Metal Ion complexing properties of 2,2’-pyridyl-1,10-phenanthroline, a more Preorganized analogue of Terpyridyl. A Crystallographic, Fluorescence, and Thermodynamic study
Academic Article
Overview
Additional document info
View All
Overview
Abstract
Some metal ion complexing properties of the ligand MPP (2-(pyrid-2'-yl)-1,10-phenanthroline) are reported. MPP is of interest in that it is a more preorganized version of terpy (2,2'; 6,2''-terpyridine). Protonation constants (pK1 = 4.60, pK2 = 3.35) for MPP were determined by monitoring the intense π-π* transitions of 2 x 10-5 M solutions of the ligand as a function of pH at ionic strength zero and 25 °C. Formation constants (log K1) at ionic strength zero and 25 °C were obtained by monitoring the π-π* transitions of MPP titrated with solutions of the metal ion, or 1:1 solutions of MPP and the metal ion were titrated with acid. It was found that large metal ions such as Ca(II) or La(III) showed increases of log K1 of about 1.5 log units as compared with the less preorganized analogue terpy. Small metal ions such as Zn(II) and Ni(II) showed little increase in log K1 for MPP compared to the terpy complexes, which is attributed to the presence of five-membered chelate rings in the MPP complexes, which favor large metal ions with an ionic radius (r+) of 1.0 Å. The structure of [Cd(MPP)(H2O)(NO¬3)2] (1) is reported: Monoclinic, P21/c , a = 7.4940(13) Å, b = 12.165(2) Å, c = 20.557(4) Å, β = 96.271(5)°, V = 1864.47(9) Å3; Z = 4, final R = 0.0XX. The Cd in 1 is eight-coordinate, comprising the three donor atoms of the MPP, a coordinated water, a monodentate and a bidentate NO3-. Cd(II) is a fairly large metal ion, with r+ = 0.96 Å, slightly too small for coordination with MPP. The effect of this size-matching in terms of structure is discussed. Fluorescence spectra of 2 x 10-7 M MPP in aqueous solution are reported. The non-protonated MPP ligand fluoresces only weakly, which is attributed to a PET (photo-induced electron transfer) effect. The CHEF (chelation enhanced fluorescence) effect induced by some metal ions is presented, and the trends of the CHEF effect which is Ca(II) > Zn(II) > Cd(II) > Hg(II) are discussed in terms of factors that control the CHEF effect.
