Auxiliary Ligand-Assisted Structural Variation of Cd(II) Metal–Organic Frameworks Showing 2D → 3D Polycatenation and Interpenetration: Synthesis, Structure, Luminescence Properties, and Selective Sensing of Trinitrophenol

Four
new metal–organic frameworks (MOFs) of Cd­(II) ion,
[Cd­(fdc)­(bipy)1.5]­(1), [{Cd­(fdc)­(bpee)1.5}.3­(H2O)]­(2), [Cd­(fdc)­(3bpdb)­(H2O)]­(3), and [{Cd2(fdc)2(4bpdb)3}.1.5­(4bpdb).2­(H2O)]­(4) (where, fdc = 2,5-furandicarboxylate dianion, bipy = 4,4′-bipyridine,
bpee = 1,2-bis­(4-pyridyl)­ethylene, 3bpdb = 1,4-bis­(3-pyridyl)-2,3-diaza-1,3-butadiene,
and 4bpdb = 1,4-bis­(4-pyridyl)-2,3-diaza-1,3-butadiene) have been
successfully synthesized using mixed ligand systems and characterized
by single crystal X-ray analysis and other physicochemical studies.
Structural determination revealed that compounds 1 and 4 possess a rare 2D → 3D polycatenated pillared-bilayer
structure with {48.62} SP 2-periodic
net topology. Compound 2 has a novel threefold interpenetrating
3D hexagonal framework structure with {46.64} sqc-net topology, whereas compound 3 features
a 2D Zig-Zag network with {44.62} sql-net topology. Interestingly, compounds 1 and 4 are the first examples of Cd­(II) MOFs based on fdc ligand
and bipy/4bpdb spacers exhibiting an unusual 2D → 3D polycatenation
of bilayers. Photoluminescence investigation revealed emissions from
compounds 1–4 owing to ligand based
charge transfer (n → π* and π → π*)
transitions. The luminescent emission of 2 can be quenched
by addition of trace amount of 2,4,6-trinitrophenol (TNP), demonstrating
its potential application for highly selective and sensitive detection
of TNP. The influence of N,N′-donor spacers on the structure,
topology, and the functionality of the resulting MOFs has been demonstrated.