The Effect of Substitution Pattern on Binding Ability in Regioisomeric Ion Pair Receptors Based on an Aminobenzoic Platform

• Main Authors: Damian Jagleniec, Krzysztof Ziach, Kajetan Dąbrowa, Jan Romański
• Format: Article
• Language: English
• Published: MDPI AG 2019-08-01
• Series: Molecules
• Subjects: ion pair receptors; salt binding; molecular recognition; substitution effects; NMR spectroscopy; UV-Vis spectroscopy
• Online Access: https://www.mdpi.com/1420-3049/24/16/2990

A series of ditopic ion pair receptors equipped with 4-nitrophenylurea and 1-aza-18-crown-6-ether linked by ortho-(<b>1</b>), meta-(<b>2</b>), and para-(<b>3</b>) substituted benzoic acid were readily synthesized in three steps from commercially available materials. The binding properties of these regioisomeric receptors were determined using UV-vis and ¹H NMR spectroscopy in MeCN and in the solid state by single-crystal X-ray diffraction crystallography. The solution studies revealed that, apart from carboxylates, all the anions tested formed stronger complexes in the presence of sodium cations. Receptors <b>2</b> and <b>3</b> were found to interact with ion pairs with remarkably higher affinity than <i>ortho</i>-substituted <b>1</b>. ¹H NMR titration experiments showed that both urea NH protons interacted with anions with comparable strength in the case of receptors <b>2</b> and <b>3</b>, but only one of the NHs was effective in anion binding in the case of receptor <b>1</b>. X-ray analysis of the crystal structure of receptor <b>1</b> and <b>1</b>&#183;NaPF₆ complex showed that binding was hampered due to the formation of an intramolecular hydrogen bond. Analysis of the crystal structures of <b>2</b>&#183;NaBr and <b>3</b>&#183;NaBr complexes revealed that proper mutual orientation of binding domains was responsible for the improved binding of the sodium salts.