Anion binding studies of tris(2-aminoethyl)amine based amide receptors with nitro functionalized aryl substitutions: A positional isomeric effect

Abstract

Syntheses and crystal structures of tren-based amide, L 1 , N, N′,N″-tris[(2-amino-ethyl)-4-nitro-benzamide] and L 2 , N, N′, N″-tris[(2-amino-ethyl)-2-nitro-benzamide] are reported and compared with previously published tripodal amide receptor L 3 , N, N′, N″-tris[(2-amino-ethyl)-3-nitro-benzamide]. The crystallographic results show intramolecular and intermolecular hydrogen-bonding interactions between two arms of the tripodal receptor and two other adjacent molecules in cases of L 1 and L 2 whereas in addition to the above interactions an aromatic π ···π stacking among tripodal arms is also observed in L 3 . Receptors L 1 , L 2 and L 3 having electron withdrawing – NO 2 substituted ( para, ortho and meta, respectively) phenyl moieties are explored toward their solution state anion binding properties and selectivity studies. The substantial changes in chemical shifts are observed for the amide protons (–N H) and aromatic protons (–C H) with F − and Cl − in cases of L 1 and L 3 , and only with F − for L 2 , indicating the participation of –NH and –CH protons in the solution state binding events. Binding constants for the above cases are calculated by 1H NMR titration upon monitoring the –NH signal. Receptor L 2 shows exclusive selectivity toward F − in dimethyl sulfoxide (DMSO). The structural aspects of binding I −, ClO 4 − and SiF 6 2− with the monoprotonated L 1 , L 1 H +·I −·DMF ( 1), L 1 H +·ClO 4 −·DMF ( 2) and L 1 H +·0.5SiF 6 2−·H 2O ( 3), respectively are examined crystallographically. Anion binding with multiple receptor units is observed via amide N–H···anion as well as aryl C–H···anion hydrogen-bonding interactions in all the complexes as observed in cases of previously reported crystal structures of anionic complexes of protonated L 3 . The aryl group having nitro functionality that contributes to solution state anion binding with the neutral receptor and solid state coordination in complexes 1– 3 through CH···anion interactions is noteworthy.