Dative and Noncovalent Tetrel Bonding in Hemidirected Pb(II) Complexes with Thiosemicarbazone and Hydrazone Ligands: Significance in Crystal Packing and Computational Insights into Pb•••N/S/O Interactions

Abstract

Supramolecular architectures in lead(II) coordination compounds are increasingly recognized for their reliance on unconventional noncovalent interactions, particularly tetrel bonding involving the heavy tetrel atom Pb(II). In this work, two new Pb(II) complexes, [Pb(HL¹)(H₂L¹)]ClO₄ (1) and [Pb₂(L²)₂]•2H₂O•DMSO (2) were synthesised using thiosemicarbazone-based and hydrazone-based Schiff base ligands (H₂L¹ and H₂L², respectively). Both complexes were characterized by elemental analysis, FT-IR, ¹H NMR, and single-crystal X-ray diffraction. Single-crystal X-ray diffraction reveals that complex 1 contains a [Pb(HL¹)(H₂L¹)]⁺ cationic moiety in which the Pb(II) centre is coordinated by one deprotonated anionic ligand (HL¹)^– in a bidentate N,S fashion and one neutral ligand (H₂L¹) in a monodentate S fashion, charge-balanced by a perchlorate anion. In contrast, complex 2 contains two crystallographically independent Pb(II) centres (Pb1 and Pb2) coordinated by the doubly deprotonated ligand (L²)^2– in a tridentate ONO fashion with pyridine nitrogen bridging, affording discrete dinuclear Pb₂ units. Both complexes exhibit hemidirected coordination geometries around the Pb(II) centers, which facilitate the formation of dative coordinate bond (Pb←N) and noncovalent tetrel bonding interactions (Pb•••S/Pb•••O) alongside conventional hydrogen bonds (C–H•••O/S, N–H•••O/N). These interactions drive the assembly of three-dimensional supramolecular frameworks. Electrostatic potential maps confirm the σ-hole character at the Pb(II) centers, while computational calculations (M06-2X/def2-TZVP) and QTAIM analysis quantified the strength and the electron-density topology of these interactions. This study highlights the pivotal role of Pb(II)-centered tetrel bonding in directing the supramolecular organization of Pb(II) complexes, offering valuable insights for crystal engineering and the design of functional materials.