Green synthesis, optical and magnetic properties of a MnII metal-organic framework (MOF) that exhibits high heat of H2 adsorption

Green synthesis of a 3D, Mn(II) metal-organic framework (MOF) composed of a trimeric Mn(II) cluster and NDC linker formulated as [Mn-3(NDC)(3)(DMA)(4)](n) (1) (where, NDC = 2,6 napthalene dicarboxylic acid, DMA = N,N-dimethylacetamide) has been achieved by employing mechanochemical and sonochemical routes. Interestingly, MOF 1 undergoes reversible structural transformation upon desolvation and solvation of DMA molecules. The desolvated framework, 1' containing two unsaturated Mn(II) sites exhibits interesting H-2 and CO2 uptake properties with isosteric heats of adsorption (Q(st)) for H-2 and CO2 of 11.8 and 29.2 kJ mol(-1), respectively. Remarkably, the Q(st) of H-2 estimated for 1' is found to be the highest value amongst the Mn(II) MOFs reported so far and the high value has been attributed to the stronger interaction of H-2 with the unsaturated Mn(II) centers. Further, variable temperature magnetic susceptibility measurements of 1 revealed weak antiferromagnetic coupling interactions between the adjacent Mn(II) ions. The thermal stability of 1 has also been examined and was found to be highly stable. Photoluminescence investigation revealed that the emission from MOF 1 is owed to ligand based charge transfer transitions. Furthermore, compound 1 undergoes temperature induced solid-state conversion into phase pure MnO nanocrystals of about 10-18 nm in size embedded in a carbonaceous layer to form MnO-C nanocomposite (NC). The MnO-C NC has been characterized by PXRD, FE-SEM, EDAX and TEM analyses.