A thermodynamic and crystallographic study of complexes of the highly preorganized ligand 8-hydroxyquinoline-2-carboxylic acid

The metal ion complexing properties of the ligand HQC (8-hydroxyquinoline-2-carboxylic acid) are reported. The structures of [Zn(HQCH)2] · 3H2O (1) and [Cd(HQCH)2] · 3H2O (2) were determined (HQCH = HQC with phenol protonated). Both 1 and 2 are triclinic, space group P1¯, with Z = 2. For 1a = 7.152(3), b = 9.227(4), c = 15.629(7) Å, α = 103.978(7)°, β = 94.896(7)°, γ = 108.033(8)°, R = 0.0499. For 2a = 7.0897(5), b = 9.1674(7), c = 16.0672(11) Å, α = 105.0240(10)°, β = 93.9910(10)°, γ = 107.1270(10)°, R = 0.0330. In 1 the Zn has a distorted octahedral coordination geometry, with Zn–N of 2.00 and 2.15 Å, and Zn–O to the protonated phenolic oxygens of 2.431 and 2.220 Å. The structure of 2 is similar, with Cd–N bonds of 2.220 and 2.228 Å, with Cd–O bonds to the protonated phenolate oxygens of 2.334 and 2.463 Å. The structures of 1 and 2, and isomorphous Ni(II) and Co(II) HQC complexes reported in the literature, show very interesting short (<2.5 Å) O–O distances in H-bonds involving the protons on the coordinated phenolates and lattice water molecules. These are discussed in relation to the possible role of short low-energy H-bonds in alcohol dehydrogenase in mediating the transfer of the hydroxyl proton of the alcohol to an adjacent serine oxygen.The formation constants for HQC are determined by UV–Visible spectroscopy at 25 °C in 0.1 M NaClO4 with Mg(II), Ca(II), Sr(II), Ba(II), La(III), Gd(III), Zn(II), Cd(II), Ni(II), Cu(II), and Pb(II). These show greatest stabilization with metal ions with an ionic radius above 1.0 Å. This is as would be expected from the fact that HQC forms two five-membered chelate rings on complex-formation, which favors larger metal ions. The ligand design concept of using rigid aromatic backbones in ligands to achieve high levels of preorganization, and hence the high log K values (for a tridentate ligand) and strong metal ion selectivities observed for HQC, is discussed.

Crystal structures and formation constants in aqueous solution indicate the high level of preorganization of the ligand 8-hydroxyquiniline-2-carboxylic acid, imparted by its rigid aromatic backbone, which predisposes it to form complexes of high thermodynamic stability with larger metal ions such as Cd(II) and Gd(III).Figure optionsDownload as PowerPoint slide