catena-Poly[sodium(I)-[mu]-tetra­ethoxy­borato]

The title compound, [Na(C₈H₂₀BO₄)]_n, has twofold crystallographic symmetry, with the Na⁺ cations bound by four O atoms [Na-O = 2.251 (3) Å]. The tetra­ethoxy­borate anion acts as a bridging ligand to form one-dimensional polymers running along the twofold crystal axis. The crystal was treated as a racemic twin.

S1. Comment

This study is part of a programme aimed at investigating boron diolates and alkoxides particularly those containing sodium cations [Cambridge Structural Database (Allen, 2002) refcodes GAKLAG (Gainsford & Kemmitt, 2004) and FEFHOO (Gainsford & Kemmitt, 2005a); Gainsford & Kemmitt, 2005b; Bishop et al., 2000]. Although borates form a range of diolate complexes (Bachelier & Verchere, 1995; Springsteen & Wang, 2002), structural data are sparse. There are no reported structures for tetraethoxyborate salts (Allen, 2002), though there are a large number of tetramethoxyborate salts reported (see Gainsford & Kemmitt, 2005b) [two guanidinium salts have since been reported, viz. FEWJOH and FEWJUN (Abrahams et al., 2005)].

The basic polymeric fragment of the title compound, (I), with asymmetric unit formula [Na_0.5(C₄H₁₀B_0.5O₂)], has twofold crystallographic symmetry with the symmetry (c) axis passing through the Na and B atoms. The molecules crystallized in a racemically twinned crystal. Enantiomeric resolution was not expected from the synthesis or from the small anomalous dispersion effects.

The sodium cations are four-coordinate in a highly distorted tetrahedral arrangement, with mean Na—O distances of 2.251 (3) Å, and O—Na—O angles of 138.18 (8) and 60.62 (12)°. This Na—O distance is at the short end of the range of those reported when the bound O atoms are not structurally constrained; one example of the latter case is POGDAQ01 (Caselli et al., 2000), where the Na—O distances range from 2.243 to 2.355 Å in a niobium–tetraoxycalix(4)arene compound. The Na1/O1/O1C/B1 and Na1/O2A/O2B/B1B coordination planes (see Fig. 1) are at right angles [89.98 (12)°]. These parameters are at the extremes of values for four-coordinate O-bound sodium cations, with the inter-planar angles usually in the range 55–80°, for example, bis(µ₂-triphenylmethoxo)dihydrido-bis(tetrahydrofuran)aluminiumsodium (JISHIC; Noth et al., 1998) with interplanar angle 64.7° and Na—O distances of 2.289–2.350 Å. When the four-coordinate arrangement approaches square planar because of the constrained O-atom geometry, the Na atoms usually have another one (or more) longer `intramolecular' contacts. making a pseudo-square-pyramidal or octahedral description more appropriate [e.g. in the calix(4)arene–sodium compound DONSAA (Bott et al., 1986), with Na—O = 2.274–2.332 Å and a fifth interaction, Na···C = 2.663 Å.] However, in some cases, the fifth available Na<sup>+</sup> site is shielded by parts of the bound molecule, `preserving' the approximate square-planar geometry (e.g. in POGDAQ01).

The B—O and C—O bond lengths average to 1.416 (4) and 1.467 (4) Å, and the mean B—O—C angle is 117.8 (2)°, values that fall within normal ranges as reported in GAKLAG. The O—B—O angles are distorted from pure tetrahedral values (Table 1) and also compared with those in the bis(1,1,1-trihydroxymethylpropane)borate salt (XOCHOM; Zviedre & Belsky, 2001) of 108.6–109.9°. The borate anions bridge the sodium cations, forming one-dimensional polymers running along the twofold-symmetry c axis direction (Fig. 2). This packing mode was also observed in catena-[µ₃bis(ethylenedioxy)borato]sodium(I) (GAKLAG), where the polymers were aligned with the 2₁ screw axis.

S2. Experimental

Sodium metaborate (10 g) was dehydrated in a domestic microwave oven (1000 W) at full power for 10 min. A sample of the resulting NaBO₂ (2.64 g, 0.04 mol) was refluxed in a 2:1 mixture of ethanol and toluene (150 ml), under a nitrogen atmosphere, running the distillate through molecular sieves to remove liberated water. After 8 h, the mixture was filtered and the volume was reduced by distillation. On cooling, the product appeared as platey crystals. These were filtered and dried (yield 7.1 g, 83%).

S3. Refinement

Friedel pairs were merged after a full data set refinement gave a Flack (1983) parameter that was indeterminate. All H atoms were constrained to their expected geometries (C—H = 0.99 and 0.98 Å) and refined with U_iso (H) vaues of 1.2 times U_eq of their carrier atoms.

Computing details

Data collection: SMART (Bruker, 2001); cell refinement: SMART; data reduction: SAINT (Bruker, 2001) and SADABS (Sheldrick, 1996); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 2003) and ORTEP-3 (Farrugia, 1997).

Figures

	Fig. 1. An ORTEP-3 (Farrugia, 1997) view showing the asymmetric unit and part of the polymeric unit of (I), with 50% probability displacement ellipsoids. Labels of asymmetric unit atoms, plus symmetry-generated atoms O2A, O2B, O1C and B1B, are shown. [Symmetry codes: (a) 2 − x, −y, z + 1; (b) x, y, z + 1; (c) 2 − x, −y, z.
	Fig. 2. PLATON view (Spek, 2003) of the cell contents of (I), along the a axis, showing the one-dimensional polymers along the twofold c axis. Na atoms are shown as globes (other atoms can be deduced from Fig. 1).

Poly[sodium(I)-µ₂-tetraethoxyborato]

Crystal data

[Na(C8H20BO4)]	F(000) = 232
Mr = 214.04	Dx = 1.062 Mg m^−3
Orthorhombic, P21212	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2 2ab	Cell parameters from 1438 reflections
a = 7.633 (3) Å	θ = 2.7–26.0°
b = 15.272 (6) Å	µ = 0.11 mm^−1
c = 5.744 (2) Å	T = 98 K
V = 669.6 (4) Å^3	Plate, colourless
Z = 2	0.57 × 0.23 × 0.07 mm

Data collection

Bruker SMART CCD area-detector diffractometer	829 independent reflections
Radiation source: fine-focus sealed tube	648 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.068
Detector resolution: 8.192 pixels mm^-1	θmax = 26.3°, θmin = 2.7°
ϕ and ω scans	h = −9→5
Absorption correction: multi-scan (Blessing, 1995)	k = −18→19
Tmin = 0.812, Tmax = 0.993	l = −7→7
3881 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)] = 0.056	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.105	H-atom parameters constrained
S = 1.12	w = 1/[σ^2(Fo^2) + (0.0421P)^2 + 0.1167P] where P = (Fo^2 + 2Fc^2)/3
829 reflections	(Δ/σ)max < 0.001
67 parameters	Δρmax = 0.18 e Å^−3
0 restraints	Δρmin = −0.22 e Å^−3

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 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	1.0000	0.0000	0.7432 (3)	0.0265 (5)
O1	0.8763 (3)	0.04140 (13)	0.4048 (3)	0.0203 (5)
O2	1.0828 (3)	0.06180 (14)	0.0814 (3)	0.0210 (6)
C1	0.7310 (4)	0.0866 (2)	0.3100 (6)	0.0274 (9)
H1A	0.6456	0.0442	0.2459	0.033*
H1B	0.7702	0.1253	0.1821	0.033*
C2	0.6462 (6)	0.1400 (3)	0.4980 (8)	0.0613 (14)
H2A	0.6009	0.1010	0.6193	0.074*
H2B	0.5495	0.1740	0.4316	0.074*
H2C	0.7329	0.1799	0.5658	0.074*
C3	1.1704 (5)	0.1349 (2)	0.1787 (6)	0.0280 (9)
H3A	1.2417	0.1159	0.3136	0.034*
H3B	1.0835	0.1782	0.2341	0.034*
C4	1.2874 (5)	0.1763 (3)	−0.0024 (7)	0.0424 (10)
H4A	1.3758	0.1339	−0.0528	0.051*
H4B	1.3456	0.2277	0.0646	0.051*
H4C	1.2167	0.1944	−0.1365	0.051*
B1	1.0000	0.0000	0.2435 (9)	0.0197 (10)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Na1	0.0350 (11)	0.0397 (11)	0.0048 (8)	−0.0007 (10)	0.000	0.000
O1	0.0203 (12)	0.0283 (12)	0.0123 (11)	0.0020 (9)	0.0014 (10)	−0.0010 (10)
O2	0.0277 (13)	0.0237 (12)	0.0115 (11)	−0.0053 (10)	0.0010 (10)	0.0016 (9)
C1	0.0231 (19)	0.034 (2)	0.0248 (18)	0.0023 (17)	0.0014 (15)	0.0014 (15)
C2	0.046 (3)	0.088 (4)	0.050 (2)	0.036 (3)	−0.005 (2)	−0.021 (3)
C3	0.034 (2)	0.0269 (19)	0.0229 (17)	−0.0074 (17)	0.0014 (16)	−0.0015 (14)
C4	0.048 (3)	0.044 (2)	0.035 (2)	−0.016 (2)	0.003 (2)	0.0068 (19)
B1	0.025 (3)	0.024 (3)	0.010 (2)	0.001 (3)	0.000	0.000

Geometric parameters (Å, º)

Na1—O2^i	2.250 (3)	C2—H2A	0.9800
Na1—O2^ii	2.250 (3)	C2—H2B	0.9800
Na1—O1	2.252 (3)	C2—H2C	0.9800
Na1—O1^iii	2.252 (3)	C3—C4	1.510 (5)
O1—C1	1.415 (4)	C3—H3A	0.9900
O1—B1	1.466 (4)	C3—H3B	0.9900
O2—C3	1.416 (4)	C4—H4A	0.9800
O2—B1	1.469 (4)	C4—H4B	0.9800
O2—Na1^iv	2.250 (3)	C4—H4C	0.9800
C1—C2	1.500 (5)	B1—O1^iii	1.466 (4)
C1—H1A	0.9900	B1—O2^iii	1.469 (4)
C1—H1B	0.9900
O2^i—Na1—O2^ii	60.64 (11)	C1—C2—H2C	109.5
O2^i—Na1—O1	138.18 (8)	H2A—C2—H2C	109.5
O2^ii—Na1—O1	138.17 (8)	H2B—C2—H2C	109.5
O2^i—Na1—O1^iii	138.17 (8)	O2—C3—C4	109.7 (3)
O2^ii—Na1—O1^iii	138.18 (8)	O2—C3—H3A	109.7
O1—Na1—O1^iii	60.62 (12)	C4—C3—H3A	109.7
C1—O1—B1	118.2 (2)	O2—C3—H3B	109.7
C1—O1—Na1	142.90 (19)	C4—C3—H3B	109.7
B1—O1—Na1	98.88 (19)	H3A—C3—H3B	108.2
C3—O2—B1	117.4 (3)	C3—C4—H4A	109.5
C3—O2—Na1^iv	143.51 (19)	C3—C4—H4B	109.5
B1—O2—Na1^iv	99.02 (19)	H4A—C4—H4B	109.5
O1—C1—C2	109.1 (3)	C3—C4—H4C	109.5
O1—C1—H1A	109.9	H4A—C4—H4C	109.5
C2—C1—H1A	109.9	H4B—C4—H4C	109.5
O1—C1—H1B	109.9	O1^iii—B1—O1	101.6 (3)
C2—C1—H1B	109.9	O1^iii—B1—O2^iii	113.63 (12)
H1A—C1—H1B	108.3	O1—B1—O2^iii	113.60 (13)
C1—C2—H2A	109.5	O1^iii—B1—O2	113.60 (13)
C1—C2—H2B	109.5	O1—B1—O2	113.63 (12)
H2A—C2—H2B	109.5	O2^iii—B1—O2	101.3 (3)
O2^i—Na1—O1—C1	−52.1 (4)	Na1—O1—B1—O1^iii	0.0
O2^ii—Na1—O1—C1	46.3 (4)	C1—O1—B1—O2^iii	−55.6 (3)
O1^iii—Na1—O1—C1	177.1 (4)	Na1—O1—B1—O2^iii	122.4 (2)
O2^i—Na1—O1—B1	130.78 (12)	C1—O1—B1—O2	59.6 (3)
O2^ii—Na1—O1—B1	−130.80 (12)	Na1—O1—B1—O2	−122.4 (2)
O1^iii—Na1—O1—B1	0.0	C3—O2—B1—O1^iii	−60.7 (3)
B1—O1—C1—C2	−167.3 (3)	Na1^iv—O2—B1—O1^iii	122.2 (2)
Na1—O1—C1—C2	15.9 (5)	C3—O2—B1—O1	54.8 (3)
B1—O2—C3—C4	164.5 (2)	Na1^iv—O2—B1—O1	−122.2 (2)
Na1^iv—O2—C3—C4	−20.5 (5)	C3—O2—B1—O2^iii	177.0 (3)
C1—O1—B1—O1^iii	−178.0 (3)	Na1^iv—O2—B1—O2^iii	0.0

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