The crystal structures of methyl prop-2-ynoate, dimethyl fumarate and their protonated species

Methyl prop-2-ynoate and dimethyl fumarate have been crystallized, the former for the first time. The protonated species of both were isolated for the first time and identified by single-crystal X-ray diffraction and Raman spectroscopy.

Abstract

Methyl prop-2-ynoate (C₄H₄O₂) was investigated in the binary superacidic system HF/MF₅ (M = Sb or As) and dimethyl fumarate (C₆H₈O₄) in the superacidic system HF/SbF₅, as well as HF/BF₃. The starting materials methyl prop-2-ynoate and dimethyl fumarate were crystallized, the former for the first time. The protonated species of these esters, namely, (1-meth­oxy­prop-2-yn-1-yl­idene)oxidanium hexa­fluoro­arsenate, C₄H₅O₂⁺ AsF₆⁻, 1,4-dimeth­oxy-4-oxo­but-2-en-1-yl­idene]oxidanium tetra­fluoro­borate bis(hy­dro­gen fluoride), C₆H₉O₄⁺ BF₄⁻ 2HF, and hemi{[1,4-dimeth­oxy-4-oxidaniumylidenebut-2-en-1-yl­idene]oxi­dan­ium} undeca­fluoro­diantimonate, 0.5C₆H₁₀O₄²⁺ Sb₂F₁₁⁻, were characterized by single-crystal X-ray diffraction and Raman spectroscopy. The protonated species were recrystallized from anhydrous hy­dro­gen fluoride. In the solid state of the monoprotonated species of methyl prop-2-ynoate and the diprotonated species of dimethyl fumarate, strong intra­molecular O—H⋯F hy­dro­gen bonds build a three-dimensional network. The monoprotonated species of dimethyl fumarate builds chains by strong O—H⋯O hy­dro­gen bonds between the cations.

Introduction

Protonated esters have occur in two conformations, namely, syn–anti and syn–syn (Hogeveen, 1967 ▸; Olah et al., 1967 ▸, 2009 ▸). The syn–anti conformation is more stable and is therefore consistent with the protonation of carb­oxy­lic acids (Olah et al., 2009 ▸; Hollenwäger et al., 2024b ▸; Hogeveen, 1968 ▸). The two conformers were observed in solution by NMR spectroscopy (Olah et al., 2009 ▸). It has not yet been possible to crystallize the syn–syn conformer of protonated esters in the solid state; the example of prop-2-ynoic acid (propiolic acid) shows that this could be achieved with a H/D exchange and solid-state effects through a larger anion (Hollenwäger et al., 2024b ▸). In magic acid (FSO₃H/SbF₅), the esters show the unimolecular cleavage of methanol by warming to 20 °C (Olah et al., 2009 ▸). An exception was observed with glycine methyl ester, which is still stable in magic acid even at 93 °C (Hollenwäger, et al., 2024a ▸).

The isolation of protonated esters enables the characterization of an important inter­mediate that is present in solution in every acid-catalyzed esterification process. The selected esters also offer the possibility of further functionalization steps due to the double and triple bonds present. This prompted us to investigate methyl prop-2-ynoate and dimethyl fumarate in the binary superacidic media HF/MF₅.

Experimental

Synthesis and crystallization

[C₄H₅O₄][MF₆] (M = Sb or As)

The Lewis acids (SbF₅: 433 mg, 2 mmol; AsF₅: 340 mg, 2 mmol) were each condensed into a fluorine-passivated FEP reactor. Anhydrous hy­dro­gen fluoride (0.5 l) was added as reactant and solvent at −196 °C. The mixture was homogenized at room tem­per­a­ture. Methyl prop-2-ynoate (83.6 µl, 84.1 mg, 1.0 mmol) was added at −196 °C under nitro­gen. The mixture was allowed to warm to room tem­per­a­ture. The solvent was removed overnight at −78 °C. The protonated species II and III were obtained as white solids (Scheme 1).

[C₆H₉O₄][MF_y] (M = Sb or B; y = 6 or 4)

The Lewis acids (SbF₅: 216 mg, 1 mmol; BF₃: 67 mg, 1 mmol) were each condensed into a fluorine-passivated FEP reactor. Anhydrous hy­dro­gen fluoride (0.5 l) was added as reactant and solvent at −196 °C. The mixture was homogenized at room tem­per­a­ture. Dimethyl fumarate (144 mg, 0.5 mmol) was added at −196 °C under nitro­gen. The mixture was allowed to warm to room tem­per­a­ture. The solvent was removed overnight at −78 °C. The protonated species V and VI were obtained as white solids (Scheme 1). A clean Raman spectrum of monoprotonated species VI could not be obtained because it contains impurities of either IV or VII.

[C₆H₁₀O₄][Sb₂F₁₁]₂ and [C₆H₁₀O₄][BF₄]₂

The Lewis acids (SbF₅: 216 mg, 1 mmol; BF₃: 67 mg, 1 mmol) were each condensed into a fluorine-passivated FEP reactor. Anhydrous hy­dro­gen fluoride (0.5 l) was added as reactant and solvent at −196 °C. The mixture was homogenized at room tem­per­a­ture. Dimethyl fumarate (48 mg, 0.33 mmol) was added at −196 °C under nitro­gen. The mix­ture was allowed to warm to room tem­per­a­ture. The solvent was removed overnight at −78 °C. The protonated species VII and VIII were obtained as white solids.

Single-crystal X-ray diffraction and Raman spectroscopic analysis

Com­pounds I, III, VI and VII were characterized by single-crystal X-ray diffraction. Com­plete data and devices for the X-ray measurements are listed in the CIF in the supporting information. Low-tem­per­a­ture Raman spectroscopic analysis was performed for I–VIII using a Bruker MultiRAM FT–Raman spectrometer with Nd:YAG laser excitation (λ = 1064 cm⁻¹) under vacuum. For the measurements, the synthesized com­pound was transferred to a cooled glass cell.

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 1 ▸. The successful protonation of the target mol­ecules was confirmed by the charge of the asymmetric unit, as well as the interatomic distances. C—O distances have become nearly equal after protonation, as the charge can formally be localized on the C atom resulting in the loss of double-bond character. The positions of the H atoms were identified by Q-peaks on the difference Fourier map and by evaluation of the contacts (Figs. 1 ▸–3 ▸ ▸). Methyl, methylene and acetylenic H atoms were refined under restrictions and the proton positions were modulated.

Table 1. Experimental details.

Experiments were carried out with Mo Kα radiation using a Rigaku Xcalibur Sapphire3 diffractometer. Absorption was corrected for by multi-scan methods (CrysAlis PRO; Rigaku OD, 2020 ▸).

	I	III	IV
Crystal data
Chemical formula	C4H4O2	C4H5O2+·AsF6−	C6H8O4
M r	84.07	274.00	144.12
Crystal system, space group	Monoclinic, P21/n	Monoclinic, P21/n	Triclinic, P
tem­per­a­ture (K)	111	111	112
a, b, c (Å)	3.8409 (5), 15.593 (2), 7.6149 (10)	6.9609 (5), 8.9319 (7), 13.7189 (9)	3.8726 (11), 5.6546 (10), 8.3778 (18)
α, β, γ (°)	90, 99.910 (12), 90	90, 91.664 (7), 90	100.642 (16), 100.42 (2), 105.73 (2)
V (Å3)	449.27 (10)	852.60 (11)	168.30 (7)
Z	4	4	1
μ (mm−1)	0.10	4.06	0.12
Crystal size (mm)	1.00 × 0.56 × 0.31	0.90 × 0.21 × 0.15	0.52 × 0.46 × 0.35
Data collection
Tmin, Tmax	0.457, 1.000	0.582, 1.000	0.579, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	3885, 1060, 891	4205, 2303, 1900	1403, 825, 703
R int	0.023	0.029	0.015
θmax (°)	27.9	29.1	28.3
(sin θ/λ)max (Å−1)	0.658	0.685	0.667
Refinement
R[F2 > 2σ(F2)], wR(F2), S	0.036, 0.099, 1.05	0.039, 0.099, 1.05	0.048, 0.144, 1.04
No. of reflections	1060	2303	825
No. of parameters	71	123	51
No. of restraints	0	0	0
H-atom treatment	All H-atom parameters refined	H atoms treated by a mixture of independent and constrained refinement	H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å−3)	0.18, −0.15	0.87, −0.92	0.52, −0.24

	VI	VII
Crystal data
Chemical formula	C6H9O4+·BF4−·2HF	0.5C6H10O42+·Sb2F11−
M r	271.96	525.57
Crystal system, space group	Orthorhombic, Pbca	Orthorhombic, Pbca
tem­per­a­ture (K)	101	101
a, b, c (Å)	12.8759 (5), 11.8899 (4), 14.6252 (7)	7.8461 (6), 15.1531 (11), 19.5536 (17)
α, β, γ (°)	90, 90, 90	90, 90, 90
V (Å3)	2239.02 (16)	2324.8 (3)
Z	8	8
μ (mm−1)	0.19	4.79
Crystal size (mm)	0.28 × 0.18 × 0.11	0.25 × 0.11 × 0.05
Data collection
Tmin, Tmax	0.885, 1.000	0.807, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	7514, 1717, 1478	13316, 3784, 2878
R int	0.026	0.053
θmax (°)	23.8	32.2
(sin θ/λ)max (Å−1)	0.568	0.750
Refinement
R[F2 > 2σ(F2)], wR(F2), S	0.040, 0.100, 1.05	0.036, 0.071, 1.04
No. of reflections	1717	3784
No. of parameters	176	169
No. of restraints	1	0
H-atom treatment	H atoms treated by a mixture of independent and constrained refinement	H atoms treated by a mixture of independent and constrained refinement
Δρmax, Δρmin (e Å−3)	0.77, −0.35	1.09, −1.01

com­puter programs: CrysAlis PRO (Rigaku OD, 2020 ▸), SHELXT (Sheldrick, 2015a ▸), SHELXL2018 (Sheldrick, 2015b ▸), ORTEP-3 for Windows (Farrugia, 2012 ▸) and PLATON (Spek, 2020 ▸).

Figure 1.

A difference Fourier map of III without the H atom between O1 and F1. The green solid lines and red dotted lines show positive and negative density distribution, respectively.

Figure 2.

A difference Fourier map of VI without the H atom between O1 and F1. The green solid lines and red dotted lines show positive and negative density distribution, respectively.

Figure 3.

A difference Fourier map of VII without the H atom between O1 and F1. The green solid lines and red dotted lines show positive and negative density distribution, respectively.

Results and discussion

Single-crystal X-ray diffraction

Crystal structure of methyl prop-2-ynoate (I)

Com­pound I crystallizes in the monoclinic space group P2₁/n with one formula unit per unit cell. Fig. 4 ▸ displays the asymmetric unit. The C1—C2 bond length [1.4466 (15) Å] is significantly elongated com­pared to an average Csp¹—Csp² hybridized bond (1.427 Å) determined by X-ray diffraction (Allen et al., 1987 ▸). The C2≡C3 triple bond [1.1780 (16) Å] is in the same range as average terminal C≡C bonds (1.181 Å; Allen et al., 1987 ▸). The C1=O1 bond [1.1972 (14) Å] is in the same range as other C=O bonds (1.196 Å) in esters (Allen et al., 1987 ▸). The C1—O2 bond [1.3195 (12) Å] is significantly shortened com­pared to other esters (1.337 Å; Allen et al., 1987 ▸). The C4—O2 bond [1.4463 (13) Å] is significantly elongated com­pared to average CH₃—O bonds in esters (1.418 Å; Allen et al., 1987 ▸).

Figure 4.

The asymmetric unit of I, with displacement ellipsoids drawn at the 50% probability level.

The crystal structure of I displays a layered structure built of weak C3—H1⋯O1 hy­dro­gen bonds, according to the classification of Jeffrey (1997 ▸). The layered structure is connected via weak C4—H2A⋯O1 hy­dro­gen bonds into a three-dimensional network (Fig. 5 ▸), according to the classification of Jeffrey (1997 ▸).

Figure 5.

Hy­dro­gen bonds in the crystal structure of I, with displacement ellipsoids drawn at the 50% probability level. [Symmetry codes: (i) x − , −y + , z + ; (ii) −x + 1, −y + 1, −z + 2; (iii) x + , −y + , z − .]

Crystal structure of (1-meth­oxy­prop-2-yn-1-yl­idene)oxidanium hexa­fluoro­arsenate (III)

Salt III crystallizes in the monoclinic space group P2₁/n with four formula units per unit cell. Fig. 6 ▸ displays the asymmetric unit of III. The C1—O1 bond [1.261 (4) Å] is significantly elongated by 0.064 Å due to the protonation com­pared to I. The C1—O2 bond [1.270 (3) Å] is significantly shortened by 0.049 Å com­pared to the starting material I. Due to the protonation, the C4—O2 bond [1.484 (4) °] is elongated by 0.038 Å com­pared to the neutral com­pound I. The C2≡C3 triple bond is not significantly influenced by the protonation.

Figure 6.

The asymmetric unit of III, with displacement ellipsoids drawn at the 50% probability level.

The three-dimensional network of III (Fig. 7 ▸) is built by a strong O1—H3⋯F1 and three weak C3—H1⋯F5, C3—H1⋯F6 and C4—H2B⋯F3 hy­dro­gen bonds, according to the classification of Jeffrey (1997 ▸). Additionally, the crystal structure forms two inter­atomic contacts (C1⋯F2 and C1⋯F5) which are 8% shorter than the sum of the van der Waals radii.

Figure 7.

The intra­molecular inter­actions in the crystal structure of III, with displacement ellipsoids drawn at the 50% probability level. [Symmetry codes: (i) −x + , y − , −z + ; (ii) −x, −y + 1, −z + 1; (iii) −x + 1, −y + 1, −z + 1; (iv) x, y − 1, z; (v) −x + , y − , −z + .]

Crystal structure of dimethyl (E)-but-2-enedioate (IV)

The determined crystal structure is in the same range as that reported by Kooijman et al. (2004 ▸). The formula unit is shown in Fig. 8 ▸ and the crystal structure exhibits the same three-dimensional network (Fig. 9 ▸).

Figure 8.

The formula unit of IV, with displacement ellipsoids drawn at the 50% probability level. [Symmetry code: (i) −x + 1, −y + 2, −z + 1.]

Figure 9.

The intra­molecular inter­actions in the crystal structure of IV, with displacement ellipsoids drawn at the 50% probability level. [Symmetry codes: (i) −x + 1, −y + 2, −z + 1; (ii) x − 1, y − 1, z; (iii) −x, −y + 1, −z + 1; (iv) x − 1, y, z; (v) −x, −y + 2, −z + 1; (vi) x + 1, y, z; (vii) x + 1, y + 1, z; (viii) −x + 2, −y + 2, −z + 1; (ix) −x + 2, −y + 3, −z + 1.]

Crystal structure of 1,4-dimeth­oxy-4-oxobut-2-en-1-yl­idene]oxidanium tetra­fluoro­borate–hy­dro­gen fluoride (1/2) (VI)

The crystal structure of the monoprotonated species VI of dimethyl fumarate crystallizes in the ortho­rhom­bic space group Pbca with eight formula units per unit cell. Fig. 10 ▸ displays the asymmetric unit. Similar to fumaric acid and acetyl­enedi­carb­oxy­lic acid, the monoprotonated type forms extended chains of cations that are connected via strong O3—H9⋯O1 hy­dro­gen bonds (Jessen & Kornath, 2022 ▸; Bayer et al., 2020 ▸; Jeffrey, 1997 ▸). Due to the protonation, the C1—O1 [1.248 (3) Å] and C4—O3 [1.247 (3) Å] bonds are significantly elongated com­pared to the neutral com­pound [1.205 (2) Å]. The C1—O2 and C4—O4 bonds [both 1.289 (3) Å] are shortened by 0.052 Å com­pared to the starting material IV. The CH₃—O and C2=C3 bonds are not significantly influenced by the monoprotonation.

Figure 10.

The asymmetric unit of VI, with displacement ellipsoids drawn at the 50% probability level.

Besides the strong hy­dro­gen bonding between the cations, the crystal structure forms a medium–strong C6—H6⋯F6 hy­dro­gen bond [3.198 (3) Å] and two weak C5—H3⋯F4 [3.242 (3) Å] and C6—H7⋯F2 [3.261 (3) Å] hy­dro­gen bonds. Furthermore, the crystal structure forms six inter­atomic C⋯F contacts, i.e. C1⋯F1 [2.900 (3) Å], C1⋯F6 [2.943 (3) Å], C2⋯F6 [2.987 (3) Å], C3⋯F6 [3.048 (3) Å], C4⋯F2 [3.020 (3) Å] and C4⋯F5 [2.974 (3) Å], which are shorter than the sum of the van der Waals radii (3.17 Å). The inter­actions in the crystal structure of VI are shown in Fig. 11 ▸.

Figure 11.

The intra­molecular inter­actions in the crystal structure of VI, with displacement ellipsoids drawn at the 50% probability level. [Symmetry codes: (i) −x + , y − , z; (ii) −x + , y + , z; (iii) x − , y + 1, −z + ; (iv) x, y + 1, z; (v) −x + 1, −y + 1, −z + 1; (vi) x, −y + , z + ; (vii) x − , −y + , −z + 1.]

Crystal structure of hemi{[1,4-dimeth­oxy-4-oxidan­iumylidenebut-2-en-1-yl­idene]oxidanium} undeca­fluoro­diarsenate (VII)

The crystal structure of the diprotonated species VII of dimethyl fumarate crystallizes in the ortho­rhom­bic space group Pbca with eight formula units per unit cell. Fig. 12 ▸ displays the formula unit. The crystal structure of the diprotonated species has a C1—O1 bond [1.277 (5) Å] significantly elongated by 0.072 Å com­pared to the starting material [1.205 (2) Å]. The C1—O2 bond [1.281 (5) Å] is shortened by 0.060 Å com­pared to the neutral com­pound [1.341 (1) Å]. The C3—O2 bond [1.489 (6) Å] is elongated by 0.034 Å com­pared to IV [1.455 (2) Å].

Figure 12.

The formula unit of VIII, with displacement ellipsoids drawn at the 50% probability level. [Symmetry code: (i) −x + 1, −y + 1, −z + 1.]

The three-dimensional network is formed by a strong O1—H1⋯F1 hy­dro­gen bond and seven inter­atomic inter­actions (Jeffrey, 1997 ▸). The inter­atomic inter­actions are C1⋯F4 [3.061 (4) Å], C1⋯F7 [3.001 (4) Å], C2⋯F3 [2.958 (5) Å], C2⋯F7 [3.008 (5) Å], C3⋯F2 [3.050 (6) Å], O1⋯F7 [2.985 (4) Å] and O1⋯F11 [2.864 (4) Å]. Fig. 13 ▸ displays the inter­atomic distances in the crystal structure of VII.

Figure 13.

The intra­molecular inter­actions in the crystal structure of VIII, with displacement ellipsoids drawn at the 50% probability level. [Symmetry codes: (i) −x + 1, −y + 1, −z + 1; (ii) −x + , y + , z; (iii) x + , −y + , −z + 1; (iv) −x + 2, −y + 1, −z + 1; (v) x − , −y + , −z + 1; (vi) −x + , y + , z; (vii) x − , −y + , −z + 1; (viii) x − 1, y, z.]

Raman spectroscopy

Raman spectra of I, II and III

Fig. 14 ▸ displays the low-tem­per­a­ture Raman spectra of I, II and III. The first evidence for succesful protonation is the significantly red-shifted C=O oscillation from 1700 cm⁻¹ in the starting material I to 1617 (in II) and 1615 cm⁻¹ (in III). Due to the protonation, the C—O oscillation is blue-shifted in the Raman spectra to 1414 (in II) and 1413 cm⁻¹ (in III) com­pared to the starting material (1278 cm⁻¹). The H₃C—O oscillation is red-shifted by 42 cm⁻¹ to 948 (in II) and 951 cm⁻¹ (in III) com­pared to I (990 cm⁻¹). The oscillation of the triple bond is only slightly affected from 2107 cm⁻¹ to 2141 (in II) and 2140 cm⁻¹ (in III).

Figure 14.

The low-tem­per­a­ture Raman spectra of I (black), II (red) and III (blue).

Raman spectra of IV, V, VII and VIII

Fig. 15 ▸ shows the low-tem­per­a­ture Raman spectra of IV, V, VII and VIII. The C=O oscillation is red-shifted by 48 cm⁻¹ to 1611 cm⁻¹ com­pared to IV (1659 cm⁻¹). Due to the pro­ton­ation, the C—O oscillation is blue-shifted in the Raman spectra to 1401 cm⁻¹ (V) com­pared to the neutral com­pound (1217 cm⁻¹). The C=C oscillation is only slightly red shifted to 1705 cm⁻¹ com­pared to the starting material (1725 cm⁻¹).

Figure 15.

The low-tem­per­a­ture Raman spectra of IV (black), V (red), VII (green) and VIII (blue).

The diprotonation is characterized by a red-shifted C=O oscillation by 46 cm⁻¹ to 1613 cm⁻¹ in the spectra of VII and VIII com­pared to the Raman spectrum of IV (1659 cm⁻¹). The C=C oscillation is significantly red-shifted by 38 cm⁻¹ to 1686 cm⁻¹ in the spectra of VII and VIII com­pared to the Raman spectrum of IV (1724 cm⁻¹). Due to the protonation, the C—O oscillation is blue-shifted in the Raman spectra to 1419 (in VII) and 1425 cm⁻¹ (in VIII) com­pared to the neutral com­pound (1217 cm⁻¹).

Conclusion

We present herein the first single-crystal X-ray diffraction and Raman spectroscopy study of the monoprotonated species of methyl prop-2-ynoate and mono- and diprotonated species of dimethyl fumarate. All three protonated species crystallize in the more stable syn–anti conformation. Furthermore, the first single-crystal structure of methyl prop-2-ynoate is reported. The protonated species are important inter­mediates of acid-catalyzed reactions.

Supplementary Material

CCDC references: 2407012, 2407013, 2407014, 2407015, 2407016

Funding Statement

Funding for this research was provided by: Ludwig-Maximilians-University; F-Select GmbH; Deutsche Forschungsgemeinschaft.