Methyl 2,6-dicarboxybenzoate

Regioselective methyl­ation of hemimellitic acid in basic MeOH yields the title 2-methyl ester, C₁₀H₈O₆, of the parent acid. The asymmetric unit contains one complete mol­ecule, which packs with the methyl ester groups lying between zigzag chains produced by hydrogen bonding, utilizing the head-to-tail carboxylic acid-acid dimer motif.

S1. Comment

Derivatives of hemimellitic acid (benzene-1,2,3-tricarboxylic acid) have been studied little using single-crystal X-ray diffraction, with only eight crystal structures in the Cambridge Structural Database (September 2002 update; Allen, 2002), of which three are determinations of the commercially available dihydrate (Fornies-Marquina et al., 1972; Takusagawa & Shimada, 1973; Mo & Adman, 1975). The successful use of many of the other members of the benzenepolycarboxylic acid family, such as trimesic and terephthalic acids, in the formation of supramolecular organic (e.g. Sharma & Zaworotko, 1996) and inorganic (e.g. Groeneman et al., 1999) networks has inspired us to investigate the relatively inexpensive hemimellitic acid as a potential supramolecular building block and metal ligand. Hemimellitic acid possesses three adjacent carboxylic acid groups, providing the opportunity for coordination to metal cations as well as the creation of extended networks through hydrogen bonding.

The reaction of hemimellitic acid with 0.5 equivalents of M₂CO₃ (M is Li, Na or K) in the mixed solvent MeOH:H₂O (9:1) under reflux conditions affords, surprisingly, regioselective methylation at the 2-position. The title compound, (I), is produced as X-ray quality colourless crystals in almost quantitative yield from the slow evaporation of the cooled solution. In contrast, similar reactions with phthalic acid in pure H₂O have been shown to yield the expected group 1 metal salts (Smith, 1975a,b). \sch

Compound (I) has previously been synthesized from the reaction of hemimellitic anhydride with MeOH and characterized only by ¹H NMR spectroscopic data (Hurst & Bender, 1971), but here, both the single-crystal X-ray structure and ¹³C NMR data are presented. NMR spectroscopic analysis of the bulk sample indicated a single product, showing that the isomeric 1-ester and the possible di- and tri-esters (Kasina & Nematollaki, 1974; Revial et al., 1983) had not been produced.

The asymmetric unit of (I) contains one complete molecule (Fig. 1). The external carboxylic acid substituents at C1 and C3 form hydrogen bonds to one symmetry-related neighbour each, via the expected head-to-tail R₂²(8) carboxylic acid-acid synthon (Etter, 1990; Etter & MacDonald, 1990; Bernstein et al., 1995) (Table 2), producing zigzag chains (Fig. 2), reminiscent of the packing of isophthalic acid (Derissen, 1974). One outer Is this text change OK? carboxylic acid group of the parent acid dihydrate (Takusagawa & Shimada, 1973) adopts the R₂²(8) dimer motif, but, in contrast to (I), the other also forms a hydrogen bond to a water of crystallization. Protection of the central Is this text change OK? carboxylic acid group as the methyl ester prevents its involvement in strong hydrogen bonding and also modifies the angle of rotation of this carboxyl group with respect to the plane of the aromatic ring, from 86.8° in the parent acid to 76.7° in (I), allowing the Me group to lie between the chains. Weak C—H···O interactions [C···O 3.2193 (18)–3.9409 (16) Å; Desiraju & Steiner, 1999] between the zigzag chains produce sheets, and interactions between these sheets produce a loosely held three-dimensional structure.

S2. Experimental

Hemimellitic acid dihydrate (1 equivalent) was refluxed with M₂CO₃ (M is Li, Na or K, 0.5 equivalents) in a 9:1 mixture of MeOH:H₂O overnight. The solutions were allowed to cool and then left to evaporate. X-ray quality colourless crystals formed (m.p. 478–482 K; literature value 478–483 K; Hurst & Bender, 1971) and single-crystal X-ray analysis indicated that compound (I) had been synthesized in all three cases. Spectroscopic analysis: IR (KBr, ν_max, cm⁻¹): 3500–2500 (br, OH), 3087, 3046 and 3006 (aromatic C—H), 2948, 2884 and 2837 (Csp³—H), 1742 (C═O, ester), 1711 and 1692 (C═O, acid), 1586(C═C, aromatic), 1466, 1453, 1430 and 1415 (Csp³—H), 1308, 1275, 1123 and 1072 (C—O), 776 (aromatic C—H), 688; ¹H NMR (400 MHz, d⁶-DMSO, δ, p.p.m.): 8.13 (2H, d, J = 8 Hz), 7.68 (1H, t, J = 7.6 Hz), 3.77 (3H, s, Me); ¹³C NMR (100 MHz, d⁶-DMSO, δ, p.p.m.): 167.9 (C_quat, C═O, ester), 166.17 (2C_quat, C═O, acid), 135.6 (C_quat, Ar), 133.4 (2CH, Ar), 129.8 (CH, Ar), 129.6 (2C_quat, Ar), 52.2 (CH₃). The ¹H NMR results correspond to the previously reported data (Hurst & Bender, 1971).

S3. Refinement

Aromatic H atoms were placed in geometric positions (C—H distance of 0.95 Å) using a riding model, while the coordinates of Me and OH H atoms were freely refined. U_iso values were set to 1.2U_eq for aryl H atoms, and 1.5U_eq for methyl H and hydroxy H atoms.

Computing details

Data collection: SMART (Siemens, 1994); cell refinement: SAINT (Siemens, 1994); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Bruker, 1997); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL and local programs.

Figures

	Fig. 1. A view of the molecule of (I), showing the atom-labelling scheme and the selective methylation at the 2-position. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. A packing plot for (I), viewed down the crystallographic a axis, showing the head-to-tail carboxylic acid-acid hydrogen-bonded dimers forming chains. H atoms not involved in the hydrogen bonding have been omitted for clarity. Hydrogen bonding between molecules is indicated by dashed lines.

Methyl 2,6-dicarboxybenzoate

Crystal data

C10H8O6	F(000) = 464
Mr = 224.16	Dx = 1.574 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 3989 reflections
a = 5.0242 (3) Å	θ = 2.8–28.5°
b = 14.6263 (10) Å	µ = 0.13 mm^−1
c = 12.9024 (9) Å	T = 150 K
β = 93.835 (2)°	Column, colourless
V = 946.02 (11) Å^3	0.37 × 0.16 × 0.13 mm
Z = 4

Data collection

Bruker SMART1000 CCD area-detector diffractometer	2287 independent reflections
Radiation source: sealed tube	1847 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.017
ω rotation with narrow frames scans	θmax = 29.0°, θmin = 2.1°
Absorption correction: multi-scan (SADABS; Sheldrick, 2001)	h = −6→6
Tmin = 0.910, Tmax = 0.983	k = −19→18
8271 measured reflections	l = −16→16

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.033	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.094	w = 1/[σ^2(Fo^2) + (0.049P)^2 + 0.304P] where P = (Fo^2 + 2Fc^2)/3
S = 1.03	(Δ/σ)max < 0.001
2287 reflections	Δρmax = 0.35 e Å^−3
160 parameters	Δρmin = −0.19 e Å^−3
0 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.

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
C1	0.1473 (2)	0.85643 (8)	0.57870 (9)	0.0185 (3)
C2	−0.0190 (2)	0.80717 (8)	0.64147 (9)	0.0171 (2)
C3	−0.1562 (2)	0.85413 (8)	0.71625 (9)	0.0187 (3)
C4	−0.1256 (3)	0.94863 (9)	0.72779 (9)	0.0219 (3)
H4	−0.2229	0.9803	0.7772	0.026*
C5	0.0458 (3)	0.99649 (9)	0.66769 (10)	0.0228 (3)
H5	0.0695	1.0605	0.6771	0.027*
C6	0.1823 (2)	0.95049 (9)	0.59374 (9)	0.0214 (3)
H6A	0.3008	0.9831	0.5529	0.026*
C7	0.2753 (2)	0.81085 (8)	0.49142 (9)	0.0187 (2)
O1	0.4629 (2)	0.86057 (6)	0.45225 (8)	0.0281 (2)
H1	0.527 (4)	0.8315 (13)	0.3967 (15)	0.042*
O2	0.20699 (17)	0.73548 (6)	0.45825 (7)	0.0219 (2)
C8	−0.0442 (2)	0.70463 (8)	0.63100 (9)	0.0178 (2)
O3	0.12083 (18)	0.65121 (6)	0.66524 (7)	0.0240 (2)
O4	−0.27597 (17)	0.68290 (6)	0.58015 (7)	0.0212 (2)
C10	−0.3313 (3)	0.58629 (9)	0.57218 (12)	0.0283 (3)
H10A	−0.207 (4)	0.5573 (13)	0.5293 (14)	0.043*
H10B	−0.509 (4)	0.5815 (12)	0.5400 (14)	0.043*
H10C	−0.324 (4)	0.5587 (13)	0.6419 (15)	0.043*
C9	−0.3305 (2)	0.80378 (9)	0.78586 (9)	0.0190 (2)
O5	−0.31048 (19)	0.72128 (6)	0.80112 (7)	0.0251 (2)
O6	−0.50342 (19)	0.85625 (7)	0.82979 (7)	0.0264 (2)
H6	−0.605 (4)	0.8211 (13)	0.8697 (14)	0.040*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0178 (6)	0.0192 (6)	0.0186 (6)	−0.0002 (4)	0.0028 (4)	0.0005 (4)
C2	0.0161 (5)	0.0178 (6)	0.0173 (5)	−0.0007 (4)	0.0011 (4)	−0.0001 (4)
C3	0.0181 (6)	0.0210 (6)	0.0172 (5)	−0.0010 (4)	0.0034 (4)	0.0000 (4)
C4	0.0247 (6)	0.0215 (6)	0.0201 (6)	0.0004 (5)	0.0056 (5)	−0.0036 (5)
C5	0.0276 (6)	0.0168 (6)	0.0245 (6)	−0.0017 (5)	0.0043 (5)	−0.0016 (5)
C6	0.0225 (6)	0.0208 (6)	0.0213 (6)	−0.0035 (5)	0.0056 (5)	0.0016 (5)
C7	0.0187 (6)	0.0191 (6)	0.0187 (6)	0.0008 (4)	0.0045 (4)	0.0027 (5)
O1	0.0336 (5)	0.0216 (5)	0.0317 (5)	−0.0051 (4)	0.0206 (4)	−0.0032 (4)
O2	0.0230 (5)	0.0217 (5)	0.0216 (4)	−0.0025 (3)	0.0072 (3)	−0.0032 (3)
C8	0.0190 (6)	0.0196 (6)	0.0155 (5)	−0.0014 (4)	0.0075 (4)	0.0002 (4)
O3	0.0225 (5)	0.0218 (5)	0.0281 (5)	0.0029 (3)	0.0051 (4)	0.0047 (4)
O4	0.0229 (4)	0.0169 (4)	0.0239 (4)	−0.0024 (3)	0.0014 (3)	−0.0013 (3)
C10	0.0315 (7)	0.0177 (6)	0.0357 (8)	−0.0050 (5)	0.0017 (6)	−0.0031 (5)
C9	0.0184 (6)	0.0220 (6)	0.0170 (5)	−0.0013 (4)	0.0035 (4)	−0.0022 (4)
O5	0.0297 (5)	0.0206 (5)	0.0264 (5)	−0.0017 (4)	0.0130 (4)	0.0000 (4)
O6	0.0289 (5)	0.0241 (5)	0.0280 (5)	0.0034 (4)	0.0158 (4)	0.0038 (4)

Geometric parameters (Å, º)

C1—C6	1.3989 (17)	C7—O2	1.2233 (15)
C1—C2	1.4014 (16)	C7—O1	1.3177 (14)
C1—C7	1.4911 (16)	O1—H1	0.91 (2)
C2—C3	1.4024 (16)	C8—O3	1.2019 (15)
C2—C8	1.5104 (17)	C8—O4	1.3359 (15)
C3—C4	1.3976 (17)	O4—C10	1.4424 (16)
C3—C9	1.4902 (16)	C10—H10A	0.960 (19)
C4—C5	1.3862 (17)	C10—H10B	0.96 (2)
C4—H4	0.9500	C10—H10C	0.984 (19)
C5—C6	1.3861 (17)	C9—O5	1.2256 (15)
C5—H5	0.9500	C9—O6	1.3156 (15)
C6—H6A	0.9500	O6—H6	0.907 (19)
C6—C1—C2	119.99 (11)	O2—C7—O1	123.67 (11)
C6—C1—C7	119.20 (11)	O2—C7—C1	122.81 (11)
C2—C1—C7	120.71 (11)	O1—C7—C1	113.50 (10)
C1—C2—C3	119.04 (11)	C7—O1—H1	110.1 (12)
C1—C2—C8	120.56 (10)	O3—C8—O4	125.67 (11)
C3—C2—C8	120.38 (10)	O3—C8—C2	124.12 (11)
C4—C3—C2	120.12 (11)	O4—C8—C2	110.20 (10)
C4—C3—C9	119.31 (11)	C8—O4—C10	115.18 (10)
C2—C3—C9	120.55 (11)	O4—C10—H10A	110.1 (11)
C5—C4—C3	120.53 (11)	O4—C10—H10B	105.8 (11)
C5—C4—H4	119.7	H10A—C10—H10B	109.9 (15)
C3—C4—H4	119.7	O4—C10—H10C	110.0 (11)
C6—C5—C4	119.62 (12)	H10A—C10—H10C	111.0 (15)
C6—C5—H5	120.2	H10B—C10—H10C	110.1 (15)
C4—C5—H5	120.2	O5—C9—O6	123.68 (11)
C5—C6—C1	120.63 (11)	O5—C9—C3	122.56 (11)
C5—C6—H6A	119.7	O6—C9—C3	113.76 (11)
C1—C6—H6A	119.7
C6—C1—C2—C3	2.39 (17)	C6—C1—C7—O2	−162.62 (12)
C7—C1—C2—C3	−173.89 (10)	C2—C1—C7—O2	13.69 (18)
C6—C1—C2—C8	−175.86 (11)	C6—C1—C7—O1	15.70 (16)
C7—C1—C2—C8	7.87 (17)	C2—C1—C7—O1	−167.99 (11)
C1—C2—C3—C4	−0.26 (17)	C1—C2—C8—O3	76.86 (16)
C8—C2—C3—C4	177.99 (11)	C3—C2—C8—O3	−101.36 (14)
C1—C2—C3—C9	−178.75 (11)	C1—C2—C8—O4	−104.08 (12)
C8—C2—C3—C9	−0.50 (17)	C3—C2—C8—O4	77.69 (13)
C2—C3—C4—C5	−1.79 (18)	O3—C8—O4—C10	3.45 (17)
C9—C3—C4—C5	176.72 (12)	C2—C8—O4—C10	−175.59 (10)
C3—C4—C5—C6	1.69 (19)	C4—C3—C9—O5	−158.93 (12)
C4—C5—C6—C1	0.48 (19)	C2—C3—C9—O5	19.58 (18)
C2—C1—C6—C5	−2.53 (18)	C4—C3—C9—O6	20.24 (16)
C7—C1—C6—C5	173.80 (11)	C2—C3—C9—O6	−161.26 (11)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O1—H1···O5^i	0.91 (2)	1.71 (2)	2.6131 (13)	172 (2)
O6—H6···O2^ii	0.91 (2)	1.74 (2)	2.6433 (13)	173 (2)
C4—H4···O3^iii	0.95	2.65	3.2681 (15)	123
C5—H5···O1^iv	0.95	3.19	3.6564 (16)	112
C5—H5···O3^iii	0.95	2.66	3.2725 (15)	122
C5—H5···O5^iii	0.95	2.65	3.5594 (16)	160
C6—H6A···O1^iv	0.95	2.58	3.3627 (15)	140
C10—H10B···O6^v	0.96 (2)	2.86 (2)	3.2975 (18)	108 (2)
C10—H10C···O6^vi	0.98 (2)	3.11 (2)	3.7083 (17)	120 (2)

Symmetry codes: (i) x+1, −y+3/2, z−1/2; (ii) x−1, −y+3/2, z+1/2; (iii) −x, y+1/2, −z+3/2; (iv) −x+1, −y+2, −z+1; (v) x, −y+3/2, z−1/2; (vi) −x−1, y−1/2, −z+3/2.