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

Abstract

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.

Keywords: BET analysis; TGA; XRD; borate; diaminoalkane salts; oxidipolyborate; pentaborate(1-); tetraborate(2-).