The stability of nickel(II) complexes of tetra-aza macrocycles
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 NiII as shown by m.m. calculations and numerous crystallographic studies. Possible causes of this are discussed.