This hypothesis is supported by the obtained hydrogen adsorption data for Cu₃[Ir(CN)₆]₂, Cu ₃[Fe(CN)₆]₂, Cu₂[Fe(CN) ₆], Cu[Pt(CN)₆], and Cu_3-xMn _x[Co(CN)₆]₂, where 0 ≤ x ≤ 3, and also by the estimated values for the involved adsorption heats. The studied samples were previously characterized using X-ray diffraction, thermogravimetry, and infrared and Mössbauer spectroscopies.