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Several tetrahydroxidohexaoxidopentaborate(1-) salts of N-substituted diimidazolium cations or N-substituted dipyrrolidinium cations linked through N-C₆-N chains have been synthesized and characterized spectroscopically (NMR, IR) and by single-crystal XRD studies: [R(NC₃H₃N)(CH₂)₆(NC₃H₃N)R][B₅O₆(OH)₄]₂·xH₂O (R = Me, x = 0 (1); R = Et, x = 3 (2); [Me(NC₃H₃N)(CH₂(C₆H₄)CH₂)(NC₃H₃N)Me][B₅O₆(OH)₄]₂ (3), [(C₄H₈N)(R)(CH₂)₆(R)(NC₄H₈)][B₅O₆(OH)₄]₂·xB(OH)₃ (R = Me, x = 0 (4, two polymorphs); R = Et, x = 0 (5); R = Bu, x = 4 (6); R = allyl, x = 0 (7)). Representative samples (1 and 7) were also characterized by thermal (TGA/DSC) studies; compounds are thermally decomposed to B₂O₃ in air. Numerous anion-anion H-bonding interactions are present in the solid-state structures of 1–5 and 7 as giant anionic networks. Unusually, in 6 there are no R₂²(8) anion-anion interactions as the co-crystallized B(OH)₃ bridges between all pentaborate anions. H-bonding interactions in 1–7 have been examined using Etter graph set analysis; C(8), C₃³(18), R₂²(8), R₂²(12) and R₄⁴(12) motifs have been identified. Full article

The synthesis and characterization of six new substituted guanidium tetrahydroxidohexaoxidopentaborate(1-) salts are reported: [C(NH₂)₂(NHMe)][B₅O₆(OH)₄]·H₂O (1), [C(NH₂)₂(NH{NH₂})][B₅O₆(OH)₄] (2), [C(NH₂)₂(NMe₂)][B₅O₆(OH)₄] (3), [C(NH₂)(NMe₂)₂][B₅O₆(OH)₄] (4), [C(NHMe)(NMe₂)₂][B₅O₆(OH)₄]·B(OH)₃ (5), and [TBDH][B₅O₆(OH)₄] (6) (TBD = 1,5,7-triazabicyclo [4.4.0]dec-5-ene). Compounds 1–6 were prepared as crystalline salts from basic aqueous solution via self-assembly processes from B(OH)₃ and the appropriate substituted cation. Compounds 1–6 were characterized by spectroscopic (NMR and IR) and by single-crystal XRD studies. A thermal (TGA) analysis on compounds 1–3 and 6 demonstrated that they thermally decomposed via a multistage process to B₂O₃ at >650 °C. The low temperature stage (<250 °C) was endothermic and corresponded to a loss of H₂O. Reactant stoichiometry, solid-state packing, and H-bonding interactions are all important in assembling these structures. An analysis of H-bonding motifs in known unsubstituted guanidinium salts [C(NH₂)₃]₂[B₄O₅(OH)₄]·2H₂O, [C(NH₂)₃][B₅O₆(OH)₄]·H₂O, and [C(NH₂)₃]₃[B₉O₁₂(OH)₆] and in compounds 1–6 revealed that two important H-bonding R₂²(8) motifs competed to stabilize the observed structures. The guanidinium cation formed charge-assisted pincer cation–anion H-bonded rings as a major motif in [C(NH₂)₃]₂[B₄O₅(OH)₄]·2H₂O and [C(NH₂)₃]₃[B₉O₁₂(OH)₆], whereas the anion–anion ring motif was dominant in [C(NH₂)₃][B₅O₆(OH)₄]·H₂O and in compounds 1–6. This behaviour was consistent with the stoichiometry of the salt and packing effects also strongly influencing their solid-state structures. Full article