Chelate ring size effect as a factor of selective fluorescent recognition of $Zn^{2+}$ ions by pyrrolo[2,3-b]quinoxaline with a substituted 2-oyridyl group receptor

Analysis of the spectral properties and structural di ff erences of two turn-on ratiometric fl uorescent receptors for Zn 2+ and Cd 2+ 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 ole fi ns 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 charac- terized by X-ray structural analysis. The formation of complexes by ( E / Z )- 2 with acetates and acetylacetonates of Zn 2+ and Cd 2+ was monitored by UV − vis, fl uorescence, and 1 H NMR titrations in acetonitrile, respectively. X-ray structural analysis for isolated [( E )- 2 ] 2 Zn 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 e ff ect increases the ligand a ffi nity for Zn 2+ (log β 12 = 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 fl exible ethylene linker between the fl uorophore and ionophore pyridyl groups in 3 signi fi cantly a ff ects 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 e ff ective overlap of zinc and donor orbitals, precluding the coordination of a cadmium ion in the same manner as zinc.