Triborate and pentaborate salts of non-metal cations derived from N-substituted piperazines: synthesis, structural (XRD) and thermal properties

Abstract

The synthesis and characterization of a triborate salt, [H₂N(CH₂CH₂)₂NH₂][B₃O₃(OH)₄]₂ (1), and four pentaborate salts, [H₂N(CH₂CH₂)₂NH][B₅O₆(OH)₄] (2a), [MeHN(CH₂CH₂)₂NH][B₅O₆(OH)₄] (2b), [MeHN(CH₂CH₂)₂NMe][B₅O₆(OH)₄] (2c) and [Me₂N(CH₂CH₂)₂NMe₂][B₅O₆(OH)₄]₂ (2d) are described. TGA and DSC analysis (in air, 25–1000 °C) indicate that triborate 1 decomposes to B₂O₃via a multistage process, with the first stage (<250 °C) being dehydration to condensed polymeric hexaborate of approximate composition: [H₂N(CH₂CH₂)₂NH₂][B₆O₁₀]. The pentaborates (2a–2d) are thermally decomposed to B₂O₃via a 2 stage process involving polymeric [NMC][B₅O₈]. The anhydrous polyborates were amorphous. BET analysis of materials derived from the thermolysis of 1 at 250, 400, 600, and 1000 °C, were all non-porous (surface area <1.8 m² g⁻¹). A single-crystal X-ray diffraction study of 1 showed that it contains isolated triborate(1−) anions in a structure comprised of alternating cationic and anionic layers held together via extensive H-bonds. Single-crystal XRD structural studies on pentaborate salts 2c and 2d are also reported.