Sodium di­phenyl­acetate, an infinite columnar structure

In the title compound, Na⁺·C₁₄H₁₁O₂^-, the di­phenyl­acetate ions have a conformation where the two phenyl rings and the carboxyl­ate group are oriented like the blades of a propeller, each ion having a well defined helicity. The crystal structure of the title compound is achiral, although non-centrosymmetric (space group ); thus, ions with both (+) and (-) helicities are present in the crystal. Each Na⁺ ion is coordinated by four carboxyl­ate O atoms at distances in the range 2.207 (2)-2.467 (3) Å to form cubes of Na and O atoms which are linked via the carboxyl­ate C atoms into a columnar structure along the rotoinversion axis.

S1. Comment

The molecule of diphenylacetic acid is achiral, but due to the rotational mobility of the phenyl rings the molecule may assume a helical chiral conformation where the two phenyl rings and the carboxylate group are oriented like the blades of a propeller. When forming molecular compounds with other achiral molecules, such a conformation can induce a chiral crystal structure. An interesting example of this situation occurs in the 1:1 adduct of diphenylacetic acid and acridine, which crystallizes from acetonitrile in a chiral crystal structure (space group P2₁2₁2₁) and spontaneously resolves the (+) and (-) enantiomers (Koshima et al., 1996). This compound is photoreactive and has been used in absolute asymmetric synthesis by photodecarboxylating condensation performed in the solid state. Our interest in this type of absolute asymmetric synthesis led us to a systematic study of other compounds of diphenylacetic acid, including the title compound, sodium diphenylacetate, (I), whose structure is reported here. According to a search of the Cambridge Structural Database (CSD release April 1999; Allen & Kennard, 1993), this is the first reported crystal structure of a salt of diphenylacetic acid with an alkali metal.

Compound (I) crystallizes in the achiral, although noncentrosymmetric, space-group P¯ 42₁c (Jones, 1986) and both enantiomers of the helical conformation of the diphenylacetate ion are present in the unit cell. There is an ambiguity in the absolute assignment of the x and y axes in such a high symmetry noncentrosymmetric spacegroup which can only be resolved if a sufficiently strong anomalous scatterer is present. In fact, inverting the structure in P¯ 42₁c is equivalent to the interchange of axes x → y, y → x. However, due to the low value of the anomalous dispersion of all atoms, including Na, at the Mo Kα wavelength, it was not possible to resolve in the present case the ambiguity in the absolute assignment of the x and y axes.

The coordination of the diphenylacetate ions around the metal ions is shown in Figs. 1 and 2. The carboxylate O1 atom and the Na⁺ ions alternate at the vertices of a distorted cube centered at the origin of the ¯ 4 axis. Each Na⁺ ion is coordinated by four O atoms (one O2 atom outside the cube and three O1 atoms at adjacent vertices of the `cube') at distances between 2.207 (2)–2.467 (3) Å. These distances are close to the average Na⁺O²⁻ distance [2.44 (16) Å; Bergerhoff & Brandenburg, 1995], except for the Na–O2ⁱ [symmetry code: (i) y, x, −z] distance [2.207 (2) Å], which is somewhat shorter. The Na/O core forms a columnar structure along the ¯ 4 axis, as seen in Fig. 2. The distorted cubes of Na and O1 atoms alternate along the columns, with the carboxylate groups, which act as bridges, joining adjacent cubes. Interestingly, the carboxylate O2 atom is involved solely in the bridging, while the O1 atom and its symmetry-related counterparts make up the oxygen content of the cube construction.

The carboxylate skeleton defined by atoms C1, C2, O1 and O2 is planar to within 0.003 Å. There is significant asymmetry between the bond distances C1–O1 [1.271 (3) Å] and C1–O2 [1.226 (3) Å], which deviate significantly from the average value of a typical delocalized double bond in carboxylate anions [1.254 (10) Å; Allen et al., 1987]. The asymmetry between these two bonds is probably explained by the different environments of the O1 and O2 atoms, O1 having three Na⁺ ions as near neighbours while O2 has just a single alkaline ion in the first coordination shell. This may also induce the large asymmetry between the angles C2–C1–O1 [113.6 (2)°] and C2–C1–O2 [121.4 (3)°]; however, the angle O1–C1–O2 [125.0 (3)°] is typical for a carboxylate group. The shortest distance between two metal ions is 3.265 (2) Å, which greatly exceeds the sum of two ionic radii of Na⁺.

The dihedral angle between the least-squares planes of the two phenyl rings is 89.35 (16)°. The dihedral angles between each of the rings and the bridging plane defined by atoms C2, C3 and C9 are 5.0 (3)° [C3–C8] and 84.4 (3)° [C9–C14]. The carboxylate group makes an angle of 56.7 (3)° with the bridging plane and angles of 54.61 (16)° and 78.40 (17)° with rings C3–C8 and C9–C14, respectively. The two phenyl rings and the carboxylate plane have torsions around the single bonds to C2 in the same direction, like the blades of a propeller. This is confirmed by examining the torsion angles around C2 [H2–C2–C1–O1 58.0°, H2–C2–C3–C4 62.5° and H2–C2–C9–C10 21.8°], which all have the same sign. These torsion angles should be compared with the corresponding values in the molecular crystal of diphenylacetic acid [48.2, 27.3 and 52.8°, respectively] and in the 1:1 adduct of diphenylacetic acid and acridine [33.9, 50.3 and 13.0°, respectively; Koshima et al., 1996]. This shows that the phenyl and carboxylate groups may rotate rather freely around the single bonds joining them to the central C atom. Indeed, there is evidence that these groups are sufficiently mobile in solution to allow inversion of helicity (Koshima et al., 1996).

There are no classical hydrogen bonds in this structure. Cohesion is maintained mainly by electrostatic forces and weaker van der Waals interactions. Interaction between π-clouds and metal atoms does not appear to play a major role, the shortest ring–metal distance being 3.812 (5) Å. There is, however, a relatively short distance between the H atom attached to C10 and the phenyl ring C3–C8 [2.847 (5) Å].

S2. Experimental

Compound (I) was prepared by neutralizing an ethanolic solution of diphenylacetic acid (98%, Aldrich) with sodium hydroxide. Clear transparent single crystals of prismatic form grew from the solution by slow evaporation over a period of a few weeks, from which one small crystal was selected and used for the X-ray analysis. Before data collection the quality of the crystal was checked by photographic methods.

Computing details

Data collection: CAD-4 Software (Enraf-Nonius, 1989); cell refinement: CAD-4 Software; data reduction: HELENA (Spek, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997a); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997b); molecular graphics: ORTEPII (Johnson, 1976); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. ORTEPII (Johnson, 1976) plot of (I) viewed along the ¯ 4 axis (c axis). Displacement ellipsoids are drawn at the 50% level and H atoms are shown as spheres of arbitrary radii.
	Fig. 2. Stacking of the Na/O core along the c axis showing the columnar structure. The phenyl groups have been omitted for clarity.

Sodium diphenylacetate

Crystal data

Na(C14H11O2)	Dx = 1.369 Mg m^−3
Mr = 234.22	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, P421c	Cell parameters from 25 reflections
a = 18.944 (3) Å	θ = 10.0–13.9°
c = 6.3326 (11) Å	µ = 0.12 mm^−1
V = 2272.6 (6) Å^3	T = 293 K
Z = 8	Prism, colourless
F(000) = 976	0.24 × 0.24 × 0.17 mm

Data collection

Enraf-Nonius CAD-4 diffractometer	1135 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.025
Graphite monochromator	θmax = 27.4°, θmin = 3.0°
profile data from ω–2θ scans	h = 0→18
Absorption correction: ψ-scan (North et al., 1968)	k = 0→24
Tmin = 0.923, Tmax = 0.982	l = −6→8
2596 measured reflections	3 standard reflections every 180 min
1598 independent reflections	intensity decay: 2.0%

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)] = 0.038	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.106	H-atom parameters constrained
S = 1.08	w = 1/[σ^2(Fo^2) + (0.0518P)^2 + 0.4591P] where P = (Fo^2 + 2Fc^2)/3
1598 reflections	(Δ/σ)max < 0.001
154 parameters	Δρmax = 0.20 e Å^−3
0 restraints	Δρmin = −0.17 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.

The H atoms were placed at calculated positions and refined as riding with an isotropic displacement parameter U(H)_eq = 1.2U_eq of the parent atom. Examination of the crystal structure with PLATON (Spek, 1995) showed that there are no solvent-accessible voids in the crystal lattice.

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

	x	y	z	Uiso*/Ueq
Na	0.08568 (7)	0.00910 (7)	0.20517 (17)	0.0414 (3)
O1	0.01543 (11)	−0.09003 (11)	0.1643 (3)	0.0397 (5)
O2	0.03012 (12)	−0.13453 (12)	0.4847 (3)	0.0470 (5)
C1	0.01654 (14)	−0.14054 (15)	0.2963 (4)	0.0313 (6)
C2	−0.00060 (15)	−0.21367 (15)	0.1967 (4)	0.0342 (6)
H2	0.0342	−0.2215	0.0848	0.041*
C3	0.00578 (17)	−0.27570 (15)	0.3495 (4)	0.0371 (7)
C4	0.06853 (18)	−0.28642 (18)	0.4576 (5)	0.0447 (8)
H4	0.1050	−0.2539	0.4436	0.054*
C5	0.0776 (2)	−0.34433 (18)	0.5851 (6)	0.0533 (9)
H5	0.1197	−0.3502	0.6585	0.064*
C6	0.0251 (2)	−0.39347 (18)	0.6048 (6)	0.0585 (10)
H6	0.0318	−0.4330	0.6895	0.070*
C7	−0.0373 (2)	−0.38408 (17)	0.4990 (6)	0.0542 (9)
H7	−0.0731	−0.4173	0.5129	0.065*
C8	−0.04725 (18)	−0.32574 (16)	0.3721 (5)	0.0440 (8)
H8	−0.0898	−0.3199	0.3011	0.053*
C9	−0.07202 (16)	−0.20896 (15)	0.0900 (5)	0.0355 (7)
C10	−0.08229 (19)	−0.23812 (17)	−0.1077 (5)	0.0474 (8)
H10	−0.0447	−0.2593	−0.1777	0.057*
C11	−0.1485 (2)	−0.2360 (2)	−0.2027 (6)	0.0582 (10)
H11	−0.1556	−0.2568	−0.3339	0.070*
C12	−0.20331 (19)	−0.2028 (2)	−0.1010 (7)	0.0569 (10)
H12	−0.2475	−0.2006	−0.1649	0.068*
C13	−0.1934 (2)	−0.17306 (19)	0.0930 (7)	0.0531 (10)
H13	−0.2307	−0.1504	0.1608	0.064*
C14	−0.12838 (18)	−0.17666 (16)	0.1875 (5)	0.0426 (7)
H14	−0.1222	−0.1568	0.3206	0.051*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Na	0.0486 (7)	0.0498 (7)	0.0257 (5)	−0.0052 (6)	−0.0084 (6)	−0.0012 (6)
O1	0.0555 (14)	0.0337 (10)	0.0298 (10)	−0.0036 (11)	−0.0002 (10)	0.0033 (10)
O2	0.0673 (15)	0.0506 (13)	0.0231 (9)	0.0076 (11)	−0.0037 (11)	−0.0072 (11)
C1	0.0334 (15)	0.0344 (15)	0.0260 (14)	−0.0008 (12)	0.0022 (12)	−0.0017 (14)
C2	0.0374 (14)	0.0382 (15)	0.0270 (12)	0.0047 (12)	0.0045 (15)	−0.0014 (14)
C3	0.0477 (17)	0.0345 (15)	0.0291 (15)	0.0041 (13)	−0.0017 (14)	−0.0030 (13)
C4	0.0437 (17)	0.0510 (18)	0.0395 (18)	0.0016 (14)	−0.0039 (15)	0.0026 (16)
C5	0.059 (2)	0.051 (2)	0.0493 (19)	0.0105 (17)	−0.0131 (18)	0.0031 (17)
C6	0.089 (3)	0.0368 (18)	0.049 (2)	0.0114 (18)	−0.005 (2)	0.0069 (16)
C7	0.075 (2)	0.0410 (18)	0.0471 (18)	−0.0122 (16)	0.006 (2)	0.0063 (17)
C8	0.0507 (19)	0.0394 (17)	0.0418 (18)	−0.0047 (14)	−0.0054 (16)	0.0004 (15)
C9	0.0434 (17)	0.0301 (15)	0.0331 (15)	−0.0017 (13)	−0.0035 (14)	0.0028 (14)
C10	0.054 (2)	0.051 (2)	0.0367 (16)	0.0049 (16)	0.0016 (16)	−0.0055 (16)
C11	0.074 (3)	0.064 (2)	0.0367 (18)	−0.010 (2)	−0.018 (2)	−0.0067 (19)
C12	0.045 (2)	0.061 (2)	0.064 (3)	−0.0045 (18)	−0.019 (2)	0.013 (2)
C13	0.047 (2)	0.0471 (19)	0.065 (3)	0.0018 (16)	0.0114 (19)	0.0099 (19)
C14	0.0505 (18)	0.0413 (17)	0.0360 (16)	−0.0016 (14)	0.0066 (17)	0.0007 (15)

Geometric parameters (Å, º)

Na—O2^i	2.207 (2)	C4—H4	0.9300
Na—O1	2.316 (2)	C5—C6	1.367 (5)
Na—O1^ii	2.344 (2)	C5—H5	0.9300
Na—O1^iii	2.467 (3)	C6—C7	1.370 (5)
Na—Na^iii	3.265 (3)	C6—H6	0.9300
Na—Na^iv	3.476 (2)	C7—C8	1.380 (5)
Na—Na^ii	3.476 (2)	C7—H7	0.9300
O1—C1	1.271 (3)	C8—H8	0.9300
O1—Na^iv	2.344 (2)	C9—C14	1.377 (5)
O1—Na^iii	2.467 (3)	C9—C10	1.382 (5)
O2—C1	1.226 (3)	C10—C11	1.391 (5)
O2—Na^v	2.207 (2)	C10—H10	0.9300
C1—C2	1.556 (4)	C11—C12	1.374 (6)
C2—C9	1.515 (4)	C11—H11	0.9300
C2—C3	1.527 (4)	C12—C13	1.365 (5)
C2—H2	0.9800	C12—H12	0.9300
C3—C4	1.387 (4)	C13—C14	1.372 (5)
C3—C8	1.389 (4)	C13—H13	0.9300
C4—C5	1.373 (5)	C14—H14	0.9300
O2^i—Na—O1	119.12 (9)	C4—C3—C8	118.0 (3)
O2^i—Na—O1^ii	149.79 (10)	C4—C3—C2	119.6 (3)
O1—Na—O1^ii	87.14 (8)	C8—C3—C2	122.2 (3)
O2^i—Na—O1^iii	107.87 (9)	C5—C4—C3	120.9 (3)
O1—Na—O1^iii	92.63 (9)	C5—C4—H4	119.5
O1^ii—Na—O1^iii	83.72 (8)	C3—C4—H4	119.5
O2^i—Na—Na^iii	115.84 (7)	C6—C5—C4	120.5 (3)
O1—Na—Na^iii	48.93 (7)	C6—C5—H5	119.8
O1^ii—Na—Na^iii	92.31 (6)	C4—C5—H5	119.8
O1^iii—Na—Na^iii	45.05 (6)	C5—C6—C7	119.6 (3)
O2^i—Na—Na^iv	158.97 (8)	C5—C6—H6	120.2
O1—Na—Na^iv	42.07 (5)	C7—C6—H6	120.2
O1^ii—Na—Na^iv	45.16 (6)	C6—C7—C8	120.5 (3)
O1^iii—Na—Na^iv	85.32 (6)	C6—C7—H7	119.8
Na^iii—Na—Na^iv	61.99 (2)	C8—C7—H7	119.8
O2^i—Na—Na^ii	143.75 (8)	C7—C8—C3	120.5 (3)
O1—Na—Na^ii	87.59 (6)	C7—C8—H8	119.8
O1^ii—Na—Na^ii	41.46 (6)	C3—C8—H8	119.8
O1^iii—Na—Na^ii	42.36 (5)	C14—C9—C10	118.3 (3)
Na^iii—Na—Na^ii	61.99 (2)	C14—C9—C2	121.3 (3)
Na^iv—Na—Na^ii	56.02 (4)	C10—C9—C2	120.4 (3)
C1—O1—Na	121.83 (17)	C9—C10—C11	120.5 (3)
C1—O1—Na^iv	133.15 (19)	C9—C10—H10	119.8
Na—O1—Na^iv	96.46 (9)	C11—C10—H10	119.8
C1—O1—Na^iii	114.31 (17)	C12—C11—C10	119.5 (3)
Na—O1—Na^iii	86.02 (9)	C12—C11—H11	120.3
Na^iv—O1—Na^iii	92.48 (9)	C10—C11—H11	120.3
C1—O2—Na^v	150.1 (2)	C13—C12—C11	120.5 (3)
O2—C1—O1	125.0 (3)	C13—C12—H12	119.8
O2—C1—C2	121.4 (3)	C11—C12—H12	119.8
O1—C1—C2	113.6 (2)	C12—C13—C14	119.7 (4)
C9—C2—C3	113.5 (2)	C12—C13—H13	120.2
C9—C2—C1	108.3 (2)	C14—C13—H13	120.2
C3—C2—C1	114.3 (2)	C13—C14—C9	121.5 (3)
C9—C2—H2	106.7	C13—C14—H14	119.2
C3—C2—H2	106.7	C9—C14—H14	119.2
C1—C2—H2	106.7
O2^i—Na—O1—C1	−16.1 (2)	O2—C1—C2—C3	−3.7 (4)
O1^ii—Na—O1—C1	148.2 (2)	O1—C1—C2—C3	175.7 (2)
O1^iii—Na—O1—C1	−128.26 (19)	C9—C2—C3—C4	179.8 (3)
Na^iii—Na—O1—C1	−116.3 (2)	C1—C2—C3—C4	−55.3 (4)
Na^iv—Na—O1—C1	151.7 (2)	C9—C2—C3—C8	5.2 (4)
Na^ii—Na—O1—C1	−170.3 (2)	C1—C2—C3—C8	130.2 (3)
O2^i—Na—O1—Na^iv	−167.79 (10)	C8—C3—C4—C5	−0.8 (5)
O1^ii—Na—O1—Na^iv	−3.53 (9)	C2—C3—C4—C5	−175.6 (3)
O1^iii—Na—O1—Na^iv	80.05 (9)	C3—C4—C5—C6	1.3 (5)
Na^iii—Na—O1—Na^iv	92.05 (9)	C4—C5—C6—C7	−1.1 (6)
Na^ii—Na—O1—Na^iv	37.98 (8)	C5—C6—C7—C8	0.4 (6)
O2^i—Na—O1—Na^iii	100.16 (10)	C6—C7—C8—C3	0.0 (5)
O1^ii—Na—O1—Na^iii	−95.58 (8)	C4—C3—C8—C7	0.1 (5)
O1^iii—Na—O1—Na^iii	−12.00 (10)	C2—C3—C8—C7	174.8 (3)
Na^iv—Na—O1—Na^iii	−92.05 (9)	C3—C2—C9—C14	83.6 (3)
Na^ii—Na—O1—Na^iii	−54.07 (7)	C1—C2—C9—C14	−44.5 (3)
Na^v—O2—C1—O1	55.5 (6)	C3—C2—C9—C10	−95.5 (3)
Na^v—O2—C1—C2	−125.1 (4)	C1—C2—C9—C10	136.4 (3)
Na—O1—C1—O2	29.2 (4)	C14—C9—C10—C11	−1.4 (4)
Na^iv—O1—C1—O2	169.0 (2)	C2—C9—C10—C11	177.8 (3)
Na^iii—O1—C1—O2	−71.8 (3)	C9—C10—C11—C12	1.8 (5)
Na—O1—C1—C2	−150.22 (18)	C10—C11—C12—C13	−1.0 (5)
Na^iv—O1—C1—C2	−10.5 (4)	C11—C12—C13—C14	−0.4 (5)
Na^iii—O1—C1—C2	108.8 (2)	C12—C13—C14—C9	0.9 (5)
O2—C1—C2—C9	123.9 (3)	C10—C9—C14—C13	0.0 (5)
O1—C1—C2—C9	−56.6 (3)	C2—C9—C14—C13	−179.1 (3)

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