Conformational changes in Cmethyl-resorcinarene pyridine N-oxide inclusion complexes in the solid state

Rakesh Puttreddy,Kari Rissanen,Ngong Kodiah Beyeh

doi:10.1039/c6ce00240d

Abstract

Aromatic N-oxides interact with Cmethyl-resorcinarene resulting in marked changes in the conformation of the host resorcinarene. In the solid state, 2- and 3-methylpyridine N-oxides form pseudo-capsular 2 : 2 endo host–guest complexes with Cmethyl-resorcinarene stabilized by C–H⋯π interactions. The Cmethyl-resorcinarene·2-methylpyridine N-oxide complex has a C4v crown conformation, while the Cmethyl-resorcinarene·3-methylpyridine N-oxide complex has a slightly open C2v boat conformation. On the contrary, other para-substituted and benzo-fused pyridine N-oxides form only exo complexes with Cmethyl-resorcinarene. In the exo complexes, the asymmetry of the guest, conformational flexibility and preferred inter-host hydrogen bonding of Cmethyl-resorcinarenes exclude endo complexation. All the exo complexes form robust 1-D hydrogen bonded chains between the host hydroxyl groups assisted by the guest N–O groups, resulting in a C2v boat conformation.

Highlights

Other unusual conformations such as the rarely observed rcct-diamond Cmethyl-resorcinarene,[3] the rcct-boat Cmethyl-2nitroresorcinarene[4] and the rcct-crown C4t-pyrogallarene (4t = tert-butyl)[5] have been reported
Complexes of 2-MePyNO and 3-MePyNO with host 1 resulted in mixed endo/exo complexes, 2-MePyNO·1 [ formally : (2MePyNO)·1 and 3-MePyNO·1 (Fig. 2), which are remarkably similar to our previous results with Cethyl-2methylresorcinarene.[21]
The hydrogen atoms para to the N–O and –CH3 groups of 2-MePyNO manifest C–H⋯π interactions with the aromatic rings of host 1 with distances ranging between 2.7 Å and 3.1 Å (Fig. 3a)

Summary

Introduction

The hydrogen atoms para to the N–O and –CH3 groups of 2-MePyNO manifest C–H⋯π interactions with the aromatic rings of host 1 with distances ranging between 2.7 Å and 3.1 Å (Fig. 3a). The increased conformational flexibility of host 1 and the larger guest size in the endo complexation via the C–H⋯π interactions between the host and the guest distort the parent C4v crown conformation to a nearly ideal C2v boat conformation (Fig. 3d).

Results

Conclusion