Dinuclear zinc(II) complexes of tetraiminodiphenol macrocycles and their interactions with carboxylate anions and amino acids. Photoluminescence, equilibria, and structure

Abstract

The reaction equilibria [H₄L]²⁺ + Zn(OAc)₂⇔ [Zn(H₂L)]²⁺ + 2HOAc (K₁) and [Zn(H₂L)]²⁺ + Zn(OAc)₂⇔ [Zn₂L]²⁺ + 2HOAc (K₂), involving zinc acetate and the perchlorate salts of the tetraiminodiphenol macrocycles [H₄L^1-3](ClO₄)₂, the lateral (CH₂)_n chains of which vary between n = 2 and n = 4, have been studied by spectrophotometric and spectrofluorimetric titrations in acetonitrile. The photoluminescence behavior of the complexes [Zn₂L¹](ClO₄)₂, [Zn₂L²(H₂O)₂](ClO₄)₂₊, [Zn₂L²(μ-O₂CR)](ClO₄) (R = CH₃, C₆H₅, p-CH₃C₆H₄, p-OCH₃C₆H₄, p-ClC₆H₄, p-NO₂C₆H₄), and [Zn₂L³(μ-OAc)](ClO₄) have been investigated. The X-ray crystal structures of the complexes [Zn₂L²(H₂O)₂](ClO₄)₂₊, Zn₂L³(μ-OAc)](ClO₄), and [Zn₂L²(μ-OBz)(OBz)(H₃O)](ClO₄) have been determined. The complex [Zn₂L²(μ-OBz)(OBz)(H₃O)](ClO₄) in which the coordinated water molecule is present as the hydronium ion (H₃O⁺) on deprotonation gives rise to the neutral dibenzoate-bridged compound [Zn₂L²(μ-OBz)₂]·H₂O. The equilibrium constants (K) for the reaction [Zn₂L²(H₂O)₂]²⁺ + A^- ⇔ [Zn₂L²A]⁺ + 2H₂O (K), where A^- = acetate, benzoate, or the carboxylate moiety of the amino acids glycine, l-alanine, l-histidine, l-valine, and l-proline, have been determined spectrofluorimetrically in aqueous solution (pH 6-7) at room temperature. The binding constants (K) evaluated for these systems vary in the range (1-8) × 10⁵.