Self-assembly and selective guest binding of three-dimensional open-framework solids from a macrocyclic complex as a trifunctional metal building block.

Abstract

The nickel(II) hexaazamacrocyclic complex (1) containing pendant pyridine groups has been synthesized by the one-pot template condensation reaction of amine and formaldehyde. From the self-assembly of 1 with deprotonated cis,cis-1,3,5-cyclohexanetricarboxylic acid, H2CTC- and CTC3-, three-dimensional supramolecular open-frameworks of [Ni(C20H32N8)][C6H9(COOH)2(COO)]2 x 4H2O (2) and [Ni(C20H32N8)]3[C6H9(COO)3]2 x 16H2O (3), respectively, have been constructed. The solids 2 and 3 are insoluble in all solvents. X-ray crystal structure of 2 indicates that each nickel(II) macrocyclic complex binds two H2CTC- ions in trans position and two pendant pyridine groups of the macrocyclic complex are involved in hydrogen-bonding interactions with the hydroxy groups of H2CTC- belonging to the neighboring macrocyclic complexes, which provides the beltlike one-dimensional chain composed of rectangular synthons. The one-dimensional chains are linked together through lattice water molecules by the hydrogen-bonding interactions to generate two-dimensional networks, which are again connected to each other by the offset pi-pi stacking interactions between the pendant pyridine rings to give rise to a three-dimensional structure in which channels are present. The X-ray crystal structure of 3 indicates that each nickel(II) macrocyclic unit binds two CTC3- ions in trans position and each CTC3- ion coordinates three nickel(II) macrocyclic complexes to form a two-dimensional layer, in which pendant pyridine rings are involved in the hydrogen bonding and the herringbone pi-pi interaction. Between the layers, the pendant pyridine rings belonging to the neighboring layers participate in the offset pi-pi stacking interactions, which gives rise to a three-dimensional network structure. The network creates channels running parallel to the a, b, and c axes, which are filled with guest water molecules. The X-ray powder diffraction patterns indicate that the frameworks of 2 and 3 are deformed upon removal of water guests but restored upon rebinding of water. The host solids 2 and 3 bind [Cu(NH3)4](ClO4)2 in MeCN with a binding constant (Kf) of 210 M(-1) and 710 M(-1), respectively, while they do not bind [Cu(en)2](ClO4)2 (en = ethylenediamine). The dried solids of 2 and 3 do not interact with benzene and toluene, but they differentiate methanol, ethanol, and phenol in toluene solvent with the Kf values of 42, 14, and 12 M(-1), respectively, for 2, and 13, 8.2, and 8.9 M(-1), respectively, for 3. In terms of binding sites for guest molecules, the solid 3 has greater capacity than the solid 2.