X-Ray, Crystal Structure and Solution Phase Studies of a Polymeric SrII Compound

Abstract

<em>In the crystal structure of the title polymeric compound, [C₄₂H₃₈N₆O₃₃Sr₅.2(H₂O)]_n, five independent metal atoms (Sr1-Sr5) have different coordination environments. The Sr1 and Sr5 atoms are nine coordinated and feature distorted tricapped trigonal-prismatic and capped square-antiprismatic geometries, respectively. The rest Sr^II atoms have eight coordination numbers. These units are connected via the carboxylate O atoms of mono- and di anionic forms of pyridine-2,6-dicarboxylic acid, (pydcH₂), and bridging water molecules that resulted in the formation of polymeric layers in 3-D. In the crystal structure, non-covalent interactions consisting of hydrogen bonds (O—H···O and C—H···O), π–π [interplanar distances of 3.413(2); 3.601(2) Å] and C═O···π [O···π distances of 3.249(3); 3.275(3) Å] stacking interactions play an important roles in stabilizing the structure. The protonation constants of propane-1, 2-diamine (1, 2-pn) and pyridine-2,6-dicarboxylate ion (pydc)^2–, the equilibrium constants for the pydc-1,2-pn proton transfer system and the stoichiometry and stability of this system with Sr²⁺ ion in aqueous solution were investigated by potentiometric pH titrations. The stoichiometry of one of the most abundant complexed species in solution was found to be the same as that of the crystalline Sr^II complex.</em>