Metal Ion Complexing Properties of a Non-macrocyclic Ligand of the Highest Levels of Preorganization. Dipyridoacridine, a Tridentate 1,10-phenanthroline Homologue
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Abstract
Some metal ion complexing properties of DPA (dipyrido[4,3-b;5,6-b]acridine ), a tridentate homologue of phen (1,10-phenanthroline) in aqueous solution are reported. UV-visible spectroscopy, following the intense π-π* transitions of DPA as a function of pH, gave protonation constants (pK) at ionic strength (μ) = 0 and 25 oC 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 oC and μ = 0, and log K1 values at μ = 0.1 were obtained: Mg(II), 0.7(1); Sr(II), 2.20(1); Ba(II) 1.1(1). The log K1 values show that the high levels of preorganization of PDA lead to complexes some 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 mechanics calculations (MM) are used to show that the best-fit M-N length for coordination with DPA is 2.60 Å, accounting for the high stability of Ca(II) or La(III) complexes of DPA, which are found to have close to this M-N length in complexes such as phen complexes. A fluorescence study of 2 x 10-7 M DPA shows that as the free ligand it fluoresces very strongly. The complexes of DPA with a variety of metal ions show weaker fluorescence than the free DPA ligand. This is discussed in terms of factors that control photo-induced electron transfer and chelation enhanced fluorescence.
