2-(3-Benzoyl-1-pyridinio)-3,4-dioxocyclo­butenolate

The title compound, C₁₆H₉NO₄, also known as the 3-benzoyl­pyridinium betaine of squaric acid, exhibits a dipolar electronic ground-state structure with a positively charged pyridinium fragment and a negatively charged squarate moiety. In the mol­ecule, the two aromatic rings are twisted by 56.03 (2)° relative to one another. The three-dimensional packing of the mol­ecules is stabilized by C-H...O short contacts.

S1. Comment

The substituted pyridinium betaines of squaric acid are of particular interest as potential organic nonlinear optical (NLO) materials (Chemla & Zyss, 1987; Nalwa et al., 1997; Wolff & Wortmann, 1999). In the course of our detailed study of squaric acid derivatives, the syntheses and structural characterizations of the 4-benzoyl (Kolev et al., 2001), 4-dimethylamino (Kolev, Yancheva et al., 2002) and 4-methoxy analogues (Kolev, Wortmann et al., 2004) have been published. In view of the very interesting chemical structure of this type of molecules, density functional theory calculations of the electronic structure and UV–vis spectroscopic studies have been carried out (Kolev et al., 2003; Kolev, Stamboliyska et al., 2004; Kolev, Yancheva & Stoyanov, 2004). The electro-optical absorption measurements (EOAM) demonstrate that the studied series of compounds possess hyperpolarizabilities exceeding that of the reference substance, p-nitroaniline, both in absolute and relative values. These results are due to be published shortly. In this paper, we report the structural characteristics of the 3-benzoylpyridinium-betaine of squaric acid, (I).

The molecular structure of (I) is dipolar, with the positive and the negative charges situated on the pyridinium and squarate groups, respectively (Fig. 1). One of the C—O bonds within the squarate system has a value of 1.204 (4) Å, typical for carbonyl group. The remaining two C—O bonds are longer and have similar values [1.219 (3) and 1.220 (4) Å], indicating that the negative charge is equally distributed between atoms O2 and O3. This charge repartition also affects the cyclobutene C—C distances, with two of them being shorter than the other two, with respective values of 1.426 (4)–1.430 (4) and 1.527 (5)–1.533 (4) Å. A similar deformation of the squarate moiety is known for the 4-benzoyl derivative (Kolev et al., 2001). The squarate and pyridinium rings are nearly coplanar, with a dihedral angle of 4.1 (2)°. A similar value of this angle was observed in previously described structures of this type. The dihedral angle between the aromatic rings is 56.0 (1)° [56.03 (2)° in Abstract?]. This is comparable with the corresponding value in 4-dimethylamino-4'-nitrobenzophenone (Kolev, Schurmann et al., 2002) and differs significantly from that in the 4-benzoyl derivative (Kolev et al., 2001), where the aromatic rings are nearly perpendicular.

The molecule of (I) is Λ shaped, i.e. the rings are mutually twisted (Nalwa et al., 1997). In the crystal, the molecules face each other in an alternate end-to-end fashion, so that opposite shoulders of the molecules are adjacent to each other. There are a number of intermolecular interactions stabilizing the three-dimensional packing of the molecules. We consider two of them as non-classical hydrogen bonds, namely the contacts between the negatively charged squarate O atoms and the pyridinium H atoms (Table 1). These contacts are likely to occur due to the greater mobility of H atoms within the pyridinium ring, caused by the electron-withdrawing N atom. A similar manner of hydrogen bonding is known in the previously reported 4-benzoyl analogue, where the molecules are linked to form ribbons. In contrast, the molecules of the title compound are two-dimensionally connected (Fig. 2).

It is interesting to note that the coplanarity between squarate and pyridinium rings determine two additional short contacts C11···O3 and C12···O2 which could be explained as intramolecular hydrogen bonds. Similar short contacts are found in the 4-benzoyl analog.

S2. Experimental

The synthesis of (I) is described in our previous article (Kolev, Yancheva & Stoyanov, 2004). The crystals were grown by slow evaporation from an acetonitrile solution over a period of a week. Spectroscopic analysis: IR (KBr pellet, ν, cm⁻¹). The ν_C—H vibrations of the pyridyl ring and benzoyl group appear at 3138 (w), 3130 (w), 3122 (w) and 3107 (w), and at 3092 (w) and 3033 (w), respectively. The very strong band at 1783 (s) is assigned to ν_C═O of the isolated carbonyl group of the squaric acid ring, while those at 1748 (s) and 1625 (s) correspond to the symmetric and asymmetric modes of the semicarbonyl groups. The pyridinium ring vibration 8a lies in the massive Not clear of the broad band at 1625 (s).

S3. Refinement

H atoms were placed in idealized positions (C—H = 0.93 Å) and constrained to ride on their parent atoms, with U_iso(H) = 1.2U_eq(C).

Computing details

Data collection: CAD-4 EXPRESS (Enraf-Nonius, 1994); cell refinement: CAD-4 EXPRESS; data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures

	Fig. 1. Molecular structure of (I), with atom-numbering scheme and 50% probability displacement ellipsoids.
	Fig. 2. The molecular packing with C—H···O short contacts indicated by dotted lines. [symmetry codes i. 1/2 + x, 1/2 − y, 1/2 + z; ii. −x 1 − y 1 − z]

2-(3-Benzoyl-1-pyridinio)-3,4-dioxocyclobutenolate

Crystal data

C16H9NO4	F(000) = 576
Mr = 279.24	Dx = 1.459 Mg m^−3
Monoclinic, P21/n	Melting point: 418 K
Hall symbol: -P 2yn	Mo Kα radiation, λ = 0.71073 Å
a = 7.9319 (11) Å	Cell parameters from 22 reflections
b = 13.6577 (14) Å	θ = 18.4–19.6°
c = 12.0890 (12) Å	µ = 0.11 mm^−1
β = 103.903 (12)°	T = 290 K
V = 1271.3 (3) Å^3	Cubic, red
Z = 4	0.24 × 0.24 × 0.24 mm

Data collection

Enraf-Nonius CAD-4 diffractometer	θmax = 28.0°, θmin = 2.3°
Radiation source: fine-focus sealed tube	h = 0→10
non–profiled ω/2θ scans	k = −18→18
6407 measured reflections	l = −15→15
3063 independent reflections	3 standard reflections every 120 min
1558 reflections with I > 2σ(I)	intensity decay: none
Rint = 0.093

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.067	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.236	H-atom parameters constrained
S = 1.04	w = 1/[σ^2(Fo^2) + (0.1235P)^2] where P = (Fo^2 + 2Fc^2)/3
3063 reflections	(Δ/σ)max < 0.001
190 parameters	Δρmax = 0.29 e Å^−3
0 restraints	Δρmin = −0.34 e Å^−3

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.

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

	x	y	z	Uiso*/Ueq
C1	−0.1537 (4)	0.2684 (2)	0.7565 (3)	0.0501 (8)
H1	−0.2144	0.2439	0.6865	0.060*
C2	−0.0726 (4)	0.3590 (2)	0.7617 (2)	0.0450 (7)
C3	0.0186 (4)	0.3943 (2)	0.8674 (3)	0.0557 (9)
H3	0.0744	0.4545	0.8722	0.067*
C4	0.0264 (5)	0.3403 (3)	0.9648 (3)	0.0645 (10)
H4	0.0866	0.3645	1.0351	0.077*
C5	−0.0545 (5)	0.2507 (3)	0.9586 (3)	0.0644 (10)
H5	−0.0484	0.2144	1.0244	0.077*
C6	−0.1450 (5)	0.2147 (3)	0.8541 (3)	0.0597 (9)
H6	−0.1999	0.1541	0.8498	0.072*
C7	−0.0922 (4)	0.4227 (2)	0.6605 (2)	0.0498 (8)
C8	−0.1268 (4)	0.3790 (2)	0.5435 (2)	0.0468 (7)
C9	−0.2293 (4)	0.4294 (2)	0.4507 (3)	0.0575 (9)
H9	−0.2784	0.4892	0.4625	0.069*
C10	−0.2581 (5)	0.3926 (3)	0.3436 (3)	0.0642 (10)
H10	−0.3294	0.4260	0.2830	0.077*
C11	−0.1815 (4)	0.3056 (3)	0.3247 (3)	0.0571 (9)
H11	−0.1985	0.2805	0.2513	0.069*
C12	−0.0538 (4)	0.2919 (2)	0.5224 (2)	0.0457 (7)
H12	0.0149	0.2567	0.5824	0.055*
C13	−0.0073 (4)	0.1671 (2)	0.3952 (2)	0.0482 (8)
C14	−0.0232 (5)	0.1106 (2)	0.2940 (3)	0.0552 (9)
C15	0.0966 (4)	0.0350 (3)	0.3663 (3)	0.0523 (8)
C16	0.1077 (4)	0.1018 (2)	0.4696 (2)	0.0470 (7)
N1	−0.0810 (3)	0.25700 (17)	0.41463 (19)	0.0455 (6)
O1	−0.0839 (4)	0.51134 (16)	0.6689 (2)	0.0685 (7)
O2	0.1785 (3)	0.09874 (17)	0.57090 (18)	0.0602 (7)
O3	−0.1006 (4)	0.11820 (18)	0.19422 (19)	0.0845 (10)
O4	0.1552 (4)	−0.04321 (19)	0.3487 (2)	0.0762 (8)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0590 (19)	0.0438 (17)	0.0464 (16)	−0.0001 (15)	0.0107 (14)	−0.0049 (13)
C2	0.0474 (17)	0.0409 (16)	0.0453 (16)	0.0086 (14)	0.0086 (13)	−0.0047 (12)
C3	0.0571 (19)	0.0496 (19)	0.0543 (19)	0.0082 (16)	0.0013 (15)	−0.0119 (15)
C4	0.076 (2)	0.072 (3)	0.0417 (18)	0.018 (2)	0.0056 (16)	−0.0080 (16)
C5	0.078 (3)	0.067 (3)	0.0506 (19)	0.025 (2)	0.0200 (17)	0.0080 (16)
C6	0.077 (2)	0.050 (2)	0.058 (2)	0.0038 (17)	0.0270 (17)	0.0033 (15)
C7	0.0578 (19)	0.0409 (18)	0.0494 (18)	0.0007 (15)	0.0103 (15)	−0.0030 (13)
C8	0.0516 (18)	0.0416 (17)	0.0454 (17)	−0.0036 (14)	0.0082 (13)	0.0063 (12)
C9	0.064 (2)	0.0438 (18)	0.062 (2)	0.0027 (16)	0.0085 (17)	0.0119 (15)
C10	0.074 (2)	0.060 (2)	0.049 (2)	−0.0025 (19)	−0.0045 (17)	0.0178 (16)
C11	0.066 (2)	0.057 (2)	0.0398 (16)	−0.0110 (17)	−0.0037 (14)	0.0104 (14)
C12	0.0525 (18)	0.0461 (17)	0.0356 (15)	−0.0026 (14)	0.0046 (12)	0.0060 (12)
C13	0.066 (2)	0.0427 (17)	0.0334 (14)	−0.0112 (15)	0.0070 (13)	−0.0012 (11)
C14	0.080 (2)	0.0471 (18)	0.0397 (16)	−0.0191 (17)	0.0161 (15)	−0.0039 (13)
C15	0.058 (2)	0.0520 (19)	0.0512 (17)	−0.0142 (16)	0.0222 (15)	−0.0093 (14)
C16	0.0584 (19)	0.0448 (17)	0.0375 (15)	−0.0105 (15)	0.0110 (13)	−0.0021 (12)
N1	0.0550 (15)	0.0427 (14)	0.0350 (12)	−0.0094 (12)	0.0032 (10)	0.0042 (10)
O1	0.0944 (19)	0.0363 (13)	0.0738 (16)	0.0009 (12)	0.0178 (14)	−0.0027 (10)
O2	0.0718 (15)	0.0600 (15)	0.0436 (13)	0.0066 (12)	0.0038 (11)	0.0000 (10)
O3	0.147 (3)	0.0612 (16)	0.0353 (13)	−0.0209 (16)	0.0015 (14)	−0.0074 (10)
O4	0.0841 (19)	0.0626 (17)	0.0831 (19)	0.0035 (14)	0.0225 (14)	−0.0252 (14)

Geometric parameters (Å, º)

C1—C6	1.377 (4)	C9—C10	1.356 (5)
C1—C2	1.389 (4)	C9—H9	0.9300
C1—H1	0.9300	C10—C11	1.378 (5)
C2—C3	1.394 (4)	C10—H10	0.9300
C2—C7	1.478 (4)	C11—N1	1.357 (4)
C3—C4	1.379 (5)	C11—H11	0.9300
C3—H3	0.9300	C12—N1	1.355 (4)
C4—C5	1.374 (5)	C12—H12	0.9300
C4—H4	0.9300	C13—N1	1.404 (4)
C5—C6	1.385 (5)	C13—C14	1.426 (4)
C5—H5	0.9300	C13—C16	1.430 (4)
C6—H6	0.9300	C14—O3	1.220 (4)
C7—O1	1.216 (4)	C14—C15	1.527 (5)
C7—C8	1.498 (4)	C15—O4	1.204 (4)
C8—C12	1.374 (4)	C15—C16	1.533 (4)
C8—C9	1.398 (4)	C16—O2	1.219 (3)
C6—C1—C2	120.6 (3)	C9—C10—C11	119.8 (3)
C6—C1—H1	119.7	C9—C10—H10	120.1
C2—C1—H1	119.7	C11—C10—H10	120.1
C3—C2—C1	118.8 (3)	N1—C11—C10	119.1 (3)
C3—C2—C7	118.9 (3)	N1—C11—H11	120.4
C1—C2—C7	122.0 (3)	C10—C11—H11	120.4
C4—C3—C2	120.3 (3)	N1—C12—C8	120.4 (3)
C4—C3—H3	119.8	N1—C12—H12	119.8
C2—C3—H3	119.8	C8—C12—H12	119.8
C5—C4—C3	120.3 (3)	N1—C13—C14	131.5 (3)
C5—C4—H4	119.9	N1—C13—C16	132.0 (2)
C3—C4—H4	119.9	C14—C13—C16	96.5 (3)
C6—C5—C4	120.0 (3)	N1—C13—C15	179.1 (3)
C6—C5—H5	120.0	O3—C14—C13	136.2 (3)
C4—C5—H5	120.0	O3—C14—C15	136.0 (3)
C5—C6—C1	120.0 (3)	C13—C14—C15	87.8 (2)
C5—C6—H6	120.0	O4—C15—C14	135.4 (3)
C1—C6—H6	120.0	O4—C15—C16	136.3 (3)
O1—C7—C2	121.6 (3)	C14—C15—C16	88.2 (2)
O1—C7—C8	118.0 (3)	O4—C15—C13	179.1 (3)
C2—C7—C8	120.4 (3)	O2—C16—C13	136.4 (3)
C12—C8—C9	117.8 (3)	O2—C16—C15	136.1 (3)
C12—C8—C7	122.0 (3)	C13—C16—C15	87.4 (2)
C9—C8—C7	120.1 (3)	C12—N1—C11	121.7 (3)
C10—C9—C8	121.1 (3)	C12—N1—C13	119.5 (2)
C10—C9—H9	119.5	C11—N1—C13	118.8 (3)
C8—C9—H9	119.5

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C9—H9···O3^i	0.93	2.59	3.225 (4)	125
C10—H10···O2^ii	0.93	2.60	3.215 (3)	124
C11—H11···O3	0.93	2.50	3.152 (5)	127
C12—H12···O2	0.93	2.54	3.191 (4)	127

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