Methylazacalixpyridines: Remarkable Bridging Nitrogen‐Tuned Conformations and Cavities with Unique Recognition Properties

Abstract

Methylazacalix[n]pyridines (n = 4, 8) and methylazacalix[m]arene[n]pyridines (m = n = 2, 4) have been synthesized by a convenient fragment coupling approach starting from 2,6-dibromopyridine, 2,6-diaminopyridine, and benzene-1,3-diamine. Thanks to the intrinsic electronic nature of nitrogen, which can adopt mainly sp(2) hybridization, allowing it variously to conjugate, partially conjugate, or not conjugate with the adjacent one or two pyridine rings, the resulting nitrogen-bridged calixpyridine derivatives act as a unique class of macrocyclic host molecules with intriguing conformational structures offering fine-tunable cavities and versatile recognition properties. Whilst in solution it is fluxional, in the solid state methylazacalix[4]pyridine adopts a 1,3-alternate conformation with a C(2v) symmetry in which every two bridging nitrogen atoms conjugate with one pyridine ring. After protonation, the methylazacalix[4]pyridinium species has a different conjugation system of its four bridging nitrogen atoms, yielding the similar twisted 1,3-alternate conformations with an approximate S(4) symmetry. The cavity of each protonated methylazacalix[4]pyridine, however, varies finely to accommodate guest species of different size and geometry, such as planar DMF or HO(2)CCO(2) (-) ion, a twisted HO(2)CCO(2) (-) ion, and a tetrahedral ClO(4) (-) ion. As giant macrocyclic hosts, both methylazacalix[8]pyridine and methylazacalix[4]arene[4]pyridine interact efficiently with fullerenes C(60) and C(70) through van der Waals forces. Their ease of preparation, versatile conformational structures, and recognition properties make these multinitrogen-containing calixarenes or cyclophanes unique and powerful macrocyclic hosts in supramolecular chemistry.

Solution structure of a pentachromium(ii) single molecule magnet from DFT calculations, isotopic labelling and multinuclear NMR spectroscopy

“…Literature data on the reactivity of 2,6dibromopyridine with 2,6-diaminopyridine (Scheme S1 †) confirm this view. 41,42 The available procedures for the synthesis of H 2 tpda indeed follow path (a) by reacting 2,6diaminopyridine with overstoichiometric 2-halopyridine (Hal = Cl, Br) and an added base (tBuOK or NaH) in refluxing benzene or THF. 40,[43][44][45][46] Although 2-chloropyridine gives better results, yields never exceed 60% even using a large excess of arylant.…”

Heterocalixarenes and Calixnaphthalenes

“…By varying the reaction conditions, between one and all four of the pyrrole rings could be converted to chloropyridines. 127 Both benzene and pyridine rings could be utilised to build the macrocycle, with both cyclic tetramers and octamers being isolated ( Figure 5.54a-d) along with linear species. This means that isomeric mixtures are often obtained (we have only shown one isomer for each compound).…”

Recognition of amino acids and anions by a Zn(II)-methylazacalix[4]pyridine complex

“…The introduction of hetero atoms, especially nitrogen atoms, on the bridges results in the formation of different conformations and cavities, owning to the fact that bridging nitrogen atoms can adopt sp 2 or sp 3 or both electronic configurations and can form various degrees of conjugations with the adjacent aromatic rings [38][39][40][41][42][43]48] . More importantly, when they interact with guest, the heteroatom-bridged calix(hetero)aromatic compounds can self regulate their conformations to best fit the guest, which enables their good recognition to both neutral molecules, such as diols, fullerenes, and metal cations [39][40][41]48] . We have demonstrated that methyazacalix [4] pyridine (MACP-4) (Figure 1), an outstanding example of heteroatom-bridged calix(hetero) aromatic compounds, was a powerful and selective ligand to metal ions.…”

“…Heteroatom-bridged calix(hetero)aromatic compounds are an emerging generation of macrocyclic host molecules [35][36][37][38][39][40][41][42][43][44][45][46][47][48] . The introduction of hetero atoms, especially nitrogen atoms, on the bridges results in the formation of different conformations and cavities, owning to the fact that bridging nitrogen atoms can adopt sp 2 or sp 3 or both electronic configurations and can form various degrees of conjugations with the adjacent aromatic rings [38][39][40][41][42][43]48] .…”