Acid–base and metal ion binding properties of pyridine-type ligands in aqueous solution.: Effect of ortho substituents and interrelation between complex stability and ligand basicity

The stability constants of the complexes formed between Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+ and Cd2+ (=M2+) and two sets of pyridine-type ligands (=L) were determined by potentiometric pH titration in aqueous solution (25 °C; I=0.5 M, NaNO3). One set consists of the simple and at the N1 site sterically unhindered pyridine-type ligands 3-chloropyridine, 4-bromopyridine, 4-(chloromethyl)pyridine, pyridine, β-picoline (=3-methylpyridine) and 3,5-lutidine (=3,5-dimethylpyridine); the other set includes the following pyridine derivatives with an ortho substituent, 2-methyl-5-bromopyridine, 2-amino-5-bromopyridine, tubercidin (=7-deazaadenosine), α-picoline (=2-methylpyridine) and 2-aminopyridine. The acidity constants of the monoprotonated ligands H(L)+ were also measured. Plots of versus pKH(L)H give straight lines for each mentioned set of pyridine derivatives. The equations for the corresponding least-squares lines allow calculation of the expected stability constant for a complex of any pyridine-type ligand (with or without an ortho substituent) provided its pKH(L)H value is known (in the pKa range 3–7). The differences between the plots for these two sets of ligands reflect the steric influence of the ortho substituent on metal ion binding at the N1 site of pyridine. It is shown that the steric effects of amino and methyl groups are equal. The extent of the steric inhibition depends on the metal ion; it is most pronounced for Ni2+ and nearly not existent for the alkaline earth ions. Furthermore, for the latter ions complex stability is independent of the basicity of the pyridine nitrogen and this indicates that in these instances outersphere complexes dominate. In the case of the divalent transition metal ions, the slopes of the straight lines are smaller for the complexes of the ortho-substituted ligands, except for the Cu2+ complexes where the slopes are identical; this indicates that Cu2+ forms with both sets of ligands mainly innersphere complexes, whereas for the other metal ions and their complexes with ortho-substituted pyridine-type ligands outersphere binding becomes important. The present results permit in addition the determination of the extent of the steric inhibition of the (C6)NH2 group on metal ion binding at N1 of the adenine residue.