The complex formation of indium(III) acetate with tetra-, octa- and decamethyl-substituted 3,3′-bis(dipyrromethene)s (H2L) in DMF (298.15 K) was studied by means of spectrophotometric titration. For the first time, the most probable mechanism of the complexation process of indium(III) with bis(dipyrromethene) ligands was proposed, taking into account the role of the solvating medium. It was found that the coordination process of the H2L ligands by the In3+ ions includes the successive stages of the formation of intermediate mono- and binuclear heteroligand complexes of the compositions [In(AcO)2HL], [(In(AcO)2)2L] and [(InAcO)2L2] and of the final most of a stable binuclear triple-helix helicate of composition [In2L3]. It was established that the effect of the methylation features of bis(dipyrromethene) ligands manifests itself in a significant increase (~1.4–1.7 times) of the complexation processes thermodynamic constants logarithms (lgКo) for [In2L3] helicates with unsubstituted α-positions of the terminal pyrrole rings of ligands compared to completely methylated analogue. It is shown that the ligands of 3,3′-bis(dipyrrometene)s can be recommended as new effective highly sensitive “naked eye” sensors of trace amounts (up to ~31·0−7 mol/l) of In3+ cations in organic media.
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