Chelate Ring Size Effect as a Factor of Selective Fluorescent Recognition of Zn2+ Ions by Pyrrolo[2,3‑b]quinoxaline with a Substituted 2‑Pyridyl Group Receptor

Analysis of the spectral properties and structural differences of two turn-on ratiometric fluorescent receptors for Zn2+ and Cd2+ ions, derivatives of pyrrolo­[2,3-b]­quinoxaline (2), and earlier published 3 (Ostrowska et al. CrystEngComm 2015, 17, 498–502) was performed. Both ligands are E/Z push–pull olefins interconverting at room temperature, with barriers to rotation about enamine double bonds, from E to Z isomers of 19.3 ± 0.1 and 16.9 ± 0.3 kcal/mol and from Z to E of 16.9 ± 0.3 and 15.7 ± 0.2 kcal/mol, respectively. Diastereoisomers (E)-2 and (Z)-2 were isolated and characterized by X-ray structural analysis. The formation of complexes by (E/Z)-2 with acetates and acetylacetonates of Zn2+ and Cd2+ was monitored by UV–vis, fluorescence, and 1H NMR titrations in acetonitrile, respectively. X-ray structural analysis for isolated [(E)-2]2Zn in relation to earlier published (E)-3-ZnOAc revealed the formation of a six-coordinated zinc ion with six- and four-membered bis-chelate rings by (E)-2. The chelate effect increases the ligand affinity for Zn2+ (log β12 = 12.45) and causes the elongation of nitrogen–metal bonds. Extension of the coordination cavity size allows coordination of a cadmium ion. The introduction of a flexible ethylene linker between the fluorophore and ionophore pyridyl groups in 3 significantly affects the selectivity of zinc-ion recognition. The distorted tetrahedral geometry of (E)-3-ZnOAc with a four-coordinated zinc ion appears to be the most preferred because of the short donor–zinc distance with a 1:1 binding mode. The formation of the small coordination cavity size with six-membered bis-chelate rings provides an effective overlap of zinc and donor orbitals, precluding the coordination of a cadmium ion in the same manner as zinc.