Versatile Framework Solids Constructed from Divalent Transition Metals and Citric Acid: Syntheses, Crystal Structures, and Thermal Behaviors

The reactions of citric acid (C6H8O7 = H4cit) with various divalent transition metals (CdII, ZnII, CuII, and CoII) in
acid solution or under hydrothermal conditions (pH = 1−4) have resulted in the isolation of five new coordination polymers with
versatile multidimensional frameworks. The five metal citrate complexes, namely, [Cd3(Hcit)2(H2O)5·H2O]n (1), [Cd3(Hcit)2(H2O)2]n
(2), [Zn3(Hcit)2(H2O)2]n (3), [Cu2(cit)(H2O)2]n (4), and [Co(H2cit)(H2O)]n (5), have been characterized by elemental analysis, FT-IR
spectra, thermal analysis, and single-crystal X-ray diffraction studies. While complex 1 was obtained by evaporation of an EtOH−H2O solution of the reaction mixture, complexes 2−5 were all prepared under hydrothermal conditions. The crystal structures of the
five complexes are remarkably distinct from each other because of the different synthetic conditions or coordination nature of the
metal ions. Complex 1 features a 2D multilayered framework which contains an infinite 1D ladder-like structure. Complexes 2 and
3 are almost isostructural, and both exhibit 2D rhombus-grid frameworks. The copper(II) citrate complex 4 displays a new alkyloxyl-bridging coordination mode, different from other divalent metal citrate complexes. Consequently, the complex forms a 3D open-framework polymer with large void channels. Complex 5 crystallizes in a chiral space group and forms a 1D infinite helical chain
along a 21 axis. The thermal behaviors of the five coordination complexes have been investigated and discussed correlated with the
crystal structures in detail. Particularly, the open-framework structure of 4 is responsible for the reversible water desorption−adsorption process.