Enhanced Metal Ion Selectivity of 2,9-Di-(pyrid-2-yl)-1,10-phenanthroline and Its Use as a Fluorescent Sensor for Cadmium(II)

Abstract

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 A. Metal ion complexation and accompanying selectivity of DPP is enhanced by the rigidity of the aromatic backbone of the ligand. Cd2+, with an ionic radius of 0.96 A, 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 that cause a significant CHEF effect with DPP are Ca2+ (10(-3) M) and Na+ (1.0 M), whereas metal ions such as Zn2+, Pb2+, and Hg2+ cause no CHEF effect with DPP. The lack of a CHEF effect for Zn2+ relates to the inability 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 the Cd-N bonds to the outer pyridyl groups stretched by steric clashes between the o-hydrogens on these outer pyridyl groups and the central aromatic ring of the second DPP ligand. The 8-coordinate Pb(II) in 2 has 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 two coordinated ClO4(-) ions, with long Pb-O bonds to ClO4(-) oxygens, typical of a sterically active lone pair on Pb(II). The Hg(II) in 3 shows an 8-coordinate structure with the Hg(II) forming short Hg-N bonds to the outer pyridyl groups of DPP, whereas the other Hg-N and Hg-O bonds are rather long. The structures are discussed in terms of the fit of large metal ions to DPP with minimal steric strain. The UV-visible studies of the equilibria involving DPP and metal ions gave formation constants that show that DPP has a higher affinity for metal ions with an ionic radius close to 1.0 A, particularly Cd(II), Gd(III), and Bi(III), and low affinity for small metal ions such as Ni(II) and Zn(II). The complexes of several metal ions, such as Cd(II), Gd(III), and Pb(II), showed an equilibrium involving deprotonation of the complex at remarkably low pH 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 pH is discussed in terms of the large hydrophobic surface of the coordinated DPP ligand destabilizing the hydration of coordinated water molecules and the build-up of charge on the metal ion in its DPP complex because of the inability of the coordinated DPP ligand to hydrogen bond with the solvent.

Highly Fluorescent Group 13 Metal Complexes With Cyclic, Aromatic Hydroxamic Acid Ligands

“…This phenomenon is known in the literature as "chelation-enhanced fluorescence (CHEF)" and is in most cases due to the suppression of low-lying charge-transfer transitions upon metal binding. 13 A preliminary solvent screening revealed that the emission efficiency for all complexes was generally high in relatively unpolar solvents (such as CHCl 3 , toluene, THF, etc.) and decreased in more polar media such as DMF and CH 3 CN.…”

Highly Efficient Separation of Actinides from Lanthanides by a Phenanthroline-Derived Bis-triazine Ligand

“…14 For example, it has been reported that the pre-organized, phenanthroline-based ligand 2 15 exhibits higher levels of complex stability and selectivity towards Cd(II) than related ligands such as quaterpyridine. 16 In addition, phenanthrolines have larger dipole moments than the 2,2'-bipyridines, 17 and often coordinate strongly to water via hydrogen bonding. 18 These properties could lead to improved extraction kinetics in a separation process since the ligand may interact more favourably with the organic/water interface.…”

Metal Ion Coordinating Properties of the Highly Preorganized Tetradentate Ligand 1,10‐Phenanthroline‐2,9‐dicarboxaldehyde‐2,9‐dioxime

“…After heating at reflux for one hour to ensure complete solution of the PDOX, and storage for a few days, colorless crystals of 1 precipitated out. C 18 …”

“…Familiar examples of preorganized ligands are crown ethers, [12] cryptands, [13] and nitrogen-donor (Ndonor) macrocycles. [14] More recently, the metal ion coordinating properties of highly preorganized ligands based on the phen (see Figure 1 for key to ligand abbreviations) backbone have been reported for ligands such as PDA, [15][16][17] DPP, [18] PDALC, [19,20] and PDAM. [21] The rigidity of the phen backbone has led to widespread use of the phen moiety in ligand design.…”