Supramolecular aggregation featuring Hg⋯S secondary-bonding interactions in crystals of mercury(ii) species augmented by computational chemistry calculations

Abstract

The Cambridge Structural Database was surveyed for crystals featuring Hg⋯S secondary-bonding interactions for mercury(II) compounds (excluding organomercury compounds). In all, 43 examples of crystals including intermolecular Hg⋯S interactions operating largely in isolation from other directional interactions were detected with the Hg⋯S interactions within zero-, one- and two-dimensional aggregation patterns. Over 50% of the aggregates are one-dimensional with topologies ranging from linear, zigzag and helical chains, and ribbons to tapes; 30% of the crystals exhibit zero-dimensional aggregates. Crucially, selected examples were subjected to DFT calculations with mapping of the molecular electrostatic potentials (MEPs) of the molecules as well as quantum theory of atoms in molecules (QTAIM), non-covalent interaction plot (NCIPLOT) and natural bond order (NBO) investigations. This study confirms the presence of σ-/π-holes or positive belts at the mercury atom depending on the coordination geometry, defined by covalently and non-covalently bound donor atoms. In molecules coordinated by sulphur donors, the σ-hole at the sulphur atom merges with the positive belt around the mercury atoms, assisting in the formation of spodium bonds and providing higher directionality. The QTAIM/NCIPLOT analysis supports both the existence and non-covalent nature of the spodium bonds. Finally, the NBO analysis reveals charge transfer effects indicating the existence of LP → σ* orbital interactions in the studied molecules; LP is a lone-pair. The energy of stabilisation provided by spodium bonds ranges from a low 3 kcal mol⁻¹ to a high 19 kcal mol⁻¹, i.e. values often greater than those by conventional hydrogen-bonding interactions.