abstract
A new heteroditopic macrobicyclic compound (t(2)pN(5)O(3)) containing two separate polyoxa and polyaza compartments was synthesized in good yield through a [1 + 1] "tripod-tripod coupling" strategy. The X-ray crystal structure of H(3)t(2)pN(5)O(3)(3+) revealed the presence of one encapsulated water molecule accepting two hydrogen bonds from two protonated secondary amines and donating a hydrogen bond to one amino group. The acid-base behavior of the compound was studied by potentiometry at 298.2 K in aqueous solution and at ionic strength 0.10 M in KCl. The results revealed unusual protonation behavior, namely a surprisingly low fourth protonation constant contrary to what was expected for the compound. H-1 NMR and DOSY experiments, as well as molecular modeling studies, showed that the water encapsulation and the conformation observed in the solid state are retained in solution. The strong binding of the encapsulated water molecule, reinforced by the cooperative occurrence of a trifurcated hydrogen bond at the polyether compartment of the macrobicycle, account for the very low log K-4(H) value obtained.
keywords
HETERODITOPIC CRYPTANDS; MOLECULAR RECOGNITION; CRYSTAL-STRUCTURES; COMPLEXATION PROPERTIES; PROTONATION CONSTANTS; DIELECTRIC-PROPERTIES; AQUEOUS-SOLUTION; LIGAND-BINDING; NMR; COORDINATION
subject category
Chemistry
authors
Mateus, P; Delgado, R; Groves, P; Campos, SRR; Baptista, AM; Brandao, P; Felix, V
our authors
Paula Cristina Ferreira da Silva Brandão
Assistant Researcher
Vítor Félix
Associate Professor with Aggregationacknowledgements
We acknowledge Pedro Lamosa for fruitful discussions and M. C. Almeida for providing elemental analysis and ESI-MS data from the Elemental Analysis and Mass Spectrometry Service at ITQB. P.M. thanks the Fundacao para a Ciencia e a Tecnologia (FCT) for the grant (SFRH/BPD/79518/2011). We acknowledge FCT and POCI, with coparticipation of the European Regional Development Fund (FEDER), for financial support under project PTDC/QUI/67175/2006. The NMR spectrometers are part of The National NMR Network (REDE/1517/RMN/2005), supported by