Synthesis, Structure and Molecular Recognition of Functionalised Tetraoxacalix[2]arene[2]triazines

Functionalised dialkoxy-substituted tetraoxacalix[2]arene[2]triazine macrocycles 6 have been readily synthesised by the fragment coupling approach using methyl 3,5-dihydroxy-4-alkoxybenzoates and cyanuric chloride as the starting materials under very mild conditions. AlCl3-mediated deallylation and debenzylation reactions afforded the lower-rim dihydroxy-substituted tetraoxacalix[2]arene[2]triazine derivatives 11 and 13 in good yields. Although dialkoxy-substituted tetraoxacalix[2]arene[2]triazine macrocycles are fluxional in solution on the NMR spectroscopy timescale, they adopt a symmetric or slightly distorted 1,3-alternate conformation with the bridging oxygen atoms conjugated with the triazine rings. The dihydroxylated tetraoxacalix[2]arene[2]triazine 13b, which gives a mixture of monomer and dimer in solution according to a diffusion NMR spectroscopy study, adopts a 1,3-alternate conformation and forms a cyclic tetrameric assembly in the solid state due to the formation of intermolecular hydrogen-bonding networks. This dihydroxylated macrocyclic host molecule, a hydrogen-bond donor macrocycle with a V-shaped cleft, interacts with 2,2'-bipyridine, 4,4'-bipyridine and 1,10-phenanthroline guests. Although in solution they form the corresponding 1:1 complexes with binding constants ranging from 37.7 to 213 M-1, 2:2 host-guest complexes were observed in the crystalline state. Hydrogen-bonding interactions, along with other non-covalent interactions, such as lone-pair-electron-pi and C-H center dot center dot center dot pi interactions, were found to be the driving force for the formation of host-guest complexes.