Complexes of Greatly Enhanced Thermodynamic Stability and Metal Ion Size-Based Selectivity, Formed by the Highly Preorganized Non-Macrocyclic Ligand 1,10-Phenanthroline-2,9-dicarboxylic Acid. A Thermodynamic and Crystallographic Study

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Abstract
The metal ion-complexing properties of 1,10-phenanthroline-2,9-dicarboxylic acid (PDA) are reported. The protonation constants (pK1 = 4.75, pK2 = 2.53) and formation constants (log K1) for PDA with Mg(II) (3.53), Ca(II) (7.3), Sr(II) (5.61), Ba(II) (5.43), La(III) (13.5), Gd(III) (16.1), Zn(II) (11.0), Cd(II) (12.8), Pb(II) (11.4), and Cu(II) (12.8) were determined by UV−vis spectroscopy in 0.1 M NaClO4 at 25 °C. The log K1 values for most of these metal ions were high enough that they were not displaced from their PDA complexes even at pH 2. The log K1 values were determined using the UV spectra to monitor the competition with EDTA (or DTPA; EDTA = ethylendiamine tetraacetic acid, DTPA = diethylenetriamine pentaacetic acid) as a function of pH according to the equilibrium: M(EDTA) + PDA + nH+ = M(PDA) + EDTAHn. The log K1 values indicate that the rigid extended aromatic backbone of PDA leads to high levels of ligand preorganization and selectivity toward large metal ions (e.g., Ca(II), Cd(II), Gd(III)) with an ionic radius of about 1.0 Å and greatly enhanced thermodynamic stability as compared to similar ligands without the reinforcing aromatic backbone. The structure of [Ca(PDA)(H2O)2]·2H2O (1) is reported: orthorhombic, Fdd2, a = 44.007(9) Å, b = 18.945(4) Å, c = 7.2446(14) Å, V = 6040(2) Å3, Z = 16, R = 0.0882. The Ca(II) ion has a coordination number of eight, lying in the plane of the tetradentate PDA, with Ca−N bonds averaging 2.55 Å and Ca−O bonds to the two acetate groups of PDA averaging 2.45 Å. These are very close to the normal Ca−L bonds of this type, supporting the idea that a metal ion the size of Ca(II) (ionic radius ≈ 1.0 Å) will fit into PDA in a low-strain manner. The remaining four coordination sites on Ca(II) in 1 come from two coordinated water molecules and a chelating carboxylate bridging from an adjacent [Ca(PDA)(H2O)2]·2H2O complex. Potential applications of PDA as a ligand in biomedical applications such as Gd(III) contrast agents in MRI are discussed.