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

The ditopic organic molecule 3-(pyridin-4-yl)acetylacetone (HacacPy) acts as a pyridine-type ligand towards HgX (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 HgCl, namely a ligand-rich mononuclear complex, HgCl(HacacPy), and a ligand-deficient one-dimensional chain polymer, [Hg(μ-Cl)(HacacPy)], with five-coordinated Hg cations. Two compounds are also observed for HgBr, a molecular complex isomorphous to the chloride derivative and a chain polymer with the composition [Hg(μ-Br)Br(HacacPy)], 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 HgI does not depend on the ligand-to-cation ratio but corresponds to [Hg(μ-I)I(HacacPy)], 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) and Al(acacPy) complexes in the first step. These mononuclear building blocks act as bridging poly(pyridine) ligands towards Hg 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 halides acting as linkers. In the alternative unprecedented product, both the tris(ligand) complexes and the [HgX(μ-X)HgX] groups act as equivalent triconnected nodes. They form a uninodal two-dimensional coordination network with vertex symbol 4.8 and fes topology.