Lithium and Sodium Benzimidazolate Coordination Networks: Syntheses, Structures, and Thermal Properties

Alkali metal imidazolates are important compounds, serving as intermediates in organic synthesis and additives in alkali ion electrolytes. However, their solid-state structures and thermal behaviors remain largely unexplored. In this study, we present the synthesis, structural analysis, and thermal characterization of lithium and sodium benzimidazolate (bim–). The crystal structures of these microcrystalline materials, determined by 3D electron diffraction, reveal closely related layered coordination networks. In these structures, 4-fold N-coordinated alkali ions are bridged in two dimensions by bim– linkers, with the networks’ surfaces decorated by the phenyl rings of the bim– linkers, stacking atop one another in the solid state. Differential scanning calorimetry combined with variable temperature X-ray powder diffraction indicates that both materials melt above 450 °C. Additionally, Na(bim) undergoes a displacive phase transition from an ordered α-phase to a highly disordered β-phase before melting. Structural variations, primarily attributable to the differing ionic radii of Li+ and Na+, result in distinct coordination environments of the alkali metal ions and varying orientations of the bim– linkers. These differences lead to markedly distinct thermal behaviors: Li(bim) exhibits positive thermal expansion along all crystal axes, whereas Na(bim) switches from area negative thermal expansion (NTE) to linear NTE during the α → β phase transition.