Divalent metal phosphonate coordination polymers constructed from a dipiperidine-based bisphosphonate ligand

Abstract

The ligand 4,4′-dipiperidine-N,N′-bis(methylenephosphonic acid), H₄L, has been reacted with divalent metal salts under solvothermal conditions to yield seven new metal phosphonate coordination polymers. The compounds have been characterized by elemental analyses and their structures determined by single-crystal X-ray diffraction. Zn₂2(L)(H₂2O)₂2 and Co₂2(L)(H₂2O)₂2 have (different) layered structures, while Mn₂2(L)(H₂2O)₃3 has a chain motif. In these compounds, the N atoms of the ligand bind to the metal ions. α-Co₂2Cl₂2(H₂2L), formed from CoCl₂·6H₂O and H₄L in ethanol, is also layered but the N atoms of the ligand are protonated. The Co atoms are tetrahedral, coordinated by three phosphonate oxygen atoms and a chloride ion. A polymorph of this compound, β-Co₂2Cl₂2(H₂2L), was obtained from a mixed ionic liquid under microwave irradiation. The primary difference between the polymorphs is the orientation of the phosphonate group relative to the dipiperidine. When reacted hydrothermally with Sn^IIC₂O₄, H₄L partially decomposes, producing phosphate ions which are incorporated into the structure of Sn₆6O₂2(H₂2L)(PO₄4)₂2·4H₂2O. In this compound, the N atoms of the ligand are protonated, and two oxide anions are incorporated for charge balance. A second phase is obtained from the same reaction, which was determined to be Sn₇7O(L)₃3. This compound has a layered structure which contains relatively large voids within the inorganic portion of the layer. These structures are discussed, as well as factors influencing the state of protonation in the final compounds. The choice of solvent and temperature were found to have a significant influence on the type of structure obtained.