Intramolecular hydrogen-bond formation in nickel(II) complexes with macrocyclic ligands containing non-co-ordinating nitrogen atom(s): new ditopic receptors differentiating hydrogensulfate and hydrogenphosphates

Abstract

A comparative study of a series of nickel(II) complexes with structurally related tetradentate 14-membered tetra-, penta- and hexa-aza macrocycles has shown that the non-co-ordinating (remote) nitrogen atoms in the ligand backbones drastically influence the solution equilibria between the four and six-co-ordinated forms of the complexes. This phenomenon is interpreted in terms of the formation of intramolecular, along with co-ordinative, hydrogen bonds between the axial ligand and remote nitrogen. Such specific two-centred interactions result in relatively high formation constants of mono(hydrogensulfate) complexes [NiL(HSO₄)(H₂O)]⁺. For the tricyclic complex, with L = 1,3,6,9,11,14-hexaazatricyclo[12.2.1.1^6,9]octadecane, the formation of both mono- and bis-(hydrogensulfate) adducts has been detected with K₁= 7 ± 1 and K₂= 36 ± 5 dm³ mol^–1, thus indicating the allosteric effect in this system. The high differentiating effect for hydrogensulfate versus hydrogenphosphates, which is especially evident for the tricyclic complex, has been established.