Fluorescence and Metal-Binding Properties of the Highly Preorganized Tetradentate Ligand 2,2′-Bi-1,10-phenanthroline and Its Remarkable Affinity for Cadmium(II)

The metal-ion-complexing properties of the tetradentate ligand 2,2′-bi-1,10-phenanthroline (BIPHEN) in 50% CH3OH/H2O are reported for a variety of metal ions. BIPHEN (with two reinforcing benzo groups in the backbone) was compared to other tetrapyridyls, 2,9-di­(pyrid-2-yl)-1,10-phenanthroline (DPP; with one benzo group) and 2,2′:6′,2″:6″,2‴- quaterpyridine (QPY; with no benzo groups), with levels of preorganization BIPHEN > DPP > QPY. Formation constants were determined by following the variation of the intense π → π* transitions in the absorbance spectra of BIPHEN in the presence of metal ion as a function of the pH. The log K 1 values show that the increased level of preorganization produced by the two benzo groups, reinforcing the backbone of the BIPHEN ligand, leads to increased complex stability with large metal ions (an ionic radius greater than 0.9 Å) compared to the less preorganized tetrapyridines DPP and QPY. In particular, the large CdII ion [log K 1(BIPHEN) = 12.7] shows unusual selectivity over the small ZnII ion [log K 1(BIPHEN) = 7.78]. The order of levels of preorganization BIPHEN > DPP > QPY leads to enhanced selectivity for SmIII over GdIII with increased preorganization, which is of interest in relation to separating AmIII from GdIII in the treatment of radioactive waste. AmIII is very close in ionic radius to SmIII, so that the size-based selectivity produced by the enhanced preorganization of BIPHEN should translate into enhanced AmIII/GdIII selectivity. The chelation-enhanced fluorescence (CHEF) effect in BIPHEN complexes is discussed. The CHEF effect in the ZnII complex is somewhat smaller than that for CdII, which is discussed in terms of decreased overlap in the Zn–N bonds formed by the too small ZnII, leading to a partial photoinduced-electron-transfer quenching of fluorescence. The structure of the complex [Cd­(BIPHEN)2]­(ClO4)2 is reported and shows that the Cd–N bonds are largely normal for the unusual 8-coordination observed, except that steric clashes between the terminal pyridyl groups of each of the BIPHEN ligands, and the rest of the orthogonal BIPHEN ligand, lead to some stretching of the outer Cd–N bonds.