Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270103015750/ga1023sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270103015750/ga1023Isup2.hkl |
CCDC reference: 221073
Compound (I) was synthesized and purified according to the method described by Bu et al. (2001). Light-yellow cubic single crystals of the title compound suitable for X-ray diffraction were obtained by recrystallization from CH3OH/ CH2Cl2 solution.
All H atoms were found in a difference electron-density map but were then placed in calculated positions (C—H = 0.93 and 0.96 Å for the aromatic system and methoxy group, respectively) and included in the final refinement using the riding-model approximation, with displacement parameters derived from the C atoms to which they were bonded [Uiso(H) = 1.2Ueq(C) and 1.5Ueq(C) for aromatic and methoxy groups, respectively].
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).
Fig. 1. The molecular structure of (I), showing the atom-numbering scheme and displacement ellipsoids at the 30% probability level. |
C20H16N4O2 | F(000) = 720 |
Mr = 344.37 | Dx = 1.372 Mg m−3 |
Monoclinic, P21/n | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -p 2yn | Cell parameters from 6907 reflections |
a = 7.0626 (11) Å | θ = 1.9–25.0° |
b = 17.355 (3) Å | µ = 0.09 mm−1 |
c = 13.674 (2) Å | T = 293 K |
β = 95.958 (4)° | Block, light yellow |
V = 1667.0 (5) Å3 | 0.40 × 0.30 × 0.15 mm |
Z = 4 |
BRUKER SMART 1000 diffractometer | 2125 reflections with I > 2σ(I) |
ω scans | Rint = 0.032 |
Absorption correction: multi-scan SAINT (Bruker 1998) and SADABS (Sheldrick, 1997) | θmax = 25.0° |
Tmin = 0.967, Tmax = 0.986 | h = −5→8 |
6907 measured reflections | k = −20→20 |
2954 independent reflections | l = −16→15 |
Refinement on F2 | w = 1/[σ2(Fo2) + (0.06P)2] where P = (Fo2 + 2Fc2)/3 |
R[F2 > 2σ(F2)] = 0.038 | (Δ/σ)max = 0.001 |
wR(F2) = 0.108 | Δρmax = 0.20 e Å−3 |
S = 1.03 | Δρmin = −0.15 e Å−3 |
2954 reflections | Extinction correction: SHELXL97 |
236 parameters | Extinction coefficient: 0.0213 (19) |
H-atom parameters constrained |
C20H16N4O2 | V = 1667.0 (5) Å3 |
Mr = 344.37 | Z = 4 |
Monoclinic, P21/n | Mo Kα radiation |
a = 7.0626 (11) Å | µ = 0.09 mm−1 |
b = 17.355 (3) Å | T = 293 K |
c = 13.674 (2) Å | 0.40 × 0.30 × 0.15 mm |
β = 95.958 (4)° |
BRUKER SMART 1000 diffractometer | 2954 independent reflections |
Absorption correction: multi-scan SAINT (Bruker 1998) and SADABS (Sheldrick, 1997) | 2125 reflections with I > 2σ(I) |
Tmin = 0.967, Tmax = 0.986 | Rint = 0.032 |
6907 measured reflections |
R[F2 > 2σ(F2)] = 0.038 | 236 parameters |
wR(F2) = 0.108 | H-atom parameters constrained |
S = 1.03 | Δρmax = 0.20 e Å−3 |
2954 reflections | Δρmin = −0.15 e Å−3 |
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 |
x | y | z | Uiso*/Ueq | ||
O1 | 0.18361 (18) | 0.81596 (6) | 1.04210 (8) | 0.0534 (3) | |
O2 | 0.28628 (18) | 1.13369 (6) | 1.05086 (8) | 0.0535 (3) | |
N4 | 0.0698 (2) | 0.93792 (8) | 0.62812 (10) | 0.0485 (4) | |
N3 | 0.4780 (2) | 1.01488 (8) | 0.66412 (10) | 0.0487 (4) | |
N1 | 0.20574 (18) | 0.89644 (7) | 0.87457 (9) | 0.0384 (3) | |
N2 | 0.29296 (17) | 1.05406 (7) | 0.88160 (9) | 0.0381 (3) | |
C19 | 0.0012 (3) | 0.90003 (10) | 0.54635 (12) | 0.0526 (5) | |
H19A | −0.0655 | 0.9281 | 0.4960 | 0.063* | |
C20 | 0.0237 (3) | 0.82220 (10) | 0.53278 (13) | 0.0521 (5) | |
H20A | −0.0284 | 0.7983 | 0.4753 | 0.063* | |
C21 | 0.1248 (3) | 0.78043 (10) | 0.60589 (13) | 0.0522 (5) | |
H21A | 0.1440 | 0.7278 | 0.5983 | 0.063* | |
C17 | 0.1971 (2) | 0.81749 (9) | 0.69029 (12) | 0.0444 (4) | |
H17A | 0.2664 | 0.7903 | 0.7406 | 0.053* | |
C18 | 0.1657 (2) | 0.89574 (9) | 0.69959 (11) | 0.0377 (4) | |
C2 | 0.2254 (2) | 0.93604 (9) | 0.79402 (11) | 0.0363 (4) | |
C3 | 0.2851 (2) | 1.01481 (8) | 0.79898 (11) | 0.0364 (4) | |
C12 | 0.3585 (2) | 1.05558 (9) | 0.71461 (11) | 0.0383 (4) | |
C13 | 0.3196 (2) | 1.13279 (9) | 0.69752 (12) | 0.0458 (4) | |
H13A | 0.2354 | 1.1588 | 0.7339 | 0.055* | |
C14 | 0.4066 (3) | 1.17048 (11) | 0.62630 (14) | 0.0581 (5) | |
H14A | 0.3811 | 1.2222 | 0.6129 | 0.070* | |
C15 | 0.5321 (3) | 1.13040 (13) | 0.57511 (14) | 0.0643 (6) | |
H15A | 0.5946 | 1.1547 | 0.5270 | 0.077* | |
C16 | 0.5639 (3) | 1.05355 (13) | 0.59627 (13) | 0.0598 (5) | |
H16A | 0.6498 | 1.0270 | 0.5615 | 0.072* | |
C9 | 0.2274 (2) | 0.93443 (9) | 0.96214 (11) | 0.0367 (4) | |
C8 | 0.2074 (2) | 0.89405 (9) | 1.05107 (12) | 0.0415 (4) | |
C11 | 0.1486 (3) | 0.77585 (12) | 1.12863 (14) | 0.0674 (6) | |
H11C | 0.1343 | 0.7219 | 1.1143 | 0.101* | |
H11B | 0.0341 | 0.7951 | 1.1520 | 0.101* | |
H11A | 0.2536 | 0.7834 | 1.1782 | 0.101* | |
C7 | 0.2098 (2) | 0.93465 (10) | 1.13619 (12) | 0.0447 (4) | |
H7A | 0.1950 | 0.9086 | 1.1943 | 0.054* | |
C6 | 0.2342 (2) | 1.01535 (10) | 1.13850 (12) | 0.0457 (4) | |
H6A | 0.2333 | 1.0414 | 1.1979 | 0.055* | |
C5 | 0.2592 (2) | 1.05585 (9) | 1.05547 (12) | 0.0404 (4) | |
C10 | 0.2855 (3) | 1.17478 (10) | 1.14060 (13) | 0.0599 (5) | |
H10C | 0.3058 | 1.2286 | 1.1291 | 0.090* | |
H10B | 0.3852 | 1.1556 | 1.1874 | 0.090* | |
H10A | 0.1649 | 1.1679 | 1.1660 | 0.090* | |
C4 | 0.2583 (2) | 1.01555 (9) | 0.96475 (11) | 0.0373 (4) |
U11 | U22 | U33 | U12 | U13 | U23 | |
O1 | 0.0728 (8) | 0.0438 (7) | 0.0446 (7) | −0.0018 (6) | 0.0113 (6) | 0.0066 (5) |
O2 | 0.0743 (9) | 0.0428 (7) | 0.0441 (7) | 0.0013 (6) | 0.0097 (6) | −0.0089 (5) |
N4 | 0.0552 (9) | 0.0427 (8) | 0.0457 (8) | 0.0001 (7) | −0.0038 (7) | 0.0002 (7) |
N3 | 0.0500 (9) | 0.0574 (9) | 0.0397 (8) | −0.0015 (7) | 0.0089 (7) | −0.0084 (7) |
N1 | 0.0395 (7) | 0.0388 (7) | 0.0370 (7) | 0.0015 (6) | 0.0046 (6) | −0.0004 (6) |
N2 | 0.0381 (7) | 0.0395 (7) | 0.0370 (7) | 0.0014 (6) | 0.0047 (6) | −0.0023 (6) |
C19 | 0.0561 (11) | 0.0553 (11) | 0.0441 (10) | −0.0001 (9) | −0.0059 (9) | −0.0025 (9) |
C20 | 0.0537 (11) | 0.0539 (11) | 0.0476 (10) | −0.0069 (9) | −0.0001 (9) | −0.0135 (9) |
C21 | 0.0604 (12) | 0.0415 (10) | 0.0551 (11) | 0.0001 (8) | 0.0075 (9) | −0.0098 (9) |
C17 | 0.0528 (10) | 0.0389 (9) | 0.0416 (9) | 0.0012 (8) | 0.0050 (8) | −0.0005 (7) |
C18 | 0.0383 (8) | 0.0374 (9) | 0.0385 (8) | −0.0036 (7) | 0.0082 (7) | 0.0002 (7) |
C2 | 0.0352 (8) | 0.0385 (8) | 0.0356 (8) | 0.0034 (7) | 0.0055 (7) | −0.0009 (7) |
C3 | 0.0344 (8) | 0.0387 (9) | 0.0361 (8) | 0.0021 (7) | 0.0031 (7) | −0.0022 (7) |
C12 | 0.0399 (9) | 0.0413 (9) | 0.0333 (8) | −0.0057 (7) | 0.0016 (7) | −0.0042 (7) |
C13 | 0.0505 (10) | 0.0406 (9) | 0.0468 (10) | −0.0049 (8) | 0.0073 (8) | −0.0004 (8) |
C14 | 0.0642 (12) | 0.0515 (11) | 0.0579 (11) | −0.0123 (9) | 0.0020 (10) | 0.0128 (9) |
C15 | 0.0605 (13) | 0.0833 (16) | 0.0502 (11) | −0.0208 (11) | 0.0109 (10) | 0.0130 (11) |
C16 | 0.0554 (11) | 0.0813 (15) | 0.0448 (10) | −0.0055 (10) | 0.0155 (9) | −0.0087 (10) |
C9 | 0.0321 (8) | 0.0406 (9) | 0.0375 (9) | 0.0030 (7) | 0.0042 (7) | −0.0013 (7) |
C8 | 0.0391 (9) | 0.0430 (9) | 0.0423 (9) | 0.0032 (7) | 0.0031 (7) | 0.0022 (8) |
C11 | 0.0895 (16) | 0.0567 (12) | 0.0588 (12) | −0.0076 (11) | 0.0222 (11) | 0.0138 (10) |
C7 | 0.0445 (9) | 0.0534 (10) | 0.0362 (9) | 0.0049 (8) | 0.0044 (7) | 0.0055 (8) |
C6 | 0.0454 (10) | 0.0545 (11) | 0.0366 (9) | 0.0095 (8) | 0.0021 (7) | −0.0053 (8) |
C5 | 0.0386 (9) | 0.0402 (9) | 0.0421 (9) | 0.0051 (7) | 0.0026 (7) | −0.0037 (7) |
C10 | 0.0753 (13) | 0.0518 (11) | 0.0534 (11) | 0.0010 (10) | 0.0106 (10) | −0.0166 (9) |
C4 | 0.0328 (8) | 0.0417 (9) | 0.0375 (9) | 0.0038 (7) | 0.0034 (7) | 0.0002 (7) |
O1—C8 | 1.3696 (19) | C12—C13 | 1.383 (2) |
O1—C11 | 1.416 (2) | C13—C14 | 1.370 (2) |
O2—C5 | 1.3667 (19) | C13—H13A | 0.9300 |
O2—C10 | 1.4198 (19) | C14—C15 | 1.375 (3) |
N4—C19 | 1.344 (2) | C14—H14A | 0.9300 |
N4—C18 | 1.346 (2) | C15—C16 | 1.378 (3) |
N3—C16 | 1.340 (2) | C15—H15A | 0.9300 |
N3—C12 | 1.3447 (19) | C16—H16A | 0.9300 |
N1—C2 | 1.3178 (19) | C9—C8 | 1.423 (2) |
N1—C9 | 1.3617 (18) | C9—C4 | 1.424 (2) |
N2—C3 | 1.3155 (18) | C8—C7 | 1.359 (2) |
N2—C4 | 1.3628 (18) | C11—H11C | 0.9600 |
C19—C20 | 1.375 (2) | C11—H11B | 0.9600 |
C19—H19A | 0.9300 | C11—H11A | 0.9600 |
C20—C21 | 1.373 (2) | C7—C6 | 1.411 (2) |
C20—H20A | 0.9300 | C7—H7A | 0.9300 |
C21—C17 | 1.373 (2) | C6—C5 | 1.362 (2) |
C21—H21A | 0.9300 | C6—H6A | 0.9300 |
C17—C18 | 1.384 (2) | C5—C4 | 1.424 (2) |
C17—H17A | 0.9300 | C10—H10C | 0.9600 |
C18—C2 | 1.490 (2) | C10—H10B | 0.9600 |
C2—C3 | 1.430 (2) | C10—H10A | 0.9600 |
C3—C12 | 1.492 (2) | ||
C8—O1—C11 | 116.23 (13) | C14—C15—C16 | 118.85 (17) |
C5—O2—C10 | 116.33 (13) | C14—C15—H15A | 120.6 |
C19—N4—C18 | 116.68 (15) | C16—C15—H15A | 120.6 |
C16—N3—C12 | 116.47 (15) | N3—C16—C15 | 123.66 (18) |
C2—N1—C9 | 118.06 (13) | N3—C16—H16A | 118.2 |
C3—N2—C4 | 117.86 (13) | C15—C16—H16A | 118.2 |
N4—C19—C20 | 123.79 (16) | N1—C9—C8 | 120.06 (14) |
N4—C19—H19A | 118.1 | N1—C9—C4 | 120.23 (13) |
C20—C19—H19A | 118.1 | C8—C9—C4 | 119.59 (14) |
C21—C20—C19 | 118.62 (16) | C7—C8—O1 | 125.47 (15) |
C21—C20—H20A | 120.7 | C7—C8—C9 | 118.87 (15) |
C19—C20—H20A | 120.7 | O1—C8—C9 | 115.65 (13) |
C17—C21—C20 | 118.98 (17) | O1—C11—H11C | 109.5 |
C17—C21—H21A | 120.5 | O1—C11—H11B | 109.5 |
C20—C21—H21A | 120.5 | H11C—C11—H11B | 109.5 |
C21—C17—C18 | 119.20 (16) | O1—C11—H11A | 109.5 |
C21—C17—H17A | 120.4 | H11C—C11—H11A | 109.5 |
C18—C17—H17A | 120.4 | H11B—C11—H11A | 109.5 |
N4—C18—C17 | 122.70 (15) | C8—C7—C6 | 121.63 (15) |
N4—C18—C2 | 116.72 (14) | C8—C7—H7A | 119.2 |
C17—C18—C2 | 120.42 (14) | C6—C7—H7A | 119.2 |
N1—C2—C3 | 121.01 (13) | C5—C6—C7 | 121.31 (15) |
N1—C2—C18 | 115.74 (13) | C5—C6—H6A | 119.3 |
C3—C2—C18 | 123.04 (13) | C7—C6—H6A | 119.3 |
N2—C3—C2 | 121.44 (14) | C6—C5—O2 | 125.41 (15) |
N2—C3—C12 | 115.57 (13) | C6—C5—C4 | 118.91 (15) |
C2—C3—C12 | 122.77 (13) | O2—C5—C4 | 115.69 (14) |
N3—C12—C13 | 123.14 (15) | O2—C10—H10C | 109.5 |
N3—C12—C3 | 115.79 (14) | O2—C10—H10B | 109.5 |
C13—C12—C3 | 120.74 (14) | H10C—C10—H10B | 109.5 |
C14—C13—C12 | 119.11 (17) | O2—C10—H10A | 109.5 |
C14—C13—H13A | 120.4 | H10C—C10—H10A | 109.5 |
C12—C13—H13A | 120.4 | H10B—C10—H10A | 109.5 |
C13—C14—C15 | 118.74 (18) | N2—C4—C5 | 120.07 (14) |
C13—C14—H14A | 120.6 | N2—C4—C9 | 120.28 (13) |
C15—C14—H14A | 120.6 | C5—C4—C9 | 119.62 (14) |
C18—N4—C19—C20 | 0.0 (3) | C13—C14—C15—C16 | 0.9 (3) |
N4—C19—C20—C21 | −1.1 (3) | C12—N3—C16—C15 | −1.7 (3) |
C19—C20—C21—C17 | 1.0 (3) | C14—C15—C16—N3 | 0.4 (3) |
C20—C21—C17—C18 | 0.2 (3) | C2—N1—C9—C8 | 179.99 (14) |
C19—N4—C18—C17 | 1.3 (2) | C2—N1—C9—C4 | 3.9 (2) |
C19—N4—C18—C2 | −174.19 (14) | C11—O1—C8—C7 | 3.6 (2) |
C21—C17—C18—N4 | −1.4 (2) | C11—O1—C8—C9 | −175.21 (15) |
C21—C17—C18—C2 | 173.88 (14) | N1—C9—C8—C7 | −173.19 (15) |
C9—N1—C2—C3 | 5.7 (2) | C4—C9—C8—C7 | 2.9 (2) |
C9—N1—C2—C18 | −169.16 (13) | N1—C9—C8—O1 | 5.7 (2) |
N4—C18—C2—N1 | 138.91 (15) | C4—C9—C8—O1 | −178.20 (13) |
C17—C18—C2—N1 | −36.7 (2) | O1—C8—C7—C6 | −179.66 (15) |
N4—C18—C2—C3 | −35.8 (2) | C9—C8—C7—C6 | −0.9 (2) |
C17—C18—C2—C3 | 148.59 (15) | C8—C7—C6—C5 | −1.0 (3) |
C4—N2—C3—C2 | 5.6 (2) | C7—C6—C5—O2 | −179.10 (15) |
C4—N2—C3—C12 | −169.17 (13) | C7—C6—C5—C4 | 0.7 (2) |
N1—C2—C3—N2 | −11.0 (2) | C10—O2—C5—C6 | −0.9 (2) |
C18—C2—C3—N2 | 163.45 (14) | C10—O2—C5—C4 | 179.34 (14) |
N1—C2—C3—C12 | 163.31 (14) | C3—N2—C4—C5 | −178.05 (14) |
C18—C2—C3—C12 | −22.2 (2) | C3—N2—C4—C9 | 4.1 (2) |
C16—N3—C12—C13 | 1.6 (2) | C6—C5—C4—N2 | −176.51 (14) |
C16—N3—C12—C3 | −171.81 (14) | O2—C5—C4—N2 | 3.3 (2) |
N2—C3—C12—N3 | 132.85 (15) | C6—C5—C4—C9 | 1.3 (2) |
C2—C3—C12—N3 | −41.8 (2) | O2—C5—C4—C9 | −178.83 (12) |
N2—C3—C12—C13 | −40.8 (2) | N1—C9—C4—N2 | −9.2 (2) |
C2—C3—C12—C13 | 144.59 (16) | C8—C9—C4—N2 | 174.70 (13) |
N3—C12—C13—C14 | −0.4 (2) | N1—C9—C4—C5 | 172.93 (13) |
C3—C12—C13—C14 | 172.77 (15) | C8—C9—C4—C5 | −3.2 (2) |
C12—C13—C14—C15 | −0.9 (3) |
Experimental details
Crystal data | |
Chemical formula | C20H16N4O2 |
Mr | 344.37 |
Crystal system, space group | Monoclinic, P21/n |
Temperature (K) | 293 |
a, b, c (Å) | 7.0626 (11), 17.355 (3), 13.674 (2) |
β (°) | 95.958 (4) |
V (Å3) | 1667.0 (5) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.09 |
Crystal size (mm) | 0.40 × 0.30 × 0.15 |
Data collection | |
Diffractometer | BRUKER SMART 1000 diffractometer |
Absorption correction | Multi-scan SAINT (Bruker 1998) and SADABS (Sheldrick, 1997) |
Tmin, Tmax | 0.967, 0.986 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 6907, 2954, 2125 |
Rint | 0.032 |
(sin θ/λ)max (Å−1) | 0.595 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.038, 0.108, 1.03 |
No. of reflections | 2954 |
No. of parameters | 236 |
No. of restraints | ? |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.20, −0.15 |
Computer programs: SMART (BRUKER, 1998), SMART, SAINT (BRUKER, 1998), SHELXS97 (Sheldrick, 1997), SHELXL97 (Sheldrick, 1997), XP (Bruker, 1998).
O1—C8 | 1.3696 (19) | N3—C12 | 1.3447 (19) |
O2—C5 | 1.3667 (19) | N1—C2 | 1.3178 (19) |
N4—C19 | 1.344 (2) | N1—C9 | 1.3617 (18) |
N4—C18 | 1.346 (2) | N2—C3 | 1.3155 (18) |
N3—C16 | 1.340 (2) | N2—C4 | 1.3628 (18) |
C19—N4—C18 | 116.68 (15) | C2—N1—C9 | 118.06 (13) |
C16—N3—C12 | 116.47 (15) | C3—N2—C4 | 117.86 (13) |
Polypyridyl bridging ligands have attracted considerable interest in recent years because of their potential as building blocks for supramolecualr assemblies under complexation with transition metal ions (Leininger et al., 2000; Bu et al., 2001) and the photophysical and redox properties of their functional complexes (Balzani et al., 1996; Scott et al., 1999; Veroni et al., 2003), which make them useful as light-harvesting and photonic molecular devices. Some of the polypyridyl compounds have been intensively studied [such as 2,3-bis(2-pyridyl)quinoxaline and its derivatives, which represent an important class of chelating agents]? because of the potential functionality of their metal complexes as molecular devices and DNA probes (Holmlin & Barton 1995; Balzani & Campagana et al., 1998; Balzani & Gomez-Lopez et al., 1998). In our efforts to systematically investigate the syntheses, crystal structures and coordination chemistry of such compounds, we have reported the crystal structures of 5,6-bis(2-pyridyl)-2,3-pyrazine-dicarbonitrile (Du et al., 2001) and? the so-called proton-sponge compounds 2,3-di-2-pyridinio-5,8-dimethoxyquinoxaline dinitrate (Liu et al., 2001) and 2,3-di-2-pyridinio-5-nitroquinoxaline diperchlorate (Xu et al., 2002). In the present paper, we report the crystal structure of the title compound, (I), namely 2,3-bis(2-pyridyl)-5,8-dimethoxyquinoxaline.
The title compound consists of a quinoxaline ring substituted with two pyridyl rings and two methoxy groups. Selected bond distances and angles are given in Table 1. Such compounds have the potential to generate three possible configurations under appropriate conditions, viz. cis–cis, cis–trans and trans–trans (the relations of the pyridyl N atoms to the quinoxaline ring; Fig. 1). The crystal structure of (I) exhibits the cis–cis conformational arrangement of the pyridyl rings (see Scheme). A cis–cis conformational arrangement of the pyridyl rings was also observed in the crystal structure of the N-protonated nitrate of the title compound, viz. 2,3-bis(2-pyridinio)-5,8-dimethoxyquinoxaline dinitrate (Liu et al., 2001), and this similarity? is consistent with other related compounds (Wozniak, 1991; Kruger et al., 2001). However, it should be noted that a variety of conformational arrangements of the pyridyl rings for (I) are observed in its transition metal complexes, namely trans–trans in the ZnII complex [Zn(I)(H2O)(NO3)2]·2CH3CN)] (Bu et al., 2002); cis–trans in the AgI complex [Ag(I)(CH3CN)2]2(ClO4)2 (Bu et al., 2001), the CuI complex [Cu(I)(CH3CN)]2(ClO4)2, the NiII complex [Ni(I)(NO3)(H2O)2]·2CH3CN·(NO3) and the CoII complex [Co(I)Cl2(H2O)] (Bu et al., 2002); and cis–cis in the CuII complex [Cu(I)(NO3)2]·CH3CN (Bu et al., 2002). These results indicate that the configuration of the title compound can spontaneously convert when coordinating to different metal centers.
The two planar pyridyl rings are not coplanar with each other or with the quinoxaline ring because of steric clashes between the H atoms of the pyridyl rings. For the pyridine N4/C17–C21 ring, the maximum deviation of any atoms from the best-fit plane is 0.008 (2) Å, with average deviation being 0.005 (1) Å. For the N3/C12–C16 ring, the corresponding values are 0.010 (1) and 0.006 (2) Å. The dihedral angle subtended by the two pyridyl rings is 67.6 (1)°, and the intramolecular Npy···Npy separation is 3.169 (2) Å, which are comparable to the values in 2,3-bis(2-pyridyl)-6,7-dimethylquinoxaline [59.3 (1)° and 2.891 (2) Å; Wozniak, 1991]. The existence of the adjacent pyridine substituents causes substantial out-of-plane twist in the quinoxaline ring so that the C2—C3—C12—C18 torsion angle [−22.2 (2)°] is similar to that observed in the metal complexes [in the 7.8 (3) to −20.3 (5)° region; Bu et al., 2002).
In the quinoxaline system, the mean deviation of any atom from the best-fit plane describing it is 0.054 (2) Å, with a maximum deviation of 0.096 (1) Å. The system can also be better described as two rings; one is planar [C4–C9, the mean and maximum deviations being 0.009 (2) and 0.018 (2) Å, respectively], while the other (N1/C2/C3/N2/C4/C9) adopts a twist-boat conformation, albeit with only a small distortion [the mean and maximum deviations are 0.035 (2) and 0.054 (2) Å, respectively]. The planes of the two rings in the quinoxaline system make a dihedral angle of 5.3 (2)°, and the mean? plane of the quinoxaline ring makes dihedral angles of 39.0 (1) and 43.4 (2)° with the planes of the two pyridyl rings. It is concluded that the non-planarity of the quinoxaline ring is caused by the intramolecular non-bonding contacts of the pyridyl rings, as this conformation is not observed in monopyridine-substituted quinoxaline compounds (Veroni et al., 2003). The C2—N1 and C3—N2 bond distances [mean 1.317 (2) Å] are noticeably shorter than the N1—C9 and N2—C4 distances [mean 1.362 (2) Å], which is a typical geometry for the quinoxaline system (Rasmussen et al., 1990). All N—C bond lengths are well within the range of values normally considered standard for single C—N (1.47 Å) and double C═N bonds (1.28 Å).
Analysis of the crystal packing of (I) showed no weak hydrogen-bonding interactions of the C—H···N or C—H···O types. However, the neighboring parallel molecules in the unit cell show substantial π–π-stacking interactions; the closest approach between the quinoxaline systems is ca 3.4 Å, with the molecular stacks stretching along the a direction. In addition, three weak C—H···π interactions were also observed, involving both pyridyl and quinoxaline rings. The H···Cg (Cg is the centroid of the aromatic ring) and H···Perp (the normal distance of the H atom to the aromatic ring) distances are in the ranges 3.10–3.19 and 2.89–3.10 Å, respectively, slightly longer than those found previously (Desiraju & Steiner, 1999). Examination of the structure with PLATON (Spek, 2003) showed that there were no solvent-accessible voids.