Metal-Ion-Complexing Properties of 2-(Pyrid-2′-yl)-1,10-phenanthroline, a More Preorganized Analogue of Terpyridyl. A Crystallographic, Fluorescence, and Thermodynamic Study

Some metal-ion-complexing properties of the ligand 2-(pyrid-2′-yl)-1,10-phenanthroline (MPP) are reported. MPP is of interest in that it is a more preorganized version of 2,2′;6,2″-terpyridine (tpy). Protonation constants (pK1 = 4.60; pK2 = 3.35) for MPP were determined by monitoring the intense π–π* transitions of 2 × 10–5 M solutions of the ligand as a function of the pH at an ionic strength of 0 and 25 °C. Formation constants (log K1) at an ionic strength of 0 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. Large metal ions such as CaII or LaIII showed increases of log K1 of about 1.5 log units compared to that of tpy. Small metal ions such as ZnII and NiII showed little increase in log K1 for MPP compared to the tpy complexes, which is attributed to the presence of five-membered chelate rings in the MPP complexes, which favor large metal ions. The structure of [Cd­(MPP)­(H2O)­(NO3)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(7)°, V = 1864.67(9) Å3, Z = 4, and final R = 0.0786. The Cd in 1 is seven-coordinate, comprising the three donor atoms of MPP, a coordinated water, a monodentate, and a bidentate NO3–. CdII 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 the structure is discussed. Fluorescence spectra of 2 × 10–7 M MPP in aqueous solution are reported. The nonprotonated MPP ligand fluoresces only weakly, which is attributed to a photoinduced-electron-transfer effect. The chelation-enhanced-fluorescence (CHEF) effect induced by some metal ions is presented, and the trend of the CHEF effect, which is CaII > ZnII > CdII ∼ LaIII > HgII, is discussed in terms of factors that control the CHEF effect, such as the heavy-atom effect.