Host-Guest Interactions of Sodiumsulfonatomethyleneresorcinarene and Quaternary Ammonium Halides : An Experimental-Computational Analysis of the Guest Inclusion Properties

Abstract

The molecular recognition of nine quaternary alkyl- and aryl-ammonium halides (Bn) by two different receptors, Calkyl-tetrasodiumsulfonatomethyleneresorcinarene (An), were studied in solution using 1H NMR spectroscopy. Substitution of methylenesulfonate groups at 2-positions of resorcinol units resulted in an increase of cavity depth by ∼2.80 Å and a narrow cavity aperture compared to Calkyl-2-H-resorcinarenes. The effect of alkyl chain lengths on the endo-complexation, that is the ability to incorporate other than N-methyl chains inside the cavities, were investigated using ammonium cations of the type ⁺NH2(R1)(R2), (R1 = Me, Et, Bu, R2 = Bu, Ph, Bz ). The C−H⋯ interactions between guests and hosts are the key driving forces for 14 out of 16 observed endo-complexes. In case of N-butyl-N-benzylammonium cation, the hydrogen bonding between -NH2 and sulfonate oxygens and the larger size hamper the N-butyl and N-benzyl groups from entering the host cavity. Association constants derived from isothermal calorimetry titrations confirm 1:1 host-guest complexes highlighting guest affinity, based on size and orientation. X-Ray crystallographic analysis revealed two types of complexes viz sodium-containing co-crystals, [(An)4⁻･m(Bn)⁺･qNa⁺], and sodium-free, [(An)4⁻･4(Bn)⁺]. Both types accommodate (Bn⁺) guests in their cavities. The N-methylated heterocycle guests and host form capsule-like structures in which the two-halves were joined by O−Na coordination bonds and self-assembles in to 2-D polymeric sheets. From the crystal structures, different conformations of methylenesulfonate groups with respect to cavity arising due to tetrahedral geometry of methylene linker were observed. Density Functional Theory (DFT) computations were used to analyze the effects of endo-guests on host conformations and to estimate the relative strengths of host-guest interactions.