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

Abstract

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.

Synthesis and Thermal Studies of Two Phosphonium Tetrahydroxidohexaoxidopentaborate(1-) Salts: Single-Crystal XRD Characterization of [iPrPPh3][B5O6(OH)4]·3.5H2O and [MePPh3][B5O6(OH)4]·B(OH)3·0.5H2O

“…The TGA plot of 2 had a similar profile, with loss of 4.0 × H2O in the first stage (100-275 °C) and the formation of anhydrous [MePPh3][B6O9.5]. Since 2 is a 1:1 B(OH)3/pentaborate(1-) co-crystal, this material is formulated as an anhydrous hexaborate [18,33,34]. This endothermic dehydration step for 2 is a two-stage process involving the loss of interstitial H2O and partial condensation/cross-linking of the pentaborate B-OH groups (2.0 × H2O, 100-150 °C,) with further condensation/cross-linking of the pentaborate B-OH groups (2.0 × H2O, 150-275 °C).…”

“…Crystallization of the crude products 1 and 2 from H 2 O afforded crystals suitable for sc-XRD studies (Section 2.4). The co-crystallization of B(OH) 3 with pentaborate(1-) anions, as in 2, has occasionally been observed in structures with large organic cations [18,[33][34][35][36].…”

“…These crystallographic studies are in agreement with the formulation of 1 and 2 as ionic phosphonium(1+)/pentaborate(1-) salts, as indicated by their spectroscopic and thermal analysis. The co-crystallization of B(OH) 3 is not uncommon in recrystallized samples of pentaborate(1-) salts containing bulky cations [18,[33][34][35][36]. Drawings of the structures of 1 and 2 showing atomic numbering are shown in Figures 2 and 3, respectively.…”

“…The B(OH)3 containing B31 forms a R2 2 (8) 'pincer' ring with the B(OH)3 containing B21, and likewise this B(OH)3 forms a 'pincer' R2 2 (8) interaction with the pentaborate containing B11 (Figure 6a). These interactions allow for the two co-crystallised B(OH)3 molecules to function as 'spacer' units to expand the lattice and replace what would otherwise be a simpler pentaborate/pentaborate R2 2 (8) interaction [18,[33][34][35][36]. A view along the a axis of 2 (along the plane shown in Figure 6a) is shown in Figure 6b.…”