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 N,N,N′,N′-tetraisopropyl/tetraisobutyl-3,4-pyridinedicarboxamide L1–L2 and six of their Hg­(II)­X2 complexes (where X = Cl–, Br–, and I–) have been synthesized and characterized.
Single crystal X-ray structures of three complexes of L1 (1–3) with HgCl2/Br2/I2 and two complexes of L2 (4–5) with HgCl2/Br2 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 6, of L2 with HgI2, 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 L1. 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 6, 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 1 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.