Tuning Metal-Dihydrogen Interaction in Metal-Organic Frameworks for Hydrogen Storage.

Inspired by a recently reported metal-organic framework (MOF), V 2 Cl 2.8 (btdd) [H 2 btdd = bis(1 H -1,2,3-triazolo[4,5- b ],[4',5'- i ])dibenzo[1,4]dioxin], that shows a greatly improved H 2 adsorption enthalpy, we employ density functional theory to probe how the number of d electrons and the mixed valences influence the M-H 2 interaction inside the M 2 Cl x (btdd) MOFs. We find a cliff in the H 2 adsorption energy: the interaction strength remains strong from Sc to V and then falls sharply at Cr. Our results confirm V 2 Cl 2.8 (btdd) as one of the best performing hydrogen adsorbents and predict that Ti 2 Cl 2.8 (btdd) is equally promising while Sc 2 Cl 2 (btdd) and Ti 2 Cl 2 (btdd) may be even better. Our analysis indicates that an empty d x 2 - y 2 orbital is the key to the much stronger binding of H 2 at the open M(II) site (M = Sc, Ti, or V), whereas a partially filled d x 2 - y 2 orbital in Cr(II) and later M(II) greatly weakens H 2 binding. Our findings will be useful in designing MOFs to enhance H 2 adsorption.