Enhanced Metal Ion Selectivity of 2,9-Di-(pyrid-2-yl)-1,10-phenanthroline and Its Use as a Fluorescent Sensor for Cadmium(II)
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- 作者：Gregory M. Cockrell ; Gang Zhang ; Donald G. VanDerveer ; Randolph P. Thummel ; Robert D. Hancock
- 刊名：Journal of the American Chemical Society
- 出版年：2008
- 出版时间：January 30, 2008
- 年：2008
- 卷：130
- 期：4
- 页码：1420 - 1430
- 全文大小：372K
- 年卷期：v.130,no.4(January 30, 2008)
- ISSN：1520-5126
文摘
The metal ion complexing properties of the ligand DPP (2,9-di-(pyrid-2-yl)-1,10-phenanthroline)were studied by crystallography, fluorimetry, and UV-visible spectroscopy. Because DPP forms five-membered chelate rings, it will favor complexation with metal ions of an ionic radius close to 1.0 Å. Metalion complexation and accompanying selectivity of DPP is enhanced by the rigidity of the aromatic backboneof the ligand. Cd2+, with an ionic radius of 0.96 Å, exhibits a strong CHEF (chelation enhanced fluorescence)effect with 10-8 M DPP, and Cd2+ concentrations down to 10-9 M can be detected. Other metal ions thatcause a significant CHEF effect with DPP are Ca2+ (10-3 M) and Na+ (1.0 M), whereas metal ions such asZn2+, Pb2+, and Hg2+ cause no CHEF effect with DPP. The lack of a CHEF effect for Zn2+ relates to theinability of this small ion to contact all four donor atoms of DPP. The structures of [Cd(DPP)2](ClO4)2 (1),[Pb(DPP)(ClO4)2H2O] (2), and [Hg(DPP)(ClO4)2] (3) are reported. The Cd(II) in 1 is 8-coordinate with theCd-N bonds to the outer pyridyl groups stretched by steric clashes between the o-hydrogens on theseouter pyridyl groups and the central aromatic ring of the second DPP ligand. The 8-coordinate Pb(II) in 2has two short Pb-N bonds to the two central nitrogens of DPP, with longer bonds to the outer N-donors.The coordination sphere around the Pb(II) is completed by a coordinated water molecule, and twocoordinated ClO4- ions, with long Pb-O bonds to ClO4- oxygens, typical of a sterically active lone pair onPb(II). The Hg(II) in 3 shows an 8-coordinate structure with the Hg(II) forming short Hg-N bonds to theouter pyridyl groups of DPP, whereas the other Hg-N and Hg-O bonds are rather long. The structuresare discussed in terms of the fit of large metal ions to DPP with minimal steric strain. The UV-visiblestudies of the equilibria involving DPP and metal ions gave formation constants that show that DPP has ahigher affinity for metal ions with an ionic radius close to 1.0 Å, particularly Cd(II), Gd(III), and Bi(III), andlow affinity for small metal ions such as Ni(II) and Zn(II). The complexes of several metal ions, such asCd(II), Gd(III), and Pb(II), showed an equilibrium involving deprotonation of the complex at remarkably lowpH values, which was attributed to deprotonation of coordinated water molecules according to: [M(DPP)(H2O)]n+ 
[M(DPP)(OH)](n-1)+ + H+. The tendency to deprotonation of these DPP complexes at low pHis discussed in terms of the large hydrophobic surface of the coordinated DPP ligand destabilizing thehydration of coordinated water molecules and the build-up of charge on the metal ion in its DPP complexbecause of the inability of the coordinated DPP ligand to hydrogen bond with the solvent.
[M(DPP)(OH)](n-1)+ + H+. The tendency to deprotonation of these DPP complexes at low pHis discussed in terms of the large hydrophobic surface of the coordinated DPP ligand destabilizing thehydration of coordinated water molecules and the build-up of charge on the metal ion in its DPP complexbecause of the inability of the coordinated DPP ligand to hydrogen bond with the solvent.