Synthesis, structure and metal binding property of internally 1,3-arylene-bridged azacalix[6]aromatics

Three internally 1,3-arylene-bridged azacalix[6]aromatics 1-3 were synthesized by the Pd-catalyzed macrocyclic fragment coupling reaction between a stellated dibrominated pentamer and N(2),N(6)-dimethylpyridine-2,6-diamine or N(1),N(3)-dimethylbenzene-1,3-diamine. Single-crystal X-ray analysis revealed that these bimacrocyclic compounds all adopt a triply pillared groove-shaped conformation, conceptually being derived from the fusion of two 1,3-alternate macrocycles. Four host/metal discrete complexes of the all-pyridine host 1, {Co(1)(CH(3)OH)(2)}(CoCl(4))·3(CH(3)OH), {Ni(1)(CH(3)OH)(2)}(NiCl(4)), {Ni(1)(CH(3)OH)(2)}(ClO(4))(2)·CH(3)OH and {Cd(2)(1)(CH(3)CN)(4)(H(2)O)(4)}(ClO(4))(4), have been structurally characterized by X-ray crystallographic analysis, exploring two different metal binding modes. The metal complexation property of the host 1 in solution was then investigated by the UV-vis and NMR titration. With variation of the radii of the tested metal ions (Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), Cd(2+), and Hg(2+)), the host 1 can trap one or two metal ions via four different modes by using its two coordination cavities or two marginal pyridine rings. Such metal binding diversity of internally bridged heteracalixaromatics spotlights their potential applications in metal ion transport, ion channel, and metallo-enzyme mimics.