Synthesis of para-linked azacalix[n]pyridine[n]pyrazines and their uranyl ion binding properties

Based on the fragment coupling strategy, the novel macrocycles, para-linked azacalix[n]pyridine[n]pyrazines (n = 2 and 4) with coexisting different heteroaromatics, were synthesized readily starting from 2,5-dibromopyrazine and 2,6-dibromopyridine. According to the X-ray diffraction analysis, the macrocycle azacalix[4]pyridine[4]pyrazine adopted a unique boat-type conformation with a large rectangular cavity in the solid state. The complexation between the azacalix[n]pyridine[n]pyrazines and uranyl ions was investigated by DFT calculations and H-1 NMR and UV-vis titration experiments. It was found that they were strong host molecules to form 1 : 1 complexes with uranyl ions with an association constant of up to (1.38 +/- 0.04) x 10(5) M-1.