Triborate and pentaborate salts of non-metal cations derived from N-substituted piperazines: synthesis, structural (XRD) and thermal properties

The synthesis and characterization of a triborate salt, [H 2 N(CH 2 CH 2 ) 2 NH 2 ][B 3 O 3 (OH) 4 ] 2 ( 1 ), and four pentaborate salts, [H 2 N(CH 2 CH 2 ) 2 NH][B 5 O 6 (OH) 4 ] ( 2a ), [MeHN(CH 2 CH 2 ) 2 NH][B 5 O 6 (OH) 4 ] ( 2b ), [MeHN(CH 2 CH 2 ) 2 NMe][B 5 O 6 (OH) 4 ] ( 2c ) and [Me 2 N(CH 2 CH 2 ) 2 NMe 2 ][B 5 O 6 (OH) 4 ] 2 ( 2d ) are described. TGA and DSC analysis (in air, 25–1000 u C) indicate that triborate 1 decomposes to B 2 O 3 via a multistage process, with the first stage ( , 250 u C) being dehydration to condensed polymeric hexaborate of approximate composition: [H 2 N(CH 2 CH 2 ) 2 NH 2 ][B 6 O 10 ]. The pentaborates ( 2a–2d ) are thermally decomposed to B 2 O 3 via a 2 stage process involving polymeric [NMC][B 5 O 8 ]. The anhydrous polyborates were amorphous. BET analysis of materials derived from the thermolysis of 1 at 250, 400, 600, and 1000 u C, were all non-porous (surface area , 1.8 m 2 g 2 1 ). A single-crystal X-ray diffraction study of 1 showed that it contains isolated triborate(1 2 ) anions in a structure comprised of alternating cationic and anionic layers held together via extensive H-bonds. Single-crystal XRD structural studies on pentaborate salts 2c and 2d are also reported.


Introduction
There has been increased interest in recent years in non-metal cation (NMC) polyborate, notably pentaborate, salts as a result of their potential applications as porous 1-5 or NLO 6 piezoelectric 7 or fluorescent 8 materials. The chemistry of boric acid in aqueous solution is complicated with monomeric and various polymeric borate ions existing in equilibrium concentrations 9 which are dependent upon B concentration, B/[OH 2 ] ratio, and temperature. Despite this, pentaborate salts of NMCs are generally readily synthesized as crystalline solids from aqueous solution by the interaction of a free base, or its hydroxide salt, with B(OH) 3 in a 1 : 5 ratio. 2,4,10 Structurally, NMC pentaborates are best described as supramolecular networks of H-bonded isolated [B 5 O 6 (OH) 4 ] 2 anions, and the formation of this extended lattice is clearly a driving force in product formation. The NMCs occupy 'cavities' within the network, and usually (if possible) add further stabilizing H-bond cation-anion interactions. 1 42 . 17 More forcing solvothermal methods have led to NMC salts of isolated or condensed polyborate sheets, which are formally derived from isolated anions. 18 We are currently undergoing a research programme investigating the structure directing effects of NMCs in borate chemistry 4,14,19 and have recently isolated, by this method from aqueous solution, 20 22 and [B 7 O 9 (OH) 5 ] 22 . As a continuation of this study we have investigated substituted piperazine based NMCs and now report the synthesis of an unusual triborate salt, as well as further examples of pentaborate salts. The structures of the anions present are illustrated in Fig. 1. The thermal properties of the triborate salt (TGA/DSC) has been investigated in detail and results compared with those of the pentaborate derivatives. Porosity (BET surface area measurements) of materials derived from the thermal decomposition of the triborate salt are also described. These results are compared with those from other known NMC polyborate salts.  (3). Eqn (1) and (2) indicate that a piperazine : B(OH) 3 ratio of 1 : 6 would be ideal for the preparation of 1, whereas a similar 1 : 5 ratio would be ideal for 2a. We obtained 1 in high yield from a 1 : 3 ratio reaction mixture in which half of the piperazine remained unreacted. We believe the function of the unreacted piperazine is to raise pH of the solution allowing the more basic triborate anion 5,15 to crystallize.

Synthesis and characterization
With the exception of 2d (obtained in 50%) products were readily obtained in high yield (86-97%) and were of good analytical purity by crystallization induced by removal of the solvents. Compounds 1 and 2a-d were characterized spectroscopically (NMR, IR) and X-ray diffraction (powder) techniques. Powder diffraction analysis showed that all samples were prepared crystalline, and crystals from 1, 2c and 2d were selected for a single-crystal structural determination (section 2.4). Unsubstituted piperazine salts usually contain dications, with monocationic salts occurring less frequently. 21 11 B NMR spectra of salts 1 and 2a-d are all very similar, despite the different stoichiometry for 1, and showed peaks at +18 ppm, +13 ppm and +1 ppm. The similarity in these spectra may be explained by the existence of a complex series of equilibria existing in aqueous solution which link monomeric and oligomeric borate species. 9 Similar spectra have been obtained from other pentaborate salts. 4,14,19 1 H and 13 C NMR spectra, obtained in D 2 O solution, were in full accord with their structures, with signals associated with B-OH and NH protons producing a single broadened single at y4.7 ppm due to rapid exchange. The IR spectrum of 1 (Fig. 2) 22 band at y925 cm 21 is apparent in the pentaborate samples 2a-d, but significantly is absent from 1.

Thermal properties of 1
The thermal properties of NMC pentaborate salts, which contain intricate 3D H-bonded networks (section 2.3), have been well investigated and the thermal behaviour of 2a-2d are consistent with earlier reports on pentaborate salts. 1-5 The layered structure for 1 (see section 2.3) might suggest that its thermal properties would be different and therefore was worthy of further and more detailed investigation.
Heating a sample of the triborate salt (1) in air resulted in thermal decomposition and the eventual formation of a glassy black solid. During this process the sample 'chars' and intumesces prior to collapsing down to the glass. This behaviour is similar to that observed for non-metal cation pentaborates. 1-5 A thermal study of the one previously known NMC triborate, [HOCH 2 CMe 2 NH 3 ][B 3 O 3 (OH) 4 ], 11 suggested dehydration (2 mole equivalent per triborate) by 120 uC to a viscous liquid by either condensation of anions or anion/ cation esterification. The process for 1 was followed in more detail by DSC/TGA (Fig. 3). Closer inspection of the TGA curve shows that the decomposition is a multistep process, with a series of endothermic processes occurring below 275 uC, and two exothermic processes occurring at 400 uC and 600 uC. The weight loss below 275 uC is consistent with dehydration as shown in eqn (4). The final residue (55.4%) is in close agreement with that calculated (54.2%) for B 2 O 3 according to eqn (5).
It was of interest to examine the physical properties (crystallinity and porosity) of materials derived from the aerobic thermolysis of 1 and a furnace procedure was used to attempt to replicate the above TGA results on a small synthetic scale. Accordingly, samples (y1.0 g) were heated to 275 uC (1a), 500 uC (1b), 600 uC (1c), or 1000 uC (1d) and weighed and characterized. Elemental analytical data of 1a were consistent with the dehydrated product (eqn (4)) and the residual weight associated with formation of 1d was in good agreement with the residual weight calculated expected for formation of B 2 O 3 . Elemental analysis data for 1b and 1c demonstrated residual presence of C, H and N with lower amounts present in 1c indicating that the oxidation process was still incomplete at this temperature/time. All thermally produced samples were amorphous by powder XRD studies. and SEM pictures of ground samples were all remarkably similar (exemplified for 1b in Fig. 4) and were consistent with glassy shards.
The porosity of the thermally derived materials were determined by multipoint BET analysis. 24 Essentially all samples were 'non-porous' with the isolated pentaborates having porosities of 0.12-0.85 m 2 g 21 , condensed pentaborates 0.04-0.86 m 2 g 21 , intumesced materials 0.12-1.59 m 2 g 21 , and the final glassy B 2 O 3 , 0.18 m 2 g 21 . The densities of 1b and 1c, determined as 'tipped solids', were extremely low (0.1, 0.2 g cm 23 respectively) and are consistent with the observation that the materials intumesced during oxidation to leave expanded materials. The low porosity of the final glassy black solid (yB 2 O 3 ) was expected, but the porosity of the intumesced materials were lower than expected and indicated macroporous aerogel-like structures, in which gaseous decomposition products were trapped within the intumesced materials. 25

XRD structural studies on 1, 2c and 2d
The structure of 1 as a triborate salt of a dication was confirmed by single-crystal XRD studies. A drawing of 1 showing the ions present and the atomic numbering scheme is shown in Fig. 5. The structure contains two identical [B 3 O 3 (OH) 4 ] 2 anions, as illustrated in Fig. 1(i 4 ] 2 anion appears in significant concentrations in aqueous solution at pH 5-12 but rarely is crystalized from such solutions by non-metal cations which generally favour partnering pentaborate anions. As noted above, the solutions had excess base present, and the pH was higher than normal. 5   Compound 1 has a giant H-bonded network involving both anion-anion and cation-anion interactions and where all four NH and all four OH groups participate as donors. For convenience, the O atom acceptor sites in 1 may be labelled, by analogy with pentaborate chemistry and distance from the 4 co-ordinate B1, a-, bor c-sites. 2 The OH sites on B1 can be labelled d. The triborate anions form infinite zig-zag chains through alternating c and a R 2 2 (8) rings (Etter 31 Fig. 6 and 7, respectively. Both compounds contain isolated [B 5 O 6 (OH) 4 ] 2 anions, as illustrated in Fig. 1(ii). B-O bond lengths and bond angles at B and O are not significantly different from those observed in other pentaborate structures containing nonmetal cations, and related systems. [1][2][3][4]14,19,32 The anions in both compounds form remarkably similar (despite the stoichiometry) extended H-bonded lattices, and the H-bond interactions for both 2c and 2d can be described as a,a,a,b. The a,a,a,b configuration is commonly found in 'brickwall' or 'herringbone' structures 1,14 but the extended lattice found in 2c and 2d is not one of these types. Each pentaborate anion forms 3 reciprocal-pair R 2 2 (8) a-interactions with neighboring pentaborates, and 1 reciprocal-pair R 2 2 (12) b-interaction (Etter 31

Thermolysis experiments of 1
Samples of 1 were individually placed within open topped ceramic bowls and positioned within a furnace (air atmosphere). The furnace temperature was set to increase from room temperature to either 250 uC (1a), 500 uC (1b) or 600 uC (1c) at 10 uC per minute. After the samples had reached the required temperature they were allowed to cool to room temperature, isolated and powdered in a mortar and pestle.
Sample 1a was pale yellow. Samples 1b and 1c were black and had increased their volume y10 fold.  sphere). The furnace temperature was set to increase from room temperature to 1000 uC at 10 uC per minute, and then held at 1000 uC for 3 h, and then allowed to cool to room temperature, yielding a glassy black solid (1d), which was powdered in a mortar and pestle and weighed (0.2759 g, 44.2% weight loss). 1d: Elem anal. Found: C 0.1, H 0.4, N 0.1%. IR: n max /cm 21 : 3200(s), 1445(s), 1194(s), 795(m), 720(m), 550(m). BET: surface area 0.16 m 2 g 21 .

X-ray crystallography
The crystallographic data collection of compounds 1, 2c and 2d were performed using a Nonius Kappa CCD diffractometer with Mo-Ka radiation (l = 0.71073 Å) controlled by the Collect 33 software package and an Oxford Cryosystem N 2 open flow cryostat at 120(2) K. The data were processed using Denzo 34 and absorption corrections were applied using SORTAV. 35 Crystallographic data are in Table 1. The structures were solved by direct methods and refined by full-matrix leastsquare procedures on F 2 using SHELXS-97 and SHELXL-97 respectively. 36 All non-hydrogen atoms were fixed using a standard riding model. Crystallographic data for 1, 2c, 2d have been deposited with the Cambridge Crystallographic Data Centre with CCDC 930889-930891. Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ (fax: +44 1223 336033) or email: deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk.

Conclusion
Salts containing cations derived from N-substituted piperazines, partnered with either triborate(12) or pentaborate(12) anions have been prepared. The triborate salt 1 possesses an alternating cationic/anionic layered solid-state structure and contains extensively H-bonded triborate sheets. Compound 1 is thermally decomposed in air via a multistage process to glassy B 2 O 3 . Materials thermally derived from 1 were amorphous and non-porous. Similar thermal behaviour was observed for the pentaborate salts (2a-2d) which possess three dimensional H-bonded lattices.