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.


Comment
The crystal structure of the title compound was previously studied by Morimoto (1956)  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) (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)

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.
supplementary materials sup-2 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. Fig. 1

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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.