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.


Introduction
Salts and neutral species containing polyoxidoborate (polyborate) anions occur as minerals and many synthetic derivatives have now also been prepared [1][2][3][4][5]. These compounds are structurally diverse, with the polyborate anions generally comprised of trigonal planar 3-coordinate and tetrahedral 4-coordinate boron centres, linked via oxygen bridges with as many terminal OH or O − groups as is necessary to complete the required geometry at boron [6][7][8][9][10][11]. Some polyborate salts are sold as bulk chemicals for use in the glass/vitreous, fire retardant, and agricultural industries [12][13][14], whilst other synthetic polyborates have interesting and specialized physical e.g., semiconducting, luminescent, non-linear optical (NLO) properties [5,15]. We are interested in synthetic polyborate chemistry and recently have started to investigate non-metal cation stabilized silicate/borate solutions as potential

Synthesis
Synthetic strategies to prepare pentaborate(1-) salts partnered with organic cations include the reaction of a free base amine with B(OH)3 in aqueous solution or use of OHactivated ion-exchange resins with quaternary ammonium halide salts [5]. Schubert and co-workers [19] have successfully applied the former method to α,ω-diaminoalkanes and have isolated a number of pentaborate(1-) salts containing a +2 diammonium cations of composition [H3N(CH2)xNH3][B5O6(OH)4]2 (x = 5, 6,[8][9][10][11][12]. In this study, the pentaborate (1-) (6) were readily prepared as crystalline solids from aqueous solution in yields of 87-99% by the reaction of 10 equivalents of B(OH)3 with the diaminoalkane in aqueous solution at room temperature. Compounds 2, 3, 5 and 6 are previously unreported. Compound 1 has recently been prepared serendipitously in low yield (~10%) as a by-product from the reaction of a fluorinated borane CF3(CF2)5CH2(OLi)CH2PPh2 . BH3 with [PtCl2(COD)] in the presence of Me2N(CH2)2NMe2 [20]. Likewise, 4 has been previously obtained in very low yield (a few crystals) as a by-product of a reaction of B(OH)3 with B2(NMe2)4 followed by crystallization from water [21]. N,N,N'N'-Tetraakyldiaminomethane cations are less stable than the tetrasubstituted diaminoethane or diaminopropane cations in aqueous solution and the formation of the corresponding dialkylammonium pentaborate(1-) salts 3 and 4 in high yields is not unexpected and further demonstrates the ready cleavage of the C-N aminal bonds [22][23][24][25]. Further characterization data for 1 and 4 are reported within this manuscript. Scheme 1 contains all the reactions studied in this work.

Synthesis
Synthetic strategies to prepare pentaborate(1-) salts partnered with organic cations include the reaction of a free base amine with B(OH) 3 in aqueous solution or use of OHactivated ion-exchange resins with quaternary ammonium halide salts [5]. Schubert and co-workers [19] have successfully applied the former method to α,ω-diaminoalkanes and have isolated a number of pentaborate(1-) salts containing a +2 diammonium cations of composition [H 3 N(CH 2 ) x NH 3 ][B 5 O 6 (OH) 4 ] 2 (x = 5, 6,[8][9][10][11][12]. In this study, the pentaborate (1-) (6) were readily prepared as crystalline solids from aqueous solution in yields of 87-99% by the reaction of 10 equivalents of B(OH) 3 with the diaminoalkane in aqueous solution at room temperature. Compounds 2, 3, 5 and 6 are previously unreported. Compound 1 has recently been prepared serendipitously in low yield (~10%) as a by-product from the reaction of a fluorinated borane CF 3 (CF 2 ) 5 [20]. Likewise, 4 has been previously obtained in very low yield (a few crystals) as a by-product of a reaction of B(OH) 3 with B 2 (NMe 2 ) 4 followed by crystallization from water [21]. N,N,N'N'-Tetraakyldiaminomethane cations are less stable than the tetrasubstituted diaminoethane or diaminopropane cations in aqueous solution and the formation of the corresponding dialkylammonium pentaborate(1-) salts 3 and 4 in high yields is not unexpected and further demonstrates the ready cleavage of the C-N aminal bonds [22][23][24][25]. Further characterization data for 1 and 4 are reported within this manuscript. Scheme 1 contains all the reactions studied in this work.   2 with B(OH) 3 at this 1:3 ratio were not explored within this study. Compound 9 contains a tetraborate(2-) rather than a pentaborate(1-) anion, observed for 1-8, and this anion is drawn schematically in Figure 1(b). Spectroscopic (IR and NMR) and thermal analysis data, including porosity measurements on materials obtained thermally, for these compounds 1-9 are described in Section 2.2 and XRD studies on 2, 3, 7, 8 and 9 are described in Section 2.3.

Thermal Analysis, Porosity Measurements and Spectroscopic Data
Compounds 1-9 gave satisfactory C, H, N (combustion) elemental analysis data and thermal gravimetric analysis (TGA) data consistent with their formulations. Non-metal cation polyborates have been previously reported to decompose thermally (in air) to afford glassy B 2 O 3 residues [5,19,29]. This thermal decomposition is usually a two-step processes with the lower temperature step associated with dehydration and cross-linking of hydroxyl groups of the polyborate anions to give condensed polyborate salts. The higher temperature step involves oxidation of the cations to afford the glassy residue. Thus for example, 1, dehydrates to a condensed polyborate, [Me 2 NH(CH 2 ) 2 NHMe 2 ][B 10 O 16 ] in the temperature range 150-300 • C with loss of 4H 2 O then with oxidation of organic cation to leave residual 5B 2 O 3 at 300-650 • C. Compound 9, which is formulated as a tetraborate(2-) salt with 2 interstitial B(OH) 3 and 2 interstitial H 2 O molecules afforded 3B 2 O 3 by a similar two-step process. Selected samples of 1-6 were either heated to 250 • C for 1h (1, 2, 5, 6) or 600 • C for 1 h (1-3, 6) to generate samples approximating to the condensed borate salt or the pyrolysed glassy solid that could be used for porosity measurements by use of the Brunauer-Emmett-Teller (BET) method [30]. Measured porosities of the thermally treated samples were in the range 0.06-0.94 m 2 /g indicating that they were all essentially non-porous and in accord with previous work [31,32]. 1 H-, 13 C-and 11 B-NMR spectra were recorded on compounds 1-9 (D 2 O solvent) and 1 H-and 13 C-spectra were in accord with the cations present. Exchangeable NH and B-OH protons were absent as specific signals and it is assumed were observed in the HOD signal present at +4.79 ppm as has been observed previously in related non-metal cation systems [31][32][33]. The pentaborate(1-) anion, although very stable in the solid-state and is readily templated by many organic cations [34], decomposes in aqueous solution as various equilibrium determined borate species are rapidly obtained and influenced by the boron concentration, the temperature and the pH of the aqueous solution [35,36]. However, 11 B spectra of 1-8 were obtained and these samples all showed three peaks centred at 1 (~5) 13 (~35%) and 18 (~60%) ppm in a pattern typical of solutions arising from pentaborate(1-) salts [31][32][33][34]37]. The 11 B spectra of 9, originating from a tetraborate(2-) salt, was significantly different with three peaks now at 1.3 (5%), 7.4 (43%) and 11.7 (52%).The upfield shift of the 'average' 11 B chemical shift on going from a pentaborate (1-) anion (1-8) to a tetraborate(2-) anion (9) is in accord with the average B/charge ratio changing from 5:1 to 2:1 [34].
IR spectra were obtained on all compounds and strong B-O stretches in the fingerprint region 1450-650 cm −1 are readily observed [38]. A band assigned to an asymmetric B-O stretch associated with the tetrahedral boron centre at~925 cm −1 and usually diagnostic of a pentaborate(1-) anion [30,39] was clearly evident in samples 1-8. There are many fewer reports of IR spectra of tetraborate(2-) species and some have been tabulated [38] but diagnostic bands have not been reported. It should be noted that the tetraborate(2-) salt 9 also displayed a stretch at 927 cm −1 in the region considered diagnostic for a pentaborate(1-) salt.
The tetraborate(2-) anion in 9 is found co-crystallized with 2B(OH) 3   The fourth R 2 2 (12) interaction is not visible from this view.   The structural characterization of 9 is considered in more detail in this manuscript since it contains a tetraborate(2-) anion and there are far fewer crystallographic reports in the literature featuring this anion; the anion is found partnered with 2Na + in the well-known borax [47,48] but is less commonly found with non-metal cation [40,[49][50][51][52] or transition metal cation salts [53][54][55][56].
The tetraborate(2-) anion in 9 is found co-crystallized with 2B(OH) 3  for the components of highest site occupancy is given in Figure 5. Bond lengths and bond angles found for C, N atoms within the [Me 3 N(CH 2 ) 2 NMe 3 ] 2+ cations are as expected and similar to those reported for the related pentaborate (1-)  numerous H-bond interactions within the structure and a giant supramolecular anionic H-bonded lattice is formed, with cations situated within the cavities. However, there are no direct anion-anion interactions within the lattice and the B(OH)3 units serve to bridge tetraborate(2-) anions by acting as ′spacers′ to expand the lattice so that it can accommodate the relatively large cation, [Me3N(CH2)2NMe3]. This spacer role for B(OH)3 has been observed before [32,33,39,[55][56][57][58]. With reference to Figure 6 it can be seen that the H-bond structure can be envisaged as ′horizontal′ chains of alternating [B4O5(OH)4] 2-/B(OH)3 units held together by R2 2 (8) interaction, crosslinked by ′vertical′ C2 2 (8) chains (involving O8,H8 … O12,H12 … O5,B1,O2,B3) in a regular 2D arrangement. These planes are further linked by additional R2 2 (8) interactions into a 3D lattice. Further details of the interactions around the tetraborate(2-) anion is shown in Figure 7 and full details are available in the Supplementary Information. The O1 site is a double H-bond acceptor in two R2 2 (8) interactions: O21H21 … O1 and O13H13 … O1. The H2O of crystallization further help to H-bond the structure together. The cation is unable to get involved with H-bonding.  The tetraborate(2-) anion has four H-bond donor sites and nine potential H bond acceptor sites, and the B(OH) 3 molecules each have three donor and three potential acceptor sites. Again, there are numerous H-bond interactions within the structure and a giant supramolecular anionic H-bonded lattice is formed, with cations situated within the cavities. However, there are no direct anion-anion interactions within the lattice and the B(OH) 3 units serve to bridge tetraborate(2-) anions by acting as spacers to expand the lattice so that it can accommodate the relatively large cation, [Me 3 N(CH 2 ) 2 NMe 3 ]. This spacer role for B(OH) 3 has been observed before [32,33,39,[55][56][57][58]. With reference to Figure 6

General
All chemicals were obtained commercially. TGA/DSC analysis (in air) were undertaken on an SDT Q600 V4.1 Build 59 instrument (TA Instruments, New Castle, DE, USA) using Al 2 O 3 crucibles, between 10-800 • C (ramp temperature rate of 10 • C min −1 ). FTIR spectra were obtained (KBr pellets) on a Perkin-Elmer 100 FTIR spectrometer (Perkin-Elmer, Seer Green, UK). NMR spectra were obtained on a Bruker Avance 400 spectrometer (Bruker, Coventry, UK) and reported in ppm with positive chemical shifts (δ) to high frequency (downfield) of TMS ( 1 H, 13 C) and BF 3 . OEt 2 ( 11 B). BET analysis were performed ion a Gemini 2375 analyser (Micromeritics Instrument Corporation, Norcross, GA, USA) with N 2 gas as the adsorbent. CHN analysis were obtained from OEA Laboratories Ltd. (Callington, UK). See Section 3.11 for single-crystal XRD methods.

Synthesis, Spectroscopic, and Analytical Data for 1
B(OH) 3 (6.2 g; 100.2 mmol) was dissolved in H 2 O (100 mL). To this solution was added Me 2 N(CH 2 ) 2 NMe 2 (1.18 g; 10.0 mmol) and the reaction mixture was stirred for 1 h. The solvent was partially removed by rotary evaporation to give white crystals. The crude product of 1 (5.5 g; 99%) was obtained after oven drying at 60 • C for 24 h.

X-Ray Crystallography
Single-crystal X-ray crystallography (sc-XRD) was undertaken at the EPSRC National Crystallography Service at the University of Southampton. Selected crystals were mounted on a MITIGEN holder in perfluoroether oil. Suitable crystals of 2, 3 and 8 were placed on a FRE+ instrument Rigaku (Wilmington, MA, USA) equipped with HF Varimax confocal mirrors and an AFC12 goniometer and HG Saturn 724+ detector diffractometer. Suitable crystals of 7 were placed on a Rigaku 007HF equipped with HF Varimax confocal mirrors and an AFC11 goniometer and HyPix 6000 detector diffractometer. Suitable crystals of 9 were placed on a Rigaku FRE+ equipped with VHF Varimax confocal mirrors and an AFC12 goniometer and HyPix 6000 diffractometer. The crystals were kept at T = 100(2) K during data collection. The structures were solved with ShelXT [59] using Olex2 [60]. The models were refined with ShelXL [61] (using Least Squares minimisation). CCDC: 1968475 (2), 1968476 (3), 1968477 (7), 1968478 (8) and 1968479 (9) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: deposit@ccdc.cam.ac.uk)

Conclusions
Six new pentaborate(1-) and one new tetraborate(2-) salts were synthesized by templated crystallization reactions from aqueous solutions containing B(OH) 3