3-(Pyridin-4-yl)acetylacetone: Cd and Hg compete for nitrogen coordination.

3-(Pyridin-4-yl)acetylacetone (HacacPy) acts as a pyridine-type ligand towards Cd and Hg halides. With CdBr, the one-dimensional polymer [Cd(μ-Br)(HacacPy)Cd(μ-Br)(HacacPy)] is obtained in which five- and six-coordinated Cd cations alternate in the chain direction. Reaction of HacacPy with HgBr results in [Hg(μ-Br)Br(HacacPy)], a polymer in which each Hg centre is tetracoordinated. In both compounds, each metal(II) cation is N-coordinated by at least one HacacPy ligand. Equimolar reaction between these Cd and Hg derivatives, either conducted in ethanol as solvent or via grinding in the solid state, leads to ligand redistribution and the formation of the well-ordered bimetallic polymer catena-poly[[bromidomercury(II)]-μ-bromido-[aquabis[4-hydroxy-3-(pyridin-4-yl)pent-3-en-2-one]cadmium(II)]-di-μ-bromido], [CdHgBr(CHNO)(HO)] or [{HgBr}(μ-Br){(HacacPy)Cd(HO)}(μ-Br)]. Hg and Cd cations alternate in the [100] direction. The HacacPy ligands do not bind to the Hg cations, which are tetracoordinated by three bridging and one terminal bromide ligand. The Cd centres adopt an only slightly distorted octahedral coordination. Three bromide ligands link them in a (2 + 1) pattern to neighbouring Hg atoms; two HacacPy ligands in a cis configuration, acting as N-atom donors, and a terminal aqua ligand complete the coordination sphere. Classical O-H...Br hydrogen bonds stabilize the polymeric chain. O-H...O hydrogen bonds between aqua H atoms and the uncoordinated carbonyl group of an HacacPy ligand in a neighbouring strand in the c direction link the chains into layers in the (010) plane.