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

摘要

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 CO _(2) uptake properties with isosteric heats of adsorption ( Q _(st) ) for H _(2) and CO _(2) 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.