Mn(II)-Based Porous Metal–Organic Framework Showing Metamagnetic Properties and High Hydrogen Adsorption at Low Pressure

Abstract

A Mn­(II)-based homometallic porous metal–organic framework, Mn₅(btac)₄(μ₃-OH)₂(EtOH)₂·DMF·3EtOH·3H₂O (<b>1</b>, btac <b>=</b> benzotriazole-5-carboxylate), has been solvothermally synthesized and structurally characterized by elemental analysis, thermogravimetric analysis, and X-ray crystallographic study. <b>1</b> is a 3D neutral framework featuring 1D porous channels constructed by {Mn–OH–Mn}_<i>n</i> chains and btac linkers. Magnetic studies show that <b>1</b> is a 3D metamagnet containing 1D {Mn–OH–Mn}_<i>n</i> ferrimagnetic chains. High-pressure H₂ adsorption measurement at 77 K reveals that activated <b>1</b> can absorb 0.99 wt % H₂ at 0.5 atm and reaches a maximum of 1.03 wt % at 5.5 atm. The steep H₂ absorption at lower pressure (98.2% of the storage capacity at 0.5 atm) is higher than the corresponding values of some MOFs (MIL-100 (16.1%), MOF-177 (57.1%), and MOF-5 (22.2%)). Furthermore, activated <b>1</b> can adsorb CO₂ at room temperature and 275 K. The adsorption enthalpy is 22.0 kJ mol^–1, which reveals the high binding ability for CO₂. Detailed gas sorption implies that the exposed Mn­(II) coordination sites in the activated <b>1</b> play an important role to improve its adsorption capacities