Nature of the Hydrogen Binding in Metal Organic Frameworks with Exposed Transition Metal Sites

MOFs with exposed transition metal (TM) sites were recently found to exhibit significantly larger experimental heat of H$_{2}$ adsorption than classical MOFs, thus attracted great attention. [1, 2] Understanding the nature of the H$_{2}$ interaction with the exposed metal sites is of critical importance for the further development of these materials. Using Mn$_{4}$Cl-MOF as an example, here we show that the H$_{2}$ binding with the exposed TM site is not of the expected Kubas type, in strong contrast to ``similar'' systems investigated previously (e.g., ref. [3] and [4]). In Mn$_{4}$Cl-MOF, there are a) no charge transfer from TM to H$_{2}$, b) no significant H-H bond elongation, and c) no evidence of any H$_{2}$-\textit{$\sigma $}$^{\ast }$ Mn-$d$ orbital hybridization. We also study the H$_{2}$ binding as a function of Mn$_{4}$-magnetic spin configurations, and find no significant effect of the magnetic state on the binding energy. We further reveal that the major contribution to the overall binding is classical Coulomb interaction arising from the small charge overlap of H$_{2}$-\textit{$\sigma $} and Mn-$d$ orbitals. This coulomb interaction is very anisotropic, and when the quantum nature of H$_{2}$-orientation is taken into account, the actual binding energy is significantly reduced from the calculated classical binding energy. [1] J. Am. Chem. Soc. 128, 16876 (2006). [2] Angew. Chem. Int. Ed. 46, 1419 (2007). [3] Phys. Rev. Lett. 94, 087205 (2005). [4] Phys. Rev. B 76, 085434 (2007).