Pentaborate(1−) salts templated by substituted pyrrolidinium cations: synthesis, structural characterization, and modelling of solid-state H-bond interactions by DFT calculations

Abstract

The synthesis and characterization of a series of pentaborate(1−) salts of substituted pyrrolidinium cations [C₄H₈NH₂][B₅O₆(OH)₄] (1), [C₄H₈NMe₂][B₅O₆(OH)₄] (2) [C₄H₈NMeH][B₅O₆(OH)₄] (3), [(2-CH₂OH)C₄H₇NH₂][B₅O₆(OH)₄] (4) is reported. All compounds were characterized by single-crystal XRD studies with 3 (1/2CH₃COCH₃) and 4 (1/2H₂O) solvated. TGA/DSC analysis of the pentaborates 1–4 showed that they thermally decomposed in air at 800 °C to 2.5 B₂O₃, in a 2 step process involving dehydration (<250 °C) and oxidative decomposition (250–600 °C). BET analysis of materials derived thermally from the pentaborates 1 and 2 had internal porosities of <1 m² g⁻¹, indicating they were non-porous. All compounds show extensive supramolecular H-bonded anionic lattices. H-bond interactions are described in detail and motifs found in these and in other pentaborate structures have been examined and modelled by DFT calculations. These calculations confirm that H-bonds interactions in pentaborates are moderately strong (ca. −10 to −21 kJ mol⁻¹) and are likely to dominate the energetics of their templated syntheses.