Synthesis and X‑ray Structures of Novel Macrocycles and Macrobicycles Containing N,N-Di(pyrrolylmethyl)‑N‑methylamine Moiety: Preliminary Anion Binding Study Rajnish Kumar, Tapas Guchhait, and Ganesan Mani* Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India * S Supporting Information ABSTRACT: The [2 + 2] Schiff base condensation reactions between the newly synthesized dialdehyde, N,N-di(α-for- mylpyrrolyl-α-methyl)-N-methylamine), and ethylenediamine or p-phenylenediamine dihydrochloride readily afforded the 30- and 34-membered large size macrocycles in very high yields. Subsequent reduction reactions of these macrocycles with NaBH 4 gave the corresponding saturated macrocyclic hexaamines in good yields. The analogous reaction of the new dialdehyde with a triamine molecule afforded the [3 + 2] Schiff base macrobicycle in high yield, which was then reduced by reaction with NaBH 4 to give the saturated macrobicycle. All these compounds were characterized by spectroscopic methods. The anion binding properties of the saturated macrocycles having the ethylene and the phenylene linkers in CDCl 3 were studied by NMR titration methods. Although they have similar pyrrolic and amine NH groups their binding properties are different and interesting, owing to the conformational flexibility or rigidness rendered by the ethylene or phenylene groups, respectively. The macrocycle having the ethylene linkers binds anions in a 1:1 fashion, while the other receptor having the phenylene linkers prefers to bind anions in a sequential 1:2 fashion and has a multiple equilibria between a 1:1 and a 1:2 complexes, as shown by their binding constants, curve fittings by EQNMR, and Job plots. The X-ray structures of the 1:2 methanol, the aqua and the benzoate anion complexes of the macrocycles show two cavities in which the guests are bound, correlating with the high affinity found for the formation of stable 1:2 complexes in solution. The X-ray structure showed that the macrobicycle Schiff base adopts an eclipsed paddle-wheel shaped conformation and exhibits an out-out configuration at the bridgehead nitrogen atoms. ■ INTRODUCTION The driving force for design and synthesis of artificial synthetic receptors for anions, inspired by selective anion recognition in nature, comes from biological, environmental, and industrial point of views. 1 Because of the inherent nature of the anions, developing synthetic receptors that recognize anions primarily by means of covalent or noncovalent interactions is a challenging task and remains an active area of research. 2,3 Among the various types of systems, pyrrole-based receptors in which Sessler and co-workers pioneered 4 have attracted attention as they form strong hydrogen bonding interactions with anions because of the acidic nature of the NH group. 5 Typically these pyrrole-based receptors have been synthesized by making use of the active α-positions of the pyrrole ring which has led to various sizes of macrocycles 6 and macrobi- cycles 7 incorporating both steric and electronic factors for recognizing and binding various types of anions. Alternatively, anion receptors have also been synthesized by anion-templated 8 [2 + 2] Schiff base condensation reactions of pyrrole systems, which often give the desired large size Schiff bases in excellent yields which is essential for further applications. 9 In addition, these Schiff bases are reduced to give receptor molecules carrying additional hydrogen bonding NH groups for strongly interacting with an anion. 10 Usually large macrocyclic receptors are flexible and preorganized for ready binding with anions. One of the key strategies for synthesizing a large size pyrrole-based anion receptor which might have favorable conformation suitable for a specific anion is through varying the spacer unit between two pyrrolide units, A as shown in Chart 1. Macrocycles and macrobicycles 11 formed by these pyrrole systems have been used in studies such Received: June 4, 2012 Published: August 7, 2012 Chart 1. Dialdehyde Derivatives for Schiff Base Condensation Reactions Article pubs.acs.org/IC © 2012 American Chemical Society 9029 dx.doi.org/10.1021/ic3011822 | Inorg. Chem. 2012, 51, 9029-9038