First Report on Crystal Engineering of Hg(II) Halides with Fully Substituted 3,4-Pyridinedicarboxamides: Generation of Two-Dimensional Coordination Polymers and Linear Zig-Zag Chains of Mercury Metal Ions

Two new ligands <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetraisopropyl/tetraisobutyl-3,4-pyridinedicarboxamide <b>L1</b>–<b>L2</b> and six of their Hg­(II)­X<sub>2</sub> complexes (where X = Cl<sup>–</sup>, Br<sup>–</sup>, and I<sup>–</sup>) have been synthesized and characterized. Single crystal X-ray structures of three complexes of <b>L1</b> (<b>1</b>–<b>3</b>) with HgCl<sub>2</sub>/Br<sub>2</sub>/I<sub>2</sub> and two complexes of <b>L2</b> (<b>4</b>–<b>5</b>) with HgCl<sub>2</sub>/Br<sub>2</sub> show that these are two-dimensional (2D) coordination polymers, with three different (one new) topologies and five coordinated Hg­(II) ions, in square pyramidal coordination. Complex <b>6</b>, of <b>L2</b> with HgI<sub>2</sub>, is a dimer with a four coordinated tetrahedral Hg­(II) ion. The ligands behave as two- or three-connecting linkers for forming coordination polymers (CPs) but only one-connector in the dimer. Various types of hydrogen bonding and other noncovalent interactions have been calculated, analyzed, and discussed for all the complexes and the ligand <b>L1</b>. Although the large size and soft character of iodide coupled with the steric effects of large isobutyl groups are mainly responsible for a change in the primary structure of <b>6</b>, a significant role of the semilocalized LP···π and C–H···O noncovalent interactions into this has also been found. The latter transform this dimer into a stable 2D hydrogen-bonded network instead of a coordination polymer. The unique halide bridged 2D structure of <b>1</b> forms one-dimensional zig-zag chains of metal ions owing to the mercurophilic interactions. Weaker interactions of the same kind further extend along the basic skeleton of 2D CP and are facilitated by LP···π, strong C–H···Cl and C–H···N­(py) interactions.