Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270111030538/eg3074sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270111030538/eg3074Isup2.hkl | |
MDL mol file https://doi.org/10.1107/S0108270111030538/eg3074Isup4.mol | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270111030538/eg3074IIsup3.hkl | |
MDL mol file https://doi.org/10.1107/S0108270111030538/eg3074IIsup5.mol | |
Chemical Markup Language (CML) file https://doi.org/10.1107/S0108270111030538/eg3074Isup6.cml | |
Chemical Markup Language (CML) file https://doi.org/10.1107/S0108270111030538/eg3074IIsup7.cml |
CCDC references: 846637; 846638
2,5-Dimethoxyterephthalic acid, (I), was synthesized according to a modification of the literature procedure of Passaniti et al. (2002) by refluxing diethyl 2,5-dihydroxyterephthalate (1.0 g, 4.1 mmol) and methyl iodide (1.3 ml, 20.7 mmol) in a suspension of K2CO3 (2.85 g, 20.7 mmol) and dry acetone (20 ml) for 48 h. To remove excess methyl iodide, methanol (15 ml) was added and the suspension refluxed for a further 48 h. After cooling the reaction mixture to room temperature, the remaining solid residue was filtered off and diethyl 2,5-dimethoxyterephthalate was precipitated by the addition of water. Compound (I) was obtained by refluxing diethyl 2,5-dimethoxyterephthalate in a tenfold amount of an aqueous 30% KOH solution for 12 h. After cooling the reaction mixture to room temperature, 6 M HCl was added to cause precipitation of the product, which was washed with water and dried at 373 K for 12 h (yield 76%, m.p. = 538 K). Spectroscopic analysis: 1H NMR (DMSO-d6, 500.13 MHz, δ, p.p.m.): 3.79 (s, 6H, CH3), 7.30 (s, 2H, Ar–H), 13.02 (s, br, 2H, OH); 13C NMR (DMSO-d6, 125.76 MHz, δ, p.p.m.): 56.7 (CH3), 114.8 (ArC–COOH), 125.4 (ArC–H), 151.4 (ArC–OCH3), 167.1 (COOH).
2,5-Diethoxyterephthalic acid, (II), was synthesized according to the procedure described above but using ethyl iodide (1.7 ml, 20.7 mmol) instead of methyl iodide for the alkylation step (yield 81%, m.p. = 523 K). Spectroscopic analysis: 1H NMR (DMSO-d6, 500.13 MHz, δ, p.p.m.): 2.28 (t, 6H, CH3, 3JHH = 7.1 Hz), 4.20 (q, 4H, CH2, 3JHH = 7.0 Hz), 7.24 (s, 2H, Ar–H), 12.95 (s, br, 2H, OH); 13C NMR (DMSO-d6, 125.76 MHz, δ, p.p.m.): 15.1 (CH3), 65.4 (CH2), 116.1 (ArC–COOH), 126.1 (ArC–H), 150.7 (ArC–OCH2CH3), 167.3 (COOH).
Crystals of (I) and (II) suitable for X-ray crystallographic determinations were afforded by slow evaporation of the solvent from solutions in acetone.
For both compounds, H atoms were positioned geometrically and allowed to ride on their respective parent atoms, with C—H = 0.98 Å and Uiso(H) = 1.5Ueq(C) for methyl, C—H = 0.99 Å and Uiso(H) = 1.2Ueq(C) for methylene, C—H = 0.95 Å and Uiso(H) = 1.2Ueq(C) for aryl, and O—H = 0.84 Å and Uiso(H) = 1.5Ueq(O) for carboxy H atoms.
For both compounds, data collection: APEX2 (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT (Bruker, 2007); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2009).
C10H10O6 | F(000) = 472 |
Mr = 226.18 | Dx = 1.573 Mg m−3 |
Monoclinic, C2/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -C 2yc | Cell parameters from 2686 reflections |
a = 11.6424 (3) Å | θ = 2.6–32.0° |
b = 10.7368 (3) Å | µ = 0.13 mm−1 |
c = 7.6554 (2) Å | T = 153 K |
β = 93.804 (2)° | Piece, colourless |
V = 954.83 (4) Å3 | 0.45 × 0.18 × 0.13 mm |
Z = 4 |
Bruker Kappa APEXII CCD area-detector diffractometer | 985 independent reflections |
Radiation source: fine-focus sealed tube | 869 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.025 |
ϕ and ω scans | θmax = 26.5°, θmin = 2.6° |
Absorption correction: multi-scan (SADABS; Bruker, 2007) | h = −14→14 |
Tmin = 0.915, Tmax = 0.983 | k = −13→13 |
5259 measured reflections | l = −8→9 |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.029 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.084 | H-atom parameters constrained |
S = 1.10 | w = 1/[σ2(Fo2) + (0.0531P)2 + 0.1541P] where P = (Fo2 + 2Fc2)/3 |
985 reflections | (Δ/σ)max < 0.001 |
75 parameters | Δρmax = 0.28 e Å−3 |
0 restraints | Δρmin = −0.18 e Å−3 |
C10H10O6 | V = 954.83 (4) Å3 |
Mr = 226.18 | Z = 4 |
Monoclinic, C2/c | Mo Kα radiation |
a = 11.6424 (3) Å | µ = 0.13 mm−1 |
b = 10.7368 (3) Å | T = 153 K |
c = 7.6554 (2) Å | 0.45 × 0.18 × 0.13 mm |
β = 93.804 (2)° |
Bruker Kappa APEXII CCD area-detector diffractometer | 985 independent reflections |
Absorption correction: multi-scan (SADABS; Bruker, 2007) | 869 reflections with I > 2σ(I) |
Tmin = 0.915, Tmax = 0.983 | Rint = 0.025 |
5259 measured reflections |
R[F2 > 2σ(F2)] = 0.029 | 0 restraints |
wR(F2) = 0.084 | H-atom parameters constrained |
S = 1.10 | Δρmax = 0.28 e Å−3 |
985 reflections | Δρmin = −0.18 e Å−3 |
75 parameters |
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. |
x | y | z | Uiso*/Ueq | ||
O1 | 0.27472 (6) | 1.05685 (8) | 0.24544 (10) | 0.0335 (2) | |
O2 | 0.26176 (6) | 0.85310 (8) | 0.22894 (10) | 0.0308 (2) | |
H2 | 0.2146 | 0.7997 | 0.1881 | 0.046* | |
O3 | 0.07663 (6) | 0.76201 (7) | 0.07749 (9) | 0.0278 (2) | |
C1 | 0.10580 (8) | 0.97913 (9) | 0.09275 (11) | 0.0202 (2) | |
C2 | 0.03630 (8) | 0.87848 (9) | 0.03664 (12) | 0.0203 (2) | |
C3 | −0.06876 (8) | 0.90021 (9) | −0.05539 (11) | 0.0207 (2) | |
H3 | −0.1158 | 0.8318 | −0.0932 | 0.025* | |
C4 | 0.22036 (8) | 0.96682 (10) | 0.19445 (13) | 0.0232 (3) | |
C5 | 0.00797 (10) | 0.65632 (10) | 0.02415 (14) | 0.0276 (3) | |
H5A | −0.0664 | 0.6611 | 0.0767 | 0.041* | |
H5B | 0.0479 | 0.5798 | 0.0630 | 0.041* | |
H5C | −0.0046 | 0.6557 | −0.1037 | 0.041* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O1 | 0.0229 (4) | 0.0328 (5) | 0.0429 (5) | −0.0063 (3) | −0.0119 (3) | 0.0015 (3) |
O2 | 0.0207 (4) | 0.0289 (5) | 0.0413 (5) | 0.0009 (3) | −0.0094 (3) | 0.0001 (3) |
O3 | 0.0234 (4) | 0.0201 (4) | 0.0382 (4) | −0.0015 (3) | −0.0096 (3) | 0.0006 (3) |
C1 | 0.0152 (5) | 0.0265 (5) | 0.0187 (4) | −0.0021 (4) | 0.0008 (3) | 0.0008 (3) |
C2 | 0.0187 (5) | 0.0216 (5) | 0.0205 (4) | −0.0011 (4) | 0.0010 (4) | 0.0010 (3) |
C3 | 0.0178 (5) | 0.0223 (5) | 0.0216 (5) | −0.0052 (4) | 0.0000 (4) | −0.0011 (4) |
C4 | 0.0173 (5) | 0.0280 (5) | 0.0240 (5) | −0.0029 (4) | −0.0014 (4) | 0.0011 (4) |
C5 | 0.0290 (6) | 0.0202 (5) | 0.0325 (5) | −0.0035 (4) | −0.0072 (4) | −0.0010 (4) |
O1—C4 | 1.2061 (12) | C1—C4 | 1.5049 (14) |
O2—C4 | 1.3327 (13) | C2—C3 | 1.3902 (14) |
O2—H2 | 0.84 | C3—C1i | 1.3891 (14) |
O3—C2 | 1.3647 (12) | C3—H3 | 0.95 |
O3—C5 | 1.4318 (12) | C5—H5A | 0.98 |
C1—C3i | 1.3891 (14) | C5—H5B | 0.98 |
C1—C2 | 1.4003 (14) | C5—H5C | 0.98 |
C4—O2—H2 | 109.5 | C2—C3—H3 | 119.6 |
C2—O3—C5 | 118.94 (7) | O1—C4—O2 | 119.67 (9) |
C3i—C1—C2 | 119.43 (9) | O1—C4—C1 | 121.68 (9) |
C3i—C1—C4 | 116.15 (9) | O2—C4—C1 | 118.65 (8) |
C2—C1—C4 | 124.41 (9) | O3—C5—H5A | 109.5 |
O3—C2—C3 | 123.21 (9) | O3—C5—H5B | 109.5 |
O3—C2—C1 | 116.99 (8) | H5A—C5—H5B | 109.5 |
C3—C2—C1 | 119.80 (9) | O3—C5—H5C | 109.5 |
C1i—C3—C2 | 120.77 (9) | H5A—C5—H5C | 109.5 |
C1i—C3—H3 | 119.6 | H5B—C5—H5C | 109.5 |
C5—O3—C2—C3 | −0.21 (13) | O3—C2—C3—C1i | 179.72 (8) |
C5—O3—C2—C1 | 179.44 (8) | C1—C2—C3—C1i | 0.08 (15) |
C3i—C1—C2—O3 | −179.74 (8) | C3i—C1—C4—O1 | 2.44 (14) |
C4—C1—C2—O3 | −0.24 (14) | C2—C1—C4—O1 | −177.08 (9) |
C3i—C1—C2—C3 | −0.07 (15) | C3i—C1—C4—O2 | −177.85 (8) |
C4—C1—C2—C3 | 179.43 (8) | C2—C1—C4—O2 | 2.64 (15) |
Symmetry code: (i) −x, −y+2, −z. |
D—H···A | D—H | H···A | D···A | D—H···A |
O2—H2···O3 | 0.84 | 1.81 | 2.5720 (10) | 150 |
C5—H5A···O1ii | 0.98 | 2.58 | 3.4634 (14) | 150 |
C5—H5B···O1iii | 0.98 | 2.46 | 3.1718 (13) | 129 |
Symmetry codes: (ii) x−1/2, y−1/2, z; (iii) −x+1/2, y−1/2, −z+1/2. |
C12H14O6 | F(000) = 268 |
Mr = 254.23 | Dx = 1.453 Mg m−3 |
Monoclinic, P21/n | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2yn | Cell parameters from 2381 reflections |
a = 3.9796 (2) Å | θ = 2.6–32.1° |
b = 15.7498 (7) Å | µ = 0.12 mm−1 |
c = 9.3833 (4) Å | T = 153 K |
β = 98.829 (1)° | Prism, colourless |
V = 581.16 (5) Å3 | 0.25 × 0.22 × 0.04 mm |
Z = 2 |
Bruker Kappa APEXII CCD area-detector diffractometer | 1331 independent reflections |
Radiation source: fine-focus sealed tube | 1134 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.030 |
ϕ and ω scans | θmax = 27.5°, θmin = 2.6° |
Absorption correction: multi-scan (SADABS; Bruker, 2007) | h = −5→5 |
Tmin = 0.926, Tmax = 0.995 | k = −20→20 |
5854 measured reflections | l = −11→12 |
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.033 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.089 | H-atom parameters constrained |
S = 1.06 | w = 1/[σ2(Fo2) + (0.0409P)2 + 0.1852P] where P = (Fo2 + 2Fc2)/3 |
1331 reflections | (Δ/σ)max < 0.001 |
84 parameters | Δρmax = 0.33 e Å−3 |
0 restraints | Δρmin = −0.19 e Å−3 |
C12H14O6 | V = 581.16 (5) Å3 |
Mr = 254.23 | Z = 2 |
Monoclinic, P21/n | Mo Kα radiation |
a = 3.9796 (2) Å | µ = 0.12 mm−1 |
b = 15.7498 (7) Å | T = 153 K |
c = 9.3833 (4) Å | 0.25 × 0.22 × 0.04 mm |
β = 98.829 (1)° |
Bruker Kappa APEXII CCD area-detector diffractometer | 1331 independent reflections |
Absorption correction: multi-scan (SADABS; Bruker, 2007) | 1134 reflections with I > 2σ(I) |
Tmin = 0.926, Tmax = 0.995 | Rint = 0.030 |
5854 measured reflections |
R[F2 > 2σ(F2)] = 0.033 | 0 restraints |
wR(F2) = 0.089 | H-atom parameters constrained |
S = 1.06 | Δρmax = 0.33 e Å−3 |
1331 reflections | Δρmin = −0.19 e Å−3 |
84 parameters |
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. |
x | y | z | Uiso*/Ueq | ||
C1 | 0.1214 (3) | 0.99431 (7) | 0.14813 (11) | 0.0145 (2) | |
C2 | −0.0960 (3) | 0.93081 (7) | 0.08127 (11) | 0.0148 (2) | |
C3 | 0.2113 (3) | 1.06213 (7) | 0.06613 (11) | 0.0153 (2) | |
H3 | 0.3564 | 1.1051 | 0.1125 | 0.018* | |
C4 | 0.2796 (3) | 0.99516 (7) | 0.30367 (11) | 0.0160 (2) | |
C5 | −0.3844 (3) | 0.79762 (7) | 0.09155 (12) | 0.0188 (3) | |
H5A | −0.2549 | 0.7684 | 0.0239 | 0.023* | |
H5B | −0.5990 | 0.8195 | 0.0364 | 0.023* | |
C6 | −0.4582 (3) | 0.73731 (8) | 0.20738 (14) | 0.0265 (3) | |
H6A | −0.2439 | 0.7154 | 0.2598 | 0.040* | |
H6B | −0.5966 | 0.6900 | 0.1632 | 0.040* | |
H6C | −0.5824 | 0.7673 | 0.2745 | 0.040* | |
O1 | 0.1948 (2) | 0.93361 (6) | 0.38440 (8) | 0.0266 (2) | |
H1 | 0.3045 | 0.9383 | 0.4680 | 0.040* | |
O2 | 0.4790 (2) | 1.05162 (5) | 0.34978 (8) | 0.0250 (2) | |
O3 | −0.1877 (2) | 0.86600 (5) | 0.16307 (8) | 0.0199 (2) |
U11 | U22 | U33 | U12 | U13 | U23 | |
C1 | 0.0157 (5) | 0.0173 (5) | 0.0101 (5) | 0.0025 (4) | 0.0006 (4) | 0.0002 (4) |
C2 | 0.0162 (5) | 0.0159 (5) | 0.0122 (5) | 0.0006 (4) | 0.0019 (4) | 0.0016 (4) |
C3 | 0.0159 (5) | 0.0168 (5) | 0.0125 (5) | −0.0009 (4) | −0.0002 (4) | −0.0014 (4) |
C4 | 0.0182 (5) | 0.0177 (5) | 0.0116 (5) | 0.0010 (4) | 0.0004 (4) | 0.0005 (4) |
C5 | 0.0212 (5) | 0.0171 (5) | 0.0174 (5) | −0.0033 (4) | 0.0007 (4) | −0.0002 (4) |
C6 | 0.0284 (6) | 0.0224 (6) | 0.0281 (6) | −0.0051 (5) | 0.0028 (5) | 0.0069 (5) |
O1 | 0.0379 (5) | 0.0290 (5) | 0.0101 (4) | −0.0128 (4) | −0.0056 (3) | 0.0050 (3) |
O2 | 0.0346 (5) | 0.0240 (4) | 0.0135 (4) | −0.0095 (4) | −0.0058 (3) | 0.0029 (3) |
O3 | 0.0265 (4) | 0.0195 (4) | 0.0122 (4) | −0.0079 (3) | −0.0014 (3) | 0.0031 (3) |
C1—C3 | 1.3947 (15) | C5—O3 | 1.4359 (13) |
C1—C2 | 1.4056 (15) | C5—C6 | 1.5059 (16) |
C1—C4 | 1.4982 (14) | C5—H5A | 0.99 |
C2—O3 | 1.3602 (13) | C5—H5B | 0.99 |
C2—C3i | 1.3932 (14) | C6—H6A | 0.98 |
C3—C2i | 1.3932 (14) | C6—H6B | 0.98 |
C3—H3 | 0.95 | C6—H6C | 0.98 |
C4—O2 | 1.2257 (13) | O1—H1 | 0.84 |
C4—O1 | 1.3061 (14) | ||
C3—C1—C2 | 119.41 (9) | O3—C5—H5A | 110.4 |
C3—C1—C4 | 115.19 (9) | C6—C5—H5A | 110.4 |
C2—C1—C4 | 125.38 (10) | O3—C5—H5B | 110.4 |
O3—C2—C3i | 122.97 (10) | C6—C5—H5B | 110.4 |
O3—C2—C1 | 118.82 (9) | H5A—C5—H5B | 108.6 |
C3i—C2—C1 | 118.21 (10) | C5—C6—H6A | 109.5 |
C2i—C3—C1 | 122.38 (10) | C5—C6—H6B | 109.5 |
C2i—C3—H3 | 118.8 | H6A—C6—H6B | 109.5 |
C1—C3—H3 | 118.8 | C5—C6—H6C | 109.5 |
O2—C4—O1 | 123.00 (9) | H6A—C6—H6C | 109.5 |
O2—C4—C1 | 120.19 (10) | H6B—C6—H6C | 109.5 |
O1—C4—C1 | 116.81 (9) | C4—O1—H1 | 109.5 |
O3—C5—C6 | 106.78 (9) | C2—O3—C5 | 118.25 (8) |
C3—C1—C2—O3 | −178.57 (10) | C2—C1—C4—O2 | 176.48 (11) |
C4—C1—C2—O3 | 3.28 (17) | C3—C1—C4—O1 | 178.48 (10) |
C3—C1—C2—C3i | 0.63 (18) | C2—C1—C4—O1 | −3.30 (16) |
C4—C1—C2—C3i | −177.52 (10) | C3i—C2—O3—C5 | 6.38 (16) |
C2—C1—C3—C2i | −0.66 (18) | C1—C2—O3—C5 | −174.46 (10) |
C4—C1—C3—C2i | 177.67 (10) | C6—C5—O3—C2 | −178.11 (9) |
C3—C1—C4—O2 | −1.74 (16) |
Symmetry code: (i) −x, −y+2, −z. |
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1···O2ii | 0.84 | 1.80 | 2.6394 (11) | 177 |
Symmetry code: (ii) −x+1, −y+2, −z+1. |
Experimental details
(I) | (II) | |
Crystal data | ||
Chemical formula | C10H10O6 | C12H14O6 |
Mr | 226.18 | 254.23 |
Crystal system, space group | Monoclinic, C2/c | Monoclinic, P21/n |
Temperature (K) | 153 | 153 |
a, b, c (Å) | 11.6424 (3), 10.7368 (3), 7.6554 (2) | 3.9796 (2), 15.7498 (7), 9.3833 (4) |
β (°) | 93.804 (2) | 98.829 (1) |
V (Å3) | 954.83 (4) | 581.16 (5) |
Z | 4 | 2 |
Radiation type | Mo Kα | Mo Kα |
µ (mm−1) | 0.13 | 0.12 |
Crystal size (mm) | 0.45 × 0.18 × 0.13 | 0.25 × 0.22 × 0.04 |
Data collection | ||
Diffractometer | Bruker Kappa APEXII CCD area-detector diffractometer | Bruker Kappa APEXII CCD area-detector diffractometer |
Absorption correction | Multi-scan (SADABS; Bruker, 2007) | Multi-scan (SADABS; Bruker, 2007) |
Tmin, Tmax | 0.915, 0.983 | 0.926, 0.995 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 5259, 985, 869 | 5854, 1331, 1134 |
Rint | 0.025 | 0.030 |
(sin θ/λ)max (Å−1) | 0.628 | 0.649 |
Refinement | ||
R[F2 > 2σ(F2)], wR(F2), S | 0.029, 0.084, 1.10 | 0.033, 0.089, 1.06 |
No. of reflections | 985 | 1331 |
No. of parameters | 75 | 84 |
H-atom treatment | H-atom parameters constrained | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.28, −0.18 | 0.33, −0.19 |
Computer programs: APEX2 (Bruker, 2007), SAINT (Bruker, 2007), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), SHELXTL (Sheldrick, 2008) and DIAMOND (Brandenburg, 2006), SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2009).
D—H···A | D—H | H···A | D···A | D—H···A |
O2—H2···O3 | 0.84 | 1.81 | 2.5720 (10) | 149.8 |
C5—H5A···O1i | 0.98 | 2.58 | 3.4634 (14) | 149.9 |
C5—H5B···O1ii | 0.98 | 2.46 | 3.1718 (13) | 128.7 |
Symmetry codes: (i) x−1/2, y−1/2, z; (ii) −x+1/2, y−1/2, −z+1/2. |
D—H···A | D—H | H···A | D···A | D—H···A |
O1—H1···O2i | 0.84 | 1.80 | 2.6394 (11) | 177.4 |
Symmetry code: (i) −x+1, −y+2, −z+1. |
Over recent years, much importance has been attached to the synthesis of di-, tri- and tetracarboxylic acids, aiming at applications in the field of structural chemistry. This may be due to the utilization of appropriate carboxylates as linker molecules for the generation of metal–organic frameworks (MOFs) (Chui et al., 1999; Eddauodi, Kim, Rosi et al., 2002; Farha et al., 2010). One point of interest is the introduction of functional groups into MOF structures, facilitating specific applications such as catalysis (Shultz et al., 2009), separation (Chen et al., 2006) or gas storage (Rosi et al., 2003). For this purpose, owing to their easy accessibility, derivatives of terephthalic acid are frequently used. An outstanding challenge is the formation of MOF structures with the same precision as practised in organic synthesis. Therefore, knowledge of the exact geometry of the linker molecules is very important, since their structures have a major influence on the topology of the framework structure formed during MOF synthesis (Böhle et al., 2011a,b). Notably, small modifications of the terephthalic acid structure, such as the introduction of space-filling substituents, can lead to a distortion of the carboxylate docking groups and thus change the geometric configuration (Eddaoudi, Kim, O'Keeffe et al., 2002). We report here the remarkable structural behaviour of two alkoxy-substituted dicarboxylic acids, namely 2,5-dimethoxyterephthalic acid, (I), and 2,5-diethoxyterephthalic acid, (II), which have already been shown to exhibit flexible coordination modes as linker molecules in MOF structures (Böhle et al., 2011a,b).
Both title compounds crystallize in monoclinic space groups, C2/c for (I) and P21/n for (II), with the asymmetric unit containing half a molecule of the respective title compound. The molecular structures of (I) and (II) are shown in Fig. 1. The bond lengths of the aromatic core are in the ranges 1.3891 (14)–1.4003 (14) for (I) and 1.3932 (14)–1.4056 (15) Å and for (II), and thus do not vary significantly from each other, which is also the case for the bond angles of the aromatic system [119.43 (9)–120.77 (9) for (I) and 118.21 (10)–122.38 (10)° for (II)]. A remarkable feature of both compounds is the nearly planar molecular geometry, characterized by maximum deviations from the least-squares planes calculated for all non-H atoms of 0.0418 (6) (atom O1) and 0.0902 (10) Å (atom C5) for (I) and (II), respectively. A completely planar arrangement of the molecules is not found as the carboxy group is slightly distorted with reference to the aromatic ring, which can be concluded from the dihedral angles between the respective mean planes [2.55 (16)° for (I) and 2.89 (14)° for (II)]. This slight deviation from an ideal coplanarity is also found for unsubstituted terephthalic acid in its different polymorphic structures [triclinic I (Bailey & Brown, 1967), triclinic II (Domenicano et al., 1990) and monoclinic (Śledź et al., 2001)].
In comparision with unsubstituted terephthalic acid, the hydrogen-bonding arrangement in (I) seems to be uncommon, as the carboxyl groups form intramolecular hydrogen bonds [S(6) graph-set motif (Etter, 1990)] with a neighbouring methoxy group (O2—H2···O3) (Fig. 1a), and do not interact with the carboxyl groups of neighbouring molecules (Fig. 2) in order to form typical hydrogen-bonded carboxylic acid dimers (Jeffrey, 1997) of R22(8) synthon mode (Etter, 1990). This is, however, the case for (II), where the molecules are linked to each other via intermolecular hydrogen-bonding interactions (O1—H1···O2ii; see Table 2 for symmetry code), leading to the formation of one-dimensional strands within the packing structure (Fig. 3). A consideration of crystal structures of similar compounds shows that the mode of interaction of the carboxyl groups does not depend on the substitutent (methoxy versus ethoxy) but is probably caused, to a greater extent, by packing effects. Thus, in the structures of 2-methoxybenzoic acid (Parvez, 1987) and 2-ethoxybenzoic acid (Gopalakrishna & Cartz, 1972), a carboxylic acid dimer is formed in the case of the methoxy derivative, while an intramolecular hydrogen bond is observed for the ethoxy-substituted benzoic acid.
For the stabilization of the packing structures of both (I) and (II), offset face-to-face π–π stacking interactions (Hunter & Sanders, 1990; Salonen et al., 2011) are of importance. Since in the packing of (I) the carboxyl groups do not contribute to the formation of a characteristic packing motif, π–π stacking, as a weaker intermolecular interaction, comes to the fore. Thus, in the packing of methoxy derivative (I) the aromatic systems are stacked in the direction of the crystallographic c axis (Fig. 2), with a centroid-to-centroid distance of 3.8277 (5) Å and a perpendicular centroid-to-plane distance of 3.415 Å. Further stabilization of the packing structure results from weak (methyl)C—H···O interactions (C5—H5A···O1iii and C5—H5B···O1iv; see Table 1 for symmetry codes) between different π-stacks (Fig. 2). The packing structure of (II) is dominated by the carboxylic acid dimer interaction discussed above. Nevertheless, offset face-to-face π–π stacking [centroid-to-centroid distance = 3.9796 (7) Å and perpendicular centroid-to-plane distance = 3.320 Å] occurs in the direction of the crystallographic a axis (Fig. 3).
In conclusion, two 2,5-dialkoxyterephthalic acid derivatives, being interesting linker molecules for the formation of metal–organic frameworks (MOFs), have been synthesized by an alkylation–saponification procedure starting from diethyl 2,5-dihydroxyterephthalate. Their molecular structures have been compared with reference to bond lengths and angles, as well as to the planarity of the molecules. The unexpected difference in the supramolecular interactions of the carboxyl groups (caused by a comparatively small change from methoxy to ethoxy substituents), including the packing behaviour, has been highlighted. [Sandy: remove last paragraph? Says nothing new]