Coordination Versatility and Weak Hg···X (X = Cl, O, N) Interactions in Hydrazone-Based Hg(II) Complexes: Structural Evolution from Discrete Units to Extended Architectures

Abstract

The work represents one-pot synthesis and comprehensive structural characterization of three new structural versatile Hg(II) coordination complexes, namely [Hg2Cl4(HL1)] (1), [HgCl2(HL2)] (2), and [Hg4Cl6(L2)2]n (3), derived from hydrazone Schiff base ligands HL1 = 2-pyridinecarbaldehyde isonicotinoylhydrazone and HL2 = 2-benzoylpyridine benzoylhydrazone. Single-crystal X-ray diffraction analysis revealed that the Hg(II) complexes span a range of architectures from discrete molecular species to extended polymeric networks. In complex 1, coordination of two Hg(II)Cl2 units to the HL1 ligand affords a binuclear molecular unit. Propagation of these units through weak chloride bridges and secondary Hg···O contacts results in a dimer-based one-dimensional coordination architecture, which is further organized into a three-dimensional supramolecular network by π···π contacts between pyridine rings. In contrast, complex 2 exists as a discrete mononuclear species in which the Hg(II) center is not chelated by the conventional N,N,O donor set of the HL2 ligand but instead coordinates selectively through the isonicotinic pyridine nitrogen atom. The polymeric complex 3 exhibits a more intricate coordination pattern, where Hg1 and Hg2 are chelated by the ligand via the N,N,O donor set in conjunction with bridging chloride anions, while Hg3 and Hg4 are coordinated by the isonicotinic pyridine nitrogen atoms and chloride ligands, leading to an extended coordination network. A detailed analysis using Density Functional Theory (DFT) and QTAIM methods confirms that, in addition to classical coordination bonds, secondary Hg···X (X = Cl, O, N) interactions, together with hydrogen bonding and π···π stacking, contribute to the stabilization and propagation of the extended architectures (1–3). Hirshfeld surface and topological analyses further elucidate the cooperative role of these contacts in directing crystal packing. These findings highlight the potential of Hg(II) systems as versatile platforms for exploring supramolecular assembly and crystal engineering driven by soft, highly adaptable metal centers.