Two Novel Metal Organic Frameworks of Sn(II) and Pb(II) with Pyridine-2,6-dicarboxylic Acid and 4,4ʹ-Bipyridine: Syntheses, Crystal Structures and Solution Studies

Two novel compounds with formulae [Sn2(pydcH)2(H2O)2O]n, 1, and (4,4΄-bpyH2)0.5[Pb(pydc)2(4,4΄-bpyH)].4,4΄-bpy.4H2O, 2, were obtained from a one-pot reaction between pyridine-2,6-dicarboxylic acid (pydcH2) and 4,4΄-bipyridine (4,4΄-bpy) with corresponding Sn(II) and Pb(II) salts. In compound 1 with a polymeric structure, each Sn(II) atom is six-coordinated by one water molecule, two (pydcH)- groups and one oxide group resulted in a coordination polymer. Compound 2 has a seven-coordinated environment around Pb(II) atom by two (pydc)2- groups and one (4,4΄-bpyH). The anionic complex is balanced by half a (4,4΄- bpyH2)2+ as counter ion. There are four uncoordinated water molecules and one 4,4΄-bpy in the crystal lattice. Therefore, in compound 2, we have neutral, mono- and biprotonated forms of 4,4΄-bipyridine, simultaneously. Several interactions including OH ···O, O-H···N and C-H···O hydrogen bonds, ion pairing, C-O···π (O···Cg 3.324(3) Å and 3.381(3) Å in 1 and O···Cg 3.346(4) Å in 2), C-H···π (C···Cg 3.618(4) Å in 2), and π···π stackings (with Cg ···Cg distances of 3.613(2) and 3.641 (2) Å in 2) are present to expand and stabilize the structure. The complexation reactions of bpy and pydc-bpy with Sn2+ and Pb2+ ions in aqueous solution were investigated by potentiometric pH titrations, and the resulting equilibrium constants and species distributions at various pHs for major formed complexes are described