A new polymorph of 2,6-dimeth­oxy­benzoic acid

A new crystalline form of 2,6-dimeth­oxy­benzoic acid, C₉H₁₀O₄, crystallizing in a tetra­gonal unit cell has been identified during screening for co-crystals. The asymmetric unit comprises a non-planar independent mol­ecule with a synplanar conformation of the carb­oxy group. The sterically bulky o-meth­oxy substituents force the carb­oxy group to be twisted away from the plane of the benzene ring by 65.72 (15)°. The carb­oxy group is disordered over two sites about the C—C bond [as indicated by the almost equal C—O distances of 1.254 (3) and 1.250 (3) Å], the occupancies of the disordered carboxym H atoms being 0.53 (5) and 0.47 (5). In the known ortho­rhom­bic form reported by Swaminathan et al. [Acta Cryst. (1976), B32, 1897–1900], due to the anti­planar conformation adopted by the OH group, the mol­ecular components are associated in the crystal in chains stabilized by linear O—H...O hydrogen bonds. However, in the new tetra­gonal polymorph, mol­ecules form dimeric units via pairs of O—H...O hydrogen bonds between the carb­oxy groups.

Key indicators

• Single-crystal X-ray study
• T = 298 K
• Mean [sigma](C-C) = 0.004 Å
• R factor = 0.062
• wR factor = 0.147
• Data-to-parameter ratio = 12.4

checkCIF/PLATON results

No syntax errors found



Alert level C
PLAT222_ALERT_3_C Large Non-Solvent    H    Uiso(max)/Uiso(min) ..        6.0 Ratio
PLAT245_ALERT_2_C U(iso) H1      Smaller than U(eq) O1      by ...      0.033 AngSq
PLAT245_ALERT_2_C U(iso) H2      Smaller than U(eq) O2      by ...      0.020 AngSq
PLAT340_ALERT_3_C Low Bond Precision on  C-C Bonds ...............     0.0041 Ang
PLAT910_ALERT_3_C Missing # of FCF Reflections Below Th(Min) .....          1

Alert level G
REFLT03_ALERT_4_G Please check that the estimate of the number of Friedel pairs is
            correct. If it is not, please give the correct count in the
            _publ_section_exptl_refinement section of the submitted CIF.
           From the CIF: _diffrn_reflns_theta_max           30.00
           From the CIF: _reflns_number_total               1653
           Count of symmetry unique reflns         1654
           Completeness (_total/calc)             99.94%
           TEST3: Check Friedels for noncentro structure
           Estimate of Friedel pairs measured         0
           Fraction of Friedel pairs measured     0.000
           Are heavy atom types Z>Si present         no
PLAT005_ALERT_5_G No _iucr_refine_instructions_details in CIF ....          ?
PLAT152_ALERT_1_G The Supplied and Calc. Volume s.u. Differ by ...          3 Units

   0 ALERT level A = Most likely a serious problem - resolve or explain
   0 ALERT level B = A potentially serious problem, consider carefully
   5 ALERT level C = Check. Ensure it is not caused by an omission or oversight
   3 ALERT level G = General information/check it is not something unexpected

   1 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   2 ALERT type 2 Indicator that the structure model may be wrong or deficient
   3 ALERT type 3 Indicator that the structure quality may be low
   1 ALERT type 4 Improvement, methodology, query or suggestion
   1 ALERT type 5 Informative message, check

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S1. Comment

In this paper it is reported the crystal structure of a new polymorph, (I), of 2,6-dimethoxybenzoic acid, produced unexpectedly during an attempt to synthesize cocrystals of boronic acid with of 2,6-dimethoxybenzoic acid. The known form, (II) (Fig. 3), of of 2,6-dimethoxybenzoic acid is orthorhombic in the space group P2₁2₁2₁ and crystallizes with one molecule in the asymmetric unit (Swaminathan et al., 1976; Bryan & White, 1982; Portalone, 2009). In (II), due to the antiplanar conformation adopted by the OH group, the molecular components are associated in the crystal in chains stabilized by linear O—H···O hydrogen bonds.

The title new polymorph (I) is tetragonal in the space group P4₁2₁2. The asymmetric unit of (I) comprises a non-planar independent molecule, as the o-methoxy substituents force the carboxy group to be twisted away from the plane of the phenyl ring by 65.72 (15)° (Fig. 1). The carboxy group, which adopts a synplanar conformation, is almost completely disordered, as indicated by the equal C—O distances, 1.254 (3) and 1.250 (3) Å, the C—C—O angles, 118.9 (2) and 117.8 (2)°, and by the presence of disordered H atoms with occupancy factors of 0.53 (5) and 0.47 (5) in the O···O intermolecular hydrogen bond. The pattern of bond lengths and bond angles of the phenyl ring is consistent with that reported in the structure determination of (II), and a comparison of the present results with those obtained for similar benzene derivatives in the gas phase (Schultz et al., 1993; Portalone et al., 1998) shows no appreciable effects of the crystal environment on the ring deformation induced by substituents. Analysis of the crystal packing of (I), (Fig. 2), shows that the molecular components form the conventional dimeric units observed in benzoic acids (Leiserowitz, 1976; Kanters et al., 1991; Moorthy et al., 2002). Indeed, the structure is stabilized by usual intermolecular C²₂(8) O—H···O interactions (Etter et al., 1990; Bernstein et al., 1995; Motherwell et al., 1999) (Table 1) which link the molecules into dimers through the disordered carboxy moieties [symmetry code: (i) -y + 1, -x + 1, -z + 3/2].

Related literature

For the orthorhombic polymorph of 2,6-dimethoxybenzoic acid, see: Swaminathan et al. (1976); Bryan & White (1982); Portalone (2009). For molecular packing modes of carboxylic acids, see: Leiserowitz (1976); Kanters et al. (1991); Moorthy et al. (2002). For analysis of benzene ring deformations induced by substitution, see: Schultz et al. (1993); Portalone et al. (1998); For computation of ring patterns formed by hydrogen bonds in crystal structures, see: Etter et al. (1990); Bernstein et al. (1995); Motherwell et al. (1999).

S2. Experimental

Polymorph (I) was formed during cocrystallization in a 1:1 molar ratio of 2,6-dimethoxybenzoic acid (1 mmol, Sigma Aldrich at 99% purity) and phenylboronic acid (1 mmol, Sigma Aldrich at 97% purity). The two components were dissolved in water (10 ml) and gently heated under reflux for 3 h. After cooling the solution to an ambient temperature, only one crystal suitable for single-crystal X-ray diffraction was grown by slow evaporation of the solvent after two weeks. Unfortunately, any attempts to produce more crystals of polymorph (I) by repeating the crystallization conditions were unsuccessful. Crystallization of 2,6-dimethoxybenzoic acid carried out under a wide range of different sets of conditions (different solvents, different molar ratio, different cosolute molecules) led systematically to the orthorhombic polymorph.

S3. Refinement

All H atoms were identified in difference Fourier maps, but for refinement all C-bound H atoms were placed in calculated positions, with C—H = 0.97 Å (phenyl) and 0.97–0.98 Å (methyl), and refined as riding on their carrier atoms. The U_iso values were kept equal to 1.2U_eq(C, phenyl). and to 1.5U_eq(C, methyl). The remaining two half H atoms of the carboxy group were freely refined and their occupancy factors constrained to sum to unity. In the absence of significant anomalous scattering in this light-atom study, Friedel pairs were merged.

Computing details

Data collection: CrysAlis CCD (Oxford Diffraction, 2006); cell refinement: CrysAlis RED (Oxford Diffraction, 2006); data reduction: CrysAlis RED (Oxford Diffraction, 2006); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures

	Fig. 1. The molecular structure of (I), showing the atom-labelling scheme. Displacements ellipsoids are at the 50% probability level.
	Fig. 2. Crystal packing diagram for (I) viewed approximately down a. All atoms are shown as small spheres of arbitrary radii. For the sake of clarity, H atoms not involved in hydrogen bonding have been omitted. Hydrogen bonding is indicated by dashed lines.
	Fig. 3. A scheme showing antiplanar and synplanar conformations of the carboxy group.

2,6-dimethoxybenzoic acid

Crystal data

C9H10O4	Dx = 1.319 Mg m^−3
Mr = 182.17	Mo Kα radiation, λ = 0.71069 Å
Tetragonal, P41212	Cell parameters from 4278 reflections
Hall symbol: P 4abw 2nw	θ = 2.9–32.3°
a = 8.1423 (3) Å	µ = 0.11 mm^−1
c = 27.6814 (18) Å	T = 298 K
V = 1835.20 (15) Å^3	Tablets, colourless
Z = 8	0.30 × 0.25 × 0.21 mm
F(000) = 768

Data collection

Oxford Diffraction Xcalibur S CCD diffractometer	1653 independent reflections
Radiation source: Enhance (Mo) X-ray source	1332 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.042
Detector resolution: 16.0696 pixels mm^-1	θmax = 30.0°, θmin = 2.9°
ω and φ scans	h = −11→10
Absorption correction: multi-scan (CrysAlis RED; Oxford Diffraction, 2006).	k = −8→11
Tmin = 0.878, Tmax = 0.999	l = −38→38
11616 measured reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.062	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.147	w = 1/[σ^2(Fo^2) + (0.0659P)^2 + 0.2634P] where P = (Fo^2 + 2Fc^2)/3
S = 1.19	(Δ/σ)max < 0.001
1653 reflections	Δρmax = 0.18 e Å^−3
133 parameters	Δρmin = −0.17 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.017 (3)

Special details

Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(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	Occ. (<1)
O1	0.7090 (3)	0.1661 (3)	0.71019 (8)	0.0558 (6)
H1	0.733 (5)	0.209 (5)	0.7340 (15)	0.023 (14)*	0.53 (5)
O2	0.5271 (3)	0.3665 (2)	0.70822 (7)	0.0508 (6)
H2	0.566 (7)	0.393 (7)	0.7333 (17)	0.031 (18)*	0.47 (5)
O3	0.7742 (3)	0.2435 (3)	0.60828 (6)	0.0652 (7)
O4	0.2838 (3)	0.1176 (3)	0.68526 (7)	0.0618 (6)
C1	0.5250 (3)	0.1775 (3)	0.64409 (7)	0.0362 (6)
C2	0.6221 (4)	0.1801 (3)	0.60276 (8)	0.0456 (7)
C3	0.5579 (5)	0.1225 (4)	0.55922 (9)	0.0644 (9)
H3	0.6230	0.1245	0.5299	0.077*
C4	0.3993 (6)	0.0628 (5)	0.55889 (11)	0.0749 (11)
H4	0.3547	0.0220	0.5287	0.090*
C5	0.3025 (5)	0.0583 (4)	0.59895 (12)	0.0653 (9)
H5	0.1917	0.0147	0.5972	0.078*
C6	0.3652 (4)	0.1176 (3)	0.64256 (9)	0.0447 (7)
C7	0.5920 (3)	0.2410 (3)	0.69057 (7)	0.0329 (5)
C8	0.8729 (5)	0.2678 (4)	0.56701 (12)	0.0722 (11)
H8A	0.889 (3)	0.164 (2)	0.5507 (7)	0.108*
H8B	0.978 (3)	0.312 (3)	0.5767 (3)	0.108*
H8C	0.819 (2)	0.344 (3)	0.5453 (7)	0.108*
C9	0.1147 (5)	0.0710 (6)	0.68556 (15)	0.0917 (14)
H9A	0.0518 (15)	0.147 (3)	0.6655 (11)	0.138*
H9B	0.0730 (16)	0.074 (4)	0.7186 (7)	0.138*
H9C	0.1036 (7)	−0.040 (3)	0.6728 (11)	0.138*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0679 (14)	0.0562 (13)	0.0432 (9)	0.0162 (11)	−0.0267 (10)	−0.0181 (9)
O2	0.0616 (13)	0.0497 (12)	0.0412 (9)	0.0118 (10)	−0.0217 (10)	−0.0159 (9)
O3	0.0665 (15)	0.0862 (17)	0.0428 (10)	−0.0139 (13)	0.0104 (10)	−0.0077 (11)
O4	0.0504 (13)	0.0836 (16)	0.0513 (10)	−0.0242 (12)	−0.0085 (10)	0.0041 (11)
C1	0.0506 (15)	0.0330 (12)	0.0251 (9)	−0.0008 (11)	−0.0133 (10)	−0.0031 (9)
C2	0.0618 (18)	0.0448 (15)	0.0302 (10)	0.0038 (14)	−0.0069 (12)	−0.0045 (11)
C3	0.094 (3)	0.069 (2)	0.0306 (12)	0.014 (2)	−0.0103 (15)	−0.0175 (14)
C4	0.095 (3)	0.083 (2)	0.0461 (16)	0.007 (2)	−0.0347 (18)	−0.0298 (17)
C5	0.068 (2)	0.064 (2)	0.0636 (17)	−0.0076 (16)	−0.0356 (17)	−0.0177 (17)
C6	0.0530 (17)	0.0412 (14)	0.0400 (12)	−0.0028 (13)	−0.0173 (12)	−0.0035 (11)
C7	0.0373 (13)	0.0370 (13)	0.0242 (8)	−0.0047 (10)	−0.0050 (9)	−0.0023 (9)
C8	0.095 (3)	0.057 (2)	0.0641 (18)	−0.0006 (19)	0.0319 (19)	−0.0022 (17)
C9	0.063 (3)	0.123 (4)	0.089 (3)	−0.034 (2)	−0.007 (2)	0.003 (3)

Geometric parameters (Å, º)

O1—C7	1.254 (3)	C3—C4	1.380 (6)
O1—H1	0.77 (4)	C3—H3	0.9700
O2—C7	1.250 (3)	C4—C5	1.361 (5)
O2—H2	0.79 (5)	C4—H4	0.9700
O3—C2	1.350 (4)	C5—C6	1.397 (4)
O3—C8	1.411 (4)	C5—H5	0.9700
O4—C6	1.355 (4)	C8—H8A	0.9684
O4—C9	1.428 (5)	C8—H8B	0.9684
C1—C2	1.391 (4)	C8—H8C	0.9684
C1—C6	1.391 (4)	C9—H9A	0.9766
C1—C7	1.490 (3)	C9—H9B	0.9766
C2—C3	1.395 (4)	C9—H9C	0.9766
C7—O1—H1	110 (3)	O4—C6—C1	115.5 (2)
C7—O2—H2	113 (4)	O4—C6—C5	125.1 (3)
C2—O3—C8	119.0 (2)	C1—C6—C5	119.3 (3)
C6—O4—C9	118.5 (3)	O2—C7—O1	123.3 (2)
C2—C1—C6	120.8 (2)	O2—C7—C1	117.8 (2)
C2—C1—C7	119.8 (2)	O1—C7—C1	118.9 (2)
C6—C1—C7	119.4 (2)	O3—C8—H8A	109.5
O3—C2—C1	115.7 (2)	O3—C8—H8B	109.5
O3—C2—C3	124.8 (3)	H8A—C8—H8B	109.5
C1—C2—C3	119.5 (3)	O3—C8—H8C	109.5
C4—C3—C2	118.4 (3)	H8A—C8—H8C	109.5
C4—C3—H3	120.8	H8B—C8—H8C	109.5
C2—C3—H3	120.8	O4—C9—H9A	109.5
C5—C4—C3	123.1 (3)	O4—C9—H9B	109.5
C5—C4—H4	118.5	H9A—C9—H9B	109.5
C3—C4—H4	118.5	O4—C9—H9C	109.5
C4—C5—C6	118.9 (3)	H9A—C9—H9C	109.5
C4—C5—H5	120.6	H9B—C9—H9C	109.5
C6—C5—H5	120.6
C8—O3—C2—C1	−172.8 (3)	C9—O4—C6—C5	−7.4 (5)
C8—O3—C2—C3	5.7 (5)	C2—C1—C6—O4	178.5 (3)
C6—C1—C2—O3	178.9 (2)	C7—C1—C6—O4	−2.0 (4)
C7—C1—C2—O3	−0.6 (4)	C2—C1—C6—C5	0.4 (4)
C6—C1—C2—C3	0.3 (4)	C7—C1—C6—C5	179.9 (3)
C7—C1—C2—C3	−179.2 (3)	C4—C5—C6—O4	−178.6 (3)
O3—C2—C3—C4	−179.2 (3)	C4—C5—C6—C1	−0.8 (5)
C1—C2—C3—C4	−0.8 (5)	C2—C1—C7—O2	114.4 (3)
C2—C3—C4—C5	0.4 (6)	C6—C1—C7—O2	−65.0 (3)
C3—C4—C5—C6	0.3 (6)	C2—C1—C7—O1	−66.0 (3)
C9—O4—C6—C1	174.7 (3)	C6—C1—C7—O1	114.5 (3)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···O1^i	0.77 (4)	1.87 (4)	2.632 (4)	168 (5)
O2—H2···O2^i	0.79 (5)	1.83 (5)	2.618 (4)	173 (5)

Symmetry code: (i) −y+1, −x+1, −z+3/2.