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.