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) 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.