Syntheses and Properties of One–Three-Dimensional Coordination Polymers Constructed by Metallohelicates of Purin-Containing Carboxylate

An acetate-functionalized purine-containing
ligand, 2-(6-oxo-6,9-dihydro-1H-purin-1-yl)
acetic acid (H2L), and its six new one–three-dimensional (1–3D) coordination
polymers, [La­(HL)2(NO3)­(H2O)2]n (1), [Ce­(HL)2(NO3)­(H2O)2]n (2), [Pb­(HL)2(H2O)2]n (3), [Cd­(HL)2(H2O)2]n (4), [Cd­(L)]n (5), and [Zn­(L)]n (6), were synthesized
and characterized. Single-crystal X-ray diffraction analyses reveal
that the architectures of complexes 1–6 are built from the metal–organic helixes of ligands H2L. The overall structures
range from the double helical chains of 1 and 2, and the 2D helical networks of 3 and 4, to the 3D coordination polymers of 5 and 6, containing 2D homochiral layers based on 1D metal–organic
helixes. In those meso compounds, ligands H2L display differently anionic states,
bridge modes, and especially unsymmetrical configurations with the
sharp distortion between the acetate and purin in favor of constructing
helical assemblies as expected. The plentiful hydrogen-bonding sites
and π···π interactions originating from
the bioactive purin group of ligand H2L greatly contribute to the formation of helical
structures and the structural dimension, showing an intriguing ability
in the supramolecular assembly. At room temperature, complexes 1–6 exhibit solid-state photoluminescences,
with their emission wavelengths and intensities varying according
to the central metal ions and the coordination patterns of ligand H2L. Additionally, the
thermostabilities of these new complexes have been discussed in detail.
In particular, 3D polymers 5 and 6 have
extra high thermostabilities and stronger blue emissions, indicating
potential applications in photoluminescent materials.