Correlating Solution- and Solid-State Structures of Conformationally Flexible Resorcinarenes: Significance of a...

The synthesis of tetramethoxyresorcinarene podands bearing p-toluene arms connected by -SO₃ - (1) and -CH₂ O- (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 ¹ H 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.