The first colorimetric receptor for the B4O7 2- anion based on nitro substituted phenanthroimidazole ferrocene derivatives

Four phenanthroimidazole ferrocene derivatives (2a-2d) were designed, synthesized and characterized by ¹H NMR, ¹³C NMR and high-resolution mass spectroscopy (HRMS). Recognition of 12 anions by 2a-2d was investigated by UV-Vis absorption analysis, showing that 2b and 2d sensed B₄O₇ ^2- selectively among the tested anions with an obvious color change observed. ¹H NMR titrations and theoretical calculations demonstrated that 2b binds B₄O₇ ^2- through O⋯H hydrogen bonding and O⋯B interactions between the nitro moiety and the B₄O₇ ^2- anion.