A new polytype of orthoboric acid, H₃BO₃-3T

The crystal structure of H₃BO₃-3T, a new trigonal polytype of orthoboric acid, consists of sheets of hydrogen-bonded B(OH)₃ mol­ecules similar to those found in the triclinic structure of orthoboric acid, H₃BO₃-2A. In each case, van der Waals forces connect the sheets. However, the stacking sequences of the sheets differ between the two polymorphs. In H₃BO₃-3T (space group P3₂), the sheets are stacked in the repeating sequence ABC..., whereas in H₃BO₃-2A (space group ), the sheets are stacked in the repeating sequence AB....

S1. Comment

Structures of boric acids known to date include triclinic orthoboric acid, H₃BO₃-2 A (Zachariasen, 1934), its deuterated analog D₃BO₃ (Craven & Sabine, 1966), and three modifications of metaboric acid [first described by Kracek et al. (1934)], viz. orthorhombic α-HBO₂ (Tazaki, 1940; Peters & Milberg, 1964), monoclinic β-HBO₂ (Zachariasen, 1952, 1963b) and cubic γ-HBO₂ (Zachariasen, 1963a). The crystal structures of the first three compounds contain sheets formed by B(OA₃) (A = H and D) groups in orthoboric acid and by [B₃O₃(OH)₃] groups in α-HBO₂, with hydrogen bonding between these groups within a given sheet. In β-HBO₂, two-thirds of the boron atoms have triangular coordination and one-third are tetrahedrally coordinated and linked into infinite chains, _∞¹[B₃O₄(OH)(H₂O)], arranged in sheets. In γ-HBO₂, all B atoms have tetrahedral coordination and are linked into a three-dimensional framework, _∞³[BO(OH)] [see Lima-de-Faria et al. (1990) for formula nomenclature].

As mentioned by Craven & Sabine (1966), H₃BO₃-2 A was one of the first compounds with a hydrogen-bonded crystal structure to be examined by X-ray diffraction methods (Zachariasen, 1934) and, ever since, has been a `textbook' example. In the original work of Zachariasen (1934), an approximate crystal structure of H₃BO₃-2 A was reported. The (BO₃)³⁻ groups of nearly perfect C_{3 h} symmetry were found to form pseudo-hexagonal sheets in which each (BO₃)³⁻ group has three adjacent groups. The H atoms were assumed to be positioned mid-way between the two nearest O atoms of adjacent (BO₃)³⁻ groups, but arguments favoring O—H···O hydrogen bonds in this structure were subsequently forwarded by Bernal & Megaw (1935). Studying disorder in the stacking of the sheets in thin microcrystals of H₃BO₃-2 A using electron diffraction, Cowley (1953) found that the H atoms are systematically displaced from the nearly collinear arrangement suggested by Zachariasen (1934). This model has not been verified in further X-ray (Zachariasen, 1954; Gajhede et al., 1986) and neutron (Craven & Sabine, 1966) diffraction experiments. Dorset (1992) performed electron-diffraction studies of thin microcrystals of H₃BO₃-2 A at low temperature (128 K and below) and determined that, as a result of a stacking disorder as discussed by Cowley (1953), microcrystals of orthoboric acid diffract as if single sheets were independent of one another. Amongst the most recent studies of triclinic orthoboric acid are the ab initio calculations of Zapol et al. (2000). Their theoretical results are in a good agreement with the experimental data, i.e. the binding energy of the molecules within a sheet is much higher than the interaction energy between the sheets, and this can explain the good lubricating properties of orthoboric acid (Erdemir et al., 1991).

This paper reports the crystal structure of a new polytype of orthoboric acid, which was isolated as an unexpected product during our attempts to synthesize new sodium uranyl borate compounds. As can be seen from Fig. 1, each of the two symmetrically independent B(OH)₃ molecules has nearly perfect C_{3 h} symmetry and is connected to three adjacent molecules by hydrogen bonding to form pseudo-hexagonal sheets, _∞²[B(OH)₃], which are parallel to the (001) plane. Details are given in Tables 1 and 2. The average values of the B—O, O—H and O···H bond distances and O—B—O, B—O—H and O—H···O bond angles within a sheet are 1.36 (1), 0.91 (5) and 1.88 (5) Å, and 120.0 (9), 111 (5) and 152 (6)°, respectively. These values are similar to those of H₃BO₃-2 A (1.361, 0.88 and 1.85 Å, and 120.0, 112 and 171°; Zachariasen, 1954). The unit-cell parameters parallel to the (001) plane in both structures are also comparable (Table 3). Although the sheets in the crystal structure of H₃BO₃-3 T are of the same type as those in H₃BO₃-2 A, the stacking sequences of the sheets are different in these two structures. The sheets are stacked in the sequence ABC··· in H₃BO₃-3 T (Fig. 2a), whereas in H₃BO₃-2 A, the sheets are stacked in the sequence AB··· (Fig. 2 b). The distances between the planes of two adjacent sheets, calculated as half of d₀₀₁ in H₃BO₃-2 A and one-third of d₀₀₁ H₃BO₃-3 T, are 3.182 (2) and 3.1869 (2) Å, respectively, whereas the shortest interatomic distances between B and O atoms from neighboring sheets are 3.157 and 3.195 Å in H₃BO₃-2 A, and 3.172 (6) and 3.187 (8) Å in H₃BO₃-3 T. It appears that small tilts of the B(OH)₃ molecules relative to the (001) plane in the crystal structure of H₃BO₃-3 T, almost of the same magnitude as those in H₃BO₃-2 A, are mainly due to weak interactions between B and O atoms, as mentioned by Zachariasen (1954) for H₃BO₃-2 A.

S2. Experimental

Crystals of the title co,pound were prepared by mild hydrothermal techniques. The initial solution was prepared from NaBO₂(H₂O)₂ (41 mg) and UO₃ (29 mg; molar ratio 4:1) dissolved in ultrapure water (3 ml). The pH was adjusted to 0.5 using concentrated (15.4 mol/l) nitric acid. The resulting solution was placed in a 23 ml Teflon-lined Parr bomb, heated at 453 K for 48 h and then cooled to ambient temperature. No crystals had formed after heating, so the solution was placed in a fume hood to evaporate. After 2 d, well shaped pseudo-hexagonal crystals of H₃BO₃-2 A and small colorless crystals of H₃BO₃-3 T were recovered.

S3. Refinement

The positions of most O and B atoms were determined by direct methods. The remaining non-H atoms, and approximate positions of the H atoms, were located from subsequent difference Fourier syntheses. O—H bond distances were restrained to 0.96 Å, with standard uncertainties of 0.02 Å. There are no atoms heavier than O atoms in this structure, so that the absolute configuration cannot be determined reliably. Both absolute structures were tested and gave the same results. The 764 Friedel opposites were merged before final refinement, because the absolute structure parameter (Flack, 1983) was 4(3), and therefore unreliable. The twin law [010/100/00–1] was introduced in the final cycles of structure refinement, thus reducing the R(F) index from 0.089 to 0.044. The twin-component scale factor refined to 0.278 (2). The low standard uncertainties of the cell constants indicate the internal consistency of the measurements themselves, i.e. the precision of the measurements, not their accuracy.

Computing details

Data collection: SMART (Bruker, 2001); cell refinement: SAINT-Plus (Bruker, 1999); data reduction: SAINT-Plus; program(s) used to solve structure: SHELXTL (Bruker, 1998); program(s) used to refine structure: SHELXTL; molecular graphics: ATOMS (Dowty, 2000); software used to prepare material for publication: SHELXTL and WinGX (Farrugia, 1999).

Figures

	Fig. 1. The arrangement of hydrogen-bonded B(OH)3 molecules within a single pseudo-hexagonal sheet parallel to the (001) plane in H3BO3-3 T. Displacement ellipsoids are shown at the 50% probability level, and H atoms are shown as gray spheres of arbitrary size (symmetry codes as in Table 2).
	Fig. 2. Stacking sequences of the sheets in (a) H3BO3-3 T and (b) H3BO3-2 A (Zachariasen, 1954), projected along the [001] axes. BO3 groups, shown as triangles, are connected via H atoms, shown as gray spheres.

boric acid

Crystal data

H3BO3	Dx = 1.499 Mg m^−3
Mr = 61.83	Mo Kα radiation, λ = 0.71073 Å
Trigonal, P32	Cell parameters from 1623 reflections
Hall symbol: P 32	θ = 3.3–33.8°
a = 7.0453 (4) Å	µ = 0.16 mm^−1
c = 9.5608 (7) Å	T = 297 K
V = 410.98 (4) Å^3	Triangular plate, colorless
Z = 6	0.14 × 0.08 × 0.02 mm
F(000) = 192

Data collection

Bruker APEX CCD area detector diffractometer	765 reflections with I > 2σ(I)
ω scans	Rint = 0.035
Absorption correction: empirical (using intensity measurements) (XPREP; Bruker, 1997)	θmax = 34.4°, θmin = 2.1°
Tmin = 0.851, Tmax = 0.918	h = −11→11
4615 measured reflections	k = −11→11
1143 independent reflections	l = −14→11

Refinement

Refinement on F^2	All H-atom parameters refined
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0363P)^2] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.044	(Δ/σ)max < 0.001
wR(F^2) = 0.088	Δρmax = 0.28 e Å^−3
S = 1.00	Δρmin = −0.25 e Å^−3
1143 reflections	Extinction correction: SHELXTL (Bruker, 1998), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
99 parameters	Extinction coefficient: 0.051 (6)
7 restraints

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.

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

	x	y	z	Uiso*/Ueq
B1	0.1837 (9)	0.6403 (10)	0.2849 (7)	0.0485 (17)
B2	0.5180 (5)	0.3069 (5)	0.2964 (5)	0.0277 (9)
O1	0.0621 (6)	0.7393 (7)	0.2805 (3)	0.0514 (8)
O2	0.0864 (6)	0.4195 (5)	0.2808 (3)	0.0467 (7)
O3	0.4090 (5)	0.7650 (6)	0.2869 (3)	0.0441 (8)
O4	0.6437 (6)	0.2103 (5)	0.2929 (2)	0.0413 (7)
O5	0.2950 (6)	0.1832 (6)	0.2998 (3)	0.0506 (8)
O6	0.6136 (7)	0.5294 (6)	0.2904 (3)	0.0464 (8)
H1	0.133 (4)	0.877 (3)	0.316 (3)	0.015 (7)*
H2	−0.057 (3)	0.361 (5)	0.311 (3)	0.013 (7)*
H3	0.483 (4)	0.695 (4)	0.311 (3)	0.022 (7)*
H4	0.565 (5)	0.077 (4)	0.333 (3)	0.048 (10)*
H5	0.243 (5)	0.274 (4)	0.324 (3)	0.027 (8)*
H6	0.755 (3)	0.602 (4)	0.318 (3)	0.027 (8)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
B1	0.045 (3)	0.056 (3)	0.050 (3)	0.029 (2)	−0.0005 (16)	−0.002 (2)
B2	0.0144 (12)	0.0177 (14)	0.053 (3)	0.0093 (11)	0.0007 (10)	0.0015 (11)
O1	0.0246 (14)	0.0384 (18)	0.097 (2)	0.0198 (14)	−0.0021 (15)	−0.0049 (16)
O2	0.0263 (15)	0.0278 (14)	0.0859 (19)	0.0133 (13)	−0.0038 (16)	−0.0024 (14)
O3	0.0233 (12)	0.0254 (14)	0.082 (2)	0.0108 (12)	−0.0006 (14)	−0.0032 (14)
O4	0.0279 (16)	0.0249 (15)	0.072 (2)	0.0138 (14)	−0.0007 (13)	−0.0025 (12)
O5	0.0284 (14)	0.0379 (18)	0.0902 (19)	0.0201 (13)	−0.0026 (15)	0.0010 (16)
O6	0.0307 (19)	0.0325 (15)	0.0772 (18)	0.0167 (13)	0.0070 (15)	0.0080 (13)

Geometric parameters (Å, º)

B1—O1	1.349 (5)	O1—H1	0.91 (2)
B1—O2	1.350 (6)	O2—H2	0.93 (2)
B1—O3	1.377 (6)	O3—H3	0.91 (2)
B2—O4	1.361 (4)	O4—H4	0.90 (2)
B2—O5	1.364 (4)	O5—H5	0.91 (2)
B2—O6	1.363 (5)	O6—H6	0.91 (2)
O1—B1—O2	120.4 (5)	O4—B2—O5	120.7 (3)
O1—B1—O3	119.9 (5)	O4—B2—O6	120.2 (3)
O2—B1—O3	119.6 (4)	O5—B2—O6	119.0 (2)
B1—O1—H1	113 (2)	B2—O4—H4	107 (2)
H1—O1—H6^i	111 (2)	H4—O4—H2^iii	120 (2)
B1—O1—H6^i	126 (1)	B2—O4—H2^iii	124 (1)
B1—O2—H2	108 (2)	B2—O5—H5	107 (2)
H2—O2—H5	120 (2)	H5—O5—H1^iv	124 (2)
B1—O2—H5	121 (1)	B2—O5—H1^iv	125.4 (8)
B1—O3—H3	117 (2)	B2—O6—H6	114 (2)
H3—O3—H4^ii	114 (2)	H6—O6—H3	112 (2)
B1—O3—H4^ii	123 (1)	B2—O6—H3	128 (1)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···O5^ii	0.91 (2)	1.88 (2)	2.715 (7)	153 (2)
O2—H2···O4^i	0.93 (2)	1.83 (2)	2.705 (6)	155 (2)
O3—H3···O6	0.91 (2)	1.82 (2)	2.688 (7)	159 (3)
O4—H4···O3^iv	0.91 (2)	1.95 (2)	2.719 (5)	141 (3)
O5—H5···O2	0.91 (2)	1.89 (2)	2.723 (6)	151 (2)
O6—H6···O1^iii	0.91 (2)	1.91 (2)	2.740 (6)	151 (3)

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