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

2015 
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 concentrati...
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