Tetra­methyl­ammonium pentaborate 0.25-hydrate

The title compound, tetramethylammonium 4,4',6,6'-tetrahydroxy-2,2'-spirobi(cyclotriboroxane) 0.25-hydrate, C₄H₁₂N⁺·B₅H₄O₁₀^-·0.25H₂O, was synthesized under mild solvothermal conditions. The B₅O₆(OH)₄^- clusters are connected by strong hydrogen-bonding interactions into a three-dimensional structure containing rectangular channels along the a axis, in which the C₄H₁₂N⁺ ions and water mol­ecules are located.

S1. Comment

Borates have provided a rich area of research for over 50 years because of their rich crystalline structures and potential applications in mineralogy and non-linear optical materials. So far, a number of borates exhibiting one-dimensional chains, two-dimensional layers and three- dimensional open structures have been reported (Salentine et al., 1987; Touboul et al., 1999; Menchetti et al., 1982; Nowogrocki et al., 2003; Huppertz et al., 2003), most of which were usually grown by a high-temperature flux method. Recently, hydrothermal and related techniques have been widely used for the preparation of inorganic materials, including phosphates and germanates (Yang et al., 1999; Zhou et al., 2001; Plevert, 2001). Interesting borates, such as Pb₆B₁₁O₁₈(OH)₉ (Yu et al., 2002) and Zn₆B₁₂O₂₄ (Choudhury et al., 2002), have also been obtained from hydrothermal systems. The hydrothermal technique is of perpetual interest in the construction of novel structures, because different kinds of templates, especially organic molecules, with a wide variety of shapes and sizes can be incorporated into the systems to shape diverse inorganic frameworks. Nevertheless, such research efforts have rarely been made in the borate field (Schubert et al., 2000; Weakley et al., 1985).

We report here the X-ray structure analysis of the title compound, (I) (Fig.1). Compound (I) is composed of C₄H₁₂N⁺ cations, B₅O₆(OH)₄⁻ anions and solvent water molecules. The B₅O₆(OH)₄⁻ anion consists of two B₃O₃ rings, each containing one tetrahedrally and two trigonally coordinated B atoms; two of these B₃O₃ rings are connected by sharing their tetrahedrally coordinated B-atom vertices. The B₅O₆(OH)₄⁻ clusters are further connected by strong hydrogen-bonding interactions into a three-dimensional structure. Hydrogen-bonding interactions exist between atoms O₂ and O₉, O₄ and O₆, O₇ and O₅, and O₉ and O₁₀, with O···O distances ranging from 2.671 (4) to 2.745 (3) Å. Interestingly, this three-dimensional structure possesses a rectangular channel along the a axis (Fig. 2), which is formed by six B₅O₆(OH)₄⁻ units connected via hydrogen bonds. Two C₄H₁₂N⁺ cations and one water molecule are located in each rectangular channel. The trigonally coordinated B atoms exhibit B—O bond lengths in the range 1.336 (4)–1.379 (4) Å, while the tetrahedrally coordinated B atoms have B—O distances of 1.449 (4)–1.474 (4) Å. The O—B—O angles lie in the ranges 115.8 (3)–122.9 (3)° and 108.0 (3)–110.9 (2)°, respectively.

S2. Experimental

In a typical synthesis of (I), H₃BO₃ (0.232 g) was dissolved in a mixed solvent containing pyridine (4.2 ml), distilled water (0.1 ml) and tetramethylammonium hydroxide (C₄H₁₃NO; 1.1 ml, 25%). The mixture, with a typical H₃BO₃/py/H₂O/NC₄H₁₃O molar ratio of 3.7:52.0:51.4:3.0 was stirred mechanically at room temperature to a final pH of about 8 and then placed in an autoclave at 443 K for 7 d. Colorless block-like crystals, identified as (I), were obtained in 18.21% yield (based on B atoms). The powder X-ray diffraction pattern of the bulk product is in good agreement with the calculated pattern based on the single-crystal solution, indicating the phase purity of the sample. Thermogravimetric analysis (TGA) showed that there were four steps of weight loss. The initial weight loss, of about 1.5 (calculated 1.52%) from 400 to 490 K, corresponds to the removal of the water molecule. The second step, about 9 (calculated 9.1%) from 470 to 570 K, was assigned to the removal of three hydroxy groups. The third step, from 570 to 1027 K, was assigned to the partial release of C₄NH₁₂·OH, resulting the decomposition of the framework. When heated further, the volatile boron oxide phases are partly separated? from the phase. The IR spectrum (4000–400 cm⁻¹, KBr pellets) of (I) contains the characteristic bands of the BO₃ and BO₄ groups, corresponding to two strong bands at about 1317 and 1087 cm⁻¹, respectively. The wide peak between 1350 and 1500 cm⁻¹ is assigned to the bending bands of the CH₂ and streching bands of the CN groups. The stretching vibration bands of the water OH groups are observed at about 3266 cm⁻¹ (Liu et al., 2003), and the stretching vibration of the OH groups attached to the B atom is at about 3370 cm⁻¹ (Yu et al., 2002).

S3. Refinement

H atoms bonded to O atoms were located from difference density maps. All H atoms attached to C atoms were positioned geometrically (C—H = 0.97 Å) and were allowed to ride on their parent C atoms. The solvent water molecules are located in channels, 4.08 (2) Å from the N atoms. In Fig. 2, the water molecule and N atom almost overlap each other because of the closeness of their b and c coordinates. The small solvent molecules are probably absorbed from the solution by the structure after synthesis, a phenomenon that can be explained by the fact that the borate structure is very polar as a result of the large number of hydroxy groups in the void space of the structure, as shown in Fig. 2. Such absorption of small molecules is a familiar occurance in microporous materials. For example, Na₄Ge₁₆O₂₈(OH)₁₂ (van den Berg et al., 2004) absorbs small gas molecules owing to voids in its structure. The parameter of 0.25 was determined according to the U_eq value and the refinement. The presence of hydration water molecules was established experimentally by TGA and IR spectroscopy.

Computing details

Data collection: SMART (Bruker, 1999); cell refinement: SMART; data reduction: SAINT and SHELXTL (Bruker, 1999); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.

Figures

Tetramethylammonium pentaborate hydrate

Crystal data

C4H12N^+·B5H4O10^−·0.25H2O	F(000) = 616
Mr = 296.23	Dx = 1.374 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 57 reflections
a = 9.2623 (4) Å	θ = 2.4–25.1°
b = 16.8814 (7) Å	µ = 0.12 mm^−1
c = 9.2119 (5) Å	T = 293 K
β = 96.281 (3)°	Block, colorless
V = 1431.73 (12) Å^3	0.48 × 0.40 × 0.30 mm
Z = 4

Data collection

Bruker SMART CCD area-detector diffractometer	2538 independent reflections
Radiation source: fine-focus sealed tube	1654 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.030
ϕ and ω scans	θmax = 25.1°, θmin = 2.4°
Absorption correction: empirical (using intensity measurements) (SADABS; Sheldrick, 1996)	h = −11→11
Tmin = 0.942, Tmax = 0.963	k = −20→16
4012 measured reflections	l = −10→5

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.064	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.177	w = 1/[σ^2(Fo^2) + (0.0671P)^2 + 1.0501P] where P = (Fo^2 + 2Fc^2)/3
S = 1.12	(Δ/σ)max = 0.005
2470 reflections	Δρmax = 0.28 e Å^−3
207 parameters	Δρmin = −0.21 e Å^−3
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.011 (3)

Special details

Experimental. Thermogravimetric analysis (TGA) was performed using a Mettler Toledo TGA/SDTA 851 e analyzer in dry N₂ atmosphere from 303 to 1573 K, with a heating rate of 15 K min⁻¹. The IR spectra (4000–400 cm⁻¹, KBr pellets) were recorded using an ABB Bomen MB 102 spectrometer.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
O1	0.8486 (2)	0.57705 (13)	0.1955 (2)	0.0390 (6)
O2	1.0888 (3)	0.53732 (16)	0.2093 (3)	0.0529 (7)
O3	1.0043 (2)	0.63342 (14)	0.0344 (3)	0.0462 (7)
O4	0.9200 (3)	0.72546 (18)	−0.1411 (3)	0.0591 (8)
O5	0.7570 (2)	0.67133 (13)	0.0127 (2)	0.0392 (6)
O6	0.6910 (2)	0.68090 (13)	0.2551 (2)	0.0407 (6)
O7	0.5300 (3)	0.7418 (2)	0.4023 (4)	0.0803 (11)
O8	0.4480 (2)	0.63774 (16)	0.2525 (3)	0.0597 (8)
O9	0.3570 (2)	0.53304 (17)	0.1089 (3)	0.0543 (8)
O10	0.5992 (2)	0.57430 (12)	0.0974 (2)	0.0370 (6)
OW	0.649 (2)	0.8871 (12)	0.143 (2)	0.135 (6)	0.25
B1	0.9795 (4)	0.5822 (2)	0.1457 (4)	0.0364 (9)
B2	0.8906 (4)	0.6768 (2)	−0.0314 (4)	0.0378 (9)
B3	0.7253 (4)	0.6256 (2)	0.1413 (4)	0.0321 (8)
B4	0.5600 (4)	0.6874 (3)	0.3036 (5)	0.0468 (11)
B5	0.4718 (4)	0.5813 (2)	0.1516 (4)	0.0386 (9)
N	0.2069 (4)	0.87837 (19)	0.1203 (3)	0.0565 (9)
C1	0.2401 (6)	0.8739 (3)	0.2795 (5)	0.0878 (16)
H1A	0.3415	0.8627	0.3035	0.132*
H1B	0.1835	0.8324	0.3169	0.132*
H1C	0.2170	0.9235	0.3223	0.132*
C2	0.2907 (7)	0.9435 (3)	0.0630 (6)	0.107 (2)
H2B	0.3927	0.9328	0.0838	0.160*
H2C	0.2679	0.9923	0.1087	0.160*
H2D	0.2661	0.9476	−0.0406	0.160*
C3	0.0482 (5)	0.8967 (3)	0.0870 (6)	0.0860 (15)
H3A	0.0264	0.9453	0.1340	0.129*
H3B	−0.0080	0.8545	0.1224	0.129*
H3C	0.0246	0.9019	−0.0166	0.129*
C4	0.2369 (6)	0.8033 (3)	0.0465 (7)	0.108 (2)
H4B	0.3382	0.7906	0.0660	0.163*
H4C	0.2119	0.8090	−0.0569	0.163*
H4D	0.1801	0.7615	0.0824	0.163*
H2	1.172 (4)	0.539 (2)	0.174 (4)	0.052 (11)*
H4	0.853 (5)	0.750 (3)	−0.178 (5)	0.070 (15)*
H7	0.602 (6)	0.765 (3)	0.429 (6)	0.11 (2)*
H9	0.371 (5)	0.503 (3)	0.046 (5)	0.082 (16)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0265 (11)	0.0462 (13)	0.0465 (14)	0.0025 (10)	0.0139 (10)	0.0095 (11)
O2	0.0289 (13)	0.0713 (18)	0.0612 (17)	0.0099 (12)	0.0173 (12)	0.0209 (14)
O3	0.0281 (11)	0.0608 (15)	0.0525 (15)	0.0064 (11)	0.0179 (11)	0.0169 (12)
O4	0.0407 (15)	0.077 (2)	0.0635 (18)	0.0136 (14)	0.0227 (13)	0.0331 (16)
O5	0.0279 (11)	0.0466 (13)	0.0452 (13)	0.0037 (10)	0.0131 (10)	0.0087 (11)
O6	0.0303 (12)	0.0449 (13)	0.0495 (14)	−0.0062 (10)	0.0158 (10)	−0.0124 (11)
O7	0.0434 (17)	0.099 (2)	0.105 (3)	−0.0209 (17)	0.0367 (17)	−0.065 (2)
O8	0.0314 (13)	0.0765 (18)	0.0755 (18)	−0.0136 (12)	0.0258 (12)	−0.0389 (15)
O9	0.0307 (13)	0.0680 (18)	0.0681 (18)	−0.0135 (12)	0.0235 (12)	−0.0296 (15)
O10	0.0261 (11)	0.0414 (13)	0.0458 (14)	−0.0030 (9)	0.0145 (10)	−0.0080 (10)
OW	0.127 (14)	0.149 (16)	0.121 (14)	0.004 (13)	−0.021 (11)	−0.009 (12)
B1	0.0302 (19)	0.041 (2)	0.039 (2)	0.0001 (16)	0.0106 (16)	0.0018 (17)
B2	0.034 (2)	0.041 (2)	0.040 (2)	0.0022 (17)	0.0139 (17)	0.0012 (18)
B3	0.0240 (17)	0.0354 (18)	0.039 (2)	−0.0011 (15)	0.0113 (15)	−0.0023 (17)
B4	0.031 (2)	0.058 (3)	0.054 (3)	−0.0043 (18)	0.0168 (18)	−0.017 (2)
B5	0.0280 (19)	0.047 (2)	0.043 (2)	−0.0017 (17)	0.0097 (16)	−0.0083 (18)
N	0.065 (2)	0.0523 (19)	0.056 (2)	−0.0001 (16)	0.0241 (17)	−0.0082 (16)
C1	0.098 (4)	0.105 (4)	0.061 (3)	0.017 (3)	0.015 (3)	0.012 (3)
C2	0.136 (5)	0.105 (4)	0.088 (4)	−0.044 (4)	0.050 (4)	−0.009 (3)
C3	0.081 (3)	0.089 (4)	0.089 (4)	0.024 (3)	0.015 (3)	−0.010 (3)
C4	0.093 (4)	0.075 (3)	0.161 (6)	0.011 (3)	0.033 (4)	−0.054 (4)

Geometric parameters (Å, º)

O1—B1	1.346 (4)	O10—B5	1.336 (4)
O1—B3	1.449 (4)	O10—B3	1.474 (4)
O2—B1	1.346 (4)	N—C1	1.467 (5)
O2—H2	0.87 (4)	N—C2	1.476 (6)
O3—B2	1.368 (4)	N—C4	1.479 (5)
O3—B1	1.379 (4)	N—C3	1.500 (5)
O4—B2	1.354 (4)	C1—H1A	0.9600
O4—H4	0.79 (5)	C1—H1B	0.9600
O5—B2	1.347 (4)	C1—H1C	0.9600
O5—B3	1.470 (4)	C2—H2B	0.9600
O6—B4	1.342 (4)	C2—H2C	0.9600
O6—B3	1.465 (4)	C2—H2D	0.9600
O7—B4	1.342 (5)	C3—H3A	0.9600
O7—H7	0.79 (6)	C3—H3B	0.9600
O8—B5	1.366 (4)	C3—H3C	0.9600
O8—B4	1.377 (5)	C4—H4B	0.9600
O9—B5	1.363 (4)	C4—H4C	0.9600
O9—H9	0.79 (5)	C4—H4D	0.9600
B1—O1—B3	123.4 (3)	C1—N—C4	112.7 (4)
B1—O2—H2	118 (2)	C2—N—C4	109.9 (4)
B2—O3—B1	118.8 (3)	C1—N—C3	108.0 (3)
B2—O4—H4	115 (3)	C2—N—C3	108.4 (4)
B2—O5—B3	123.3 (3)	C4—N—C3	108.2 (4)
B4—O6—B3	124.4 (3)	N—C1—H1A	109.5
B4—O7—H7	109 (4)	N—C1—H1B	109.5
B5—O8—B4	119.1 (3)	H1A—C1—H1B	109.5
B5—O9—H9	114 (3)	N—C1—H1C	109.5
B5—O10—B3	123.5 (3)	H1A—C1—H1C	109.5
O2—B1—O1	118.5 (3)	H1B—C1—H1C	109.5
O2—B1—O3	119.9 (3)	N—C2—H2B	109.5
O1—B1—O3	121.6 (3)	N—C2—H2C	109.5
O5—B2—O4	122.2 (3)	H2B—C2—H2C	109.5
O5—B2—O3	121.2 (3)	N—C2—H2D	109.5
O4—B2—O3	116.6 (3)	H2B—C2—H2D	109.5
O1—B3—O6	109.7 (3)	H2C—C2—H2D	109.5
O1—B3—O5	110.9 (2)	N—C3—H3A	109.5
O6—B3—O5	108.7 (3)	N—C3—H3B	109.5
O1—B3—O10	109.5 (3)	H3A—C3—H3B	109.5
O6—B3—O10	110.1 (2)	N—C3—H3C	109.5
O5—B3—O10	108.0 (3)	H3A—C3—H3C	109.5
O6—B4—O7	122.9 (3)	H3B—C3—H3C	109.5
O6—B4—O8	120.7 (3)	N—C4—H4B	109.5
O7—B4—O8	116.3 (3)	N—C4—H4C	109.5
O10—B5—O9	122.2 (3)	H4B—C4—H4C	109.5
O10—B5—O8	122.1 (3)	N—C4—H4D	109.5
O9—B5—O8	115.8 (3)	H4B—C4—H4D	109.5
C1—N—C2	109.6 (4)	H4C—C4—H4D	109.5

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O2—H2···O9^i	0.87 (4)	1.88 (4)	2.745 (3)	174 (4)
O4—H4···O6^ii	0.79 (5)	1.94 (5)	2.733 (4)	173 (5)
O7—H7···O5^iii	0.79 (6)	1.89 (6)	2.671 (4)	171 (6)
O9—H9···O10^iv	0.79 (5)	1.90 (5)	2.688 (3)	176 (5)

Symmetry codes: (i) x+1, y, z; (ii) x, −y+3/2, z−1/2; (iii) x, −y+3/2, z+1/2; (iv) −x+1, −y+1, −z.