Framework reduction and alkali-metal doping of a triply catenating metal-organic framework enhances and then diminishes H ₂ uptake

A permanently microporous metal-organic framework compound with the formula Zn ₂(NDC) ₂(diPyTz) (NDC = 2,6-naphthalenedicarboxylate, diPyTz = di-3,6-(4-pyridyl)-1,2,4,5-tetrazine) has been synthesized. The compound, which features a triply catenating, pillared-paddlewheel structure, was designed to be easily chemically reduced (diPyTz sites) by appropriate channel permeants. Reduction was achieved by using the naphthalenide anion, with the accompanying metal cation (Li ⁺, Na ⁺ or K ⁺) serving to dope the compound in extraframework fashion. H ₂ uptake at 1 atm and 77 K increases from 1.12 wt % for the neutral material to 1.45, 1.60, and 1.51 wt % for the Li ⁺-, Na ⁺-, and K ⁺-doped materials, respectively. The isosteric heats of adsorption are similar for all four versions of the material despite the large uptake enhancements for the reduced versions. Nitrogen isotherms were also measured in order to provide insight into the mechanisms of uptake enhancement. The primary mechanism is believed to be dopant- facilitated displacement of catenated frameworks by sorbed H ₂. More extensive cation doping decreases the H ₂ loading.