On the nature and interplay of Hg⋯O/S spodium bonding and O⋯S chalcogen bonding in one-dimensional phenylmercury(ii) 3-alkoxycyclobutene-1,2-dione-4-thiolate coordination polymers

Two one-dimensional coordination polymers, PhHg(3-alkoxycyclobutene-1,2-dione-4-thiolate) (alkoxy = OMe and OEt), have been synthesised, characterised spectroscopically, crystallographically and through computational chemistry techniques. Both species exhibit intermolecular Hg⋯O σ-hole spodium bonding as well as intramolecular Hg⋯O π-hole spodium bonding. In the ethoxy species there are complementary σ-hole O⋯S chalcogen bonds to the Hg⋯O σ-hole spodium bonding as well as orthogonal Hg⋯S π-hole spodium bonding. Each of the coordination polymers is connected into a supramolecular layer. For the methoxy derivative, the layer features π(cyclobutene)⋯π(phenyl) stacking and side-on C = O⋯π(cyclobutene)/anti-parallel carbonyl⋯carbonyl interactions. For the ethoxy species, Hg⋯S, offset π(cyclobutene)⋯π(cyclobutene) stacking and C = O⋯π(cyclobutene) interactions occur within the layer. In each case, weak C-H⋯π interactions operate between the two-dimensional arrays. The nature of the electron transfer responsible for the spodium/chalcogen bonding has been established. In terms of QTAIM energies, those associated with Hg⋯O σ-hole spodium bonds were in the range of 17.5 to 23.3 kJ mol ⁻¹, which were less than the energies computed for the intramolecular Hg⋯O π-hole spodium bonds in each case (31.0 to 43.6 kJ mol ⁻¹). The lowest energies for the intermolecular Hg⋯O spodium bonding were computed for the ethoxy species, which correlates with the compensating Hg⋯S π-hole spodium bonding (2 × 11.0 kJ mol ⁻¹) and O⋯S chalcogen bonding (12.5 kJ mol ⁻¹). The electron transfer leading to the Hg⋯O/S spodium bonding involves donation of an oxygen- or sulphur-bound lone pair of electrons to the σ*(Hg-C) orbital, with the intramolecular Hg⋯O spodium bond involving two lone pair interactions. The O⋯S chalcogen bond is assigned to LP(O) → σ*(C-S).