3-(Pyridin-4-yl)acetylacetone: a donor ligand towards mercury(II) halides and a versatile linker for complex...

The ditopic organic molecule 3-(pyridin-4-yl)acetylacetone (HacacPy) acts as a pyridine-type ligand towards HgX₂ (X = Cl, Br, I). The nature of the anion and the ligand-to-cation ratio dominate the outcome of the reaction. Two different coordination compounds form with HgCl₂, namely a ligand-rich mononuclear complex, HgCl₂(HacacPy)₂, and a ligand-deficient one-dimensional chain polymer, [Hg(μ-Cl)₂(HacacPy)]¹_∞, with five-coordinated Hg^II cations. Two compounds are also observed for HgBr₂, a molecular complex isomorphous to the chloride derivative and a chain polymer with the composition [Hg(μ-Br)Br(HacacPy)]¹_∞, in which the cations are four-coordinated. The ligand-rich mononuclear and ligand-deficient polymeric chloride and bromide complexes may be interconverted via thermal degradation and mechanochemical synthesis. In contrast to the chloride and bromide compounds, the reaction product with HgI₂ does not depend on the ligand-to-cation ratio but corresponds to [Hg(μ-I)I(HacacPy)]¹_∞, isomorphous to the bromide derivative. The N-coordinated HacacPy complexes could not be deprotonated and further crosslinked with a second cation. Synthesis of mixed-metal products could be achieved, however, by deprotonation of the acetylacetone moiety in HacacPy and formation of tris-chelated Fe(acacPy)₃ and Al(acacPy)₃ complexes in the first step. These mononuclear building blocks act as bridging poly(pyridine) ligands towards Hg^II halides and form two structure types. The first represents a one-dimensional ladder, with the tris(ligand) complexes acting as triconnected nodes and the Hg^II halides acting as linkers. In the alternative unprecedented product, both the tris(ligand) complexes and the [HgX₂(μ-X)HgX] groups act as equivalent triconnected nodes. They form a uninodal two-dimensional coordination network with vertex symbol 4.8² and fes topology.