Abstract
Covalent macrocycles and three-dimensional cages were prepared by the self-assembly of di- or tritopic anilines and 2,6-diformylpyridine subcomponents around palladium(II) templates. The resulting 2,6-bis(imino)pyridyl-PdII motif contains a tridentate ligand, leaving a free coordination site on the PdII centers, which points inward. The binding of ligands to the free coordination sites in these assemblies was found to alter the product stability, and multitopic ligands could be used to control product size. Multitopic ligands also bridged metallomacrocycles to form higher-order supramolecular assemblies, which were characterized via NMR spectroscopy, mass spectrometry, and X-ray crystallography. An efficient method was developed to reduce the imine bonds to secondary amines, leading to fully organic covalent macrocycles and cages that were inaccessible through other means.
Highlights
Covalent organic macrocycles[1] and cages[2] have found wide application
Higher yields and cleaner products may be obtained through the use of templates[8] and reversibly formed linkages[9] such as imines,[10] boronic esters,[11] and alkenes[12] or alkynes[13]. The use of such dynamic covalent bonds leads to the formation of thermodynamic products, but such products may show limited stability due to cleavage of the dynamic bonds, such as hydrolysis of imines
In order to elucidate the design principles for this class of Pd-templated architectures, we carried out a careful analysis of the crystal structures of complexes bearing a 2,6bis(imino)pyridyl-PdII moiety.[17]
Summary
Covalent organic macrocycles[1] and cages[2] have found wide application. These structures serve as hosts for guest recognition,[3] in molecular separations,[4] as catalysts,[5] for surface modification[6] and to enable the generation of new mechanically interlocked molecular architectures.[7]. The more rigid tris-anilines gave only traces of discrete species along with oligomeric products, as suggested by the presence of small sharp 1H NMR signals along with more intense broad signals (Figure S73, SI) This result is not surprising, as a fully planar tris-aniline, with an angle β of 120°, would require an angle α of either 71° or 109° to form the small or large cage structures proposed, respectively.[30] Such α values are outside of the range of angles adopted by the bis(imino)pyridyl-PdII building block studied . ESI-MS monitoring of the reaction showed effective reduction of all imine bonds, but NMR spectra of the crude product were indecipherable, as expected considering the numerous stereoisomers originating from the NH stereocenters of the reduced cages, as was observed in the cases of 7·T1 and 8·T2. Templated cage 13·T33·Cl3 was successfully prepared through aniline exchange, albeit in low yield, to the direct synthesis (Figure S80, SI)
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