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.