Redetermination of the borax structure from laboratory X-ray data at 145 K

The title compound, sodium tetraborate decahydrate (mineral name: borax), Na₂[B₄O₅(OH)₄]·8H₂O, has been studied previously using X-ray [Morimoto (1956). Miner. J. 2, 1-18] and neutron [Levy & Lisensky (1978). Acta Cryst. B34, 3502-3510] diffraction data. The structure contains tetra­borate anions [B₄O₅(OH)₄]^2- with twofold rotation symmetry, which form hydrogen-bonded chains, and [Na(H₂O)₆] octa­hedra that form zigzag chains [Na(H₂O)_4/2(H₂O)_2/1]. The O-H bond distances obtained from the present redetermination at 145 K are shorter than those in the neutron study by an average of 0.127 (19) Å.

Key indicators

• Single-crystal X-ray study
• T = 145 K
• Mean [sigma](O-B) = 0.001 Å
• R factor = 0.025
• wR factor = 0.077
• Data-to-parameter ratio = 15.5

checkCIF/PLATON results

No syntax errors found



Alert level A
REFLT03_ALERT_3_A  Reflection count < 85% complete (theta max?)
           From the CIF: _diffrn_reflns_theta_max           33.16
           From the CIF: _diffrn_reflns_theta_full          33.16
           From the CIF: _reflns_number_total               2275
           TEST2: Reflns within _diffrn_reflns_theta_max
           Count of symmetry unique reflns         2812
           Completeness (_total/calc)             80.90%

Author Response: Data is essentially complete in the shell to 2theta 50 degrees (13 reflections missing as noted in experimental).

PLAT029_ALERT_3_A _diffrn_measured_fraction_theta_full Low .......       0.81

Author Response: More than sufficient data was measured but only an incomplete shell to theta 33.2 degrees.

PLAT411_ALERT_2_A Short Inter H...H Contact  H9A    ..  H9B     ..       1.33 Ang.

Author Response: Fault of Platon due to choice of the asymmetric unit

PLAT411_ALERT_2_A Short Inter H...H Contact  H6A    ..  H6B     ..       1.35 Ang.

Author Response: Fault of Platon due to choice of the asymmetric unit

Alert level B
PLAT430_ALERT_2_B Short Inter D...A Contact  O1     ..  O8      ..       2.79 Ang.
PLAT430_ALERT_2_B Short Inter D...A Contact  O1     ..  O8      ..       2.79 Ang.
PLAT430_ALERT_2_B Short Inter D...A Contact  O5     ..  O6      ..       2.83 Ang.

Alert level C
PLAT042_ALERT_1_C Calc. and Rep. MoietyFormula Strings Differ ....          ?
PLAT063_ALERT_3_C Crystal Probably too Large for Beam Size .......       0.65 mm
PLAT430_ALERT_2_C Short Inter D...A Contact  O4     ..  O6      ..       2.86 Ang.
PLAT430_ALERT_2_C Short Inter D...A Contact  O4     ..  O9      ..       2.87 Ang.
PLAT710_ALERT_4_C Delete 1-2-3 or 2-3-4 Linear Torsion Angle ... #         16
            O6  -NA2 -O6  -NA1  -132.91  0.01   2.555   1.555   1.555   2.565
PLAT710_ALERT_4_C Delete 1-2-3 or 2-3-4 Linear Torsion Angle ... #         21
            O7  -NA1 -O7  -NA2    12.00  0.00   5.555   1.555   1.555   1.545

   4 ALERT level A = In general: serious problem
   3 ALERT level B = Potentially serious problem
   6 ALERT level C = Check and explain
   0 ALERT level G = General alerts; check

   1 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   7 ALERT type 2 Indicator that the structure model may be wrong or deficient
   3 ALERT type 3 Indicator that the structure quality may be low
   2 ALERT type 4 Improvement, methodology, query or suggestion
   0 ALERT type 5 Informative message, check

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S1. Comment

The crystal structure of the title compound was previously studied by Morimoto (1956) using X-ray and later by Levy & Lisensky (1978, hereafter LL) using neutron diffraction data.

There are 8 other compounds with free tetraborate anions [B₄O₅(OH)₄]^2- reported in the Cambridge Structural Database [C.S.D., version 5.29 with November 2007 updates (Allen, 2002)] with most containing protonated amine-based cations, e.g. DALQEN (Wang et al., 2004) and SIBDIR (Pan et al., 2007). The tetraborate anion in borax has 2-fold symmetry with the axis passing through O1 (Fig. 1) as is observed in five of the related structures. Both Na1 and Na2 cations are on special positions (centre of symmetry and 2-fold axis, respectively) so that they elegantly bind via shared water molecules in a typical zigzag cationic chain [Na(H₂O)_4/2(H₂O)_2/1] parallel to the c axis (e.g. DARNOA, Yi et al., 2005), as shown in Figure 2. As is found through a C.S.D. search of similar Na⁺/H₂O cation chains, the Na–O distances to the bridging water molecules are longer than those to non-bridging water molecules, where the trans related Na–O distances belong to non-bridging water molecules.

The results of the present study and the LL model are essentially superimposable, but do reflect expected differences associated with the H atom positions: The systematic pairwise study (Allen, 1986) gave a difference for O–H (X-ray versus neutron) of -0.155 (10) Å, while a more recent study of levoglucosan (Smrčok et al., 2006) averaged at -0.016 (6) Å. The mean O—H distance here (0.843 (17) Å) is significantly shorter than for the neutron set (0.97 (1) Å). As the O···O distances involved in the hydrogen bonding are very similar for both studies (Table 1), the observed H···O distances are correspondingly longer here than in the LL model. We also note that average Na–O distances are marginally longer (0.006 (6) Å) and the B–O distances marginally shorter (-0.005 (2) Å) in the LL model, e.g. Na–O6, B1–O2 are 2.458 (3), 1.500 (2) Å compared with 2.4441 (6), 1.5075 (8) Å, respectively, in the present study. These latter differences are barely significant given that the neutron data set was collected at 296.5 K.

Cell cohesion is provided by strong O—H···O hydrogen bonds of two types: (1) tetraborate anions "head to tail" link via the O5–H and O2 atoms (entry 1, Table 1) to form anionic chains as also seen in DALQEN (Wang et al., 2004); (2) the anionic and cationic chains crosslink through the water & tetraborate strong O–H···O hydrogen bond interactions (entries 2–10; see also Fig. 2 and diagrams in the LL study).

Related literature

For previous studies of the borax structure, see: Morimoto (1956); Levy & Lisenky (1978). For other structures listed in the Cambridge Structural Database (Allen, 2002) that contain the [B₄O₅(OH)₄]^2- anion, see: Wang et al. (2004); Pan et al. (2007). For related structures, see: Yi et al. (2005). For comparative studies of hydrogen bonds obtained from X-ray and neutron data, see: Allen (1986); Smrčok et al. (2006).

S2. Experimental

To a tetrahydrofuran (thf) solution (90 ml) of sodium tetrahydridoborate (0.31 g, 8.4 mmol) was added 0.5 g (4.2 mmol) of diaminomethane dihydrochloride. After 24 h, the solvent was removed and the remaining product dissolved in water. Methanol was added and the solution was left in a refrigerator. A small clump of colourless crystals of the title compound appeared after several days in the bottom of the flask.

S3. Refinement

A total of 13 reflections (below 50°/2θ) were not collected. In the present re-determination the same atomic labels and atomic coordinates have been used as in the previous studies (Morimoto, 1956; Levy & Lisensky, 1978). The positions of the H atoms were fully refined with isotropic thermal parameters for each H atom.

Computing details

Data collection: APEX2 (Bruker, 2006); cell refinement: APEX2 (Bruker, 2006); data reduction: APEX2 (Bruker, 2006); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997) and Mercury (Macrae et al., 2006); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and PLATON (Spek, 2003).

Figures

	Fig. 1. Molecular structure of the tetraborate anion shown with displacement ellipsoids at the 50% probability level. [Symmetry code: i) -x, y, 1/2 - z.]
	Fig. 2. Part of the crystal structure showing the zigzag [Na(H2O)4/2(H2O)2/1] chain, the hydrogen bonded tetraborate chain and some interlinking hydrogen bonds, shown as dashed lines. For clarity, only selected atoms and one of each chain is shown. [Symmetry codes: i) -x, y 1/2 - z; ii) x, 1 + y, z.]

sodium tetraborate decahydrate

Crystal data

Na2[B4O5(OH)4]·8H2O	F(000) = 792
Mr = 381.38	Dx = 1.720 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 6688 reflections
a = 11.8843 (5) Å	θ = 2.6–31.9°
b = 10.6026 (4) Å	µ = 0.22 mm^−1
c = 12.2111 (5) Å	T = 145 K
β = 106.790 (2)°	Prism, colourless
V = 1473.06 (10) Å^3	0.65 × 0.36 × 0.26 mm
Z = 4

Data collection

Bruker–Nonius APEX2 CCD area-detector diffractometer	2275 independent reflections
Radiation source: fine-focus sealed tube	2137 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.018
Detector resolution: 8.192 pixels mm^-1	θmax = 33.2°, θmin = 2.6°
ϕ and ω scans	h = −17→16
Absorption correction: multi-scan (SADABS; Bruker, 2006)	k = −15→15
Tmin = 0.813, Tmax = 0.94	l = −16→17
8429 measured reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.025	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.076	All H-atom parameters refined
S = 1.08	w = 1/[σ^2(Fo^2) + (0.0482P)^2 + 0.3901P] where P = (Fo^2 + 2Fc^2)/3
2275 reflections	(Δ/σ)max = 0.001
147 parameters	Δρmax = 0.37 e Å^−3
0 restraints	Δρmin = −0.22 e Å^−3

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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
Na1	0.0000	0.0000	0.0000	0.01676 (10)
Na2	0.0000	0.84795 (4)	0.2500	0.01796 (11)
B1	0.08552 (6)	0.34499 (6)	0.21553 (5)	0.01044 (13)
B2	0.09847 (6)	0.45643 (6)	0.39269 (6)	0.01154 (13)
O1	0.0000	0.26659 (6)	0.2500	0.01094 (13)
O2	0.15546 (4)	0.41927 (4)	0.31574 (4)	0.01276 (11)
O3	0.01964 (4)	0.43573 (4)	0.12445 (4)	0.01339 (11)
O4	0.16140 (4)	0.27014 (5)	0.16772 (4)	0.01570 (11)
O5	0.16369 (4)	0.51522 (5)	0.49130 (4)	0.01815 (12)
O6	0.12357 (5)	0.84607 (5)	0.44846 (5)	0.01851 (12)
O7	0.12296 (5)	0.00117 (5)	0.19548 (5)	0.01799 (12)
O8	0.11919 (5)	0.16556 (5)	0.46252 (5)	0.02067 (12)
O9	0.11746 (5)	0.70654 (6)	0.17227 (5)	0.02187 (12)
H4	0.7717 (12)	0.2622 (11)	0.2876 (12)	0.032 (3)*
H5	0.1187 (13)	0.4667 (13)	0.0305 (12)	0.040 (3)*
H6A	0.3089 (13)	0.3828 (14)	0.0413 (12)	0.042 (4)*
H6B	0.8662 (14)	0.2018 (16)	0.4941 (13)	0.051 (4)*
H7A	0.3098 (13)	0.4817 (11)	0.3051 (12)	0.030 (3)*
H7B	0.1304 (12)	0.0776 (14)	0.2014 (12)	0.039 (3)*
H8A	0.9099 (12)	0.1906 (13)	0.1075 (11)	0.036 (3)*
H8B	0.8131 (12)	0.1365 (12)	0.0352 (11)	0.034 (3)*
H9A	0.4018 (13)	0.1300 (15)	0.3385 (12)	0.046 (4)*
H9B	0.6140 (15)	0.2331 (15)	0.1058 (14)	0.053 (4)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Na1	0.0165 (2)	0.01812 (19)	0.0158 (2)	0.00057 (13)	0.00485 (15)	0.00047 (13)
Na2	0.0179 (2)	0.0201 (2)	0.0176 (2)	0.000	0.00773 (16)	0.000
B1	0.0100 (3)	0.0121 (3)	0.0096 (3)	0.0012 (2)	0.0035 (2)	−0.00028 (19)
B2	0.0108 (3)	0.0133 (3)	0.0111 (3)	−0.0014 (2)	0.0040 (2)	−0.0014 (2)
O1	0.0117 (3)	0.0104 (3)	0.0111 (3)	0.000	0.0037 (2)	0.000
O2	0.0103 (2)	0.0168 (2)	0.0122 (2)	−0.00223 (15)	0.00493 (16)	−0.00376 (15)
O3	0.0103 (2)	0.0180 (2)	0.0130 (2)	0.00309 (15)	0.00519 (16)	0.00559 (15)
O4	0.0119 (2)	0.0209 (2)	0.0142 (2)	0.00489 (17)	0.00365 (17)	−0.00434 (16)
O5	0.0132 (2)	0.0275 (3)	0.0150 (2)	−0.00595 (18)	0.00593 (19)	−0.00949 (18)
O6	0.0146 (2)	0.0219 (2)	0.0186 (2)	−0.00109 (18)	0.00414 (19)	0.00229 (18)
O7	0.0151 (3)	0.0160 (2)	0.0232 (3)	0.00012 (17)	0.0061 (2)	0.00003 (17)
O8	0.0185 (3)	0.0242 (3)	0.0184 (3)	0.00116 (19)	0.0039 (2)	0.00608 (19)
O9	0.0217 (3)	0.0196 (2)	0.0229 (3)	−0.00047 (19)	0.0041 (2)	−0.00352 (19)

Geometric parameters (Å, º)

Na1—O8^i	2.3815 (6)	Na2—O6^ii	2.4441 (6)
Na1—O8^ii	2.3815 (6)	Na2—O6	2.4441 (6)
Na1—O6^iii	2.3979 (5)	B1—O4	1.4451 (8)
Na1—O6^iv	2.3979 (5)	B1—O1	1.4657 (7)
Na1—O7^v	2.4121 (6)	B1—O2	1.4902 (8)
Na1—O7	2.4121 (6)	B1—O3	1.5075 (8)
Na2—O7^vi	2.4041 (6)	B2—O2	1.3655 (8)
Na2—O7^vii	2.4041 (6)	B2—O3^ii	1.3757 (8)
Na2—O9	2.4214 (6)	B2—O5	1.3784 (8)
Na2—O9^ii	2.4214 (6)
O8^i—Na1—O8^ii	180.00 (2)	O9—Na2—O6^ii	81.696 (19)
O8^i—Na1—O6^iii	90.45 (2)	O9^ii—Na2—O6^ii	97.72 (2)
O8^ii—Na1—O6^iii	89.55 (2)	O7^vi—Na2—O6	88.29 (2)
O8^i—Na1—O6^iv	89.55 (2)	O7^vii—Na2—O6	92.34 (2)
O8^ii—Na1—O6^iv	90.45 (2)	O9—Na2—O6	97.72 (2)
O6^iii—Na1—O6^iv	180.00 (3)	O9^ii—Na2—O6	81.70 (2)
O8^i—Na1—O7^v	91.717 (19)	O6^ii—Na2—O6	179.07 (3)
O8^ii—Na1—O7^v	88.283 (19)	O4—B1—O1	111.72 (5)
O6^iii—Na1—O7^v	89.177 (19)	O4—B1—O2	110.91 (5)
O6^iv—Na1—O7^v	90.823 (19)	O1—B1—O2	109.42 (4)
O8^i—Na1—O7	88.283 (19)	O4—B1—O3	107.71 (5)
O8^ii—Na1—O7	91.717 (19)	O1—B1—O3	108.56 (5)
O6^iii—Na1—O7	90.823 (19)	O2—B1—O3	108.43 (5)
O6^iv—Na1—O7	89.177 (19)	O2—B2—O3^ii	122.44 (6)
O7^v—Na1—O7	180.000 (16)	O2—B2—O5	117.78 (6)
O7^vi—Na2—O7^vii	94.98 (3)	O3^ii—B2—O5	119.78 (6)
O7^vi—Na2—O9	172.90 (2)	B1^ii—O1—B1	110.90 (6)
O7^vii—Na2—O9	81.05 (2)	B2—O2—B1	116.59 (5)
O7^vi—Na2—O9^ii	81.05 (2)	B2^ii—O3—B1	120.25 (5)
O7^vii—Na2—O9^ii	172.90 (2)	Na1^vi—O6—Na2	90.952 (19)
O9—Na2—O9^ii	103.49 (3)	Na2^viii—O7—Na1	91.581 (19)
O7^vi—Na2—O6^ii	92.34 (2)	Na2^viii—O7—H7B	134.8 (10)
O7^vii—Na2—O6^ii	88.29 (2)	Na1—O7—H7B	96.4 (10)
O4—B1—O1—B1^ii	−172.74 (6)	O7^vi—Na2—O6—Na1^vi	−0.355 (19)
O2—B1—O1—B1^ii	64.05 (4)	O7^vii—Na2—O6—Na1^vi	94.56 (2)
O3—B1—O1—B1^ii	−54.12 (3)	O9—Na2—O6—Na1^vi	175.84 (2)
O3^ii—B2—O2—B1	−4.91 (9)	O9^ii—Na2—O6—Na1^vi	−81.56 (2)
O5—B2—O2—B1	174.23 (5)	O6^ii—Na2—O6—Na1^vi	−132.911 (15)
O4—B1—O2—B2	−156.88 (5)	O8^i—Na1—O7—Na2^viii	−89.93 (2)
O1—B1—O2—B2	−33.19 (7)	O8^ii—Na1—O7—Na2^viii	90.07 (2)
O3—B1—O2—B2	85.06 (6)	O6^iii—Na1—O7—Na2^viii	179.640 (19)
O4—B1—O3—B2^ii	136.95 (6)	O6^iv—Na1—O7—Na2^viii	−0.360 (19)
O1—B1—O3—B2^ii	15.82 (7)	O7^v—Na1—O7—Na2^viii	−122 (44)
O2—B1—O3—B2^ii	−102.97 (6)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O5—H5^ix···O3^ix	0.836 (15)	1.895 (15)	2.7300 (7)	176.3 (15)
O4^x—H4···O9^xi	0.828 (14)	2.049 (14)	2.8658 (8)	168.4 (12)
O6—H6A^xii···O5^xiii	0.868 (16)	1.978 (16)	2.8323 (8)	167.9 (14)
O6^xiv—H6B···O4^x	0.846 (16)	2.040 (15)	2.8624 (8)	163.9 (16)
O7^xii—H7A···O2	0.827 (16)	1.989 (16)	2.8135 (8)	174.1 (12)
O7—H7B···O4	0.816 (15)	2.135 (15)	2.9233 (8)	162.3 (14)
O8^x—H8A···O1^xv	0.866 (13)	1.936 (13)	2.7865 (6)	167.0 (14)
O8^x—H8B···O5^xvi	0.855 (15)	2.341 (14)	3.1320 (8)	154.2 (12)
O9—H9A^xii···O3	0.843 (16)	2.253 (16)	3.0894 (8)	171.7 (15)
O9—H9B^xvii···O8^iii	0.849 (17)	2.069 (16)	2.9034 (8)	167.4 (15)

Symmetry codes: (iii) x, −y+1, z−1/2; (ix) x, −y+1, z+1/2; (x) −x+1, y, −z+1/2; (xi) x+1/2, y−1/2, z; (xii) −x+1/2, y+1/2, −z+1/2; (xiii) −x+1/2, −y+3/2, −z+1; (xiv) −x+1, −y+1, −z+1; (xv) x+1, y, z; (xvi) x+1/2, −y+1/2, z−1/2; (xvii) x−1/2, y+1/2, z.