Structural (XRD) Characterization and an Analysis of H-Bonding Motifs in Some Tetrahydroxidohexaoxidopentaborate(1-) Salts of N-Substituted Guanidinium Cations †

The synthesis and characterization of six new substituted guanidium tetrahydroxidohexaoxidopentaborate(1-) salts are reported: [C(NH(2))(2)(NHMe)][B(5)O(6)(OH)(4)]·H(2)O (1), [C(NH(2))(2)(NH{NH(2)})][B(5)O(6)(OH)(4)] (2), [C(NH(2))(2)(NMe(2))][B(5)O(6)(OH)(4)] (3), [C(NH(2))(NMe(2))(2)][B(5)O(6)(OH)(4)] (4), [C(NHMe)(NMe(2))(2)][B(5)O(6)(OH)(4)]·B(OH)(3) (5), and [TBDH][B(5)O(6)(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 B(2)O(3) at >650 °C. The low temperature stage (<250 °C) was endothermic and corresponded to a loss of H(2)O. 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(NH(2))(3)](2)[B(4)O(5)(OH)(4)]·2H(2)O, [C(NH(2))(3)][B(5)O(6)(OH)(4)]·H(2)O, and [C(NH(2))(3)](3)[B(9)O(12)(OH)(6)] and in compounds 1–6 revealed that two important H-bonding R(2)(2)(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(NH(2))(3)](2)[B(4)O(5)(OH)(4)]·2H(2)O and [C(NH(2))(3)](3)[B(9)O(12)(OH)(6)], whereas the anion–anion ring motif was dominant in [C(NH(2))(3)][B(5)O(6)(OH)(4)]·H(2)O 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.