Tunable Strategies Involving Flexibility and Angularity of Dual Linkers for a 3D Metal–Organic Framework Capable of Multimedia Iodine Capture

The
widespread use of nuclear power poses severe health and environmental
risks owing to the nonregulated release and disposal of radioactive
wastes in the environment. Among these wastes, the capture and removal
of radioactive iodine poses a big challenge. To develop a novel material
for capturing molecular iodine, we have strategically synthesized
a nitrogen-rich three-dimensional (3D) metal–organic framework
(MOF), {[Mn2(oxdz)2(tpbn)­(H2O)2]·2C2H5OH}n (1), utilizing a bent heterocyclic dicarboxylate linker
(H2oxdz: (4,4′-(1,3,4-oxadiazole-2,5-diyl)­dibenzoic
acid)) and a flexible bis­(tridentate) ligand (tpbn: N, N′, N″, N‴-tetrakis­(2-pyridylmethyl)-1,4-diaminobutane).
Based on its single-crystal structure, 1 is an eightfold
interpenetrated 3D framework, consisting of a unique 4-connected {Mn2(tpbn)} subunit, in which the pores line up with the nitrogen
atoms of the oxadiazole moiety. This can be considered as a big leap
for the development of 3D MOFs using flexible bis­(tridentate) ligands.
To emphasize the role of the flexible methylene chain length in such
ligand in the dimensionality of the resultant framework, the tphn
(N, N′, N″, N‴-tetrakis­(2-pyridylmethyl)-1,6-diaminohexane)
ligand with two additional methylene groups provides a one-dimensional
(1D) CP {[Mn2(oxdz)2(tphn)­(H2O)]·CH3OH}n (2). This spacer
chain lengthening has a profound effect on the coordination of such
ligand with Mn­(II), further affecting the binding of oxdz. The inherent
polarizable nature of the oxadiazole moiety and the presence of permanent
pore of dimensions (19.122 × 19.253 Å2) in 1 have been exploited for the capture/removal of iodine not
only from vapor and an organic solution but also from an aqueous media.
It exhibits competent 100% reversible sorption of iodine with an uptake
capacity of (1.1 ± 0.05) g/g of 1. The uptake value
has been corroborated by both gravimetric and titrimetric analyses.
The interaction of iodine with 1 has been notably studied
with molecular simulations, kinetic models of sorption, field emission
scanning electron microscopy (FESEM), and energy-dispersive X-ray
spectroscopy (EDX) analysis. Moreover, 1 is highly stable
and is recyclable without much loss of sorption capability up to five
cycles.