Metal Ion Coordinating Properties of the Highly Preorganized Tetradentate Ligand 1,10-Phenanthroline-2,9-dicarboxaldehyde-2,9-dioxime

Use of UV/Vis spectroscopy to measure protonation and formation constants of PDOX (1,10-phenanthroline-2,9-dicarboxaldehyde-2,9-dioxime) is reported. All complexes all show a protonation constant of an oxime that is quite close to pK(a1) for the free ligand, that is, for Zn-II logK for ZnPDOX + H+ reversible arrow ZnPDOXH+. This is in agreement with literature crystal structures, which show that, for small metal ions such as Zn-II, one oxime group of PDOX is left non-coordinated. Only for Pb-II is the protonation constant of the PDOX complex significantly lowered, to a value of 8.2. In order to investigate the high logK(1) value for the Pb-II/PDOX complex, and its relatively low protonation constant, the crystal structure of [Pb(PDOXH2)(DMSO)(2)Cl(ClO4)] (1) was determined.