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

Gainsford, G.J.; Kemmitt, T.; Higham, C.

doi:10.1107/S1600536808010441

inorganic compounds

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Volume 64| Part 5| May 2008| Pages i24-i25

doi:10.1107/S1600536808010441

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

Graeme J. Gainsford,^a ^* Tim Kemmitt ^a and Caleb Higham ^a

^aIndustrial Research Limited, PO Box 31-310, Lower Hutt, New Zealand
^*Correspondence e-mail: g.gainsford@irl.cri.nz

(Received 26 March 2008; accepted 15 April 2008; online 23 April 2008)

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.

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 ).

Experimental

Crystal data

Na₂[B₄O₅(OH)₄]·8H₂O
M_r = 381.38
Monoclinic, C 2/c
a = 11.8843 (5) Å
b = 10.6026 (4) Å
c = 12.2111 (5) Å
β = 106.790 (2)°
V = 1473.06 (10) Å³
Z = 4
Mo Kα radiation
μ = 0.22 mm⁻¹
T = 145 (2) K
0.65 × 0.36 × 0.26 mm

Data collection

Bruker–Nonius APEX2 CCD area-detector diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2006 ) T_min = 0.813, T_max = 0.94
8429 measured reflections
2275 independent reflections
2137 reflections with I > 2σ(I)
R_int = 0.018

Refinement

R[F² > 2σ(F²)] = 0.025
wR(F²) = 0.076
S = 1.08
2275 reflections
147 parameters
All H-atom parameters refined
Δρ_max = 0.37 e Å⁻³
Δρ_min = −0.22 e Å⁻³

Table 1
Selected bond lengths (Å)

Na1—O8ⁱ	2.3815 (6)
Na1—O6ⁱⁱ	2.3979 (5)
Na1—O7	2.4121 (6)
Na2—O7ⁱⁱⁱ	2.4041 (6)
Na2—O9	2.4214 (6)
Na2—O6	2.4441 (6)
B1—O4	1.4451 (8)
B1—O1	1.4657 (7)
B1—O2	1.4902 (8)
B1—O3	1.5075 (8)
B2—O2	1.3655 (8)
B2—O3ⁱ	1.3757 (8)
B2—O5	1.3784 (8)
Symmetry codes: (i) $[-x, y, -z+{\script{1\over 2}}]$ ; (ii) $[-x, y-1, -z+{\script{1\over 2}}]$ ; (iii) $[-x, y+1, -z+{\script{1\over 2}}]$ .

Table 2
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O5—H5^iv⋯O3^iv	0.836 (15)	1.895 (15)	2.7300 (7)	176.3 (15)
O4^v—H4⋯O9^vi	0.828 (14)	2.049 (14)	2.8658 (8)	168.4 (12)
O6—H6A^vii⋯O5^viii	0.868 (16)	1.978 (16)	2.8323 (8)	167.9 (14)
O6^ix—H6B⋯O4^v	0.846 (16)	2.040 (15)	2.8624 (8)	163.9 (16)
O7^vii—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^v—H8A⋯O1^x	0.866 (13)	1.936 (13)	2.7865 (6)	167.0 (14)
O8^v—H8B⋯O5^xi	0.855 (15)	2.341 (14)	3.1320 (8)	154.2 (12)
O9—H9A^vii⋯O3	0.843 (16)	2.253 (16)	3.0894 (8)	171.7 (15)
O9—H9B^xii⋯O8^xiii	0.849 (17)	2.069 (16)	2.9034 (8)	167.4 (15)
Symmetry codes: (iv) $[x, -y+1, z+{\script{1\over 2}}]$ ; (v) $[-x+1, y, -z+{\script{1\over 2}}]$ ; (vi) $[x+{\script{1\over 2}}, y-{\script{1\over 2}}, z]$ ; (vii) $[-x+{\script{1\over 2}}, y+{\script{1\over 2}}, -z+{\script{1\over 2}}]$ ; (viii) $[-x+{\script{1\over 2}}, -y+{\script{3\over 2}}, -z+1]$ ; (ix) -x+1, -y+1, -z+1; (x) x+1, y, z; (xi) $[x+{\script{1\over 2}}, -y+{\script{1\over 2}}, z-{\script{1\over 2}}]$ ; (xii) $[x-{\script{1\over 2}}, y+{\script{1\over 2}}, z]$ ; (xiii) $[x, -y+1, z-{\script{1\over 2}}]$ .

Data collection: APEX2 (Bruker, 2006); cell refinement: APEX2; data reduction: APEX2; 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 and PLATON (Spek, 2003 ).

Supporting information

Crystal structure: contains datablocks global, I. DOI: 10.1107/S1600536808010441/wm2174sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S1600536808010441/wm2174Isup2.hkl

checkCIF report

Comment top

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 top

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).

Experimental top

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.

Computing details top

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 top

	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 top

Crystal data top

Na₂[B₄O₅(OH)₄]·8H₂O	F(000) = 792
M_r = 381.38	D_x = 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) Å³	0.65 × 0.36 × 0.26 mm
Z = 4

Data collection top

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

Refinement top

Refinement on F²	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F² > 2σ(F²)] = 0.025	Hydrogen site location: inferred from neighbouring sites
wR(F²) = 0.076	All H-atom parameters refined
S = 1.08	w = 1/[σ²(F_o²) + (0.0482P)² + 0.3901P] where P = (F_o² + 2F_c²)/3
2275 reflections	(Δ/σ)_max = 0.001
147 parameters	Δρ_max = 0.37 e Å⁻³
0 restraints	Δρ_min = −0.22 e Å⁻³

Crystal data top

Na₂[B₄O₅(OH)₄]·8H₂O	V = 1473.06 (10) Å³
M_r = 381.38	Z = 4
Monoclinic, C2/c	Mo Kα radiation
a = 11.8843 (5) Å	µ = 0.22 mm⁻¹
b = 10.6026 (4) Å	T = 145 K
c = 12.2111 (5) Å	0.65 × 0.36 × 0.26 mm
β = 106.790 (2)°

Data collection top

Bruker–Nonius APEX2 CCD area-detector diffractometer	2275 independent reflections
Absorption correction: multi-scan (SADABS; Bruker, 2006)	2137 reflections with I > 2σ(I)
T_min = 0.813, T_max = 0.94	R_int = 0.018
8429 measured reflections

Refinement top

R[F² > 2σ(F²)] = 0.025	0 restraints
wR(F²) = 0.076	All H-atom parameters refined
S = 1.08	Δρ_max = 0.37 e Å⁻³
2275 reflections	Δρ_min = −0.22 e Å⁻³
147 parameters

Special details top

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 (Å²) top

	x	y	z	U_iso*/U_eq
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 (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
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) top

Na1—O8ⁱ	2.3815 (6)	Na2—O6ⁱⁱ	2.4441 (6)
Na1—O8ⁱⁱ	2.3815 (6)	Na2—O6	2.4441 (6)
Na1—O6ⁱⁱⁱ	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—O3ⁱⁱ	1.3757 (8)
Na2—O9	2.4214 (6)	B2—O5	1.3784 (8)
Na2—O9ⁱⁱ	2.4214 (6)

O8ⁱ—Na1—O8ⁱⁱ	180.00 (2)	O9—Na2—O6ⁱⁱ	81.696 (19)
O8ⁱ—Na1—O6ⁱⁱⁱ	90.45 (2)	O9ⁱⁱ—Na2—O6ⁱⁱ	97.72 (2)
O8ⁱⁱ—Na1—O6ⁱⁱⁱ	89.55 (2)	O7^vi—Na2—O6	88.29 (2)
O8ⁱ—Na1—O6^iv	89.55 (2)	O7^vii—Na2—O6	92.34 (2)
O8ⁱⁱ—Na1—O6^iv	90.45 (2)	O9—Na2—O6	97.72 (2)
O6ⁱⁱⁱ—Na1—O6^iv	180.00 (3)	O9ⁱⁱ—Na2—O6	81.70 (2)
O8ⁱ—Na1—O7^v	91.717 (19)	O6ⁱⁱ—Na2—O6	179.07 (3)
O8ⁱⁱ—Na1—O7^v	88.283 (19)	O4—B1—O1	111.72 (5)
O6ⁱⁱⁱ—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)
O8ⁱ—Na1—O7	88.283 (19)	O4—B1—O3	107.71 (5)
O8ⁱⁱ—Na1—O7	91.717 (19)	O1—B1—O3	108.56 (5)
O6ⁱⁱⁱ—Na1—O7	90.823 (19)	O2—B1—O3	108.43 (5)
O6^iv—Na1—O7	89.177 (19)	O2—B2—O3ⁱⁱ	122.44 (6)
O7^v—Na1—O7	180.000 (16)	O2—B2—O5	117.78 (6)
O7^vi—Na2—O7^vii	94.98 (3)	O3ⁱⁱ—B2—O5	119.78 (6)
O7^vi—Na2—O9	172.90 (2)	B1ⁱⁱ—O1—B1	110.90 (6)
O7^vii—Na2—O9	81.05 (2)	B2—O2—B1	116.59 (5)
O7^vi—Na2—O9ⁱⁱ	81.05 (2)	B2ⁱⁱ—O3—B1	120.25 (5)
O7^vii—Na2—O9ⁱⁱ	172.90 (2)	Na1^vi—O6—Na2	90.952 (19)
O9—Na2—O9ⁱⁱ	103.49 (3)	Na2^viii—O7—Na1	91.581 (19)
O7^vi—Na2—O6ⁱⁱ	92.34 (2)	Na2^viii—O7—H7B	134.8 (10)
O7^vii—Na2—O6ⁱⁱ	88.29 (2)	Na1—O7—H7B	96.4 (10)

O4—B1—O1—B1ⁱⁱ	−172.74 (6)	O7^vi—Na2—O6—Na1^vi	−0.355 (19)
O2—B1—O1—B1ⁱⁱ	64.05 (4)	O7^vii—Na2—O6—Na1^vi	94.56 (2)
O3—B1—O1—B1ⁱⁱ	−54.12 (3)	O9—Na2—O6—Na1^vi	175.84 (2)
O3ⁱⁱ—B2—O2—B1	−4.91 (9)	O9ⁱⁱ—Na2—O6—Na1^vi	−81.56 (2)
O5—B2—O2—B1	174.23 (5)	O6ⁱⁱ—Na2—O6—Na1^vi	−132.911 (15)
O4—B1—O2—B2	−156.88 (5)	O8ⁱ—Na1—O7—Na2^viii	−89.93 (2)
O1—B1—O2—B2	−33.19 (7)	O8ⁱⁱ—Na1—O7—Na2^viii	90.07 (2)
O3—B1—O2—B2	85.06 (6)	O6ⁱⁱⁱ—Na1—O7—Na2^viii	179.640 (19)
O4—B1—O3—B2ⁱⁱ	136.95 (6)	O6^iv—Na1—O7—Na2^viii	−0.360 (19)
O1—B1—O3—B2ⁱⁱ	15.82 (7)	O7^v—Na1—O7—Na2^viii	−122 (44)
O2—B1—O3—B2ⁱⁱ	−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) top

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···O8ⁱⁱⁱ	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.

Experimental details

Crystal data
Chemical formula	Na₂[B₄O₅(OH)₄]·8H₂O
M_r	381.38
Crystal system, space group	Monoclinic, C2/c
Temperature (K)	145
a, b, c (Å)	11.8843 (5), 10.6026 (4), 12.2111 (5)
β (°)	106.790 (2)
V (Å³)	1473.06 (10)
Z	4
Radiation type	Mo Kα
µ (mm⁻¹)	0.22
Crystal size (mm)	0.65 × 0.36 × 0.26

Data collection
Diffractometer	Bruker–Nonius APEX2 CCD area-detector diffractometer
Absorption correction	Multi-scan (SADABS; Bruker, 2006)
T_min, T_max	0.813, 0.94
No. of measured, independent and observed [I > 2σ(I)] reflections	8429, 2275, 2137
R_int	0.018
(sin θ/λ)_max (Å⁻¹)	0.770

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.025, 0.076, 1.08
No. of reflections	2275
No. of parameters	147
H-atom treatment	All H-atom parameters refined
Δρ_max, Δρ_min (e Å⁻³)	0.37, −0.22

Computer programs: APEX2 (Bruker, 2006), SHELXS97 (Sheldrick, 2008), ORTEP-3 (Farrugia, 1997) and Mercury (Macrae et al., 2006), SHELXL97 (Sheldrick, 2008) and PLATON (Spek, 2003).

Selected bond lengths (Å) top

Na1—O8ⁱ	2.3815 (6)	B1—O1	1.4657 (7)
Na1—O6ⁱⁱ	2.3979 (5)	B1—O2	1.4902 (8)
Na1—O7	2.4121 (6)	B1—O3	1.5075 (8)
Na2—O7ⁱⁱⁱ	2.4041 (6)	B2—O2	1.3655 (8)
Na2—O9	2.4214 (6)	B2—O3ⁱ	1.3757 (8)
Na2—O6	2.4441 (6)	B2—O5	1.3784 (8)
B1—O4	1.4451 (8)

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

Hydrogen-bond geometry (Å, º) top

D—H···A	D—H	H···A	D···A	D—H···A
O5—H5^iv···O3^iv	0.836 (15)	1.895 (15)	2.7300 (7)	176.3 (15)
O4^v—H4···O9^vi	0.828 (14)	2.049 (14)	2.8658 (8)	168.4 (12)
O6—H6A^vii···O5^viii	0.868 (16)	1.978 (16)	2.8323 (8)	167.9 (14)
O6^ix—H6B···O4^v	0.846 (16)	2.040 (15)	2.8624 (8)	163.9 (16)
O7^vii—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^v—H8A···O1^x	0.866 (13)	1.936 (13)	2.7865 (6)	167.0 (14)
O8^v—H8B···O5^xi	0.855 (15)	2.341 (14)	3.1320 (8)	154.2 (12)
O9—H9A^vii···O3	0.843 (16)	2.253 (16)	3.0894 (8)	171.7 (15)
O9—H9B^xii···O8^xiii	0.849 (17)	2.069 (16)	2.9034 (8)	167.4 (15)

Symmetry codes: (iv) x, −y+1, z+1/2; (v) −x+1, y, −z+1/2; (vi) x+1/2, y−1/2, z; (vii) −x+1/2, y+1/2, −z+1/2; (viii) −x+1/2, −y+3/2, −z+1; (ix) −x+1, −y+1, −z+1; (x) x+1, y, z; (xi) x+1/2, −y+1/2, z−1/2; (xii) x−1/2, y+1/2, z; (xiii) x, −y+1, z−1/2.

Acknowledgements

We thank Dr J. Wikaira of the University of Canterbury, New Zealand, for her assistance with the data collection.

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