Electrophilic Reactivity of Tetrabromorhodamine 123 is Bromine Induced: Convergent Interpretation through Complementary Molecular Descriptors

Nucleophilic addition of water and of methanol to 3,6-diamino-2,4,5,7-tetrabromo-9-[2-(methoxycarbonyl) phenyl]-9<i>H</i>-xanthen-9-ylium, 4BrR123, yields respectively 2-(3,6-diamino-2,4,5,7-tetrabromo-9-hydroxy-9<i>H</i>-xanthen-9-yl)­xanthyl benzoate, HO4BrR123 and 2-(3,6-diamino-2,4,5,7-tetrabromo-9-methoxy-9<i>H</i>-xanthen-9-yl)­xanthyl benzoate, MeO4BrR123. The novel experimental results are addressed theoretically. The linear free energy relationship, LFER, second-order perturbation theory analysis of the natural bond orbital, NBO, and quantum theory of atoms in molecules, QTAIM, lead to the same conclusion: the electron-withdrawing effect of bonded Br atoms in 4BrR123 extremely enhances the molecular electrophilicity, as compared to 3,6-diamino-9-[2-(methoxycarbonyl) phenyl]-9<i>H</i>-xanthen-9-ylium, R123. The reactivity of these diaminoxanthylium cations is discussed in the context of local and global softness in extended conjugated systems