Homochiral metal-organic frameworks based on transition metal bisphosphonates

Homochiral metal-organic frameworks with binaphthylbisphosphonate bridging ligands [Mn(L-H-2)(MeOH)].(MeOH), 1, [Co-2(L-H-2)(2)(H2O)(3)].(H2O)(4), 2, [Ni(L-H-2)(MeOH)(4)], 3, [Cu(L-Et-2)], 4, and [Zn-3(L-H)(2)(C6H5N)(2)], 5, where L is completely deprotonated enantiopure 2,2'-diethoxy-1,1'-binaphthalene-6,6'-bisphosphonic acid, were synthesized under hydro(solvo)-thermal reactions. Single-crystal X-ray diffraction studies reveal that 3-5 adopt condensed network structures, while compounds 1 and 2 adopt robust three-dimensional open-framework structures possessing rhombohedral channels. Solid-state circular dichroism spectra indicate that compounds 1-5 made from bridging ligands of opposite handness are supramolecular enantiomers. Compounds 1, 5, and L-H-4 exhibit strong blue fluorescence in solid state, and the formation of excimers in 5 and L-H-4 in solid state correlates well with the pi-pi stacking interactions. The present work illustrates the potential of rational synthesis of robust chiral porous network solids. The chiral pockets and functionalities within crystalline chiral porous solids may find important applications in enantioselective processes.