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

In compound (I), the dimethyl ester of 5,6-bis­(pyridin-2-yl)pyrazine-2,3-di­carb­oxy­lic acid, pyridine ring B is inclined to pyrazine ring A by 44.8 (2)°. The N_pyrazine—C—C—N_pyridine torsion angle is −133.7 (4)°, with the N atoms trans to each other. Pyridine ring C is inclined to pyrazine ring A by 50.3 (2)°. Here the N_pyrazine—C—C—N_pyridine torsion angle is 50.7 (5)° and the N atoms are cis to one another. In compound (II), the diethyl ester, which possesses twofold rotation symmetry, the pyridine rings are inclined to the pyrazine ring by 40.7 (1)°, with the N atoms cis to one another.

Abstract

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.

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.

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 + , 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 + , 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.

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) ; (ii) ; (iii) .

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 + , 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⋯Cg1iv	0.97	2.56	3.409 (3)	146

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) .

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).

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 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 (10) ring motifs, forming chains propagating along [10].

Figure 5.

The chemical scheme for compound L2H₂.

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%.

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, I a	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[F 2 > 2σ(F 2)], wR(F 2), 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
Δρmax, Δρmin (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 ▸).

Supplementary Material

CCDC references: 1448182, 1448181

Additional supporting information: crystallographic information; 3D view; checkCIF report

supplementary crystallographic information

(I) Dimethyl 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate. Crystal data

C18H14N4O4	F(000) = 728
Mr = 350.33	Dx = 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

(I) Dimethyl 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate. Data collection

Stoe–Siemens AED2 diffractometer	θmax = 25.5°, θmin = 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
Rint = 0.012	intensity decay: 1%

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

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(Fo2) + (0.0759P)2 + 1.0624P] where P = (Fo2 + 2Fc2)/3
S = 1.11	(Δ/σ)max < 0.001
3028 reflections	Δρmax = 0.19 e Å−3
238 parameters	Δρmin = −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)

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

(I) Dimethyl 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate. Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
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*

(I) Dimethyl 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate. Atomic displacement parameters (Ų)

	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)

(I) Dimethyl 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate. Geometric parameters (Å, º)

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)

(I) Dimethyl 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate. Hydrogen-bond geometry (Å, º)

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. Crystal data

C20H18N4O4	Dx = 1.324 Mg m−3
Mr = 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

(II) Diethyl 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate. Data collection

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

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

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(Fo2) + (0.0925P)2] where P = (Fo2 + 2Fc2)/3
S = 1.01	(Δ/σ)max < 0.001
1851 reflections	Δρmax = 0.34 e Å−3
129 parameters	Δρmin = −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)

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

(II) Diethyl 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate. Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
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*

(II) Diethyl 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate. Atomic displacement parameters (Ų)

	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)

(II) Diethyl 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate. Geometric parameters (Å, º)

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.

(II) Diethyl 5,6-bis(pyridin-2-yl)pyrazine-2,3-dicarboxylate. Hydrogen-bond geometry (Å, º)

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···Cg1iv	0.97	2.56	3.409 (3)	146
C10—H10B···Cg1v	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.