Hydrogen Storage and Selective, Reversible O₂ Adsorption in a Metal–Organic Framework with Open Chromium(II) Sites

A chromium(II)-based metal-organic framework Cr3 [(Cr4 Cl)3 (BTT)8 ]2 (Cr-BTT; BTT(3-) =1,3,5-benzenetristetrazolate), featuring coordinatively unsaturated, redox-active Cr(2+) cation sites, was synthesized and investigated for potential applications in H2 storage and O2 production. Low-pressure H2 adsorption and neutron powder diffraction experiments reveal moderately strong Cr-H2 interactions, in line with results from previously reported M-BTT frameworks. Notably, gas adsorption measurements also reveal excellent O2 /N2 selectivity with substantial O2 reversibility at room temperature, based on selective electron transfer to form Cr(III) superoxide moieties. Infrared spectroscopy and powder neutron diffraction experiments were used to confirm this mechanism of selective O2 binding.