Correlating Solution- and Solid-State Structures of Conformationally Flexible Resorcinarenes: Significance of a Sulfonyl Group in Intramolecular Self-Inclusion.

Małgorzata Pamuła,Kaisa Helttunen,Maija Nissinen

doi:10.1002/chem.201905211

Abstract

The synthesis of tetramethoxyresorcinarene podands bearing p‐toluene arms connected by ‐SO3‐ (1) and ‐CH2O‐ (2) linkers is presented herein. In the solid state, the resorcinarene podand 1 forms an intramolecular self‐inclusion complex with the pendant p‐toluene group of a podand arm, whereas the resorcinarene podand 2 does not show self‐inclusion. The conformations of the flexible resorcinarene podands in solution were investigated by variable‐temperature experiments using 1D and 2D NMR spectroscopic techniques as well as by computational methods, including a conformational search and subsequent DFT optimisation of representative structures. The 1H NMR spectra of 1 and 2 at room temperature show a single set of proton signals that are in agreement with C 4v symmetry. At low temperatures, the molecules exist as a mixture of boat conformations featuring slow exchange on the NMR timescale. Energy barriers (ΔG ≠ 298) of 55.5 and 52.0 kJ mol−1 were calculated for the boat‐to‐boat exchange of 1 and 2, respectively. The results of the ROESY experiments performed at 193 K and computational modelling suggest that in solution the resorcinarene podand 1 adopts a similar conformation to that present in its crystal structure, whereas podand 2 populates a more versatile range of conformations in solution.

Highlights

Inclusion complexes, along with the control of self-inclusion behaviour, play an essential role in the preparation and function of self-assembled nanomaterials.[1,2,3] In supramolecular chemistry, macrocyclic hosts are important building blocks providing cavities for inclusion complexation thereby enabling the self-assembly of multiple supramolecular architectures
The results show that the solution structure of 1 correlates well with the conformation adopted by the molecule in its crystal structure, whereas 2 populates different types of low-energy conformations with the possibility of self-inclusion according to the computational structures
We have presented the synthesis of two conformationally flexible resorcinarene podands without permanent cavities that were characterised by a complete set of high-resolution spectroscopic techniques and X-ray crystallography

Summary

Introduction

Along with the control of self-inclusion behaviour, play an essential role in the preparation and function of self-assembled nanomaterials.[1,2,3] In supramolecular chemistry, macrocyclic hosts are important building blocks providing cavities for inclusion complexation thereby enabling the self-assembly of multiple supramolecular architectures. These supramolecular assemblies range from simple 1:1 inclusion complexes and pseudorotaxanes[4,5] to polymers[6,7,8,9] and particles[10,11] featuring in some cases gelation properties.[12] In particular, supramolecular polymers are attracting increasing interest due to their unique properties of self-healing,[13] stimuli-responsiveness[14] and shape memory.[15]. Crystal structures of intramolecular self-inclusion monomers are relatively rare.[6,22,23]

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