Complexation of Calcium by the Synthetic Ionophore McN-4308, [2-(4-(Diphenylmethyl)-1-piperidinyl)-2-oxoethoxy]acetic Acid. Crystal and Molecular Structure of the Free Acid and of the Calcium Complex

McN-4308 is a synthetic monocarboxylic acid that exhibits the properties of a calcium ionophore. We have studied the crystal structures of McN-4308 and its calcium complex. The free acid crystallizes in space group P2₁/c with cell dimensions a = 20.244 (4) Å, b = 5.822 (2) Å, c = 16.363 (4) Å, β = 95.85 (2)∘, and Z = 4. The diglycolamic acid moiety shows conformational flexibility about the O3—C3 and the C1—C2 bonds. The calcium complex of McN-4308 crystallizes in space group P1̄, a = 17.460 (1) Å, b = 18.089 (1) Å, c = 15.834 (1) Å, a = 97.24 (1)∘, β = 88.24 (1)∘, γ = 113.74 (1)∘ with four calcium ions, each coordinated by two ionophores, in the unit cell (Z = 4). The complex exists in the solid state as centrosymmetric dimers possessing two calcium cations, four ionophore anions, and two water molecules. The calcium ions are eight-coordinate with square antiprism geometry. The two dimers observed in the unit cell are stereoisomers but have very similar overall geometry. In contrast to the free acid, the ionophores in the complex have nearly planar diglycolamic moieties. The dimers are held together through bridging carboxylate groups of two of the ionophores.