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

A Mn­(II)-based homometallic porous metal–organic framework, Mn5(btac)4(μ3-OH)2(EtOH)2·DMF·3EtOH·3H2O (1, btac = benzotriazole-5-carboxylate), has been solvothermally synthesized and structurally characterized by elemental analysis, thermogravimetric analysis, and X-ray crystallographic study. 1 is a 3D neutral framework featuring 1D porous channels constructed by {Mn–OH–Mn}n chains and btac linkers. Magnetic studies show that 1 is a 3D metamagnet containing 1D {Mn–OH–Mn}n ferrimagnetic chains. High-pressure H2 adsorption measurement at 77 K reveals that activated 1 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 1 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 1 play an important role to improve its adsorption capacities.