Equilibrium and structural studies of complexes with a hexadentate ligand containing amide, amine and pyridylnitrogen donors. Crystal structures of copper(ii), nickel(ii) and cobalt(iii) complexes

Abstract

A novel potentially hexadentate ligand, 1,8-bis(2-pyridylcarbamoyl)-3,6-diazaoctane (pyctrien) has been synthesised as its tetrahydrochloride salt; its protonation constants and the stability constants of the copper(II) and nickel(II) chelates have been determined by potentiometry. Amide group deprotonation permits the formation of [MLH₋₁]⁺ and [MLH₋₂] species in all cases, whereas the detection of the neutral or the protonated species depends on the nature of the metal ions. The solid complexes of copper, nickel and cobalt with the neutral and the deprotonated ligand have been synthesised and characterised by IR and UV–VIS spectroscopy. The amidic groups are coordinated through the oxygen atoms in all solid complexes [ML](ClO₄)₂ (M = Cu²⁺, Ni²⁺ and Co²⁺). The complexes obtained with the deprotonated forms of the ligand imply the coordination through the nitrogen atoms of the amidic groups. In the cobalt compound, the coordination of the six nitrogen donor atoms leads to the spontaneous oxidation of Co(II) into Co(III); thus, NMR spectroscopic investigations show that this complex has an α-cis configuration. X-Ray crystal determination has shown that [CuLH₋₁(H₂O)](ClO₄) is square pyramidal with a water molecule fixed in the apical position, one picolinamide moiety remaining uncoordinated (in agreement with ESR studies), whereas [NiLH₋₂] and [CoLH₋₂](ClO₄)_0.75Cl_0.25 are both octahedral with the two pyridine rings in cis-positions.