Controlling pseudopolymorphism via robust and repetitive solvent-containing supramolecular interactions in urea-based isostructural coordination polymers

Abstract

In a systematic study, six pseudopolymorphic coordination polymers containing the ditopic 1,3-di(pyridin-4-yl)urea ligand (4bpu) constructed with d¹⁰ metal cations, possessing the formula {[M(4bpu)I₂]S}_n [(M = Zn, Cd and Hg), (S = MeOH or EtOH)], namely Zn-MeOH, Zn-EtOH, Cd-MeOH, Cd-EtOH, Hg-MeOH and Hg-EtOH were obtained. The title compounds were characterized by single-crystal X-ray diffraction analysis (SC-XRD), elemental analysis (CHN), FT-IR spectroscopy, thermogravimetric analysis (TGA), and powder X-ray diffraction (PXRD). The diffraction studies show that these compounds are isostructural 1D zig-zag chain coordination polymers which is also confirmed using XPac 2.0.2 software and the occurrence of three- and one-dimensional (3D and 1D) isostructurality was investigated in terms of their molecular assembly in solid state structures. These comparisons were performed by extracting the dissimilarity index and stretching parameters, providing quantitative insights into the structural similarity across the pseudopolymorphic coordination polymers, which exhibit robust 3D isostructurality. Additionally, the geometry of the zig-zag chain structures was thoroughly analyzed, highlighting the subtle variations and common features that contribute to isostructurality. The supramolecular architecture of these pseudopolymorphs is stabilized by robust and repetitive hydrogen bonding motifs involving N–H⋯O, O–H⋯I and C–H⋯I interactions between the framework and guest solvent molecules (MeOH or EtOH). These interactions replace the commonly observed bifurcated hydrogen bonds (α-tape motif) between urea moieties, emphasizing the pivotal role of solvent molecules in controlling pseudopolymorphism and defining the final structural assembly. This detailed understanding of hydrogen bonding provides valuable insights into tailoring intermolecular interactions, enabling the design of materials with enhanced functionalities for diverse applications. A detailed investigation of urea–CO⋯π_py interactions in urea-based compounds highlights the classification of these interactions as semilocalized (η²-SL) based on geometric parameters and reveals their significance through crystallographic and database studies. Hirshfeld surface analysis has been performed for all compounds to determine the percentage contribution of intermolecular interactions.