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

Abstract

Green synthesis of a 3D, Mn(ii) metal-organic framework (MOF) composed of a trimeric Mn(ii) cluster and NDC linker formulated as [Mn3(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 H2 and CO2 uptake properties with isosteric heats of adsorption (Qst) for H2 and CO2 of 11.8 and 29.2 kJ mol-1, respectively. Remarkably, the Qst of H2 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 H2 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.