Equilibrium, Kinetic, and Computational Studies on the Formation of Cu2+ and Zn2+ Complexes with an Indazole-Containing Azamacrocyclic Scorpiand: Evidence for Metal-Induced Tautomerism

Abstract

Cu2+ and Zn2+ coordination chemistry of a new member of the family of scorpiand-like macrocyclic ligands derived from tris(2-aminoethyl)amine (tren) is reported. The new ligand (L1) contains in its pendant arm not only the amine group derived from tren but also a 6-indazole ring. Potentiometric studies allow the determination of four protonation constants. UV−vis and fluorescence data support that the last protonation step occurs on the indazole group. Equilibrium measurements in the presence of Cu2+ and Zn2+ reveal the formation of stable [ML1]2+, [MHL1]3+, and [ML1(OH)]+ complexes. Kinetic studies on the acid-promoted decomposition of the metal complexes were carried out using both absorbance and fluorescence detection. For Zn2+, both types of detection led to the same results. The experiments suggest that [ZnL1]2+ protonates upon addition of an acid excess to form [ZnHL1]3+ within the mixing time of the stopped-flow instrument, which then decomposes with a first-order dependence on the acid concentration. The kinetic behavior is more complex in the case of Cu2+. Both [CuL1]2+ and [CuHL1]3+ show similar absorption spectra and convert within the mixing time to a new intermediate species with a band at 750 nm, the process being reverted by addition of base. The intermediate then decomposes with a secondorder dependence on the acid concentration. However, kinetic experiments with fluorescence detection showed the existence of an additional faster step. With the help of DFT calculations, an interpretation is proposed in which protonation of [CuL1]2+ to form [CuHL1]3+ would involve dissociation of the tren-based NH group in the pendant arm and coordination of a 2H-indazole group. Further protonation would lead to dissociation of coordinated indazole, which then will convert to the more stable 1H tautomer in a process signaled by fluorescence changes that would not be affecting to the d−d spectrum of the complex.