title: Resolving the Magnetic Asymmetry of the Inner Space in Self-assembled Dimeric Capsules Based on Tetraurea-calix[4]pyrrole Components.
abstract: The encapsulation of N,N, N',N'-tetramethyl-1,5-pentanediamine-N,N'-dioxide 2 in a non-chiral capsular assembly formed by dimerization of tetraurea-calix[4]pyrrole 1a produced the observation of the N-methyl groups of the encapsulated guest as two separated singlets resonating highly upfield in the (1)H NMR spectrum. In order to clarify the origin of the observed signal splitting we assembled and studied a series of structurally related dimeric capsules. We used the tetraurea-calix[4]pyrrole 1a , the enantiomerically pure tetraurea-calix[4] pyrrole R-1b and the tetraurea-bisloop calix[4]pyrrole 1c as components of the produced assemblies. The (1)H NMR spectra of the assembled encapsulation complexes with bis-N-oxide 2 evidenced diverse splitting patterns of the N-methyl groups. In addition, 2D EXSY/ROESY NMR experiments revealed the existence of chemical exchange processes involving the separated methyl signals of the encapsulated guest. The capsular assemblies were mainly stabilized by a belt of eight head-to-tail hydrogen-bonded urea groups. The interconversion between the two senses of rotation of the unidirectionally oriented urea groups was slow on the (1)H NMR timescale. These characteristics determined the appearance of a new asymmetry element (supramolecular conformational chirality) in the assemblies that accounted for some of the magnetic asymmetries featured by the capsule's inner space. The racemization of the supramolecular chirality element was fast on the EXSY timescale and produced the chemical exchange processes detected for the encapsulation complexes.
authors.affiliation: Institute of Chemical Research of Catalonia (ICIQ) The Barcelona Institute of Science and Technology Avgda. Països Catalans 16 43007 Tarragona, Spain.
authors.forename: Mónica
authors.initials: M
authors.lastname: Espelt
authors.affiliation: Institute of Chemical Research of Catalonia (ICIQ) The Barcelona Institute of Science and Technology Avgda. Països Catalans 16 43007 Tarragona, Spain.
authors.forename: Gemma
authors.initials: G
authors.lastname: Aragay
authors.affiliation: Institute of Chemical Research of Catalonia (ICIQ) The Barcelona Institute of Science and Technology Avgda. Països Catalans 16 43007 Tarragona, Spain; Catalan Institution for Research and Advanced Studies (ICREA) Passeig Lluís Companys, 23 08018 Barcelona, Spain. pballester@iciq.es.
authors.forename: Pablo
authors.initials: P
authors.lastname: Ballester
mesh_terms: D047250:Calixarenes / Q000737:chemistry*; D019281:Dimerization; D008280:Magnetics; D011166:Porphyrins / Q000737:chemistry*; D066244:Proton Magnetic Resonance Spectroscopy; D013237:Stereoisomerism
chemical_list: D011166:Porphyrins; C469811:calix(4)pyrrole; D047250:Calixarenes