Two novel metal organic frameworks of Sn(II) and Pb(II) with Pyridine-2,6-dicarboxylic Acid and 4,4′-Bipyridine: syntheses, crystal structures and solution studies

Abstract

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 O-H⋅⋅⋅ O, O-H⋅⋅⋅EN and C-H⋅⋅⋅O hydrogen bonds, ion pairing, C-O⋅⋅⋅π (O⋅⋅⋅Cg 3.324(3) A and 3.381(3) A in 1 and O⋅⋅⋅Cg 3.346(4) A in 2), C-H⋅⋅⋅π (C⋅⋅⋅Cg 3.618(4) A in 2), and π⋅⋅⋅π stackings (with Cg ⋅⋅⋅ Cg distances of 3.613(2) and 3.641 (2) A 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.