Polyborate Anions Partnered with Large Nonmetal Cations: Triborate(1–), Pentaborate(1–) and Heptaborate(2–) Salts

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

Two substituted phosphonium tetrahydoxidohexaoxidopentaborate(1-) salts, [iPrPPh3][B5O6(OH)4]·3.5H2O (1) and [MePPh3][B5O6(OH)4]·B(OH)3·0.5H2O (2), were prepared by templated self-assembly processes with good yields by crystallization from basic methanolic aqueous solutions primed with B(OH)3 and the appropriate phosphonium cation. Salts 1 and 2 were characterized by spectroscopic (NMR and IR) and thermal (TGA/DSC) analysis. Salts 1 and 2 were thermally decomposed in air at 800 °C to glassy solids via the anhydrous phosphonium polyborates that are formed at lower temperatures (<300 °C). BET analysis of the anhydrous and pyrolysed materials indicated they were non-porous with surface areas of 0.2-2.75 m2/g. Rhe recrystallization of 1 and 2 from aqueous solution afforded crystals suitable for single-crystal XRD analyses. The structure of 1 comprises alternating cationic/anionic layers with the H2O/pentaborate(1-) planes held together by H-bonds. The cationic planes have offset face-to-face (off) and vertex-to-face (vf) aromatic ring interactions with the iPr groups oriented towards the pentaborate(1-)/H2O layers. The anionic lattice in 2 is expanded by the inclusion of B(OH)3 molecules to accommodate the large cations; this results in the formation of a stacked pentaborate(1-)/B(OH)3 structure with channels occupied by the cations. The cations within the channels have vf, ef (edge-to-face), and off phenyl embraces. Both H-bonding and phenyl embrace interactions are important in stabilizing these two solid-state structures.

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(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.

Oxidoborates Templated by Cationic Nickel(II) Complexes and Self-Assembled from B(OH)3

Several oxidoborates, self-assembled from B(OH)3 and templated by cationic Ni(II) coordination compounds, were synthesized by crystallization from aqueous solution. These include the ionic compounds trans-[Ni(NH3)4(H2O)2][B4O5(OH)4].H2O (1), s-[Ni(dien)2][B5O6(OH)4]2 (dien = N-(2-aminoethyl)-1,2-ethanediamine (2), trans-[Ni(dmen)2(H2O)2] [B5O6(OH)4]2.2H2O (dmen = N,N-dimethyl-1,2-diaminoethane) (3), [Ni(HEen)2][B5O6(OH)4]2 (HEen = N-(2-hydroxyethyl)-1,2-diaminoethane) (4), [Ni(AEN)][B5O6(OH)4].H2O (AEN = 1-(3-azapropyl) -2,4-dimethyl-1,5,8-triazaocta-2,4-dienato(1-)) (5), trans-[Ni(dach)2(H2O)2][Ni(dach)2] [B7O9(OH)5]2.4H2O (dach = 1,2-diaminocyclohexane) (6), and the neutral species trans-[Ni(en)(H2O)2{B6O7(OH)6}].H2O (7) (en = 1,2-diaminoethane), and [Ni(dmen)(H2O){B6O7(OH)6}].5H2O (8). Compounds 1–8 were characterized by single-crystal XRD studies and by IR spectroscopy and 2, 4–7 were also characterized by thermal (TGA/DSC) methods and powder XDR studies. The solid-state structures of all compounds show extensive stabilizing H-bond interactions, important for their formation, and also display a range of gross structural features: 1 has an insular tetraborate(2-) anion, 2–5 have insular pentaborate(1-) anions, 6 has an insular heptaborate(2-) anion (‘O+’ isomer), whilst 7 and 8 have hexaborate(2-) anions directly coordinated to their Ni(II) centers, as bidentate or tridentate ligands, respectively. The Ni(II) centers are either octahedral (1–4, 7, 8) or square-planar (5), and compound 6 has both octahedral and square-planar metal geometries present within the structure as a double salt. Magnetic susceptibility measurements were undertaken on all compounds.

Pentaborate(1-) Salts and a Tetraborate(2-) Salt Derived from C2- or C3-Linked Bis(alkylammonium) Dications: Synthesis, Characterization, and Structural (XRD) Studies

The synthesis of a number of pentaborate(1-) salts from cations arising from N-substituted α,α-, α,β-, and α,γ-diaminoalkanes has been attempted in aqueous solution from B(OH)₃ and the appropriate diammine in a 10:1 ratio. Despite relatively mild work-up conditions the pentaborate(1-) salts prepared were not always as anticipated and the following compounds were isolated in good yield: [Me₂NH(CH₂)₂NHMe₂][B₅O₆(OH)₄]₂ (1), [Et₂NH(CH₂)₂NHEt₂][B₅O₆(OH)₄]₂ (2), [Et₂NH₂][B₅O₆(OH)₄] (3), [Me₂NH₂][B₅O₆(OH)₄] (4), [Me₂NH(CH₂)₃NHMe₂][B₅O₆(OH)₄]₂ (5), [Et₂NH(CH₂)₃NHEt₂][B₅O₆(OH)₄]₂ (6), [Me₃NCH₂CH=CH₂][B₅O₆(OH)₄] (7), and [Me₃N(CH₂)₃NMe₃] [B₅O₆(OH)₄]₂^.0.5H₂O (8). The tetraborate(2-) salt, [Me₃N(CH₂)₂NMe₃][B₄O₅(OH)₄]^.2B(OH)₃^.2H₂O (9) was obtained in moderate yield (41%) from a 3:1 reaction of B(OH)₃ with [Me₃N(CH₂)₂NMe₃](OH)₂. All compounds were characterized by spectroscopy (¹H, ¹¹B, ¹³C NMR and IR) and thermal gravimetric analysis (TGA). BET analysis on materials derived thermally from selected samples (1, 2, 6, 7) all had porosities of < 1 m²/g, demonstrating that they were non-porous. Single-crystal XRD structures were obtained for 2, 3, 7, 8 and 9 and all contain extensive H-bonded polyborate lattices.

Transition-metal complexes with oxidoborates. Synthesis and XRD characterization of [(H3NCH2CH2NH2)Zn{κ3 O,O′,O′′-B12O18(OH)6-κ1 O′′′}Zn(en)(NH2CH2CH2NH3)]·8H2O (en=1,2-diaminoethane): a neutral bimetallic zwiterionic polyborate system containing the ‘isolated’ dodecaborate(6−) anion

The title compound, [(H3NCH2CH2NH2)Zn{κ3 O,O′,O′′-B12O18(OH)6-κ1 O′′′}Zn(en)(NH2CH2CH2NH3)]·8H2O (en=1,2-diaminoethane) (1), was prepared as a crystalline solid in moderate yield from the reaction of B(OH)3 with [Zn(en)3][OH]2 in aqueous solution (15:1) ratio. The structure contains a neutral bimetallic complex comprised of a unusual dodecaborate(6−) anion ligating two [H3NCH2CH2NH2Zn(en)n]3+ centers in a monodentate (n=1) or tridentate (n=0) manner.