The stability of nickel(II) complexes of tetra-aza macrocycles

Abstract

The stability constants, log K₁, have been determined by a potentiometric and u.v.-visible spectroscopic technique for the nickel(II) complexes of the tetra-aza macrocyclic ligands 1,4,7,10-tetra-azacyclododecane (L¹) and 1,4,7,10-tetra-azacyclotridecane (L²). The value of log K₁ in 0.1 mol dm^–3 and 25 °C was found to be 16.4 ± 0.1 for L¹ and 17.98 ± 0.05 for L². The variation of log K₁ for 12- to 16-membered ring tetra-aza-macrocycles as a function of metal-ion size is discussed. Two clear trends emerge. First, the larger the metal ion, the more strongly it prefers the smallest macrocycle, L¹, and secondly, the smaller the metal ion, the more it prefers to complex with the 14-membered ring L³. This apparent paradox is explained in terms of molecular mechanics (m.m.) calculations, which show that the 12-membered macrocycle is more flexible than the 14-membered, and has effectively a larger cavity for accommodating the metal ion. A particularly puzzling feature is that low-spin nickel(II) complexes more strongly with L³ than with L², even though the cavity size in L² is a better fit for low-spin Ni^II as shown by m.m. calculations and numerous crystallographic studies. Possible causes of this are discussed.