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]­(<b>1</b>), [{Cd­(fdc)­(bpee)_1.5}.3­(H₂O)]­(<b>2</b>), [Cd­(fdc)­(3bpdb)­(H₂O)]­(<b>3</b>), and [{Cd₂(fdc)₂(4bpdb)₃}.1.5­(4bpdb).2­(H₂O)]­(<b>4</b>) (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 <b>1</b> and <b>4</b> possess a rare 2D → 3D polycatenated pillared-bilayer structure with {4⁸.6²} <b>SP</b> 2-periodic net topology. Compound <b>2</b> has a novel threefold interpenetrating 3D hexagonal framework structure with {4⁶.6⁴} <b>sqc</b>-net topology, whereas compound <b>3</b> features a 2D Zig-Zag network with {4⁴.6²} <b>sql</b>-net topology. Interestingly, compounds <b>1</b> and <b>4</b> 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 <b>1</b>–<b>4</b> owing to ligand based charge transfer (n → π* and π → π*) transitions. The luminescent emission of <b>2</b> 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