Solution and solid-state conformations of 1,5-pyridine and 1,5-phenanthroline-bridged p-tert-butylcalix[8]arene derivatives

An extension of the method for the regioselective introduction of nitrogen-containing heterocycles at the 1,5-phenolic positions of p-tert-butylcalix[8]arene previously described for 2,6-bis(chloromethyl)pyridine allowed the use of 2,9-bis(bromomethyl)-1,10-phenanthroline as a selective 1,5-dialkylating agent. The 1,5-dimethylpyridine derivative was previously characterized as a relatively rigid dicesium complex [1(4H)(2-)center dot 2Cs(+)], its metal-free counterpart 1(6H) is herein described as a fluxional species in solution, as evidenced by VT-NMR and theoretical studies. The 1,5-[2,9-bis(methylene)-1,10-phenanthroline]-p-tert-butylcalix[8]arene analogue 2(6H) was characterized spectroscopically in solution as a relatively rigid cone conformer at 390 K. The identity of both metal-free 1(6H) and the monocesium complex [2(5H)(-)center dot Cs+] was established in the solid state by X-ray crystallography, confirming the 1,5-substitution of the latter. The conformational behavior of 1(6H) and 2(6H) in solution is described in terms of the enthalpic and entropic contributions by semiempirical methods. (C) 2013 Elsevier Ltd. All rights reserved.