Electronic spectra and structures of Cu²⁺ and Ni²⁺ complexes with dioxotetraazacycloalkanediacetates and their diester derivatives

Functionalized macrocyclic ligands, dioxotetraazacycloalkanediacetic acid, with different ring sizes and their diester derivatives were synthesized, and their Cu²⁺ and Ni²⁺ chelates were characterized by electronic absorption spectra in solution. A single-crystal X-ray analysis of nickel(II) 2,9-dioxo-1,4,7,10-tetraaza-4,7-cyclotetradecanediacetate pentahydrate showed that the central metal ion had an octahedral coordination with two amine nitrogen atoms, two carboxylate oxygen atoms and an amide oxygen atom from a ligand molecule and an oxygen atom from a water molecule: the metal complex crystallized in the monoclinic space group P2^I/n with a = 9.040(1), b = 13.539(1), c = 18.168(2) Å, β = 101.895(2)° and Z =4. Electronic spectra showed that all the complexes studied had similar structures in acidic solutions. In alkaline solutions, some metal chelates had a distorted planar structure in which two deprotonated amide nitrogen atoms and two amine nitrogen atoms were coordinated to the central metal ion. The tendency of deprotonation of amide groups is dependent on the ring size of the ligand, the nature of the central metal ion and the ability of the pendant arm to coordinate with the central metal ion.