A Two-Dimensional Layered Bismuth Coordination Polymer Based on a Lone Pair-π Interaction

Metal-organic frameworks (MOFs) and coordination polymers (CPs), which are made from a wide variety of metal ions and organic ligands, constitute a broad and rapidly developing research field spanning both fundamental studies and applications. Here, we report the synthesis, crystal structure, and electronic properties of Bi(PCA)(H₂O) (H₃PCA = protocatechuic acid), the first example of a CP based on a cation-based lone pair-π interaction. This CP features a two-dimensional (2D) layered structure composed of Bi³⁺ ions and PCA^3- ligands. The central Bi³⁺ ion adopts an unusual hemispherical six-coordinated geometry, attributed to the presence of a stereochemically active 6s² lone pair. Single-crystal X-ray diffraction (XRD) analysis revealed that the 2D layers are assembled in an alternating interdigitated manner through cation-based lone pair-π interaction between the 6s² lone pairs of Bi³⁺ ions and the π-cloud of PCA^3- ligands, as well as π-π interaction between adjacent PCA^3- ligands. Furthermore, optical absorption measurements and density functional theory (DFT) calculations demonstrated the presence of a characteristic absorption band associated with the lone pair-π interactions between the 2D layers. These findings highlight the potential of exploiting unique coordination geometries of metal ions to design novel MOFs and CPs with distinctive crystal structures and electronic properties.