Abstract
The supramolecular networks derived from the complexation of polyazamacrocycles with halide anions constitute fundamental building blocks of a broad range of modern materials. This study provides insights into the conformational framework that supports the binding of protonated cyclen macrocyles (1,4,7,10-Tetraazacyclododecane) by chloride anions through NHδ+···Cl− interactions. The isolated complex comprised of two cyclen hosts linked by one Cl− anion is characterized by means of infrared action spectroscopy and ion mobility mass spectrometry, in combination with quantum chemical computations. The Cl− anion is found to act as a hinge that bridges the protonated moieties of the two macrocycles leading to a molecular tweezer configuration. Different types of conformations emerge, depending on whether the trimer adopts an open arrangement, with significant freedom for internal rotation of the cyclen moieties, or it locks in a folded conformation with intermolecular H-bonds between the two cyclen backbones. The ion mobility collision cross section supports that folded configurations of the complex are dominant under isolated conditions in the gas phase. The IRMPD spectroscopy experiments suggest that two qualitatively different families of folded conformations coexist at room temperature, featuring either peripheral or inner positions of the anion with respect to the macrocycle cavities, These findings should have implications in the growth of extended networks in the nanoscale and in sensing applications.
Highlights
The aim of this work is to contribute to the understanding of the anionic supramolecular chemistry of azamacrocycles through the characterization of benchmark aggregates involving the binding of Cl− to protonated cyclen
The present study focuses on supramolecular features that should be relevant to the modeling of azamacrocycle networks, such as the preferred coordination arrangement sustained by the Cl− anions and the relative stability of packed sandwich-like configurations vs. open chain-like arrangements
The mass spectrum displayed strong signals at the nominal masses m/z = 381/383, which were assigned to the2·Cl− ions
Summary
The supramolecular complexes of polyazamacrocycles with halide anions conform intermediate arrangements in the synthesis of a broad range of modern nanostructured materials, with typically catalytic or ion-exchange activity (Alper et al, 1991; Ilioudis and Steed, 2001; Warden et al, 2004a; Mateus et al, 2010; Park et al, 2012; Wenzel et al, 2012; Lee et al, 2013, 2015; Evans and Beer, 2014; Busschaert et al, 2015). Previous works have investigated the recognition and binding of anions by azamacrocyles and related receptors employing condensed-phase methods, typically UV-vis absorption and fluorescence, NMR spectroscopy, or crystallography techniques (Ilioudis and Steed, 2001; Wichmann et al, 2006; Wenzel et al, 2012; Evans and Beer, 2014; Busschaert et al, 2015). We employed this methodology to characterize protonated cyclen and provide insights into the structure and intramolecular interactions in the isolated macrocycle (Avilés-Moreno et al, 2018). Despite the challenges imposed by proton interactions to the accurate description of the system, it is shown that the interrelation between experimental spectroscopy and computations provides insights into fundamental supramolecular features of azamacrocycle/halide frameworks
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