Metal Ion Complexing Properties of Dipyridoacridine, a Highly Preorganized Tridentate Homologue of 1,10-Phenanthroline

DPA (dipyrido[4,3-b;5,6-b]acridine) may be considered as a tridentate homologue of phen (1,10-phenanthroline). In this paper some of the metal ion complexing properties of DPA in aqueous solution are reported. Using UV−visible spectroscopy to follow the intense π−π* transitions of DPA as a function of pH gave protonation constants at ionic strength (μ) = 0 and 25 °C of pK1 = 4.57(3) and pK2 = 2.90(3). Titration of 10−5 M solutions of DPA with a variety of metal ions gave log K1 values as follows: Zn(II), 7.9(1); Cd(II), 8.1(1); Pb(II), 8.3(1); La(III), 5.23(7); Gd(III), 5.7(1); Ca(II), 3.68; all at 25 °C and μ = 0. Log K1 values at μ = 0.1 were obtained for Mg(II), 0.7(1); Sr(II), 2.20(1); Ba(II), 1.5(1). The log K1 values show that the high level of preorganization of DPA leads to complexes 3 log units more stable than the corresponding terpyridyl complexes for large metal ions such as La(III) or Ca(II), but that for small metal ions such as Mg(II) and Zn(II) such stabilization is minimal. Molecular mech...