Formation and dissociation kinetics of Cu(ii) and Ni(ii) complexes with N2S2-macrocycles

Abstract

The kinetics of the formation and dissociation of the Cu²⁺ and Ni²⁺ complexes with a series of N₂S₂ macrocycles, in which the ring size and the geometry of the arrangement of the donor groups have been varied, have been measured at 25 °C and I = 0.5 (KNO₃). Both the deprotonated (L) and the monoprotonated (LH⁺) form of the ligands are reactive species in the formation step. In their deprotonated form, first bond formation, in some cases supported by an ICB effect, is rate determining, independently of the ring size. In the monoprotonated form, we find slower rates, due to the charge repulsion and/or conformation changes induced by hydrogen bonds. In contrast the mechanism of the dissociation is very dependent on the ring size. The complexes with the smaller rings react as flexible open-chain ligands directly with H⁺. In contrast, the complexes with the larger rings react in a similar way as rigid ligands: first the metal amine bond slowly dissociates so that the free electron pair of the amine can take the “out conformation” and then it is protonated. The 14-membered macrocycle L³ forms complexes in which the metal ions are ideally coordinated so that their dissociation becomes extremely slow.