The hydrothermal reaction of M(NO₃)₂·6H₂O (M = Ni and Zn) with benzene-1,3,5-tricarboxylic acid (H₃btc) and 1,2-bis(1,2,4-triazol-4-yl)ethane (btre) produced the mixed-ligand coordination polymers (MOFs) ³ _∞{[Ni₃(µ₃-btc)₂(µ₄-btre)₂(µ-H₂O)₂]·∼22H₂O} (1) and ³ _∞{[Zn₃(µ₄-btc)₂(µ₄-btre)(H₂O)₂]·2H₂O} (3). The compounds, characterized by single-crystal X-ray diffraction, X-ray powder diffraction and thermoanalysis feature trinuclear secondary building units (SBU) within the three-dimensional frameworks. The trinuclear nickel unit in 1 exhibits an intra-trimer together with some weak inter-trimer antiferromagnetic coupling with J = −13.88(8) cm⁻¹ from the magnetic susceptibility measurement between 1.9–300 K. The zinc coordination polymer 3 shows a strong fluorescence at 423 nm upon excitation at 323 nm (not seen in the free btre ligand). Compound 3 is thermally robust until 200 °C (ambient pressure) where loss of the water molecules starts. Careful control of the dehydration procedure (freeze-drying) for 1 and (heating to 280 °C) for 3 allowed for a solid-state reaction with single-crystal-to-single-crystal structural transformations in obtaining the largely dehydrated products ³ _∞{[Ni₃(µ₂-btc)₂(µ₄-btre)₂(µ-H₂O)₂(H₂O)₂]·4H₂O} (2) and ³ _∞{[Zn₃(µ₆-btc)₂(µ₄-btre)₂]·∼0.67H₂O} (4), respectively. In the transformation from 1 to 2 the unit cell volume is reduced to about 60%. The transition from 3 to 4 involves breakage and formation of new Zn–O bonds.

The synthesis of ferro- and ferri-magnetic systems with a tunable T_c and three-dimensional (3-D) ordering from molecular precursors implying transition metal ions is one of the active branches of molecular inorganic chemistry. The nature of the interactions between the transition metal ions (or transition metal ions and radicals) is not so easy to grasp by synthetic chemists working in this field since it may be either electrostatic (orbital) or magnetic (mainly dipolar). Therefore, the systems fulfilling the necessary requirements to present the expected magnetic properties are not so easy to design on paper and realize in the beaker. In this work we show how the design of one-, two- and three-dimensional materials can strongly benefit from the use of crystal engineering techniques, which can give rise to structures of different shapes, and how these differences can give rise to different properties. We will focus on the networks constructed by assembling malonate ligands and metal centres. The idea of using malonate (dianion of propanedioic acid, H₂mal) is that it can give rise to different coordination modes with the metal ions it binds. Extended magnetic networks of dimensionalities one (1-D), two (2-D) and three (3-D) can be chemically constructed from malonate-bridged metallic complexes. These coordination polymers behave as ferro-, ferri- or canted antiferro-magnets. We are currently trying to obtain analogous compounds using magnetically anisotropic ions, such as cobalt(II), in order to explore how structural differences influence the magnetic properties. In this case the control of the spatial arrangement of the magnetic building blocks is of paramount importance in determining the strength of the magnetic interaction. The possibility of controlling the shape of the networks depends on the coordination bond between the metal ion and the ligands and on supramolecular interactions such as stacking interactions or hydrogen bonding.