Polymorphs or solvates? Coordination of 3,5-dihydroxybenzoate to copper and zinc metal centers

Infinite polymeric 1D chains of [Cu(3,5-dhb)(pyridine)2]n (1a and 1b) and [Cu2(3,5-dhb)4]n (2) were prepared by reacting copper acetate hydrate with 3,5-dihydroxybenzoic acid in the molar ratios of pyridine to Cu(II) of 5:1 and 1.1:1, respectively. In contrast, a zinc monomer Zn(3,5-dhb)2(pyridine)2, (3), was obtained under similar reactant conditions to the synthesis of 1a, but with zinc acetate. In compounds 1, the copper centers are coordinated in a near octahedral fashion containing four 3,5-dhb ligands and two pyridine molecules. Interestingly, the Cu containing molecules in compound 1a share the same formula with compound 1b, but they are solvates with different conformations. The structure of 2 revealed a paddlewheel dicopper core bridged by four carboxylate groups in a syn,syn mode. Each copper center is coordinated in a square pyramidal fashion. In contrast, the geometry around the zinc center is distorted tetrahedral and consists of two monodentate 3,5-dhb and two pyridine ligands. Compounds 1–3 were characterized by elemental analyses, IR, UV–Vis and single crystal X-ray diffraction methods. Compounds 1a and 1b were further studied by thermogravimetric and differential scanning calorimetry (DSC) analyses and X-ray powder diffraction experiments.