Crystallization of
N,
N′-dimethylpyrazinediium bis(tetrafluoroborate), C
6H
10N
22+·2BF
4−, (I), and
N,
N′-diethylpyrazinediium bis(tetrafluoroborate), C
8H
14N
22+·2BF
4−, (II), from dried acetonitrile under argon protection has permitted their single-crystal studies. In both crystal structures, the pyrazinediium dications are located about an inversion center (located at the ring center) and each pyrazinediium aromatic ring is π-bonded to two centrosymmetrically related BF
4− anions. Strong anion–π interactions, as well as weak C—H
F hydrogen bonds, between BF
4− and pyrazinediium ions are present in both salts.
Supporting information
CCDC references: 735128; 735129
The title dialkyldipyrazinium salts were prepared according to the literature
procedure of Curphey et al. (1972) and were recrystallized from dried
accetonitrile under argon protection. Crystals are extremely sensitive to
moisture and turned black after a few hours under ambient conditions. The
highest residual electron-density peak in (II) is located on the center of the
aromatic C3—N1 bond.
Carbon-bound H atoms were placed in calculated positions (C—H =
0.95–0.99Å) and included in the refinement in the riding-model
approximation, with Uiso(H) values set at 1.2–1.5 Ueq(C).
For both compounds, data collection: SMART (Bruker, 2003); cell refinement: SAINT (Bruker, 2003); data reduction: SAINT (Bruker, 2003) and SADABS (Bruker, 2003); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL (Sheldrick, 2008); molecular graphics: XP (Bruker, 1999); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and XCIF (Bruker, 1999).
(I)
N,
N'-Dimethyldipyrazinium bis(tetrafluoroborate)
top
Crystal data top
C6H10N22+·2BF4− | F(000) = 284 |
Mr = 283.78 | Dx = 1.718 Mg m−3 |
Monoclinic, P21/c | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2ybc | Cell parameters from 2068 reflections |
a = 5.6227 (14) Å | θ = 2.7–29.5° |
b = 14.884 (4) Å | µ = 0.20 mm−1 |
c = 6.7419 (17) Å | T = 173 K |
β = 103.557 (4)° | Block, colourless |
V = 548.5 (2) Å3 | 0.14 × 0.14 × 0.10 mm |
Z = 2 | |
Data collection top
Bruker SMART diffractometer | 1617 independent reflections |
Radiation source: sealed tube | 1331 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.029 |
ω scans | θmax = 30.5°, θmin = 2.7° |
Absorption correction: multi-scan (SADABS; Sheldrick, 1996) | h = −7→7 |
Tmin = 0.776, Tmax = 1.000 | k = −20→21 |
5874 measured reflections | l = −9→9 |
Refinement top
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.048 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.131 | H-atom parameters constrained |
S = 1.08 | w = 1/[σ2(Fo2) + (0.0592P)2 + 0.2138P] where P = (Fo2 + 2Fc2)/3 |
1617 reflections | (Δ/σ)max < 0.001 |
83 parameters | Δρmax = 0.49 e Å−3 |
0 restraints | Δρmin = −0.34 e Å−3 |
Crystal data top
C6H10N22+·2BF4− | V = 548.5 (2) Å3 |
Mr = 283.78 | Z = 2 |
Monoclinic, P21/c | Mo Kα radiation |
a = 5.6227 (14) Å | µ = 0.20 mm−1 |
b = 14.884 (4) Å | T = 173 K |
c = 6.7419 (17) Å | 0.14 × 0.14 × 0.10 mm |
β = 103.557 (4)° | |
Data collection top
Bruker SMART diffractometer | 1617 independent reflections |
Absorption correction: multi-scan (SADABS; Sheldrick, 1996) | 1331 reflections with I > 2σ(I) |
Tmin = 0.776, Tmax = 1.000 | Rint = 0.029 |
5874 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.048 | 0 restraints |
wR(F2) = 0.131 | H-atom parameters constrained |
S = 1.08 | Δρmax = 0.49 e Å−3 |
1617 reflections | Δρmin = −0.34 e Å−3 |
83 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. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor
wR and goodness of fit S are based on F2, conventional
R-factors R are based on F, with F set to zero for
negative F2. The threshold expression of F2 >
σ(F2) is used only for calculating R-factors(gt) etc.
and is not relevant to the choice of reflections for refinement.
R-factors based on F2 are statistically about twice as large
as those based on F, and R- factors based on ALL data will be
even larger. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
C1 | 0.1283 (2) | 0.51841 (10) | −0.14178 (19) | 0.0263 (3) | |
H1 | 0.2181 | 0.5309 | −0.2417 | 0.032* | |
C2 | −0.0206 (2) | 0.56442 (10) | 0.1367 (2) | 0.0266 (3) | |
H2 | −0.0357 | 0.6094 | 0.2330 | 0.032* | |
C3 | 0.2166 (3) | 0.67158 (10) | −0.0107 (2) | 0.0329 (3) | |
H3A | 0.3547 | 0.6787 | 0.1076 | 0.049* | |
H3B | 0.0937 | 0.7179 | −0.0072 | 0.049* | |
H3C | 0.2744 | 0.6778 | −0.1364 | 0.049* | |
N1 | 0.1057 (2) | 0.58116 (8) | −0.00507 (17) | 0.0245 (3) | |
B1 | −0.3820 (3) | 0.63391 (13) | 0.5878 (3) | 0.0338 (4) | |
F1 | −0.2778 (2) | 0.71657 (9) | 0.6478 (2) | 0.0583 (4) | |
F2 | −0.2392 (3) | 0.58902 (10) | 0.4808 (2) | 0.0704 (5) | |
F3 | −0.39212 (18) | 0.58310 (9) | 0.75906 (16) | 0.0491 (3) | |
F4 | −0.6186 (2) | 0.64571 (8) | 0.47351 (19) | 0.0572 (4) | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
C1 | 0.0222 (6) | 0.0356 (7) | 0.0224 (6) | −0.0008 (5) | 0.0082 (4) | −0.0006 (5) |
C2 | 0.0240 (6) | 0.0350 (7) | 0.0221 (6) | −0.0004 (5) | 0.0084 (5) | −0.0030 (5) |
C3 | 0.0328 (7) | 0.0305 (7) | 0.0371 (8) | −0.0069 (6) | 0.0115 (6) | −0.0015 (6) |
N1 | 0.0201 (5) | 0.0301 (6) | 0.0235 (5) | −0.0014 (4) | 0.0059 (4) | −0.0003 (4) |
B1 | 0.0326 (8) | 0.0418 (9) | 0.0279 (8) | 0.0097 (6) | 0.0088 (6) | 0.0024 (6) |
F1 | 0.0478 (7) | 0.0569 (7) | 0.0644 (8) | −0.0056 (5) | 0.0011 (6) | −0.0073 (6) |
F2 | 0.0971 (11) | 0.0701 (9) | 0.0626 (8) | 0.0284 (7) | 0.0564 (8) | 0.0068 (6) |
F3 | 0.0307 (5) | 0.0808 (8) | 0.0376 (6) | 0.0085 (5) | 0.0115 (4) | 0.0199 (5) |
F4 | 0.0485 (6) | 0.0490 (6) | 0.0578 (7) | 0.0072 (5) | −0.0202 (5) | 0.0032 (5) |
Geometric parameters (Å, º) top
C1—N1 | 1.3389 (18) | C3—H3A | 0.9800 |
C1—C2i | 1.378 (2) | C3—H3B | 0.9800 |
C1—H1 | 0.9500 | C3—H3C | 0.9800 |
C2—N1 | 1.3411 (18) | B1—F2 | 1.373 (2) |
C2—C1i | 1.378 (2) | B1—F1 | 1.382 (2) |
C2—H2 | 0.9500 | B1—F4 | 1.383 (2) |
C3—N1 | 1.4873 (19) | B1—F3 | 1.393 (2) |
| | | |
N1—C1—C2i | 119.61 (12) | H3B—C3—H3C | 109.5 |
N1—C1—H1 | 120.2 | C1—N1—C2 | 120.83 (12) |
C2i—C1—H1 | 120.2 | C1—N1—C3 | 120.37 (12) |
N1—C2—C1i | 119.56 (12) | C2—N1—C3 | 118.78 (12) |
N1—C2—H2 | 120.2 | F2—B1—F1 | 109.10 (16) |
C1i—C2—H2 | 120.2 | F2—B1—F4 | 111.95 (15) |
N1—C3—H3A | 109.5 | F1—B1—F4 | 109.67 (14) |
N1—C3—H3B | 109.5 | F2—B1—F3 | 108.31 (14) |
H3A—C3—H3B | 109.5 | F1—B1—F3 | 109.72 (14) |
N1—C3—H3C | 109.5 | F4—B1—F3 | 108.06 (14) |
H3A—C3—H3C | 109.5 | | |
Symmetry code: (i) −x, −y+1, −z. |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
C1—H1···F2i | 0.95 | 2.43 | 2.973 (2) | 116 |
C2—H2···F2 | 0.95 | 2.26 | 2.892 (2) | 124 |
C1—H1···F3ii | 0.95 | 2.32 | 3.081 (2) | 136 |
C2—H2···F4iii | 0.95 | 2.58 | 3.053 (2) | 111 |
Symmetry codes: (i) −x, −y+1, −z; (ii) x+1, y, z−1; (iii) x+1, y, z. |
(II)
N,
N'-Diethyldipyrazinium bis(tetrafluoroborate)
top
Crystal data top
C8H14N22+·2BF4− | F(000) = 316 |
Mr = 311.83 | Dx = 1.523 Mg m−3 |
Monoclinic, P21/n | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2yn | Cell parameters from 117 reflections |
a = 9.324 (3) Å | θ = 3.7–22.5° |
b = 6.2281 (17) Å | µ = 0.17 mm−1 |
c = 11.987 (4) Å | T = 173 K |
β = 102.346 (7)° | Block, colorless |
V = 680.0 (4) Å3 | 0.20 × 0.14 × 0.12 mm |
Z = 2 | |
Data collection top
Bruker SMART diffractometer | 2000 independent reflections |
Radiation source: sealed tube | 1272 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.047 |
ω scans | θmax = 30.5°, θmin = 2.5° |
Absorption correction: multi-scan (SADABS; Bruker, 2003) | h = −12→13 |
Tmin = 0.383, Tmax = 1.000 | k = −8→8 |
7346 measured reflections | l = −16→16 |
Refinement top
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.062 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.167 | H-atom parameters constrained |
S = 1.09 | w = 1/[σ2(Fo2) + (0.0852P)2 + 0.0899P] where P = (Fo2 + 2Fc2)/3 |
2000 reflections | (Δ/σ)max < 0.001 |
92 parameters | Δρmax = 0.45 e Å−3 |
0 restraints | Δρmin = −0.22 e Å−3 |
Crystal data top
C8H14N22+·2BF4− | V = 680.0 (4) Å3 |
Mr = 311.83 | Z = 2 |
Monoclinic, P21/n | Mo Kα radiation |
a = 9.324 (3) Å | µ = 0.17 mm−1 |
b = 6.2281 (17) Å | T = 173 K |
c = 11.987 (4) Å | 0.20 × 0.14 × 0.12 mm |
β = 102.346 (7)° | |
Data collection top
Bruker SMART diffractometer | 2000 independent reflections |
Absorption correction: multi-scan (SADABS; Bruker, 2003) | 1272 reflections with I > 2σ(I) |
Tmin = 0.383, Tmax = 1.000 | Rint = 0.047 |
7346 measured reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.062 | 0 restraints |
wR(F2) = 0.167 | H-atom parameters constrained |
S = 1.09 | Δρmax = 0.45 e Å−3 |
2000 reflections | Δρmin = −0.22 e Å−3 |
92 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. |
Refinement. Refinement of F^2^ against ALL reflections. The weighted
R-factor wR and goodness of fit S are based on
F^2^, conventional R-factors R are based on F,
with F set to zero for negative F^2^. The threshold expression
of F^2^ > σ(F^2^) is used only for calculating
R-factors(gt) etc. and is not relevant to the choice of
reflections for refinement. R-factors based on F^2^ are
statistically about twice as large as those based on F, and R-
factors based on ALL data will be even larger. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top | x | y | z | Uiso*/Ueq | |
N1 | 0.60256 (16) | 0.9015 (2) | 0.45371 (12) | 0.0257 (4) | |
C1 | 0.6435 (2) | 1.0319 (3) | 0.54395 (15) | 0.0281 (4) | |
H1 | 0.7448 | 1.0546 | 0.5754 | 0.034* | |
C2 | 0.4600 (2) | 0.8676 (3) | 0.40895 (15) | 0.0281 (4) | |
H2 | 0.4311 | 0.7748 | 0.3452 | 0.034* | |
C3 | 0.7184 (2) | 0.7926 (3) | 0.40372 (17) | 0.0337 (5) | |
H3A | 0.7900 | 0.9005 | 0.3888 | 0.040* | |
H3B | 0.6722 | 0.7247 | 0.3301 | 0.040* | |
C4 | 0.7970 (2) | 0.6238 (4) | 0.4850 (2) | 0.0437 (6) | |
H4A | 0.8506 | 0.6931 | 0.5550 | 0.066* | |
H4B | 0.8663 | 0.5460 | 0.4488 | 0.066* | |
H4C | 0.7251 | 0.5228 | 0.5038 | 0.066* | |
B1 | 0.0272 (2) | 0.8102 (4) | 0.24170 (19) | 0.0336 (5) | |
F1 | −0.08722 (18) | 0.6739 (3) | 0.24404 (14) | 0.0729 (6) | |
F2 | 0.01751 (16) | 0.8796 (2) | 0.13071 (11) | 0.0527 (4) | |
F3 | 0.15932 (15) | 0.7041 (2) | 0.27718 (12) | 0.0604 (5) | |
F4 | 0.02217 (13) | 0.9881 (2) | 0.31250 (11) | 0.0463 (4) | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
N1 | 0.0273 (8) | 0.0271 (8) | 0.0217 (7) | 0.0017 (6) | 0.0030 (6) | 0.0028 (6) |
C1 | 0.0263 (9) | 0.0296 (10) | 0.0254 (9) | −0.0025 (8) | −0.0007 (7) | 0.0020 (7) |
C2 | 0.0302 (10) | 0.0286 (10) | 0.0226 (9) | −0.0005 (7) | −0.0006 (7) | 0.0018 (7) |
C3 | 0.0316 (10) | 0.0391 (12) | 0.0320 (10) | 0.0065 (8) | 0.0101 (8) | 0.0002 (9) |
C4 | 0.0400 (12) | 0.0465 (14) | 0.0426 (12) | 0.0157 (10) | 0.0043 (9) | 0.0012 (10) |
B1 | 0.0322 (12) | 0.0384 (13) | 0.0288 (11) | −0.0020 (9) | 0.0034 (9) | −0.0078 (9) |
F1 | 0.0751 (11) | 0.0804 (12) | 0.0764 (11) | −0.0469 (9) | 0.0457 (9) | −0.0445 (9) |
F2 | 0.0767 (10) | 0.0453 (9) | 0.0332 (7) | 0.0047 (7) | 0.0053 (6) | 0.0009 (6) |
F3 | 0.0592 (9) | 0.0533 (9) | 0.0545 (9) | 0.0195 (7) | −0.0193 (7) | −0.0153 (7) |
F4 | 0.0429 (7) | 0.0488 (8) | 0.0441 (7) | −0.0025 (6) | 0.0027 (6) | −0.0226 (6) |
Geometric parameters (Å, º) top
N1—C2 | 1.339 (2) | C3—H3B | 0.9900 |
N1—C1 | 1.341 (2) | C4—H4A | 0.9800 |
N1—C3 | 1.504 (2) | C4—H4B | 0.9800 |
C1—C2i | 1.370 (3) | C4—H4C | 0.9800 |
C1—H1 | 0.9500 | B1—F1 | 1.369 (3) |
C2—C1i | 1.370 (3) | B1—F3 | 1.382 (3) |
C2—H2 | 0.9500 | B1—F2 | 1.383 (3) |
C3—C4 | 1.512 (3) | B1—F4 | 1.402 (3) |
C3—H3A | 0.9900 | | |
| | | |
C2—N1—C1 | 120.36 (16) | H3A—C3—H3B | 108.1 |
C2—N1—C3 | 120.33 (16) | C3—C4—H4A | 109.5 |
C1—N1—C3 | 119.32 (15) | C3—C4—H4B | 109.5 |
N1—C1—C2i | 120.36 (17) | H4A—C4—H4B | 109.5 |
N1—C1—H1 | 119.8 | C3—C4—H4C | 109.5 |
C2i—C1—H1 | 119.8 | H4A—C4—H4C | 109.5 |
N1—C2—C1i | 119.28 (17) | H4B—C4—H4C | 109.5 |
N1—C2—H2 | 120.4 | F1—B1—F3 | 110.1 (2) |
C1i—C2—H2 | 120.4 | F1—B1—F2 | 108.80 (18) |
N1—C3—C4 | 110.33 (16) | F3—B1—F2 | 108.42 (17) |
N1—C3—H3A | 109.6 | F1—B1—F4 | 110.50 (17) |
C4—C3—H3A | 109.6 | F3—B1—F4 | 109.54 (16) |
N1—C3—H3B | 109.6 | F2—B1—F4 | 109.38 (19) |
C4—C3—H3B | 109.6 | | |
Symmetry code: (i) −x+1, −y+2, −z+1. |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
C2—H2···F2ii | 0.95 | 2.51 | 3.090 (2) | 119 |
C1—H1···F3i | 0.95 | 2.35 | 2.999 (2) | 126 |
C2—H2···F3 | 0.95 | 2.53 | 3.082 (2) | 117 |
C1—H1···F4i | 0.95 | 2.31 | 3.225 (2) | 161 |
Symmetry codes: (i) −x+1, −y+2, −z+1; (ii) −x+1/2, y−1/2, −z+1/2. |
Experimental details
| (I) | (II) |
Crystal data |
Chemical formula | C6H10N22+·2BF4− | C8H14N22+·2BF4− |
Mr | 283.78 | 311.83 |
Crystal system, space group | Monoclinic, P21/c | Monoclinic, P21/n |
Temperature (K) | 173 | 173 |
a, b, c (Å) | 5.6227 (14), 14.884 (4), 6.7419 (17) | 9.324 (3), 6.2281 (17), 11.987 (4) |
β (°) | 103.557 (4) | 102.346 (7) |
V (Å3) | 548.5 (2) | 680.0 (4) |
Z | 2 | 2 |
Radiation type | Mo Kα | Mo Kα |
µ (mm−1) | 0.20 | 0.17 |
Crystal size (mm) | 0.14 × 0.14 × 0.10 | 0.20 × 0.14 × 0.12 |
|
Data collection |
Diffractometer | Bruker SMART diffractometer | Bruker SMART diffractometer |
Absorption correction | Multi-scan (SADABS; Sheldrick, 1996) | Multi-scan (SADABS; Bruker, 2003) |
Tmin, Tmax | 0.776, 1.000 | 0.383, 1.000 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 5874, 1617, 1331 | 7346, 2000, 1272 |
Rint | 0.029 | 0.047 |
(sin θ/λ)max (Å−1) | 0.715 | 0.715 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.048, 0.131, 1.08 | 0.062, 0.167, 1.09 |
No. of reflections | 1617 | 2000 |
No. of parameters | 83 | 92 |
H-atom treatment | H-atom parameters constrained | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.49, −0.34 | 0.45, −0.22 |
Hydrogen-bond geometry (Å, º) for (I) top
D—H···A | D—H | H···A | D···A | D—H···A |
C1—H1···F2i | 0.95 | 2.43 | 2.973 (2) | 116 |
C2—H2···F2 | 0.95 | 2.26 | 2.892 (2) | 124 |
C1—H1···F3ii | 0.95 | 2.32 | 3.081 (2) | 136 |
C2—H2···F4iii | 0.95 | 2.58 | 3.053 (2) | 111 |
Symmetry codes: (i) −x, −y+1, −z; (ii) x+1, y, z−1; (iii) x+1, y, z. |
F atoms of BF4- π-interaction modes to Me2Pyz2+ topF atoms | Closest F—C distance (Å) | dcentroid (Å) | dplane(Å) |
F2 | 3.06 | 3.69 | 2.93 |
F3 | 3.00 | 2.71 | 2.69 |
Hydrogen-bond geometry (Å, º) for (II) top
D—H···A | D—H | H···A | D···A | D—H···A |
C2—H2···F2i | 0.95 | 2.51 | 3.090 (2) | 119 |
C1—H1···F3ii | 0.95 | 2.35 | 2.999 (2) | 126 |
C2—H2···F3 | 0.95 | 2.53 | 3.082 (2) | 117 |
C1—H1···F4ii | 0.95 | 2.31 | 3.225 (2) | 161 |
Symmetry codes: (i) −x+1/2, y−1/2, −z+1/2; (ii) −x+1, −y+2, −z+1. |
F atoms of BF4- π-interaction modes to Et2Pyz2+ topF atoms | Closest F—C distance (Å) | dcentroid (Å) | dplane (Å) |
F1 | 3.06 | 3.38 | 2.79 |
F2 | 3.02 | 2.82 | 2.80 |
F3 | 3.09 | 3.57 | 2.88 |
Investigations on the supramolecular chemistry of anion–π-acid interactions are relevant to anion binding in biological systems (Gamez et al., 2007), as well as to the design of new anion receptors (Beer et al., 2001; Bianchi et al., 1997). We demonstrated previously the charge-transfer (C–T) nature of anion–π interactions (Rosokha et al., 2004). Such interactions play an important role in the stabilization of ternary anion–π complexes that are responsible for the direction of crystal growth of anions and π-acids into infinite chain structures (wires) (Han et al., 2008; Lu et al., 2009). We also suggested that one-dimensional molecular wires are derived from the ternary synthons of the donor (D, anion) and acceptor (A, aromatic π-acid). We further demonstrated that triad complexes (DAD or ADA triad synthons) can be isolated by charge modulation in cationic π-acid salts (Lu et al., 2009).
In this communication, we turn to the anion–π interactions in N,N'-dimethyldipyrazinium bis(tetrafluoroborate), (I), and N,N'-diethyldipyrazinium bis(tetrafluoroborate), (II), which contain monoanionic tetrafluoroborate donors and dicationic R2Pyz2+ π-acceptors [where R = Me for dimethyldipyrazinium in (I) and R = Et for diethyldipyrazinium in (II)]. N,N'-Dialkylated (diquaternized) dipyraziniums are strong electron acceptors with interesting redox properties (Hilgers et al., 1994; Schmittel et al., 2005). They are also important precursors for generating stable radical species (Kaim et al., 1993). However, studies of the C–T behavior between dipyrazinium acceptors and various neutral donors in solution show no stable C–T band. The existence of stable anion–π triads {D-···A2+···D-} could be the main explanation for this observation. Unfortunately, so far, no crystal structures of dialkyldipyrazinium salts have been reported to confirm such an assumption.
Compounds (I) and (II) were prepared according to the literature procedure of Curphey et al. (1972) and were recrystallized in both cases as white crystals from dried accetonitrile. As shown in Fig. 1, the asymmetry unit of each salt contains half the dipyrazinium dication and one BF4- anion. The dipyrazinium dication is located about an inversion center in both salts and the pyrazine rings experience an average increase of the aromatic C—N bond lengths of σim 0.025Å, and the aromatic C—C bonds increase by σim 0.013Å relative to the neutral parent molecule.
The anion–π interaction patterns are illustrated in Fig. 2. In both compounds, every cationic π-acceptor is π-bonded with two BF4- anions, which sit centrosymmetrically above and below the aromatic ring of the cation to form a DAD triad. Careful examination of the modes of approach of the BF4- anions to the π-acceptors reveals some differences. In compound (I), a head-to-face mode can be identified since only one of the four F atoms of the anion (F3) bonds strongly with the π-acceptor ring. In (II), a face-to-face mode can be identified since three F atoms (F1, F2 and F3) of the the anion intimately bond with the π-acceptor ring on the ring surface. The relevant distances (Berryman et al., 2007) of closest F to aromatic C atoms, F to center-of-ring (dcentroid) and F to plane-of-ring (dplane) are summarized in Tables 1 and 3 for (I) and (II), respectively. The closest F to center-of-ring (dcentroid) distances in (I) and (II) are 2.71 and 2.82Å, respectively. From a literature survey (Mooibroek et al., 2008) of all available crystal data involving BF4-–π interactions, these two contact distances (dcentroid) represent unique examples of BF4-–π strong interactions.
Weak C—H···F hydrogen-bond interactions are also found among these triad units. The anions form several contacts with H atoms of the dipyrazinium group that are less than the sum of the van der Waals radii of hydrogen (1.2Å) and fluorine (1.5Å). The hydrogen-bonding information is summarized in Tables 2 and 4 for (I) and (II), respectively. It is worth noting here that there are bifurcated C—H···F hydrogen bonds in both compounds. For example, in compound (I), atom F2 bonds both to H1(—C1) and H2(—C2) of the same molecule.
In summary, we report here the first crystal structures of dialkyldipyrazinium salts. The preservation of DAD triads is found in both salts owing to weak C—H···F hydrogen bonding as well as the presence of strong electrostatic anion–cation interactions. Strong anion–π bonding in both triads effectively protect the dicationic π-acceptor from forming stable C–T complexes with additional electron donors in solution.