Recurrent supramolecular motifs in discrete complexes and coordination polymers based on mercury halides: Prevalence of chelate ring stacking and substituent effects

In recent years, the crystal engineering library has been enriched with a number of previously unrecognized or unnoticed intermolecular interactions, such as agostic, tetrel, chalcogen, pnicogen bonding and chelate ring stacking-collectively referred to as "unconventional interactions". Many open questions remain unaddressed regarding their ability to form synthon interactions, specificity, and cooperativity, for example with π-π stacking interactions. In this work, we throw light on the formation of chelate ring stacking in metal-organic assemblies of nicotinohydrazide ligands (N′-(1-(2-pyridyl)ethylidene)nicotinohydrazide (HL) and N′-(phenyl(pyridin-2-yl)methylene)nicotinohydrazide (HL₁)) with mercury(ii) halide (HgBr₂, HgI₂) salts. Their reaction produced five compounds, namely [Hg(μ-L)BrHgBr₂]_n (1), [Hg(μ-L₁)Br]_n (2), [Hg(L)I₂] (3), [Hg(HL₁)I₂]·(CH₃OH) (4), and [Hg(μ-L₁)I]_n (5). Crystal structure analysis reveals that chelate ring stackings are formed in four of the reported metal-organic compounds, and are common also in the literature precedents. The energies of chelate ring stackings and π-π heterocycle stackings have been computed and analyzed by means of DFT calculations, and the results were verified using Bader's theory of "atoms in molecules". These results provide a rationale for preferential formation of both unconventional and conventional stackings and allow us to conclude that chelate ring interaction may be considered as a synthon interaction for nicotinohydrazide metal complexes. Interpretations for packing differences imposed by the substituent effect (substitution of methyl group in HL for phenyl group in HL₁) were provided based on the Hirshfeld surface analysis and 2D fingerprint plots of the crystal structures reported here.