(C₅H₆N)Pb₂X₅ (X = Cl, Br): Hybrid Lead Halides Based on Seven-Coordinate Pb(II)

Hybrid lead halides are a diverse family of compounds, of interest for their optoelectronic properties, that vary in the dimensionality and connectivity of their inorganic substructures. The great majority of these compounds are based on lead-centered octahedra, with few examples featuring inorganic architectures containing higher coordination numbers. Here, we report the synthesis and characterization of a pyridinium lead bromide phase that is based on seven-coordinate Pb(II) centers. Through edge- and face-sharing, the polyhedra form a corrugated, two-dimensional inorganic substructure. Electronic structure calculations were used to examine the band structure and the role of the stereoactive lone pair in the inherently asymmetric, seven-coordinate Pb(II) geometry. For reference, we have visualized the role of the lone pair in the binary halide PbBr₂, which also has a seven-coordinate inner ligand sphere. A comparison of the new structure with the limited number of existing hybrid lead halides with similar inorganic architectures highlights the templating role of the organic cation for these compounds. We also contribute characterization and discussion of isomorphic pyridinium lead chloride, which had been deposited in the Cambridge Structural Database but never, to our knowledge, addressed in the literature. The compounds were synthesized using solution conditions and structures determined with single-crystal X-ray diffraction. The materials were also characterized via powder X-ray diffraction, combustion elemental analysis, and diffuse reflectance UV-vis spectroscopy. While the structures reported here are centrosymmetric, the seven-coordinate, capped trigonal prismatic geometry that we have identified is a source of local asymmetry that could be used as a component in designing globally noncentrosymmetric structures.