Metal complexes of macrocyclic ligands. Part XXXIII. Effect of the side-chain length on the on/off equilibrium in the Ni2+ and Cu2+ complexes of 11-(2-aminoethyl)- and 11-(3-aminopropyl)-1,4,7,11-tetraazacyclotetradecane

Alkylation of 1,4,7-tritosyl-1,4,7,11-tetraazacyclotetradecane with CH₂O/KCN or acrylonitrile gave the corresponding cyanomethyl or 2-cyanoethyl derivatives, which, by treatment with Na in BuOH, were detosylated and reduced to the macrocyclic pentaamines 1 and 2 . The Ni²⁺ and Cu²⁺ complexes with 1 and 2 show a reversible pH-dependent change in geometry, in which the side-chain amino group is either coordinated to the metal ion (basic form) or protonated and, thus, non-coordinated (acidic form). The length of the side chain determines the log K_a values of these protonation/deprotonation equilibria: 1.89 and ～ 2.0 for the Cu²⁺ and Ni²⁺ complexes of ligand 1 with the 2-aminoethyl side chain, and 6.17 and 7.43 for the Cu²⁺ and Ni²⁺ complexes of ligand 2 with the 3-aminopropyl side chain.