Substituent-driven supramolecular assembly in Hg(II) bromide complexes with pyridine-based hydrazone ligands: crystal structures, Hirshfeld surface analysis, and DFT/QTAIM study

Three new Hg(II) bromide complexes, HgBr2L (1–3) (L1 = N′-(pyridin-2-ylmethylene)isonicotinohydrazide, L2 = N′-(1-(pyridin-2-yl)ethylidene)isonicotinohydrazide, L3 = N′-(phenyl(pyridin-2-yl)methylene)isonicotinohydrazide), have been synthesized in good yields and thoroughly characterized. Single-crystal X-ray diffraction analyses unveil that complexes 1 and 3 are mononuclear, while complex 2 is dinuclear. All complexes further assemble into one-dimensional supramolecular chains mediated by N–H···N hydrogen bonds, which are reinforced by π···π stacking and Br···Br halogen interactions. Systematic variation of the ligand substituents (–H, –CH3, –Ph) modulates Hg–O bond lengths, Br···Br contacts, and the overall supramolecular architecture. Hirshfeld surface analysis and 2D fingerprint plots disclose clear substituent-dependent trends: H···Br/Br···H contacts dominate in 1 (37.1%) but decrease progressively in 2 (31.3%) and 3 (27.7%), mirroring the reduction in short Br···Br distances as substituent bulk increases. Conversely, H···H contributions rise from 22.4% (1) to 30.1% (3), while H···N/N···H remains significant (12.2% → 9.8% → 7.4%) across the series, underscoring the persistent role of chain-forming N–H···N bonds. Aromatic interactions show substituent-dependent variation: C···C contributions peak at 7.1% in 2 (enhanced π···π stacking between pyridyl rings, centroid···centroid 3.596(6) Å), whereas they remain modest in 1 (3.1%) and 3 (2.8%). Br···Br contacts are quantifiable but minor (2.1% in 1, 0.4% in 2, 0.7% in 3). Minor Hg⋅⋅⋅Br/Br⋅⋅⋅Hg (up to 2.1% in 2) and Hg⋅⋅⋅H contributions reflect weak secondary coordination extensions, particularly in the dinuclear motif of 2. Density functional theory (DFT) calculations and QTAIM analysis corroborate the experimental results, emphasizing the pivotal role of noncovalent contacts in directing crystal packing. This study provides new insights into halide-mediated supramolecular assembly in Hg(II) hydrazone complexes, highlighting how subtle ligand and bromide ions can be leveraged to control structural and packing features in metal–organic materials.