Redetermination of Ce[B5O8(OH)(H2O)]NO3·2H2O

The crystal structure of Ce[B5O8(OH)(H2O)]NO3·2H2O, cerium(III) aquahydroxidooctaoxidopentaborate nitrate dihydrate, has been redetermined from single-crystal X-ray diffraction data. In contrast to the previous determination [Li et al. (2003 ▶). Chem. Mater. 15, 2253–2260], the present study reveals the location of all H atoms, slightly different fundamental building blocks (FBBs) of the polyborate anions, more reasonable displacement ellipsoids for all non-H atoms, as well as a model without disorder of the nitrate anion. The crystal structure is built from corrugated polyborate layers parallel to (010). These layers, consisting of [B5O8(OH)(H2O)]2− anions as FBBs, stack along [010] and are linked by Ce3+ ions, which exhibit a distorted CeO10 coordination sphere. The layers are additionally stabilized via O—H⋯O hydrogen bonds between water molecules and nitrate anions, located at the interlayer space. The [BO3(H2O)]-group shows a [3 + 1] coordination and is considerably distorted from a tetrahedral configuration. Bond-valence-sum calculation shows that the valence sum of boron is only 2.63 valence units (v.u.) when the contribution of the water molecule (0.49 v.u.) is neglected.

The crystal structure of Ce[B 5 O 8 (OH)(H 2 O)]NO 3 Á2H 2 O, cerium(III) aquahydroxidooctaoxidopentaborate nitrate dihydrate, has been redetermined from single-crystal X-ray diffraction data. In contrast to the previous determination [Li et al. (2003). Chem. Mater. 15, 2253-2260, the present study reveals the location of all H atoms, slightly different fundamental building blocks (FBBs) of the polyborate anions, more reasonable displacement ellipsoids for all non-H atoms, as well as a model without disorder of the nitrate anion. The crystal structure is built from corrugated polyborate layers parallel to (010). These layers, consisting of [B 5 O 8 (OH)-(H 2 O)] 2À anions as FBBs, stack along [010] and are linked by Ce 3+ ions, which exhibit a distorted CeO 10 coordination sphere. The layers are additionally stabilized via O-HÁ Á ÁO hydrogen bonds between water molecules and nitrate anions, located at the interlayer space. The [BO 3 (H 2 O)]-group shows a [3 + 1] coordination and is considerably distorted from a tetrahedral configuration. Bond-valence-sum calculation shows that the valence sum of boron is only 2.63 valence units (v.u.) when the contribution of the water molecule (0.49 v.u.) is neglected.

Related literature
For a previous structural study of the title compound, see: Li et al. (2003). For the La analogue, see: Li et al. (2005). For the bond-valence method, see: Brown (2002). For related structures, see: Sun et al. (2010Sun et al. ( , 2012. For a review on geometrical parameters of borate groups, see: Zobetz (1982Zobetz ( , 1990. FBBs in borate crystal chemistry were reviewed by Burns et al. (1995).  Table 1 Selected geometric parameters (Å , ).
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2613).

Comment
Borate compounds have been extensively studied due to their diverse structural chemistry and successful industry  Li et al. (2003) with a structure model that describes disordered oxygen positions of the nitrate anion, and where the hydrogen positions could not be determined even in the ordered model of the La analogue (Li et al., 2005). Herein we report the redetermined crystal structure based on single-crystal X-ray diffraction data of Ce   (Sun et al., 2012). The Ce 3+ ion resides at the center of the 9-membered rings and adopt a distorted 10-coordination to the surrounding oxygen atoms to form a 1-6-3 crown-shaped polyhedron (Figs. 1, 3), six of them coming from the nearly planar 9-membered ring in the middle, one from a triangular [NO 3 ] anion on the top, one from an OH group (originating from a triangular [BO 2 (OH)] group from the next layer) and two from water molecules at the botton. The water molecules and the nitrate [NO 3 ] groups, located at the interlayer space, additionally stabilize the structural set-up of the title nitrate borate compound, via their O-H···O hydrogen bonds (Table 2). In the present model, H-atom H7 (attached to O14) has no acceptor atom, and none of alternative approximate positions found in difference Fourier maps for H7 were reliable because they were too close to the Ce 3+ ion. All hydrogen atoms except for H7 point to the backbone of the polyborate layers, whereas H-atom H7 points to the [100] direction (i.e. O14-H7 parallel to the polyborate layers).
In contrast to the previous report (Li et al., 2003), one of the 3-coordinated boron atoms with B-O distances less than 1.38 Å (denoted as B5 in this paper) was altered to be ′3 + 1′ coordinated to three surrounding O-atoms and a water molecule forming a highly distorted tetrahedral [BO 3 Table 1), as observed in its La counterpart (Li et al., 2005) and previous reviews on the crystal chemistry of borates (Zobetz, 1982(Zobetz, , 1990. This may be attributed to the fact that the water molecule strongly

Experimental
During our systematically investigation on rare earth borates (Sun et al., 2010;Sun et al., 2012), the title compound, [NO 3 ] . 2H 2 O, was synthesized by using molten boric acid as flux which has been firstly described by Li et al. (2003). Typically, a mixture of Ce(NO 3 ) 3 . 6H 2 O (1.00 g) and H 3 BO 3 (3.00 g) with molar ratio of Ce:B = 1:21 was prepared by thoroughly homogeneous grinding and transferred into a Teflon-lined stainless-steel autoclave (30 ml in volume), then heated to and kept at 513 K for three days. Transparent, colorless crystals of the title compound were obtained by filtration, rinsed with deionized water for several times, and dried in a desiccators. The phases of the asprepared solid products were identified by powder X-ray diffraction (PXRD) analyses. Optical microscopy was used to check the selected crystals proper for single-crystal X-ray diffraction while their chemical compositions were examined by use of an Energy Dispersive X-ray Spectrometer (EDX) (Oxford Instruments). Scanning electron microscopy (SEM) was used to document the crystal morphologies.

Refinement
Initially, all hydrogen positions were located from difference Fourier maps and refined freely. Then a common variable was used for the isotropic atomic displacement parameters (U iso ) of all hydrogen atoms while their atomic coordinates were refined. After refinement the O-H bond lengths of 2 water molecules (i.e. H3-O10-H2 and H7-O14-H6) became improper, soft restraints on U iso and on bond lengths (d(O-H) = 0.82 (2) Å) were applied.

Special details
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 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 R-factors(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq