Unique cadmium(II) dipicolinate coordination tetramers with different Cd(II) coordination environments and number of halide ions (Cl, Br)

Cadmium(II) dipicolinate (dipic) coordination tetramers, namely [Cd2Cl(dipic)(dipicH)(H2O)4]2·2H2O (1), [Cd2Br(dipic)(dipicH)(H2O)4]2·2H2O (2), [Cd2Cl2(dipicH)2(H2O)3]2·2H2O (3), [Cd2Br2(dipicH)2(H2O)3]2·2H2O (4) were prepared. The molecular and crystal structures of 1–4 were determined by single crystal X-ray structural analysis. Tetramers 3 and 4 were also characterised by powder X-ray diffraction and thermal methods (TG, DSC and hot-stage microscopy). Tetramers 1–4 were prepared at 60 °C under autogenous pressure by reaction of cadmium(II) salt (chloride or bromide) and dipicolinic acid in aqueous solution. The concentration of both reactants and the time of heating crucially effect the preparation of a particular tetramer. The tetramers 3 and 4 can be reproducibly prepared and isolated in high yield as pure compounds, whilst 1 and 2 could not be obtained in a pure form and always as the minor component in the mixture. Tetramers 1 and 2 are isostructural, containing two crystallographically independent 7-coordinated Cd(II) ions bridged by two dipicolinate ligands into a dinuclear unit. Two dinuclear units are centrosymmetrically connected via bridging water molecules into a tetrameric structure. The two 7-coordinated Cd(II) ions have different coordination environments, one having a halide ion substituted with a water molecule. Tetramers 3 and 4 are also isostructural and very similar to 1 and 2, containing 7-coordinated Cd(II) ions coordinated in the same way (including halide ion) as in 1 and 2. The difference in coordination environment of Cd(II) ions for 1 and 2, when compared to 3 and 4, is also reflected in different hydrogen bond motifs found in the crystal structures, i.e. R44(8) for 1 and 2 or R33(10) for 3 and 4.

Cadmium(II) dipicolinate (dipic) coordination tetramers, [Cd2Cl(dipic)(dipicH)(H2O)4]2·2H2O (1), [Cd2Br(dipic)(dipicH)(H2O)4]2·2H2O (2), [Cd2Cl2(dipicH)2(H2O)3]2·2H2O (3), [Cd2Br2(dipicH)2(H2O)3]2·2H2O (4) were prepared, characterised by PXRD, TG and DSC. Their structures were determined by single crystal X-ray structural analysis.Figure optionsDownload as PowerPoint slideHighlights
► Cadmium(II) dipicolinate coordination tetramers were prepared.
► Different Cd(II) coordination environments and number of halide ions (Cl, Br).
► Preparation by reaction of cadmium(II) salt and dipicolinic acid.
► Concentration of reactants and time of heating effect formation of particular tetramer.
► Different hydrogen bond motifs, R44(8) or R33(10), in the crystal structures.