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 (Q_st) for H₂ and CO₂ of 11.8 and 29.2 kJ mol⁻¹, respectively. Remarkably, the Q_st 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.