Influence of Halogen Bonding Interaction on Supramolecular Assembly of Coordination Compounds; Head-to-Tail N···X Synthon Repetitivity

In this study, N-(3-halophenyl)-2-pyrazine­carboxamide ligands, L3‑F, L3‑Cl, L3‑Br, and L3‑I, carrying a different halogen atom on the phenyl meta-position and N-phenyl-2-pyrazine­carboxamide
ligand, LH, have been employed for the synthesis of 12
mercury­(II) complexes, [HgCl2­(LH)]n, 1, [HgCl2­(L3‑Cl)]n, 2,
[Hg2Cl4­(L3‑Br)2], 3, [Hg2Cl4­(L3‑I)2], 4, [Hg2Br4­(LH)2], 5, [HgBr2­(L3‑F)], 6, [HgBr2­(L3‑Cl)], 7, [HgBr2­(L3‑Br)], 8, [HgBr2­(L3‑I)], 9, [Hg2I4­(LH)2], 10, [HgI2­(L3‑Br)], 11, and [HgI2­(L3‑I)]n, 12.
Interestingly, structural analysis clearly shows that, by the replacing
of coordinated anions from chloride with bromide and iodide in each
series containing the same ligand, the coordination geometry and structural
motif of the resulting compounds have been dramatically affected.
One of the common features in the crystal structures of these complexes
is that there is a strong tendency to form halogen bonding synthons
between adjacent halophenyl and pyrazine rings. The influence of these
halogen bonding interactions on the supramolecular assemblies has
been discussed with the help of geometrical analysis and theoretical
calculations. The X···N halogen bonding distances are
2.5–9.4% shorter than the sum of the van der Waals radii of
nitrogen and halogen atoms. Theoretical methods also show the halogen
bonding energies within a range of −27.86 to −46.15
kJ·mol–1. In all complexes synthesized here,
the pyrazine ring is coordinated to the mercury­(II) ion through the
N atom syn to the carbonyl. Therefore, the second
common feature of the crystal structures for complexes studied here
is the selectivity of the metal ion coordination site. The halogen
bond synthon repetitivity across these compounds and selectivity in
the mercury­(II) ion coordination site further point to application
in the coordination crystal engineering research field.