2,3-Bis(2-pyridinio)-5,8-di­methoxy­quinoxaline dinitrate

In the crystal structure of the title compound, C₂₀H₁₈N₄O₂²⁺·2NO₃⁻, two pyridinium rings form dihedral angles of 48.0 (2) and 33.3 (4)° with the mean quinoxaline plane, and a dihedral angle of 52.6 (2)° with each other. The orientation of the pyridinium rings is such that their N atoms face each other. Both pyridinium N—H groups participate in N—H...O hydrogen bonds involving the O atoms of the nitrate anions. There exist significant π–π interactions responsible for the formation of the stacks along the b axis of the crystal.

Key indicators

• Single-crystal X-ray study
• T = 298 K
• Mean [sigma](C-C) = 0.003 Å
• R factor = 0.042
• wR factor = 0.110
• Data-to-parameter ratio = 11.6

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

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S1. Comment

2,3-Bis(2-pyridyl)quinoxaline and its derivatives represent an important class of chelating agents which have been extensively studied over the last two decades. Most of the studies focused on the electrochemical and photochemical properties of their metal complexes (Balzani et al., 1996; Scott et al., 1999). Aromatic compounds of this type also exhibit proton-sponge properties (Staab & Saupe, 1988; Robertson et al., 1998), i.e. represent the species which can act as external proton acceptors through the formation of N—H···Y hydrogen bonds. In the present paper, we report the crystal structure of the nitrate salt of diprotonated 2,3-bis(2-pyridyl)-5,8-dimethoxyquinoxaline (Bpdq), i.e. C₂₀H₁₈N₄O₂²⁺·2NO₃^-, (I).

The cation of (I) consists of a quinoxaline ring system substituted with two protonated pyridine rings and two methoxy groups (Fig. 1). The pyridine rings cannot be coplanar with each other or with the quinoxaline system, as the planar conformation would have caused sterically unacceptable contacts between the pyridyl ortho-H atoms. In fact, the existence of the adjacent pyridinium substituents causes substantial out-of-plane twist even within the quinoxaline itself, the torsion angle C2—C3—C12—C18 being 6.9 (3)°; the mean atomic displacement from the least-squares quinoxaline plane is 0.0324 (3) Å. The pyridinium rings C12–C16/N3 and C17–C21/N4 form dihedral angles of 48.0 (2) and 33.3 (4)°, respectively, with the mean quinoxaline plane, and a dihedral angle of 52.6 (2)° with each other. The orientation of the pyridinium rings is such that their N atoms face each other. These geometrical characteristics are similar to those observed in the structures of the analogues of (I) (Rasmussen et al., 1990; Du et al., 2001).

Selected bond distances and angles are given in Table 1. The C2—N1 and C3—N2 bond distances [1.313 (2) and 1.322 (2) Å, respectively] are noticeably shorter than N1—C9 and N2—C4 [1.355 (2) and 1.358 (2) Å, respectively], which is typical for quinoxaline system geometry (Anthony et al., 1998; Rasmussen et al., 1990). All N—C bond lengths are well within the range of values normally considered standard for single C—N and double C═N bonds [1.47 (Sasada, 1984) and 1.28 Å (Wang et al., 1998), respectively].

Each pyridinium N—H group participates in N—H···O hydrogen bonds involving the nitrate O atom (Fig. 2 and Table 2). Furthermore, the neighbouring cations in the crystal of (I) show substantial π–π-stacking interactions; the closest approach between the quinoxaline rings is about 3.5 Å, the cationic stacks are stretching along the c axis of the crystal.

S2. Experimental

2,3-Bis(2-pyridyl)-5,8-dimethoxyquinoxaline (Bpdq) was prepared by the reaction of 1,4-dimethoxy-2,3-phenylenediamine (0.4 g, 2.4 mmol) and 2,2'-bipyridyl (0.5 g, 2.4 mmol) in ethanol (30 ml) at reflux for 4 h under argon, by a method similar to that reported in the literature (Waterland et al., 1998). Orange single crystals of the title compound suitable for X-ray diffraction were obtained by slow diffusion of diethyl ether into the acetonitrile solution of Bpdq in the presence of HNO₃.

S3. Refinement

H-atoms positions were calculated and included in subsequent refinement in the riding motion approximation.

Computing details

Data collection: SMART (Bruker, 1998); cell refinement: SMART; data reduction: SAINT (Bruker, 1998); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XP (Bruker, 1998).

Figures

	Fig. 1. ORTEPII (Johnson, 1976) view of diprotonated Bpdq shown with 30% probability ellipsoids.
	Fig. 2. The packing diagram showing the cationic stacks in the crystal of (I).

2,3-bis(2-pyridinio)-5,8-dimethoxyquinoxaline dinitrate

Crystal data

C20H18N4O2^2+·2NO3^−	F(000) = 976
Mr = 470.40	Dx = 1.525 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 16.1380 (13) Å	Cell parameters from 7963 reflections
b = 7.2682 (6) Å	θ = 1.3–25.0°
c = 18.0257 (15) Å	µ = 0.12 mm^−1
β = 104.344 (2)°	T = 298 K
V = 2048.4 (3) Å^3	Prism, orange
Z = 4	0.30 × 0.20 × 0.15 mm

Data collection

Bruker SMART 1000 diffractometer	2323 reflections with I > 2σ(I)
ω scans	Rint = 0.040
Absorption correction: multi-scan [SAINT (Bruker, 1998) and SADABS (Sheldrick, 1997)]	θmax = 25.0°
Tmin = 0.965, Tmax = 0.982	h = −15→19
8042 measured reflections	k = −7→8
3569 independent reflections	l = −21→21

Refinement

Refinement on F^2	w = 1/[σ^2(Fo^2) + (0.0555P)^2] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.042	(Δ/σ)max = 0.010
wR(F^2) = 0.110	Δρmax = 0.29 e Å^−3
S = 0.97	Δρmin = −0.17 e Å^−3
3569 reflections	Extinction correction: SHELXL97
309 parameters	Extinction coefficient: 0.0151 (12)
H-atom parameters constrained

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic)

treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Full-matrix

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
N1	0.40571 (10)	0.6735 (2)	1.05675 (9)	0.0341 (4)
N2	0.34234 (10)	0.7641 (2)	0.90253 (9)	0.0341 (4)
N3	0.14904 (10)	0.8475 (2)	0.94308 (9)	0.0382 (4)
H3A	0.1714	0.8905	0.9880	0.046*
N4	0.20014 (10)	0.5057 (2)	1.05107 (9)	0.0388 (4)
H4B	0.1867	0.4736	1.0037	0.047*
O1	0.57194 (9)	0.7010 (2)	1.12419 (8)	0.0511 (4)
O2	0.45021 (9)	0.8501 (2)	0.81438 (8)	0.0480 (4)
C2	0.32338 (12)	0.6700 (3)	1.02416 (11)	0.0314 (5)
C3	0.29038 (12)	0.7244 (3)	0.94658 (11)	0.0319 (5)
C4	0.42792 (12)	0.7621 (3)	0.93427 (11)	0.0333 (5)
C5	0.48624 (13)	0.8119 (3)	0.88965 (12)	0.0376 (5)
C6	0.57093 (13)	0.8219 (3)	0.92497 (13)	0.0419 (5)
H6A	0.6094	0.8540	0.8963	0.050*
C7	0.60206 (14)	0.7849 (3)	1.00354 (13)	0.0415 (5)
H7A	0.6605	0.7940	1.0256	0.050*
C8	0.54861 (13)	0.7362 (3)	1.04808 (12)	0.0372 (5)
C9	0.45968 (12)	0.7223 (3)	1.01330 (11)	0.0336 (5)
C10	0.50296 (15)	0.9484 (4)	0.77466 (13)	0.0620 (7)
H10A	0.4719	0.9683	0.7225	0.093*
H10B	0.5535	0.8779	0.7756	0.093*
H10C	0.5189	1.0648	0.7992	0.093*
C11	0.65922 (14)	0.7295 (4)	1.16291 (14)	0.0606 (7)
H11A	0.6675	0.6999	1.2162	0.091*
H11B	0.6741	0.8560	1.1579	0.091*
H11C	0.6949	0.6519	1.1408	0.091*
C12	0.19879 (12)	0.7488 (3)	0.90748 (11)	0.0331 (5)
C13	0.16333 (13)	0.6831 (3)	0.83523 (12)	0.0387 (5)
H13A	0.1963	0.6156	0.8094	0.046*
C14	0.07809 (14)	0.7180 (3)	0.80111 (13)	0.0458 (6)
H14A	0.0538	0.6747	0.7520	0.055*
C15	0.02945 (14)	0.8159 (3)	0.83947 (13)	0.0471 (6)
H15A	−0.0281	0.8379	0.8171	0.057*
C16	0.06666 (13)	0.8808 (3)	0.91116 (13)	0.0452 (6)
H16A	0.0344	0.9483	0.9377	0.054*
C17	0.29371 (13)	0.6529 (3)	1.15385 (11)	0.0400 (5)
H17A	0.3425	0.7228	1.1733	0.048*
C18	0.27060 (12)	0.6083 (3)	1.07707 (11)	0.0333 (5)
C19	0.15032 (14)	0.4520 (3)	1.09596 (13)	0.0502 (6)
H19A	0.1012	0.3839	1.0754	0.060*
C20	0.17103 (16)	0.4962 (4)	1.17200 (14)	0.0552 (7)
H20A	0.1358	0.4607	1.2032	0.066*
C21	0.24455 (15)	0.5939 (3)	1.20173 (13)	0.0482 (6)
H21A	0.2611	0.6200	1.2538	0.058*
O3	0.18556 (10)	0.3121 (2)	0.92612 (9)	0.0576 (5)
O4	0.05221 (13)	0.3601 (3)	0.90596 (16)	0.1056 (8)
O5	0.10246 (15)	0.2249 (3)	0.82050 (12)	0.1011 (8)
O6	0.21296 (16)	0.0560 (3)	0.18721 (11)	0.1036 (8)
O7	0.23531 (11)	0.0266 (3)	0.07477 (11)	0.0673 (5)
O8	0.11559 (12)	−0.0417 (4)	0.09471 (13)	0.0958 (8)
N5	0.11136 (15)	0.2981 (3)	0.88328 (13)	0.0589 (6)
N6	0.18783 (14)	0.0156 (3)	0.12064 (12)	0.0516 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0274 (9)	0.0386 (10)	0.0354 (9)	−0.0006 (8)	0.0062 (8)	−0.0035 (8)
N2	0.0309 (10)	0.0378 (10)	0.0333 (9)	−0.0014 (8)	0.0073 (8)	−0.0013 (8)
N3	0.0333 (10)	0.0463 (11)	0.0332 (9)	−0.0001 (8)	0.0046 (8)	−0.0015 (8)
N4	0.0374 (10)	0.0464 (11)	0.0329 (9)	−0.0073 (9)	0.0092 (8)	0.0011 (8)
O1	0.0294 (8)	0.0810 (12)	0.0377 (9)	−0.0023 (8)	−0.0013 (7)	0.0024 (8)
O2	0.0407 (9)	0.0675 (11)	0.0374 (8)	−0.0109 (8)	0.0126 (7)	0.0034 (8)
C2	0.0294 (11)	0.0354 (12)	0.0287 (10)	0.0001 (9)	0.0059 (9)	−0.0028 (9)
C3	0.0293 (11)	0.0360 (12)	0.0307 (11)	−0.0025 (9)	0.0080 (9)	−0.0023 (9)
C4	0.0298 (12)	0.0334 (11)	0.0370 (12)	−0.0003 (9)	0.0092 (9)	−0.0049 (9)
C5	0.0355 (12)	0.0414 (13)	0.0378 (12)	−0.0012 (10)	0.0125 (10)	−0.0026 (10)
C6	0.0336 (12)	0.0475 (14)	0.0484 (13)	−0.0038 (10)	0.0172 (10)	−0.0065 (11)
C7	0.0284 (11)	0.0424 (13)	0.0538 (14)	−0.0011 (10)	0.0102 (10)	−0.0072 (11)
C8	0.0304 (11)	0.0408 (12)	0.0373 (12)	0.0017 (9)	0.0025 (10)	−0.0061 (10)
C9	0.0303 (11)	0.0329 (11)	0.0381 (12)	0.0007 (9)	0.0093 (9)	−0.0062 (9)
C10	0.0596 (16)	0.084 (2)	0.0489 (15)	−0.0214 (15)	0.0247 (12)	0.0022 (14)
C11	0.0324 (13)	0.094 (2)	0.0495 (15)	0.0041 (13)	−0.0017 (11)	−0.0032 (14)
C12	0.0302 (11)	0.0377 (11)	0.0318 (11)	−0.0024 (9)	0.0082 (9)	0.0032 (9)
C13	0.0365 (12)	0.0433 (12)	0.0350 (12)	−0.0008 (10)	0.0065 (10)	0.0010 (10)
C14	0.0419 (13)	0.0505 (14)	0.0392 (13)	−0.0052 (11)	−0.0012 (11)	0.0023 (11)
C15	0.0300 (12)	0.0531 (15)	0.0532 (15)	−0.0001 (11)	0.0008 (11)	0.0091 (12)
C16	0.0303 (12)	0.0520 (14)	0.0526 (14)	0.0060 (11)	0.0091 (11)	0.0047 (12)
C17	0.0387 (12)	0.0437 (13)	0.0356 (12)	0.0010 (10)	0.0054 (10)	−0.0006 (10)
C18	0.0297 (11)	0.0367 (12)	0.0326 (11)	0.0019 (9)	0.0061 (9)	0.0016 (9)
C19	0.0480 (14)	0.0522 (15)	0.0543 (15)	−0.0114 (12)	0.0202 (12)	0.0033 (12)
C20	0.0674 (17)	0.0565 (16)	0.0528 (15)	−0.0054 (14)	0.0357 (13)	0.0046 (13)
C21	0.0594 (15)	0.0511 (14)	0.0379 (13)	0.0073 (13)	0.0191 (12)	0.0057 (11)
O3	0.0447 (10)	0.0757 (12)	0.0440 (9)	0.0042 (9)	−0.0050 (8)	−0.0106 (8)
O4	0.0436 (12)	0.115 (2)	0.157 (2)	−0.0016 (13)	0.0230 (14)	0.0125 (17)
O5	0.133 (2)	0.1011 (18)	0.0487 (12)	−0.0421 (14)	−0.0172 (12)	−0.0112 (11)
O6	0.173 (2)	0.0816 (16)	0.0438 (12)	−0.0015 (15)	0.0028 (13)	−0.0161 (11)
O7	0.0625 (12)	0.0721 (13)	0.0735 (13)	−0.0099 (10)	0.0283 (10)	−0.0007 (10)
O8	0.0469 (12)	0.131 (2)	0.1087 (17)	−0.0034 (13)	0.0181 (12)	−0.0244 (15)
N5	0.0544 (14)	0.0546 (14)	0.0586 (14)	−0.0148 (11)	−0.0034 (13)	0.0114 (11)
N6	0.0578 (14)	0.0458 (12)	0.0494 (13)	0.0084 (10)	0.0099 (11)	−0.0019 (10)

Geometric parameters (Å, º)

N1—C2	1.313 (2)	C6—C7	1.406 (3)
N1—C9	1.355 (2)	C7—C8	1.363 (3)
N2—C3	1.322 (2)	C8—C9	1.420 (3)
N2—C4	1.358 (2)	C12—C13	1.372 (3)
N3—C12	1.351 (3)	C13—C14	1.384 (3)
N3—C16	1.333 (2)	C14—C15	1.367 (3)
N4—C18	1.343 (2)	C15—C16	1.367 (3)
N4—C19	1.333 (3)	C17—C21	1.377 (3)
O1—C8	1.354 (2)	C17—C18	1.380 (3)
O1—C11	1.424 (2)	C19—C20	1.366 (3)
O2—C5	1.366 (2)	C20—C21	1.372 (3)
O2—C10	1.432 (3)	O3—N5	1.258 (2)
C2—C3	1.423 (3)	O4—N5	1.214 (3)
C2—C18	1.496 (3)	O5—N5	1.227 (3)
C3—C12	1.482 (3)	O6—N6	1.204 (2)
C4—C9	1.419 (3)	O7—N6	1.261 (2)
C4—C5	1.428 (3)	O8—N6	1.217 (3)
C5—C6	1.359 (3)
C2—N1—C9	118.07 (17)	N1—C9—C4	120.63 (18)
C3—N2—C4	118.35 (17)	N1—C9—C8	119.16 (18)
C16—N3—C12	122.51 (19)	C4—C9—C8	120.20 (18)
C19—N4—C18	122.47 (18)	N3—C12—C13	118.58 (18)
C8—O1—C11	117.52 (18)	N3—C12—C3	118.53 (18)
C5—O2—C10	116.03 (16)	C13—C12—C3	122.82 (18)
N1—C2—C3	121.67 (18)	C12—C13—C14	119.5 (2)
N1—C2—C18	113.16 (17)	C15—C14—C13	120.2 (2)
C3—C2—C18	125.16 (17)	C16—C15—C14	119.0 (2)
N2—C3—C2	120.81 (18)	N3—C16—C15	120.2 (2)
N2—C3—C12	113.23 (17)	C21—C17—C18	120.1 (2)
C2—C3—C12	125.93 (17)	N4—C18—C17	118.38 (18)
N2—C4—C9	120.14 (18)	N4—C18—C2	120.69 (17)
N2—C4—C5	120.23 (18)	C17—C18—C2	120.93 (18)
C9—C4—C5	119.50 (18)	N4—C19—C20	120.4 (2)
C6—C5—O2	125.84 (19)	C19—C20—C21	119.1 (2)
C6—C5—C4	118.5 (2)	C20—C21—C17	119.5 (2)
O2—C5—C4	115.68 (18)	O4—N5—O5	123.3 (3)
C5—C6—C7	121.9 (2)	O4—N5—O3	118.1 (2)
C8—C7—C6	121.5 (2)	O5—N5—O3	118.5 (3)
O1—C8—C7	126.04 (19)	O6—N6—O8	121.4 (3)
O1—C8—C9	115.52 (18)	O6—N6—O7	121.9 (2)
C7—C8—C9	118.43 (19)	O8—N6—O7	116.7 (2)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N3—H3a···O7^i	0.86	1.92	2.762 (4)	166
N4—H4b···O3	0.86	1.82	2.617 (2)	153

Symmetry code: (i) x, y+1, z+1.