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.
Highlights
There has been increased interest in recent years in non-metal cation (NMC) polyborate, notably pentaborate, salts as a result of their potential applications as porous[1,2,3,4,5] or NLO6 piezoelectric[7] or fluorescent[8] materials
As a continuation of this study we have investigated substituted piperazine based NMCs and report the synthesis of an unusual triborate salt, as well as further examples of pentaborate salts
Unsubstituted piperazine salts usually contain dications, with monocationic salts occurring less frequently.21 11B NMR spectra of salts 1 and 2a–d are all very similar, despite the different stoichiometry for 1, and showed peaks at +18 ppm, +13 ppm and +1 ppm. The similarity in these spectra may be explained by the existence of a complex series of equilibria existing in aqueous solution which link monomeric and oligomeric borate species.[9]
Summary
There has been increased interest in recent years in non-metal cation (NMC) polyborate, notably pentaborate, salts as a result of their potential applications as porous[1,2,3,4,5] or NLO6 piezoelectric[7] or fluorescent[8] materials. Pentaborate salts of NMCs are generally readily synthesized as crystalline solids from aqueous solution by the interaction of a free base, or its hydroxide salt, with B(OH)[3] in a 1 : 5 ratio.[2,4,10] Structurally, NMC pentaborates are best described as supramolecular networks of H-bonded isolated [B5O6(OH)4]2 anions, and the formation of this extended lattice is clearly a driving force in product formation. The NMCs occupy ‘cavities’ within the network, and usually (if possible) add further stabilizing H-bond cation-anion interactions.[1,2,4] Occasionally, NMC polyborate salts other than pentaborates have been obtained from aqueous solution e.g. These results are compared with those from other known NMC polyborate salts
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