Sulfonate center dot center dot center dot Pyridinium Supramolecular Synthon: A Robust Interaction Utilized to Design Molecular Assemblies

An approach for supramolecular design through a sulfonate center dot center dot center dot pyridinium synthon has been adopted to achieve the structural modulations in molecular complexes of metanilic acid and various pyridyl derivatives. Layered ionic solids [(3-ABSA)(2)(-)(4,4'-BPY-H)(2)(+)(4,4'-BPY)] (1) and [(3-ABSA) (4,4'-BPP-H)(+)] (2) have been obtained by reacting metanilic acid (3-ABSA-H) with linear bis-4-pyridines: 4,4'-bipyridine (4,4'-BPY) and 4,4'-trimethylenedipyridine (4,4'-BPP). 1a, a polymorph of 1, obtained with a slight excess of metanilic acid, also exhibits the layered ionic structure. The tendency of bis-4-pyridines to form infinite N+-H center dot center dot center dot N assisted chains and hence layered structures is also supported by the literature. Attempts to break down the layered structures by employing nonlinear bis-pyridines, 2,2'-bipyridine (2,2'-BPY) and 1,7-phenanthroline (1,7-phen), lead to the isolation of [(3-ABSA) (2,2'-BPY-H)(+)center dot H2O](2) (3) and [(3-ABSA) (1,7-Phen-H)(+) (1,7-Phen)center dot H2O] (4). Compound 3 displays two-dimensional (2-D) framework structure with bis-pyridinium cations stacked in rectangular cavities, while in 4 one-dimensional (1-D) trilayer chains associate to form 2-D sheets, which in turn undergo pi-interlocking to form a 3-D supramolecule. Monopyridyl derivative 4-phenylpyridine (4-PhPy) yields a pseudo-honeycomb framework [(3-ABSA)(-)(4-PhPy-H)(+)center dot H2O] (5), with pyridinium cations occupying the hives. [(1,5-DNSA)(2-)(4,4'-BPP-2H)(2+)center dot 2H(2)O] (6), obtained with bis-sulfonic acid (1,5-DNSA-2H) and linear bis-pyridine, is a 2-D porous solid with water-associated bis-pyridinium chains bridged by bis-sulfonates through a masked sulfonate center dot center dot center dot pyridinium synthon. Interestingly, the supramolecular synthon manifests itself variedly: ionic interaction in 1, 1a, and 2; charge assisted hydrogen bonding interaction in 4 and 5; and water-masked interaction in 3 and 6. Enhanced thermal and chemical stability in molecular salts 1-6 may be attributed to proton transfer. Incipience of mild to intense color in 1-5 is attributed to extended pi-stacking of pyridyl ligands. The products are also characterized by spectroscopic and thermogravimetric methods.