Dimethyl and diethyl esters of 5,6-bis­(pyridin-2-yl)pyrazine-2,3-di­carb­oxy­lic acid: a comparison

Alfonso, M.; Stoeckli-Evans, H.

doi:10.1107/S2056989016001080

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ISSN: 2056-9890

Volume 72| Part 2| February 2016| Pages 233-237

doi:10.1107/S2056989016001080

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Dimethyl and diethyl esters of 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylic acid: a comparison

Montserrat Alfonso ^a and Helen Stoeckli-Evans ^b ^*

^aInstitute of Chemistry, University of Neuchâtel, Av. de Bellevaux 51, CH-2000 Neuchâtel, Switzerland, and ^bInsitute of Physics, University of Neuchâtel, rue Emile-Argand 11, CH-2000 Neuchâtel, Switzerland
^*Correspondence e-mail: helen.stoeckli-evans@unine.ch

Edited by M. Gdaniec, Adam Mickiewicz University, Poland (Received 2 January 2016; accepted 18 January 2016; online 27 January 2016)

In dimethyl 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate, C₁₈H₁₄N₄O₄, (I), and diethyl 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate, C₂₀H₁₈N₄O₄, (II), the dimethyl and diethyl esters of 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylic acid, the orientation of the two pyridine rings differ. In (I), pyridine ring B is inclined to pyrazine ring A by 44.8 (2)° and the pyridine and pyrazine N atoms are trans to one another, while pyridine ring C is inclined to the pyrazine ring by 50.3 (2)°, with the pyridine and pyrazine N atoms cis to one another. In compound (II), the diethyl ester, which possesses twofold rotation symmetry, the pyridine ring is inclined to the pyrazine ring by 40.7 (1)°, with the pyridine and pyrazine N atoms trans to one another. In the crystal of (I), molecules are linked by C—H⋯N hydrogen bonds, forming chains along [001]. The chains are linked by C—H⋯π interactions, forming a three-dimensional structure. In the crystal of (II), molecules are linked via C—H⋯O hydrogen bonds, forming a three-dimensional framework. There are C—H⋯π interactions present within the framework.

Keywords: crystal structure; dimethyl; diethyl; dicarboxylic acid; pyrazine; pyridine; C—H⋯O and C—H⋯N hydrogen bonding.

CCDC references: 1448182; 1448181

1. Chemical context

5,6-Bis(pyridin-2-yl)pyrazine-2,3-dicarboxylic acid (L1H₂) was synthesized to study its coordination behaviour with first row transitions metals (Alfonso, 1999 ). It exists as a zwitterion, with the adjacent pyridine and pyridinium rings almost coplanar due to the presence of an intramolecular N—H⋯N hydrogen bond. The crystal structures of the zwitterion and different charged forms of L1H₂, viz. the HCl, HClO₄ and HPF₆ salts, and details of the hydrogen bonding have been reported (Alfonso et al., 2001 ).

Metal-catalysed hydrolysis of amino acid esters is a well documented phenomenon (Dugas, 1989 ). It has been shown previously that the reaction of copper(II) salts with the dimethyl esters of pyrazine-2,3-dicarboxylic acid (Neels et al., 1997 ) and 2,5-dimethylpyrazine-3,6-dicarboxylic acid (Wang & Stoeckli-Evans, 1998 ) resulted in the partial hydrolysis of the ligand and the formation of a two-dimensional network in the first case and a mononuclear complex in the second. Hence, metal-ion-promoted ester hydrolysis leads to the formation of new ligands and may serve as a general route to prepare new coordination compounds. The title compounds, (I) and (II), were synthesized to study the hydrolysis of these esters with first row transition metals (Alfonso, 1999), and we report herein on their syntheses and crystal structures.

2. Structural commentary

As seen in compound (I), Fig. 1, the dimethyl ester of L1H₂, pyridine ring B (N4/C10–C14) is inclined to the pyrazine ring (A; N1/N2/C1–C4) by 44.8 (2)° and the pyridine and pyrazine N atoms, N1 and N4, are trans to one another. Pyridine ring C (N3/C5–C9) is inclined to pyrazine ring A by 50.3 (2)°. However, here the pyridine and pyrazine N atoms, N2 and N3, are cis to one another. The two pyridine rings, B and C, are inclined to one another by 60.2 (2)°. The acetate groups, O1/O2/C15/C17 and O3/O4/C16/C18, are almost planar with r.m.s. deviations of 0.027 and 0.007 Å, respectively. They are inclined to the pyrazine ring by 60.3 (3) and 49.8 (3)°, respectively, and to one another by 42.4 (3)°.

Figure 1
A view of the molecular structure of compound (I)

, showing the atom labelling. Displacement ellipsoids are drawn at the 50% probability level.

Compound (II), the diethyl ester of L1H₂, possesses twofold rotation symmetry, with the twofold rotation axis bisecting the C_ar—Cⁱ_ar bonds [ar = aromatic; symmetry code (i): −x + 2, −y + $[{3\over 2}]$ , z], as shown in Fig. 2. The pyridine N atoms, N2 and N2ⁱ, face one another with an N2⋯N2ⁱ separation of 3.043 (3) Å. The two pyridine rings are inclined to one another by 55.1 (1)° and to the pyrazine ring mean plane by 40.7 (1)°, with the pyrazine and pyridine N atoms, N1 and N2, trans to one another. The acetate group, O1/O2/C8/C9 [maximum deviation of 0.012 (3) Å for atom C8] is inclined to the pyrazine ring mean plane by 38.9 (1)°, and by 47.6 (2)° to the acetate group related by the twofold rotation axis. The oxygen atoms, O2 and O2ⁱ, are separated by only 2.840 (3) Å. The pyrazine ring in (II) has a slight twist-boat conformation (r.m.s. deviation = 0.046 Å) with the N1/C1/C2 and N1ⁱ/C1ⁱ/C2ⁱ planes inclined to one another by 3.9 (3)°.

Figure 2
A view of the molecular structure of compound (II)

, showing the atom labelling. Displacement ellipsoids are drawn at the 50% probability level. Unlabelled atoms are related to labelled atoms by the symmetry code (−x + 2, −y + $[{3\over 2}]$ , z).

As noted above the differences in the structures of the two compounds lies essentially in the orientation of the pyridine rings with respect to the pyrazine ring (cf Figs. 1 and 2). It is possible that the slight distortion of the planarity of the pyrazine ring in (II), mentioned above, is related to the short N2⋯N2ⁱ contact of 3.043 (3) Å of the adjacent pyridine rings and to the even shorter O2⋯O2ⁱ contact of 2.840 (3) Å of the adjacent acetate groups.

3. Supramolecular features

In the crystal of (I), molecules are linked by C—H⋯N hydrogen bonds, forming chains along [001]; see Table 1 and Fig. 3. The chains are linked via C—H⋯π interactions (Table 1), forming a three-dimensional structure.

Table 1
Hydrogen-bond geometry (Å, °) for (I)

Cg2 is the centroid of the N3/C5–C9 pyridine ring.

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C11—H11⋯N3ⁱ	0.93	2.57	3.334 (5)	140
C7—H7⋯Cg2ⁱⁱ	0.93	2.95	3.742 (5)	144
C17—H17C⋯Cg2ⁱⁱⁱ	0.96	2.92	3.722 (6)	141
Symmetry codes: (i) $[x, -y+{\script{3\over 2}}, z-{\script{1\over 2}}]$ ; (ii) $[x+{\script{1\over 2}}, -y+1, z]$ ; (iii) $[x-{\script{1\over 2}}, y+{\script{3\over 2}}, z-{\script{1\over 2}}]$ .

Figure 3
A view along the a axis of the crystal packing of compound (I)

. The hydrogen bonds are shown as dashed lines (see Table 1

; only H atom H11 has been included).

In the crystal of (II), molecules are linked via C—H⋯O hydrogen bonds, forming a three-dimensional framework; see Table 2 and Fig. 4. Within the framework there are a number of C—H⋯π interactions present (Table 2).

Table 2
Hydrogen-bond geometry (Å, °) for (II)

Cg1 and Cg2 are the centroids of the pyrazine and pyridine rings N1/C1/C2/N1′/C1′/C2′ and N2/C3–C7, respectively [symmetry code (′): −x + 2, −y + $[{3\over 2}]$ , z].

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C7—H7⋯O1ⁱ	0.94	2.48	3.308 (3)	147
C4—H4⋯Cg2ⁱⁱ	0.94	2.92	3.739 (2)	147
C10—H10B⋯Cg1ⁱⁱⁱ	0.97	2.56	3.409 (3)	146
C10—H10B⋯Cg1^iv	0.97	2.56	3.409 (3)	146
Symmetry codes: (i) $[-y+{\script{7\over 4}}, x+{\script{1\over 4}}, z+{\script{1\over 4}}]$ ; (ii) $[y-{\script{1\over 4}}, -x+{\script{5\over 4}}, -z+{\script{1\over 4}}]$ ; (iii) -x+2, -y+1, -z; (iv) $[x, y-{\script{1\over 2}}, -z]$ .

Figure 4
A view along the a axis of the crystal packing of compound (II)

. The hydrogen bonds are shown as dashed lines (see Table 2

; only H atom H7 has been included).

4. Database survey

Besides the structures of the zwitterion and different charged forms of L1H₂, viz. the HCl, HClO₄ and HPF₆ salts (Alfonso et al., 2001), the crystal structures of two copper(II) complexes of L1H₂ have been reported, viz: catena-[[[μ₃-5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate]triaquadibromodicopper(II)] methanol solvate trihydrate] and catena-[[[μ₄-5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate)diaquadibromodicopper(II) monohydrate] (Neels et al., 2003 ).

The structure of the isoelectronic compound 3,6-bis(pyridin-2-yl)pyrazine-2,5-dicarboxylic acid (L2H₂), Fig. 5, has also been reported (Wang & Stoeckli-Evans, 2012a ). It too exists as a zwitterion and the structures of its dihydrochloride salt and the dimethyl sulfonate disolvate have also been reported (Wang & Stoeckli-Evans, 2012a). The crystal structures of the dimethyl (III) and diethyl (IV) esters of L2H₂ have been deposited as private communications (Wang & Stoeckli-Evans, 2012b ,c ) with the Cambridge Structural Database (CSD; Groom & Allen, 2014 ). Both compounds crystallize in the triclinic space group P $[\overline{1}]$ and possess inversion symmetry. The pyridine rings lie almost in the plane of the pyrazine ring and the N atoms are trans with respect to each other and to the nearest pyrazine N atom (as illustrated in Fig. 5). The ester groups are planar and in both compounds lie almost normal to the pyrazine ring. In the crystals of both compounds, inversion-related molecules are linked via pairs of C—H⋯O hydrogen bonds, enclosing R₂²(10) ring motifs, forming chains propagating along [10 $[\overline{1}]$ ].

Figure 5
The chemical scheme for compound L2H₂.

5. Synthesis and crystallization

The synthesis of 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylic acid (L1H₂) has been reported (Alfonso et al., 2001). The dimethyl and diethyl esters, compounds (I) and (II), respectively, were obtained by the usual esterification procedure in acidic medium from the diacid and an excess of the corresponding alcohol.

Synthesis of compound (I): dimethyl-5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate L1H₂

(1.00 g, 3.11 mmol) was heated under reflux in freshly distilled MeOH (40ml) containing H₂SO₄ conc. (98%, 1 ml) during 16 h. After stopping the reaction, the temperature of the solution was allowed to cool to room temperature and then poured into an aqueous solution of NaOAc (6 g in 150 ml deionized water). The resulting solution was stirred in an ice bath containing NaCl to afford a white solid which was removed by filtration, washed with cold water and dried under vacuum. Single crystals suitable for X-ray analysis were obtained by the slow diffusion technique from CH₂Cl₂ and MeOH (yield: 0.77g, 65%; m.p. 410.2–411.7 K). Selected IR bands (KBr pellet, cm⁻¹): ν = 1743(s), 1729(vs), 1339(s), 1302(s), 1283(vs), 1164(s), 1089(vs). ¹H NMR (CDCl3, 400 MHz, p.p.m.): δ = 8.34(dt, 2H, J = 4.1Hz, J = 1.0 Hz, pyH), 7.99(dt, 2H, J = 7.7 Hz, J = 1.0 Hz, pyH), 7.82(td, 2H, J = 7.7 Hz, J = 1.0 Hz, pyH), 7.26(td, 2H, J = 7.7 Hz, J = 1.0 Hz, pyH), 4.04(s, 6H, CH₃). ¹³C NMR (CDCl₃, 50 MHz, p.p.m.): δ = 165.53, 155.98, 153.35, 149.26, 142.92, 137.64, 125.41, 124.49, 54.11. DCI–MS m/z: 351(MH⁺), 318, 279, 255, 208. Analysis for C₁₈H₁₄N₄O₄ (350.33), calculated C 61.70, H 4.04, N 15.99%, found C 61.4, H 3.91, N 15.65%.

Synthesis of compound (II): diethyl-5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate

This compound was prepared by the same method as for (I). L1H₂ in freshly distilled EtOH containing catalytic amounts of H₂SO₄ conc. gave compound (II) as a white solid. Slow evaporation of an ethanolic solution afforded colourless crystals suitable for X-ray analysis (yield: 0.70g, 62%; m.p. 390.5–391.3 K). Selected IR bands (KBr pellet, cm⁻¹): ν = 3055(w), 1737(s), 1723(vs), 1368(s), 1301(s), 1276(vs), 1276(vs), 1161(s), 1086(vs). ¹H NMR (CDCl₃, 400 MHz, p.p.m.): δ = 8.33(d, 2H, J = 4 Hz, pyH), 8.01(d, 2H, J = 7.7 Hz, pyH), 7.81(t, 2H, J = 7.7 Hz, pyH), 7.24(t, 2H, J = 4.4 Hz, pyH), 4.52(m, 4H, J = 7 Hz, CH₂), 1.45(t, 6H, J = 7.4 Hz, CH₃). EI–MS m/z: 378 (34), 349 (9), 232 (95), 206 (66), 179 (25), 152 (11), 129 (9), 78(base), 46 (38). Analysis for C₂₀H₁₈N₄O₄ (378.38), calculated C 63.49, H 4.79, N 14.81%, found C 63.49, H 4.61, N 14.77%.

6. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3. For both compounds, the C-bound H-atoms were included in calculated positions and treated as riding atoms: C—H = 0.93–0.98 Å with U_iso(H) = 1.5U_eq(C-methyl) and 1.2U_eq(C) for other H atoms. For compound (I), the Flack parameter (Parsons et al., 2013 ) is = −0.2 (10), but it has no physical meaning here.

Table 3
Experimental details

	(I)	(II)
Crystal data
Chemical formula	C₁₈H₁₄N₄O₄	C₂₀H₁₈N₄O₄
M_r	350.33	378.38
Crystal system, space group	Monoclinic, Ia	Tetragonal, I4₁/a
Temperature (K)	293	223
a, b, c (Å)	8.4249 (12), 12.2465 (10), 16.2561 (13)	10.2295 (6), 10.2295 (6), 36.281 (3)
α, β, γ (°)	90, 103.730 (8), 90	90, 90, 90
V (Å³)	1629.3 (3)	3796.5 (5)
Z	4	8
Radiation type	Mo Kα	Mo Kα
μ (mm⁻¹)	0.10	0.10
Crystal size (mm)	0.70 × 0.50 × 0.38	0.65 × 0.50 × 0.50

Data collection
Diffractometer	Stoe–Siemens AED2	Stoe IPDS 1
No. of measured, independent and observed [I > 2σ(I)] reflections	3035, 3028, 2737	14760, 1851, 1153
R_int	0.012	0.043
(sin θ/λ)_max (Å⁻¹)	0.606	0.616

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.050, 0.135, 1.11	0.049, 0.149, 1.01
No. of reflections	3028	1851
No. of parameters	238	129
No. of restraints	2	0
H-atom treatment	H-atom parameters constrained	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.19, −0.21	0.34, −0.19
Computer programs: STADI4 (Stoe & Cie, 1997 ), EXPOSE, CELL and INTEGRATE in IPDS-I (Stoe & Cie, 2004 ), X-RED (Stoe & Cie, 1997), SHELXS97 (Sheldrick, 2008 ), SHELXL2014 (Sheldrick, 2015 ), PLATON (Spek, 2009 ), Mercury (Macrae et al., 2008 ), and publCIF (Westrip, 2010 ).

Supporting information

CCDC references: 1448182; 1448181

Crystal structure: contains datablocks I, II, Global. DOI: 10.1107/S2056989016001080/gk2653sup1.cif

Structure factors: contains datablock I. DOI: 10.1107/S2056989016001080/gk2653Isup2.hkl

Structure factors: contains datablock II. DOI: 10.1107/S2056989016001080/gk2653IIsup3.hkl

Supporting information file. DOI: 10.1107/S2056989016001080/gk2653Isup4.cml

Supporting information file. DOI: 10.1107/S2056989016001080/gk2653IIsup5.cml

checkCIF report

Chemical context top

5,6-Bis(pyridin-2-yl)pyrazine-2,3-dicarboxylic acid (L1H₂) was synthesized to study its coordination behaviour with first row transitions metals (Alfonso, 1999). It exists as a zwitterion, with the adjacent pyridine and pyridinium rings almost coplanar due to the presence of an intramolecular N—H···N hydrogen bond. The crystal structures of the zwitterion and different charged forms of L1H₂, viz. the HCl, HClO₄ and HPF₆ salts, and details of the hydrogen bonding have been reported (Alfonso et al., 2001).

Metal-catalysed hydrolysis of amino acid esters is a well documented phenomenon (Dugas, 1989). It has been shown previously that the reaction of copper(II) salts with the dimethyl esters of pyrazine-2,3-dicarboxylic acid (Neels et al., 1997) and 2,5-dimethylpyrazine-3,6-dicarboxylic acid (Wang & Stoeckli-Evans, 1998) resulted in the partial hydrolysis of the ligand and the formation of a two-dimensional network in the first case and a mononuclear complex in the second. Hence, metal-ion-promoted ester hydrolysis leads to the formation of new ligands and becomes an extremely interesting manner to prepare new coordination compounds. The title compounds, (I) and (II), were synthesized to study the hydrolysis of these esters with first row transition metals (Alfonso, 1999), and we report herein on their syntheses and crystal structures.

Structural commentary top

As seen in compound (I), Fig.1, the dimethyl ester of L1H₂, pyridine ring B (N4/C10–C14) is inclined to the pyrazine ring (A; N1/N2/C1–C4) by 44.8 (2)° and the pyridine and pyrazine N atoms, N1 and N4, are trans to one another. Pyridine ring C (N3/C5–C9) is inclined to pyrazine ring A by 50.3 (2)°. However, here the pyridine and pyrazine N atoms, N2 and N4, are cis to one another. The two pyridine rings, B and C, are inclined to one another by 60.2 (2)° and to the planar pyrazine ring A (r.m.s. deviation 0.032 Å) by 44.8 (2) and 50.3 (2)°, respectively. The acetate groups, O1/O2/C15/C17 and O3/O4/C16/C18, are planar with r.m.s. deviations of 0.027 and 0.007 Å, respectively. They are inclined to the pyrazine ring by 60.3 (3) and 49.8 (3)°, respectively, and to one another by 42.4 (3)°.

Compound (II), the diethyl ester of L1H₂, possesses twofold rotation symmetry, with the twofold rotation axis bisecting the C_ar—Cⁱ_ar bonds [ar = aromatic; symmetry code (i): -x + 2, -y + 3/2, z], as shown in Fig. 2. The pyridine N atoms, N2 and N2ⁱ, face one another with an N2···N2ⁱ separation of 3.043 (3) Å. The two pyridine rings are inclined to one another by 55.1 (1)° and to the pyrazine ring mean plane by 40.7 (1)°, with the pyrazine and pyridine N atoms, N1 and N2, trans to one another. The acetate group, O1/O2/C8/C9 [maximum deviation of 0.012 (3) Å for atom C8] is inclined to the pyrazine ring mean plane by 38.9 (1)°, and by 47.6 (2)° to the acetate group related by the twofold rotation axis. The oxygen atoms, O2 and O2ⁱ, are separated by only 2.840 (3) Å. The pyrazine ring in (II) has a slight twist-boat conformation (r.m.s. deviation = 0.046 Å) with the N1/C1/C2 and N1ⁱ/C1ⁱ/C2ⁱ planes inclined to one another by 3.9 (3)°.

As noted above the differences in the structures of the two compounds lies essentially in the orientation of the pyridine rings with respect to the pyrazine ring (cf Figs. 1 and 2). It is possible that the slight distortion of the planarity of the pyrazine ring in (II), mentioned above, is related to the short N2···N2ⁱ contact of 3.043 (3) Å of the adjacent pyridine rings and to the even shorter O2···O2ⁱ contact of 2.840 (3) Å of the adjacent acetate groups.

Supramolecular features top

In the crystal of (I), molecules are linked by C—H···N hydrogen bonds, forming chains along [001]; see Table 1 and Fig. 3. The chains are linked via C—H···π interactions (Table 1), forming a three-dimensional structure.

In the crystal of (II), molecules are linked via C—H···O hydrogen bonds, forming a three-dimensional framework; see Table 2 and Fig. 4. Within the framework there are a number of C—H···π interactions present (Table 2).

Database survey top

\ Besides the structures of the zwitterion and different charged forms of L1H₂, viz. the HCl, HClO₄ and HPF₆ salts (Alfonso et al., 2001), the crystal structures of two copper(II) complexes of L1H₂ have been reported, viz: catena-[[[µ₃-5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate]\ triaquadibromodicopper(II)] methanol solvate trihydrate] and catena-[[[µ₄-5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate)\ diaquadibromodicopper(II) monohydrate] (Neels et al., 2003).

The structure of the isoelectronic compound 3,6-bis(pyridin-2-yl)pyrazine-2,5-dicarboxylic acid (L2H₂), Fig. 5, has also been reported (Wang & Stoeckli-Evans, 2012a). It too exists as a zwitterion and the structures of its dihydrochloride salt and the dimethyl sulfonate disolvate have also been reported (Wang & Stoeckli-Evans, 2012a). The crystal structures of the dimethyl (III) and diethyl (IV) esters of L2H₂ have been deposited as private communications (Wang & Stoeckli-Evans, 2012b,c) with the Cambridge Structural Database (CSD; Groom & Allen, 2014). Both compounds crystallize in the triclinic space group P1 and possess inversion symmetry. The pyridine rings lie almost in the plane of the pyrazine ring and the N atoms are trans with respect to each other and to the nearest pyrazine N atom (as illustrated in Fig. 5). The ester groups are planar and in both compounds lie almost normal to the pyrazine ring. In the crystals of both compounds, inversion-related molecules are linked via pairs of C—H···O hydrogen bonds, enclosing R²₂(10) ring motifs, forming chains propagating along [101].

Synthesis and crystallization top

Synthesis of compound (I): dimethyl-5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate L1H₂ (1.00 g, 3.11 mmol) was heated under reflux in freshly distilled MeOH (40ml) containing H₂SO₄ conc. (98%, 1 ml) during 16 h. After stopping the reaction, the temperature of the solution was allowed to cool to room temperature and then poured into an aqueous solution of NaOAc (6 g in 150 ml deionized water). The resulting solution was stirred in an ice bath containing NaCl to afford a white solid which was removed by filtration, washed with cold water and dried under vacuum. Single crystals suitable for X-ray analysis were obtained by the slow diffusion technique from CH₂Cl₂ and MeOH (yield: 0.77g, 65%; m.p. 410.2–411.7 K). Selected IR bands (KBr pellet, cm^-1): ν = 1743(s), 1729(vs), 1339(s), 1302(s), 1283(vs), 1164(s), 1089(vs). ¹H NMR (CDCl3, 400 MHz, p.p.m.): δ = 8.34(dt, 2H, J = 4.1Hz, J = 1.0Hz, pyH), 7.99(dt, 2H, J = 7.7Hz, J = 1.0Hz, pyH), 7.82(td, 2H, J = 7.7Hz, J = 1.0Hz, pyH), 7.26(td, 2H, J = 7.7Hz, J = 1.0Hz, pyH), 4.04(s, 6H, CH₃). ¹³C NMR (CDCl₃, 50 MHz, p.p.m.): δ = 165.53, 155.98, 153.35, 149.26, 142.92, 137.64, 125.41, 124.49, 54.11. DCI–MS m/z: 351(MH⁺), 318, 279, 255, 208. Analysis for C₁₈H₁₄N₄O₄ (350.33), calculated C 61.70, H 4.04, N 15.99 %, found C 61.4, H 3.91, N 15.65 %.

Synthesis of compound (II): diethyl-5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate

This compound was prepared by the same method as for (I). L1H₂ in freshly distilled EtOH containing catalytic amounts of H₂SO₄ conc. gave compound (II) as a white solid. Slow evaporation of an ethanolic solution afforded colourless crystals suitable for X-ray analysis (yield: 0.70g, 62%; m.p. 390.5–391.3 K). Selected IR bands (KBr pellet, cm^-1): ν = 3055(w), 1737(s), 1723(vs), 1368(s), 1301(s), 1276(vs), 1276(vs), 1161(s), 1086(vs). ¹H NMR (CDCl₃, 400 MHz, p.p.m.): δ = 8.33(d, 2H, J = 4Hz, pyH), 8.01(d, 2H, J = 7.7Hz, pyH), 7.81(t, 2H, J = 7.7Hz, pyH), 7.24(t, 2H, J = 4.4Hz, pyH), 4.52(m, 4H, J = 7Hz, CH₂), 1.45(t, 6H, J = 7.4Hz, CH3). EI–MS m/z: 378 (34), 349 (9), 232 (95), 206 (66), 179 (25), 152 (11), 129 (9), 78(base), 46 (38). Analysis for C₂₀H₁₈N₄O₄ (378.38), calculated C 63.49, H 4.79, N 14.81 %, found C 63.49, H 4.61, N 14.77 %.

Refinement top

Crystal data, data collection and structure refinement details are summarized in Table 3. For both compounds, the C-bound H-atoms were included in calculated positions and treated as riding atoms: C—H = 0.93–0.98 Å with U_iso(H) = 1.5U_eq(C-methyl) and 1.2U_eq(C) for other H atoms. For compound (I), the Flack parameter (Parsons et al., 2013) is = -0.2 (10), but it has no physical meaning here.

Structure description top

5,6-Bis(pyridin-2-yl)pyrazine-2,3-dicarboxylic acid (L1H₂) was synthesized to study its coordination behaviour with first row transitions metals (Alfonso, 1999). It exists as a zwitterion, with the adjacent pyridine and pyridinium rings almost coplanar due to the presence of an intramolecular N—H···N hydrogen bond. The crystal structures of the zwitterion and different charged forms of L1H₂, viz. the HCl, HClO₄ and HPF₆ salts, and details of the hydrogen bonding have been reported (Alfonso et al., 2001).

Metal-catalysed hydrolysis of amino acid esters is a well documented phenomenon (Dugas, 1989). It has been shown previously that the reaction of copper(II) salts with the dimethyl esters of pyrazine-2,3-dicarboxylic acid (Neels et al., 1997) and 2,5-dimethylpyrazine-3,6-dicarboxylic acid (Wang & Stoeckli-Evans, 1998) resulted in the partial hydrolysis of the ligand and the formation of a two-dimensional network in the first case and a mononuclear complex in the second. Hence, metal-ion-promoted ester hydrolysis leads to the formation of new ligands and becomes an extremely interesting manner to prepare new coordination compounds. The title compounds, (I) and (II), were synthesized to study the hydrolysis of these esters with first row transition metals (Alfonso, 1999), and we report herein on their syntheses and crystal structures.

As seen in compound (I), Fig.1, the dimethyl ester of L1H₂, pyridine ring B (N4/C10–C14) is inclined to the pyrazine ring (A; N1/N2/C1–C4) by 44.8 (2)° and the pyridine and pyrazine N atoms, N1 and N4, are trans to one another. Pyridine ring C (N3/C5–C9) is inclined to pyrazine ring A by 50.3 (2)°. However, here the pyridine and pyrazine N atoms, N2 and N4, are cis to one another. The two pyridine rings, B and C, are inclined to one another by 60.2 (2)° and to the planar pyrazine ring A (r.m.s. deviation 0.032 Å) by 44.8 (2) and 50.3 (2)°, respectively. The acetate groups, O1/O2/C15/C17 and O3/O4/C16/C18, are planar with r.m.s. deviations of 0.027 and 0.007 Å, respectively. They are inclined to the pyrazine ring by 60.3 (3) and 49.8 (3)°, respectively, and to one another by 42.4 (3)°.

Compound (II), the diethyl ester of L1H₂, possesses twofold rotation symmetry, with the twofold rotation axis bisecting the C_ar—Cⁱ_ar bonds [ar = aromatic; symmetry code (i): -x + 2, -y + 3/2, z], as shown in Fig. 2. The pyridine N atoms, N2 and N2ⁱ, face one another with an N2···N2ⁱ separation of 3.043 (3) Å. The two pyridine rings are inclined to one another by 55.1 (1)° and to the pyrazine ring mean plane by 40.7 (1)°, with the pyrazine and pyridine N atoms, N1 and N2, trans to one another. The acetate group, O1/O2/C8/C9 [maximum deviation of 0.012 (3) Å for atom C8] is inclined to the pyrazine ring mean plane by 38.9 (1)°, and by 47.6 (2)° to the acetate group related by the twofold rotation axis. The oxygen atoms, O2 and O2ⁱ, are separated by only 2.840 (3) Å. The pyrazine ring in (II) has a slight twist-boat conformation (r.m.s. deviation = 0.046 Å) with the N1/C1/C2 and N1ⁱ/C1ⁱ/C2ⁱ planes inclined to one another by 3.9 (3)°.

As noted above the differences in the structures of the two compounds lies essentially in the orientation of the pyridine rings with respect to the pyrazine ring (cf Figs. 1 and 2). It is possible that the slight distortion of the planarity of the pyrazine ring in (II), mentioned above, is related to the short N2···N2ⁱ contact of 3.043 (3) Å of the adjacent pyridine rings and to the even shorter O2···O2ⁱ contact of 2.840 (3) Å of the adjacent acetate groups.

The structure of the isoelectronic compound 3,6-bis(pyridin-2-yl)pyrazine-2,5-dicarboxylic acid (L2H₂), Fig. 5, has also been reported (Wang & Stoeckli-Evans, 2012a). It too exists as a zwitterion and the structures of its dihydrochloride salt and the dimethyl sulfonate disolvate have also been reported (Wang & Stoeckli-Evans, 2012a). The crystal structures of the dimethyl (III) and diethyl (IV) esters of L2H₂ have been deposited as private communications (Wang & Stoeckli-Evans, 2012b,c) with the Cambridge Structural Database (CSD; Groom & Allen, 2014). Both compounds crystallize in the triclinic space group P1 and possess inversion symmetry. The pyridine rings lie almost in the plane of the pyrazine ring and the N atoms are trans with respect to each other and to the nearest pyrazine N atom (as illustrated in Fig. 5). The ester groups are planar and in both compounds lie almost normal to the pyrazine ring. In the crystals of both compounds, inversion-related molecules are linked via pairs of C—H···O hydrogen bonds, enclosing R²₂(10) ring motifs, forming chains propagating along [101].

Synthesis and crystallization top

Synthesis of compound (II): diethyl-5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate

This compound was prepared by the same method as for (I). L1H₂ in freshly distilled EtOH containing catalytic amounts of H₂SO₄ conc. gave compound (II) as a white solid. Slow evaporation of an ethanolic solution afforded colourless crystals suitable for X-ray analysis (yield: 0.70g, 62%; m.p. 390.5–391.3 K). Selected IR bands (KBr pellet, cm^-1): ν = 3055(w), 1737(s), 1723(vs), 1368(s), 1301(s), 1276(vs), 1276(vs), 1161(s), 1086(vs). ¹H NMR (CDCl₃, 400 MHz, p.p.m.): δ = 8.33(d, 2H, J = 4Hz, pyH), 8.01(d, 2H, J = 7.7Hz, pyH), 7.81(t, 2H, J = 7.7Hz, pyH), 7.24(t, 2H, J = 4.4Hz, pyH), 4.52(m, 4H, J = 7Hz, CH₂), 1.45(t, 6H, J = 7.4Hz, CH3). EI–MS m/z: 378 (34), 349 (9), 232 (95), 206 (66), 179 (25), 152 (11), 129 (9), 78(base), 46 (38). Analysis for C₂₀H₁₈N₄O₄ (378.38), calculated C 63.49, H 4.79, N 14.81 %, found C 63.49, H 4.61, N 14.77 %.

Refinement details top

Crystal data, data collection and structure refinement details are summarized in Table 3. For both compounds, the C-bound H-atoms were included in calculated positions and treated as riding atoms: C—H = 0.93–0.98 Å with U_iso(H) = 1.5U_eq(C-methyl) and 1.2U_eq(C) for other H atoms. For compound (I), the Flack parameter (Parsons et al., 2013) is = -0.2 (10), but it has no physical meaning here.

Computing details top

Data collection: STADI4 (Stoe & Cie, 1997) for (I); EXPOSE in IPDS-I (Stoe & Cie, 2004) for (II). Cell refinement: STADI4 (Stoe & Cie, 1997) for (I); CELL in IPDS-I (Stoe & Cie, 2004) for (II). Data reduction: X-RED (Stoe & Cie, 1997) for (I); INTEGRATE in IPDS-I (Stoe & Cie, 2004) for (II). For both compounds, program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: PLATON (Spek, 2009) and Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015), PLATON (Spek, 2009) and publCIF (Westrip, 2010).

Figures top

	Fig. 1. A view of the molecular structure of compound (I), showing the atom labelling. Displacement ellipsoids are drawn at the 50% probability level.
	Fig. 2. A view of the molecular structure of compound (II), showing the atom labelling. Displacement ellipsoids are drawn at the 50% probability level. Unlabelled atoms are related to labelled atoms by the symmetry code (-x + 2, -y + 3/2, z).
	Fig. 3. A view along the a axis of the crystal packing of compound (I). The hydrogen bonds are shown as dashed lines (see Table 1; only H atom H11 has been included).
	Fig. 4. A view along the a axis of the crystal packing of compound (II). The hydrogen bonds are shown as dashed lines (see Table 2; only H atom H7 has been included).
	Fig. 5. The chemical scheme for compound L2H₂.

(I) Dimethyl 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate top

Crystal data top

C₁₈H₁₄N₄O₄	F(000) = 728
M_r = 350.33	D_x = 1.428 Mg m⁻³
Monoclinic, Ia	Mo Kα radiation, λ = 0.71073 Å
a = 8.4249 (12) Å	Cell parameters from 33 reflections
b = 12.2465 (10) Å	θ = 14.1–19.6°
c = 16.2561 (13) Å	µ = 0.10 mm⁻¹
β = 103.730 (8)°	T = 293 K
V = 1629.3 (3) Å³	Rod, colourless
Z = 4	0.70 × 0.50 × 0.38 mm

Data collection top

Stoe–Siemens AED2 diffractometer	θ_max = 25.5°, θ_min = 2.1°
ω/\2q scans	h = −10→10
3035 measured reflections	k = 0→14
3028 independent reflections	l = −19→19
2737 reflections with I > 2σ(I)	2 standard reflections every 60 min
R_int = 0.012	intensity decay: 1%

Refinement top

Refinement on F²	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F² > 2σ(F²)] = 0.050	H-atom parameters constrained
wR(F²) = 0.135	w = 1/[σ²(F_o²) + (0.0759P)² + 1.0624P] where P = (F_o² + 2F_c²)/3
S = 1.11	(Δ/σ)_max < 0.001
3028 reflections	Δρ_max = 0.19 e Å⁻³
238 parameters	Δρ_min = −0.21 e Å⁻³
2 restraints	Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0065 (18)

Crystal data top

C₁₈H₁₄N₄O₄	V = 1629.3 (3) Å³
M_r = 350.33	Z = 4
Monoclinic, Ia	Mo Kα radiation
a = 8.4249 (12) Å	µ = 0.10 mm⁻¹
b = 12.2465 (10) Å	T = 293 K
c = 16.2561 (13) Å	0.70 × 0.50 × 0.38 mm
β = 103.730 (8)°

Data collection top

Stoe–Siemens AED2 diffractometer	R_int = 0.012
3035 measured reflections	2 standard reflections every 60 min
3028 independent reflections	intensity decay: 1%
2737 reflections with I > 2σ(I)

Refinement top

R[F² > 2σ(F²)] = 0.050	2 restraints
wR(F²) = 0.135	H-atom parameters constrained
S = 1.11	Δρ_max = 0.19 e Å⁻³
3028 reflections	Δρ_min = −0.21 e Å⁻³
238 parameters

Special details top

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
O1	0.8419 (5)	1.1583 (3)	0.0691 (2)	0.0470 (9)
O2	1.0295 (4)	1.1079 (3)	−0.0014 (2)	0.0395 (8)
O3	1.0067 (5)	1.1074 (3)	0.3016 (2)	0.0463 (10)
O4	1.1454 (4)	1.1588 (3)	0.2054 (2)	0.0355 (8)
N1	0.9519 (4)	0.8948 (3)	0.0567 (2)	0.0266 (8)
N2	1.0688 (4)	0.8934 (3)	0.2330 (2)	0.0272 (8)
N3	1.0738 (4)	0.6793 (3)	0.3087 (2)	0.0279 (8)
N4	0.8579 (5)	0.6232 (3)	0.0803 (2)	0.0348 (9)
C1	0.9822 (5)	0.8015 (3)	0.0992 (2)	0.0248 (9)
C2	0.9784 (5)	0.9878 (3)	0.1014 (2)	0.0259 (9)
C3	1.0319 (5)	0.9857 (3)	0.1896 (3)	0.0268 (9)
C4	1.0489 (5)	0.8006 (3)	0.1880 (2)	0.0227 (8)
C5	1.1158 (5)	0.6996 (3)	0.2355 (2)	0.0248 (9)
C6	1.2218 (5)	0.6347 (4)	0.2045 (3)	0.0311 (9)
H6	1.2486	0.6520	0.1537	0.037*
C7	1.2879 (6)	0.5430 (4)	0.2500 (3)	0.0349 (10)
H7	1.3595	0.4979	0.2303	0.042*
C8	1.2459 (6)	0.5200 (3)	0.3248 (3)	0.0345 (10)
H8	1.2879	0.4589	0.3565	0.041*
C9	1.1394 (5)	0.5903 (4)	0.3517 (3)	0.0312 (9)
H9	1.1118	0.5748	0.4025	0.037*
C10	0.9393 (5)	0.6998 (3)	0.0486 (2)	0.0255 (9)
C11	0.9763 (6)	0.6908 (4)	−0.0299 (3)	0.0334 (10)
H11	1.0322	0.7464	−0.0499	0.040*
C12	0.9291 (6)	0.5984 (4)	−0.0777 (3)	0.0389 (11)
H12	0.9543	0.5900	−0.1301	0.047*
C13	0.8443 (7)	0.5192 (4)	−0.0469 (3)	0.0413 (12)
H13	0.8098	0.4561	−0.0779	0.050*
C14	0.8113 (7)	0.5353 (4)	0.0320 (3)	0.0439 (13)
H14	0.7529	0.4814	0.0524	0.053*
C15	0.9399 (6)	1.0942 (4)	0.0546 (3)	0.0316 (10)
C16	1.0580 (5)	1.0903 (4)	0.2399 (3)	0.0287 (9)
C17	1.0131 (8)	1.2133 (4)	−0.0439 (3)	0.0525 (14)
H17A	1.0924	1.2190	−0.0774	0.079*
H17B	1.0307	1.2706	−0.0024	0.079*
H17C	0.9053	1.2199	−0.0798	0.079*
C18	1.1672 (6)	1.2674 (4)	0.2412 (3)	0.0386 (11)
H18A	1.2114	1.3143	0.2049	0.058*
H18B	1.2410	1.2645	0.2961	0.058*
H18C	1.0636	1.2955	0.2464	0.058*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
O1	0.054 (2)	0.0354 (19)	0.054 (2)	0.0149 (17)	0.0183 (17)	0.0094 (17)
O2	0.055 (2)	0.0344 (19)	0.0313 (17)	−0.0011 (15)	0.0150 (15)	0.0089 (14)
O3	0.079 (3)	0.0309 (19)	0.037 (2)	−0.0093 (17)	0.0314 (19)	−0.0065 (14)
O4	0.0460 (18)	0.0266 (17)	0.0375 (16)	−0.0064 (13)	0.0171 (14)	−0.0077 (13)
N1	0.0339 (19)	0.0234 (19)	0.0235 (18)	−0.0009 (14)	0.0084 (14)	0.0004 (13)
N2	0.0320 (19)	0.027 (2)	0.0231 (18)	−0.0006 (14)	0.0081 (15)	−0.0003 (13)
N3	0.0337 (19)	0.0282 (19)	0.0223 (18)	−0.0009 (15)	0.0077 (14)	0.0010 (14)
N4	0.049 (2)	0.0326 (19)	0.0237 (17)	−0.0122 (17)	0.0103 (16)	−0.0023 (16)
C1	0.029 (2)	0.025 (2)	0.023 (2)	0.0003 (16)	0.0110 (17)	0.0009 (16)
C2	0.027 (2)	0.027 (2)	0.024 (2)	0.0006 (17)	0.0066 (16)	0.0011 (16)
C3	0.032 (2)	0.023 (2)	0.027 (2)	0.0005 (17)	0.0108 (18)	−0.0016 (16)
C4	0.0233 (19)	0.024 (2)	0.022 (2)	−0.0001 (15)	0.0082 (15)	0.0013 (15)
C5	0.029 (2)	0.024 (2)	0.0201 (19)	−0.0017 (16)	0.0032 (16)	−0.0009 (16)
C6	0.038 (2)	0.029 (2)	0.026 (2)	0.0039 (19)	0.0090 (18)	−0.0011 (18)
C7	0.040 (2)	0.027 (2)	0.036 (2)	0.0047 (19)	0.006 (2)	−0.0035 (19)
C8	0.043 (3)	0.022 (2)	0.034 (2)	0.0019 (19)	0.0007 (19)	0.0013 (18)
C9	0.038 (2)	0.029 (2)	0.024 (2)	−0.0046 (18)	0.0027 (18)	0.0040 (18)
C10	0.031 (2)	0.025 (2)	0.020 (2)	0.0015 (17)	0.0040 (16)	0.0015 (16)
C11	0.042 (3)	0.034 (2)	0.026 (2)	−0.0032 (18)	0.0130 (18)	−0.0015 (18)
C12	0.053 (3)	0.039 (3)	0.026 (2)	0.001 (2)	0.013 (2)	−0.007 (2)
C13	0.059 (3)	0.032 (3)	0.030 (2)	−0.003 (2)	0.003 (2)	−0.0083 (19)
C14	0.066 (3)	0.031 (2)	0.035 (3)	−0.018 (2)	0.012 (2)	0.002 (2)
C15	0.038 (2)	0.030 (2)	0.023 (2)	−0.002 (2)	0.0008 (17)	0.0001 (17)
C16	0.037 (2)	0.022 (2)	0.025 (2)	0.0026 (17)	0.0038 (17)	−0.0003 (17)
C17	0.069 (4)	0.039 (3)	0.045 (3)	−0.012 (3)	0.005 (3)	0.017 (2)
C18	0.047 (3)	0.024 (2)	0.046 (3)	−0.0043 (19)	0.013 (2)	−0.006 (2)

Geometric parameters (Å, º) top

O1—C15	1.202 (6)	C6—C7	1.387 (6)
O2—C15	1.324 (6)	C6—H6	0.9300
O2—C17	1.455 (5)	C7—C8	1.373 (7)
O3—C16	1.200 (6)	C7—H7	0.9300
O4—C16	1.325 (6)	C8—C9	1.387 (7)
O4—C18	1.446 (5)	C8—H8	0.9300
N1—C1	1.329 (5)	C9—H9	0.9300
N1—C2	1.340 (5)	C10—C11	1.388 (6)
N2—C3	1.331 (6)	C11—C12	1.377 (7)
N2—C4	1.341 (5)	C11—H11	0.9300
N3—C9	1.340 (5)	C12—C13	1.368 (7)
N3—C5	1.343 (5)	C12—H12	0.9300
N4—C10	1.335 (6)	C13—C14	1.389 (7)
N4—C14	1.336 (6)	C13—H13	0.9300
C1—C4	1.419 (5)	C14—H14	0.9300
C1—C10	1.489 (6)	C17—H17A	0.9600
C2—C3	1.397 (5)	C17—H17B	0.9600
C2—C15	1.505 (6)	C17—H17C	0.9600
C3—C16	1.508 (6)	C18—H18A	0.9600
C4—C5	1.495 (6)	C18—H18B	0.9600
C5—C6	1.377 (6)	C18—H18C	0.9600

C15—O2—C17	115.7 (4)	N4—C10—C11	123.2 (4)
C16—O4—C18	116.2 (3)	N4—C10—C1	117.0 (3)
C1—N1—C2	117.5 (3)	C11—C10—C1	119.7 (4)
C3—N2—C4	116.6 (3)	C12—C11—C10	119.1 (4)
C9—N3—C5	116.7 (4)	C12—C11—H11	120.5
C10—N4—C14	116.5 (4)	C10—C11—H11	120.5
N1—C1—C4	121.1 (4)	C13—C12—C11	118.8 (4)
N1—C1—C10	116.2 (3)	C13—C12—H12	120.6
C4—C1—C10	122.7 (4)	C11—C12—H12	120.6
N1—C2—C3	120.9 (4)	C12—C13—C14	118.3 (4)
N1—C2—C15	118.3 (3)	C12—C13—H13	120.9
C3—C2—C15	120.8 (4)	C14—C13—H13	120.9
N2—C3—C2	122.5 (4)	N4—C14—C13	124.2 (4)
N2—C3—C16	116.7 (3)	N4—C14—H14	117.9
C2—C3—C16	120.8 (4)	C13—C14—H14	117.9
N2—C4—C1	121.1 (4)	O1—C15—O2	125.6 (4)
N2—C4—C5	115.8 (3)	O1—C15—C2	122.9 (4)
C1—C4—C5	122.8 (4)	O2—C15—C2	111.6 (4)
N3—C5—C6	123.2 (4)	O3—C16—O4	126.2 (4)
N3—C5—C4	117.7 (3)	O3—C16—C3	124.5 (4)
C6—C5—C4	119.1 (4)	O4—C16—C3	109.4 (3)
C5—C6—C7	119.0 (4)	O2—C17—H17A	109.5
C5—C6—H6	120.5	O2—C17—H17B	109.5
C7—C6—H6	120.5	H17A—C17—H17B	109.5
C8—C7—C6	118.9 (4)	O2—C17—H17C	109.5
C8—C7—H7	120.5	H17A—C17—H17C	109.5
C6—C7—H7	120.5	H17B—C17—H17C	109.5
C7—C8—C9	118.3 (4)	O4—C18—H18A	109.5
C7—C8—H8	120.9	O4—C18—H18B	109.5
C9—C8—H8	120.9	H18A—C18—H18B	109.5
N3—C9—C8	123.9 (4)	O4—C18—H18C	109.5
N3—C9—H9	118.1	H18A—C18—H18C	109.5
C8—C9—H9	118.1	H18B—C18—H18C	109.5

C2—N1—C1—C4	−3.6 (5)	C5—N3—C9—C8	0.1 (6)
C2—N1—C1—C10	175.4 (3)	C7—C8—C9—N3	0.4 (7)
C1—N1—C2—C3	−1.6 (5)	C14—N4—C10—C11	−0.4 (7)
C1—N1—C2—C15	−178.2 (4)	C14—N4—C10—C1	176.0 (4)
C4—N2—C3—C2	−1.4 (5)	N1—C1—C10—N4	−133.7 (4)
C4—N2—C3—C16	−178.6 (4)	C4—C1—C10—N4	45.3 (5)
N1—C2—C3—N2	4.3 (6)	N1—C1—C10—C11	42.8 (5)
C15—C2—C3—N2	−179.2 (4)	C4—C1—C10—C11	−138.2 (4)
N1—C2—C3—C16	−178.6 (4)	N4—C10—C11—C12	−0.7 (7)
C15—C2—C3—C16	−2.1 (6)	C1—C10—C11—C12	−177.0 (4)
C3—N2—C4—C1	−3.8 (5)	C10—C11—C12—C13	1.2 (7)
C3—N2—C4—C5	170.5 (3)	C11—C12—C13—C14	−0.6 (8)
N1—C1—C4—N2	6.5 (5)	C10—N4—C14—C13	1.0 (8)
C10—C1—C4—N2	−172.4 (4)	C12—C13—C14—N4	−0.5 (8)
N1—C1—C4—C5	−167.4 (4)	C17—O2—C15—O1	4.4 (7)
C10—C1—C4—C5	13.7 (5)	C17—O2—C15—C2	−174.1 (4)
C9—N3—C5—C6	−0.6 (6)	N1—C2—C15—O1	120.0 (5)
C9—N3—C5—C4	−177.9 (4)	C3—C2—C15—O1	−56.6 (6)
N2—C4—C5—N3	50.7 (5)	N1—C2—C15—O2	−61.5 (5)
C1—C4—C5—N3	−135.1 (4)	C3—C2—C15—O2	121.9 (4)
N2—C4—C5—C6	−126.6 (4)	C18—O4—C16—O3	−6.2 (7)
C1—C4—C5—C6	47.6 (6)	C18—O4—C16—C3	173.9 (4)
N3—C5—C6—C7	0.7 (7)	N2—C3—C16—O3	−50.9 (6)
C4—C5—C6—C7	177.9 (4)	C2—C3—C16—O3	131.8 (5)
C5—C6—C7—C8	−0.2 (7)	N2—C3—C16—O4	128.9 (4)
C6—C7—C8—C9	−0.4 (7)	C2—C3—C16—O4	−48.3 (5)

Hydrogen-bond geometry (Å, º) top

Cg2 is the centroid of the N3/C5–C9 pyridine ring.

D—H···A	D—H	H···A	D···A	D—H···A
C11—H11···N3ⁱ	0.93	2.57	3.334 (5)	140
C7—H7···Cg2ⁱⁱ	0.93	2.95	3.742 (5)	144
C17—H17C···Cg2ⁱⁱⁱ	0.96	2.92	3.722 (6)	141

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

(II) Diethyl 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate top

Crystal data top

C₂₀H₁₈N₄O₄	D_x = 1.324 Mg m⁻³
M_r = 378.38	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I4₁/a	Cell parameters from 5000 reflections
a = 10.2295 (6) Å	θ = 3.3–52.1°
c = 36.281 (3) Å	µ = 0.10 mm⁻¹
V = 3796.5 (5) Å³	T = 223 K
Z = 8	Block, colourless
F(000) = 1584	0.65 × 0.50 × 0.50 mm

Data collection top

Stoe IPDS 1 diffractometer	1153 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	R_int = 0.043
Plane graphite monochromator	θ_max = 26.0°, θ_min = 2.1°
φ rotation scans	h = −12→12
14760 measured reflections	k = −12→12
1851 independent reflections	l = −44→44

Refinement top

Refinement on F²	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F² > 2σ(F²)] = 0.049	H-atom parameters constrained
wR(F²) = 0.149	w = 1/[σ²(F_o²) + (0.0925P)²] where P = (F_o² + 2F_c²)/3
S = 1.01	(Δ/σ)_max < 0.001
1851 reflections	Δρ_max = 0.34 e Å⁻³
129 parameters	Δρ_min = −0.19 e Å⁻³
0 restraints	Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc^*=kFc[1+0.001xFc²λ³/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0049 (10)

Crystal data top

C₂₀H₁₈N₄O₄	Z = 8
M_r = 378.38	Mo Kα radiation
Tetragonal, I4₁/a	µ = 0.10 mm⁻¹
a = 10.2295 (6) Å	T = 223 K
c = 36.281 (3) Å	0.65 × 0.50 × 0.50 mm
V = 3796.5 (5) Å³

Data collection top

Stoe IPDS 1 diffractometer	1153 reflections with I > 2σ(I)
14760 measured reflections	R_int = 0.043
1851 independent reflections

Refinement top

R[F² > 2σ(F²)] = 0.049	0 restraints
wR(F²) = 0.149	H-atom parameters constrained
S = 1.01	Δρ_max = 0.34 e Å⁻³
1851 reflections	Δρ_min = −0.19 e Å⁻³
129 parameters

Special details top

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å²) top

	x	y	z	U_iso*/U_eq
O1	0.7424 (2)	0.7559 (3)	0.00480 (5)	0.1009 (9)
O2	0.91015 (17)	0.64422 (18)	−0.01674 (4)	0.0693 (5)
N1	0.86590 (16)	0.73125 (18)	0.07318 (4)	0.0478 (5)
N2	0.88149 (17)	0.8399 (2)	0.16340 (5)	0.0583 (6)
C1	0.9332 (2)	0.7371 (2)	0.04156 (5)	0.0473 (5)
C2	0.93113 (19)	0.7446 (2)	0.10488 (5)	0.0450 (5)
C3	0.84753 (19)	0.7504 (2)	0.13824 (5)	0.0452 (5)
C4	0.7388 (2)	0.6719 (2)	0.14134 (5)	0.0466 (5)
H4	0.7174	0.6124	0.1225	0.056*
C5	0.6613 (2)	0.6817 (2)	0.17249 (5)	0.0503 (5)
H5	0.5874	0.6282	0.1755	0.060*
C6	0.6952 (2)	0.7712 (2)	0.19868 (6)	0.0569 (6)
H6	0.6448	0.7803	0.2202	0.068*
C7	0.8042 (2)	0.8481 (3)	0.19322 (6)	0.0637 (7)
H7	0.8256	0.9098	0.2114	0.076*
C8	0.8508 (2)	0.7155 (3)	0.00796 (5)	0.0558 (6)
C9	0.8402 (3)	0.6183 (4)	−0.05106 (7)	0.0978 (11)
H9A	0.7744	0.5501	−0.0471	0.117*
H9B	0.7956	0.6977	−0.0594	0.117*
C10	0.9335 (4)	0.5760 (3)	−0.07867 (8)	0.0965 (11)
H10A	0.8877	0.5552	−0.1013	0.145*
H10B	0.9793	0.4990	−0.0699	0.145*
H10C	0.9960	0.6455	−0.0833	0.145*

Atomic displacement parameters (Å²) top

	U¹¹	U²²	U³³	U¹²	U¹³	U²³
O1	0.0662 (13)	0.187 (2)	0.0499 (11)	0.0406 (14)	−0.0132 (9)	−0.0026 (12)
O2	0.0689 (11)	0.0889 (12)	0.0500 (9)	0.0023 (9)	−0.0168 (8)	−0.0172 (8)
N1	0.0424 (9)	0.0665 (12)	0.0347 (9)	0.0012 (8)	−0.0018 (7)	0.0026 (8)
N2	0.0464 (10)	0.0866 (14)	0.0418 (9)	−0.0104 (10)	0.0042 (8)	−0.0093 (9)
C1	0.0440 (10)	0.0630 (13)	0.0348 (10)	0.0039 (10)	−0.0018 (8)	0.0012 (9)
C2	0.0418 (10)	0.0581 (13)	0.0350 (10)	−0.0016 (9)	−0.0010 (8)	0.0011 (9)
C3	0.0379 (10)	0.0629 (13)	0.0348 (10)	0.0007 (9)	−0.0026 (8)	0.0026 (9)
C4	0.0421 (11)	0.0541 (12)	0.0437 (11)	0.0028 (9)	−0.0017 (8)	0.0041 (9)
C5	0.0407 (11)	0.0630 (13)	0.0471 (12)	0.0017 (10)	0.0035 (9)	0.0086 (10)
C6	0.0450 (12)	0.0826 (17)	0.0431 (11)	0.0052 (12)	0.0067 (9)	0.0062 (11)
C7	0.0553 (14)	0.0941 (18)	0.0416 (11)	−0.0080 (13)	0.0046 (10)	−0.0127 (12)
C8	0.0467 (13)	0.0851 (17)	0.0357 (11)	0.0035 (12)	−0.0008 (9)	0.0047 (11)
C9	0.095 (2)	0.141 (3)	0.0578 (16)	−0.007 (2)	−0.0290 (15)	−0.0268 (18)
C10	0.157 (3)	0.0720 (18)	0.0605 (17)	−0.020 (2)	−0.0135 (19)	−0.0139 (14)

Geometric parameters (Å, º) top

O1—C8	1.189 (3)	C4—H4	0.9400
O2—C8	1.305 (3)	C5—C6	1.364 (3)
O2—C9	1.460 (3)	C5—H5	0.9400
N1—C2	1.337 (2)	C6—C7	1.379 (3)
N1—C1	1.339 (2)	C6—H6	0.9400
N2—C3	1.339 (3)	C7—H7	0.9400
N2—C7	1.343 (3)	C9—C10	1.450 (5)
C1—C1ⁱ	1.392 (4)	C9—H9A	0.9800
C1—C8	1.499 (3)	C9—H9B	0.9800
C2—C2ⁱ	1.413 (4)	C10—H10A	0.9700
C2—C3	1.483 (3)	C10—H10B	0.9700
C3—C4	1.377 (3)	C10—H10C	0.9700
C4—C5	1.384 (3)

C8—O2—C9	117.3 (2)	C7—C6—H6	120.4
C2—N1—C1	118.40 (17)	N2—C7—C6	123.8 (2)
C3—N2—C7	116.06 (19)	N2—C7—H7	118.1
N1—C1—C1ⁱ	120.88 (11)	C6—C7—H7	118.1
N1—C1—C8	113.63 (18)	O1—C8—O2	124.2 (2)
C1ⁱ—C1—C8	125.48 (12)	O1—C8—C1	123.5 (2)
N1—C2—C2ⁱ	120.38 (11)	O2—C8—C1	112.27 (19)
N1—C2—C3	114.72 (17)	C10—C9—O2	108.7 (3)
C2ⁱ—C2—C3	124.88 (11)	C10—C9—H9A	109.9
N2—C3—C4	123.56 (18)	O2—C9—H9A	109.9
N2—C3—C2	115.71 (18)	C10—C9—H9B	109.9
C4—C3—C2	120.65 (18)	O2—C9—H9B	109.9
C3—C4—C5	119.1 (2)	H9A—C9—H9B	108.3
C3—C4—H4	120.4	C9—C10—H10A	109.5
C5—C4—H4	120.4	C9—C10—H10B	109.5
C6—C5—C4	118.2 (2)	H10A—C10—H10B	109.5
C6—C5—H5	120.9	C9—C10—H10C	109.5
C4—C5—H5	120.9	H10A—C10—H10C	109.5
C5—C6—C7	119.20 (19)	H10B—C10—H10C	109.5
C5—C6—H6	120.4

C2—N1—C1—C1ⁱ	−3.1 (4)	C3—C4—C5—C6	1.1 (3)
C2—N1—C1—C8	177.7 (2)	C4—C5—C6—C7	0.1 (3)
C1—N1—C2—C2ⁱ	−4.4 (4)	C3—N2—C7—C6	−0.2 (4)
C1—N1—C2—C3	174.09 (19)	C5—C6—C7—N2	−0.6 (4)
C7—N2—C3—C4	1.6 (3)	C9—O2—C8—O1	3.0 (4)
C7—N2—C3—C2	178.3 (2)	C9—O2—C8—C1	−179.0 (2)
N1—C2—C3—N2	−137.7 (2)	N1—C1—C8—O1	37.9 (4)
C2ⁱ—C2—C3—N2	40.8 (4)	C1ⁱ—C1—C8—O1	−141.2 (3)
N1—C2—C3—C4	39.1 (3)	N1—C1—C8—O2	−140.1 (2)
C2ⁱ—C2—C3—C4	−142.4 (3)	C1ⁱ—C1—C8—O2	40.8 (4)
N2—C3—C4—C5	−2.1 (3)	C8—O2—C9—C10	162.0 (3)
C2—C3—C4—C5	−178.62 (19)

Symmetry code: (i) −x+2, −y+3/2, z.

Hydrogen-bond geometry (Å, º) top

Cg1 and Cg2 are the centroids of the pyrazine and pyridine rings N1/C1/C2/N1'/C1'/C2' and N2/C3–C7, respectively [symmetry code ('): -x + 2, -y + 3/2, z].

D—H···A	D—H	H···A	D···A	D—H···A
C7—H7···O1ⁱⁱ	0.94	2.48	3.308 (3)	147
C4—H4···Cg2ⁱⁱⁱ	0.94	2.92	3.739 (2)	147
C10—H10B···Cg1^iv	0.97	2.56	3.409 (3)	146
C10—H10B···Cg1^v	0.97	2.56	3.409 (3)	146

Symmetry codes: (ii) −y+7/4, x+1/4, z+1/4; (iii) y−1/4, −x+5/4, −z+1/4; (iv) −x+2, −y+1, −z; (v) x, y−1/2, −z.

Hydrogen-bond geometry (Å, º) for (I) top

Cg2 is the centroid of the N3/C5–C9 pyridine ring.

D—H···A	D—H	H···A	D···A	D—H···A
C11—H11···N3ⁱ	0.93	2.57	3.334 (5)	140
C7—H7···Cg2ⁱⁱ	0.93	2.95	3.742 (5)	144
C17—H17C···Cg2ⁱⁱⁱ	0.96	2.92	3.722 (6)	141

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

Hydrogen-bond geometry (Å, º) for (II) top

Cg1 and Cg2 are the centroids of the pyrazine and pyridine rings N1/C1/C2/N1'/C1'/C2' and N2/C3–C7, respectively [symmetry code ('): -x + 2, -y + 3/2, z].

D—H···A	D—H	H···A	D···A	D—H···A
C7—H7···O1ⁱ	0.94	2.48	3.308 (3)	147
C4—H4···Cg2ⁱⁱ	0.94	2.92	3.739 (2)	147
C10—H10B···Cg1ⁱⁱⁱ	0.97	2.56	3.409 (3)	146
C10—H10B···Cg1^iv	0.97	2.56	3.409 (3)	146

Symmetry codes: (i) −y+7/4, x+1/4, z+1/4; (ii) y−1/4, −x+5/4, −z+1/4; (iii) −x+2, −y+1, −z; (iv) x, y−1/2, −z.

Experimental details

	(I)	(II)
Crystal data
Chemical formula	C₁₈H₁₄N₄O₄	C₂₀H₁₈N₄O₄
M_r	350.33	378.38
Crystal system, space group	Monoclinic, Ia	Tetragonal, I4₁/a
Temperature (K)	293	223
a, b, c (Å)	8.4249 (12), 12.2465 (10), 16.2561 (13)	10.2295 (6), 10.2295 (6), 36.281 (3)
α, β, γ (°)	90, 103.730 (8), 90	90, 90, 90
V (Å³)	1629.3 (3)	3796.5 (5)
Z	4	8
Radiation type	Mo Kα	Mo Kα
µ (mm⁻¹)	0.10	0.10
Crystal size (mm)	0.70 × 0.50 × 0.38	0.65 × 0.50 × 0.50

Data collection
Diffractometer	Stoe–Siemens AED2	Stoe IPDS 1
Absorption correction	–	–
No. of measured, independent and observed [I > 2σ(I)] reflections	3035, 3028, 2737	14760, 1851, 1153
R_int	0.012	0.043
(sin θ/λ)_max (Å⁻¹)	0.606	0.616

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.050, 0.135, 1.11	0.049, 0.149, 1.01
No. of reflections	3028	1851
No. of parameters	238	129
No. of restraints	2	0
H-atom treatment	H-atom parameters constrained	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	0.19, −0.21	0.34, −0.19

Computer programs: STADI4 (Stoe & Cie, 1997), EXPOSE in IPDS-I (Stoe & Cie, 2004), CELL in IPDS-I (Stoe & Cie, 2004), X-RED (Stoe & Cie, 1997), INTEGRATE in IPDS-I (Stoe & Cie, 2004), SHELXS97 (Sheldrick, 2008), PLATON (Spek, 2009) and Mercury (Macrae et al., 2008), SHELXL2014 (Sheldrick, 2015), PLATON (Spek, 2009) and publCIF (Westrip, 2010).

Acknowledgements

We are grateful to the Swiss National Science Foundation and the University of Neuchâtel for financial support.

References

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research communications\(\def\hfill{\hskip 5em}\def\hfil{\hskip 3em}\def\eqno#1{\hfil {#1}}\)

Di­methyl and di­ethyl esters of 5,6-bis­­(pyridin-2-yl)pyrazine-2,3-di­carb­­oxy­lic acid: a comparison

1. Chemical context

2. Structural commentary

3. Supra­molecular features

4. Database survey

5. Synthesis and crystallization

6. Refinement

Supporting information

Acknowledgements

References

research communications

Dimethyl and diethyl esters of 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylic acid: a comparison

3. Supramolecular features