Effects of ligand chain length and terminal N-alkylation on the protonation constants and stability constants of some transition metal complexes of linear tetraaza and pentaaza ligands

A number of terminal N-alkylated linear tetraaza and pentaaza ligands were prepared. Their ligand protonation constants and some transition and post-t ransition metal (Ni2+, Cu2+, Zn2+, and Cd2+) complex stability constants we re determined by potentiometric titration methods. In general, methylation and ethylation at the terminal nitrogen atoms caused the corresponding liga nd nitrogen basicity to increase; however, the corresponding metal complex stabilities were decreased compared to the non-alkylated structural analogs , presumably due to a steric effect.