The study of 8-Hydroxyquinoline-2-Carboxyllic acid and its metal ion complexing properties

UNCW Author/Contributor (non-UNCW co-authors, if there are any, appear on document)
F. Crisp McDonald Jr. (Creator)
The University of North Carolina Wilmington (UNCW )
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Robert Hancock

Abstract: The metal ion coordinating properties of the ligand HQC (8-hydroxyquinoline-2- carboxylic acid) were studied by UV-visible spectroscopy and X-ray crystallography. The protonation constants of HQC were determined by monitoring absorbance of 5 x 10-5 M HQC at five different wavelengths in the range 210-300 nm as a function of pH. The protonation constants pK1 and pK2 were determined to be 10.14 and 3.92 at 25 oC in 0.1 M NaClO4. The experiments were repeated with 5 x 10-5 M HQC and 5 x 10-5 M metal ion solutions. These experiments produced two apparent protonation constants (pK’1 and pK’2) that were shifted to lower pH by competition with the metal ion as compared with pK1 and pK2. These were used to calculate equilibrium constants for the metal ions (M) as: M(HQC) + H+ ?? M(HQC)H (pK’1) [1] M + HQC ?? M(HQC) (log[M] + pK1 + pK2 – pK’1 – pK’2) [2] The HQC complexes also gave hydrolysis constants at higher pH according to: M(HQC) + OH- ?? M(HQC)OH [3] Log K1 (eq. 2) was obtained in 0.1 M NaClO4 at 25 oC for a selection of metal ions: Mg(II), 4.93; Ca(II), 6.16; Sr(II), 4.82; Ba(II), 4.10; La(III), 10.13; Gd(III), 9.89; Cu(II), 12.00; Zn(II), 9.10; Cd(II), 8.57; Pb(II), 11.35. The constants were remarkably high for a tridentate ligand with a donor set consisting of a pyridine nitrogen, and a phenolic and a carboxylic oxygen. These were discussed in terms of the high levels of preorganization of HQC, and contrasted with other similar ligands such as 8-hydroxyquinoline and dipicolinic acid. The high stability of HQC complexes is discussed in relation to its possible use in biomedical applications, such as the use of its Gd(III) complex as an MRI imaging agent. The attachment of a proton to the HQC complexes as in equation 1 was of some interest. Accordingly, crystals of [Zn((HQC)H)2] .2H2O (1) and [Cd((HQC)H)2] .2H2O (2) were grown and the structures determined by X-ray crystallography. Crystals of 1 were triclinic, Pi, a = 7.152(3), b = 9.227(4), c = 15.629(7) Å, a = 103.978(7), ß = 94.896(7), ? = 108.033(8)°, Z = 2, R = 0.0499. Crystals of 2 were triclinic, Pi, a = 7.0897(5), b = 9.1674(7), c = 16.0672(11) Å, a = 105.0240(10), ß = 93.9910(10), ? = 107.1270(10)°, Z = 3, R = 0.033. The crystal structures showed that the protons present were indeed attached to the phenolic oxygens, which were coordinated to the metal ions. Of considerable interest was the very short O---O distance of 2.4- 2.5 Å found for water molecules H-bonded to these protons, which is in the range for a very strong H-bond. The structures suggested that in the case of the Cd(II) complex (2) the proton was in one case actually attached to the water molecule to give a hydronium ion H-bonded to the non-protonated phenolic oxygen, while in the Zn(II) complex (1) the H-bond was actually symmetrical with the proton midway between the two oxygens. The significance of these short H-bonds is discussed in relation to their significance in biochemical processes.

Additional Information

A Thesis Submitted to the University of North Carolina Wilmington in Partial Fulfillment Of the Requirements for the Degree of Master of Science
Language: English
Date: 2009
Ligands--Analysis, Metal ions--Analysis
Ligands -- Analysis
Metal ions -- Analysis

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