A modular approach for molecular recognition by zinc dipicolinate complexes

Abstract

A series of zinc dipicolinate (2,6-pyridinedicarboxylate; pdc) complexes [H₂tmbpy][Zn(pdc)₂]·5H₂O (1) {H₂tmbpy = 1,3-bis(4-pyridinium)propane(2+)}, [H₂bpy][Zn(pdc)₂]·6H₂O (2) {H₂bpy = 4,4′-bipyridinium(2+)}, [H₂bpy][Zn(pdc)₂]·3.5(4np)·2H₂O (3) {4np = 4-nitrophenol}, [H₂tmbpy][Zn(pdc)₂]·4(2,7dhn)·3H₂O (4) {2,7dhn = 2,7-dihydroxynaphthalene}, [H₂bpy][Zn(pdc)₂]·2(2,7dhn)·5H₂O (5), [H₂bpy][Zn(pdc)₂]·2(pyrogl)·6H₂O (6) {pyrogl = pyrogallol}, and [H₂tmbpy][Zn(pdc)₂]·2(2,6dhn)·8H₂O (7) {2,6dhn = 2,6-dihydroxynaphthalene} were synthesized and characterised. Different packing patterns in these complexes arise from differences in the abilities of cations to π-stack with zinc dipicolinate units. In 2, the planar [H₂bpy]²⁺ moieties are π-stacked with adjacent 4,4′-bipyridinium cations and [Zn(pdc)₂]²⁻ anions, whereas in 1, π-stacks are formed exclusively between [Zn(pdc)₂]²⁻ moieties. 4-Nitrophenol can selectively replace the [H₂bpy]²⁺ cations to form adduct 3, thus representing a novel host–guest system for molecular recognition of 4-nitrophenol. This recognition does not occur upon treatment of 4-nitrophenol with 1 which bears a nonplanar [H₂tmbpy]²⁺ cation. Host–guest interactions of pyrogallol, 2,6-dihydroxynaphthalene, and 2,7-dihydroxynaphthalene with the parent complexes 1 and 2 were studied, resulting in the crystallization of 4–7. The crystal structures of 5–7 show that guest organic molecules are accommodated in the layers of zinc dipicolinate anions. The formation of different water clusters is also discussed. In addition, topological analysis and classification of the H-bonded patterns driven by strong hydrogen bonds between the [H₂bpy]²⁺ or [H₂tmbpy]²⁺ cations and the [Zn(pdc)₂]²⁻ anions, and organic guest molecules were carried out. This analysis revealed: (i) discrete 0D dimeric (in 1 and 2) or tetrameric (in 7) motifs with 1M2-1 or 1,3M4-1 topology, respectively, (ii) infinite 1D zigzag chains with 2C1 topology in 5, and (iii) infinite 2D layers with the hcb [Shubnikov hexagonal plane net/(6,3)] topology in 6.