2,6-Dimethyl-1,4-benzoquinone 4-monooxime

Mol­ecules of the title compound, C₈H₉NO₂, are linked into sheets by a combination of C—H...N, O—H...N and O—H...O hydrogen bonds and C—H...π inter­actions. The hydrogen bonds are arranged as described by the graph-set ring notations R₂²(7) and R₃³(5), and a C8 chain motif. There are two planar symmetry-independent mol­ecules in the asymmetric unit, with a dihedral angle of 19.24 (5)° between their least-squares mean planes.

S1. Comment

H-atom transfer in a hydrogen bond is an elementary process present in many systems of biological interest. Recently, many experimental and computational studies have addressed H-atom dynamics in hydrogen bonds (Yan et al., 2001; Mavri & Grdadolnik, 2001a,b; Rospenk et al., 2001; Došlći et al., 2001). In addition, the quinone monooximate (nitroso) complexes of transition metals are of interest in respect of their structure, reactivity and possible application as starting reagents in the synthesis of a wide variety of organic compounds (Kasumov et al., 2000). Quinone monooxime compounds are an example of systems possessing both intra- and intermolecular hydrogen bonds (Kržan & Mavri, 2002). They are key reagents in the production of azo dyes. Since they are good complexing agents, they have found a place in many analytical, synthetic and other applications (Carugo et al., 1991; Shipmen et al., 1955; Castellani & Millini, 1984; Verdoorn et al., 1994). In solution, quinone monooximes generally exist as a mixture of quinone monooxime-nitrosophenol and nitrosonaphthol tautomers (see scheme).

The nitroso–oxime tautomeric equilibrium has been extensively studied by spectroscopic methods, including UV, IR and NMR techniques (Fischer et al., 1965; Enchev et al., 1999; Ivanova & Enchev, 2001). In the classic study on nitrosophenols, it was shown that, in solution, 2-nitrosophenol exists exclusively in the phenolic form, while its 5-methoxy and 5-dimethylamino derivatives are found in solvent-dependent equilibrium with their corresponding quinonoid forms (Burawoy et al., 1955). The latter is a particularly powerful method [It is not clear what the method is] for studying the structure and dynamics of such hydrogen-bonded systems (Abilgaard et al., 1998). In general, it was established that quinonoid forms are favoured in polar solvents, while phenolic forms are favoured in nonpolar solvents (Kržan et al., 2000). Generally, o-nitrosophenols exist as the quinone-monooxime form, but p-nitrosophenols exist as the nitrosophenol form (Abilgaard et al., 1998; Kržan et al., 2000; Kržan & Mavri, 2002). As we could not find in the literature any example of a solid-state compound in the quinone monooxime form, an X-ray structure determination of the title compound, (I), was carried out and the results are presented here.

2,6-Dimethyl-1,4-benzoquinone monooxime, (I), was synthesized according to the reaction mechanism in the scheme. In the structure of (I), there are two symmetry-independent molecules in the asymmetric unit (Fig. 1). Selected bond distances and angles (I) are listed in Table 1. Compound (I) possesses normal geometrical parameters and, as expected, is essentially planar [for the non-H atoms, the largest r.m.s. deviations from the best least-squares plane are −0.008 (2) Å for the C1–C6 ring and 0.024 (2) Å for the C9–C14 ring].

The present determination reveals that (I) exists as the quinone–oxime tautomer in the solid state. This is evident from the relative contraction of the C1—O1, C4—N1, C2—C3 and C5—C6 bonds, and the relative elongation of the C1—C2, C3—C4, C4—C5, C1—C6 and N1—O2 bonds. A similar pattern of bond contractions and elongations was also observed for the C9–C14 ring (Table 1). It should be noted here that the tautomers of o-benzoquinone monoxime have been investigated by Carugo et al. (1991). The N—O bond distances in the o-benzoquinone monoxime and its nitrosophenol tautomer are reported to be 1.378 and 1.200 Å, respectively. The corresponding values in (I) are 1.362 (3) Å for N1—O2 and 1.377 (2) Å for N2—O4, showing that both molecules are in the quinone monooxime form. The orientations of the methyl groups are thought to be determined by the steric effects of the adjacent carbonyl groups. From the Coeditor: This least-squares mean plane calculation would show the following more clearly: please check and supply s.u.s. All the non-H atoms in each molecule lie in the same plane, as shown by the r.m.s. deviations of 0.012(?) and 0.029(?) Å from the corresponding least-squares mean planes.

Further examination of non-bonded contacts also reveals four intermolecular hydrogen bonds (Table 2). Hence, as shown Fig. 1, asymmetric units of (I) are linked through C11—H11···N1 and O2—H2···N2 hydrogen bonds, and the dihedral angle between the C1–C6 and C9–C14 rings is 19.24 (5)°. The arrangement of the C11—H11···N1 and O2—H2···N2 hydrogen bonds can be described by the graph-set notation R₂²(7). Other hydrogen bonds are arranged as described by R₃³(5) and a C8 chain motif in the bc plane (Fig. 2) (Bernstein et al., 1995). From the Coeditor: The O2—H2···O1 distance in Table 2 looks very long. Please look at this.

Stacking of the quinoid rings in (I) shows Cg1···Cg1ⁱ contacts [Cg1 is the centroid of the C1–C6 ring; symmetry code: (i) 1/2 + x, y, 1/2 − z] of 3.6375 (14) Å and Cg2···Cg2ⁱⁱ contacts [Cg2 is the centroid of the C9–C14 ring; symmetry code: (ii) −x,1 − y,-z] of 4.2291 (13) Å. The C9–C14 ring is also involved in an intermolecular C—H···π interaction with atoms C13 and C16 of the C1–C6 ring. This interaction is characterized by the following geometrical parameters: the distances between Cg2 and atoms H13(−x, 1 − y, −z) and H16B(1 − x, 1 − y, −z) are 3.3453 and 3.1851 Å, respectively, the distances of atoms C13(−x, 1 − y, −z) and C16(1 − x, 1 − y, −z) from the plane of the C9–C14 ring are 3.544 (2) and 3.751 (3) Å, respectively, and the C13—H13···Cg2 and C16—H16B···Cg2 angles are 94.6 and 119.4°, respectively. From the Coeditor: I would have expected these angles to be closer to linearity. Are there examples of other C—H···π interactions with such angles?

S2. Experimental

A mixture of 2,6-dimethylaniline (1.51 g, 10 mmol), water (50 ml) and concentrated hydrochloric acid (2.5 ml, 30 mmol) was heated with stirring until a clear solution was obtained. This solution was cooled to 273–278 K and a solution of sodium nitrite (0.96 g, 14 mmol) in water was added dropwise. The resulting mixture was stirred for 30 min at 278–288 K. The precipitated product was crystallized from acetonitrile to obtain crystals of 2,6-dimethyl-1,4-benzoquinone monooxime, (I) (m.p. 444–446 K).

S3. Refinement

Methyl H atoms were located from difference Fourier syntheses and refined as part of a rigid rotating group with C—H = 0.96 Å and U_iso(H) = 1.5U_eq(C). Other H atoms were placed geometrically and refined using a riding model, with Csp²—H = 0.93 Å and U_iso(H) = 1.2U_eq(C). From the Coeditor: please check amended text.

Computing details

Data collection: X-AREA (Stoe & Cie, 2002); cell refinement: X-AREA; data reduction: X-RED32 (Stoe & Cie, 2002); 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. A view of the two independent molecules of (I), with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. Dashed lines indicate the intramolecular hydrogen bonds. Please check added text.
	Fig. 2. A view of packing diagram of (I), along the Which? axis. Please provide extra information.

2,6-Dimethyl-1,4-benzoquinone monooxime

Crystal data

C8H9NO2	Dx = 1.280 Mg m^−3
Mr = 151.16	Melting point = 444–446 K
Orthorhombic, Pbca	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2ab	Cell parameters from 7762 reflections
a = 7.2505 (7) Å	θ = 1.5–25.9°
b = 16.3379 (10) Å	µ = 0.09 mm^−1
c = 26.4894 (17) Å	T = 296 K
V = 3137.9 (4) Å^3	Tetragonal prism, red
Z = 16	0.30 × 0.17 × 0.09 mm
F(000) = 1280

Data collection

Stoe IPDS-2 diffractometer	1718 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.062
Plane graphite monochromator	θmax = 26.1°, θmin = 1.5°
Detector resolution: 6.67 pixels mm^-1	h = −8→8
ω scans	k = −17→20
20873 measured reflections	l = −32→32
3089 independent reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.048	H-atom parameters constrained
wR(F^2) = 0.143	w = 1/[σ^2(Fo^2) + (0.0913P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 0.87	(Δ/σ)max < 0.001
3089 reflections	Δρmax = 0.31 e Å^−3
198 parameters	Δρmin = −0.27 e Å^−3
0 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0052 (11)

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

	x	y	z	Uiso*/Ueq
C1	0.5133 (3)	0.15696 (13)	0.21911 (7)	0.0518 (5)
C2	0.5170 (3)	0.13642 (13)	0.27310 (8)	0.0538 (5)
C3	0.5254 (3)	0.19684 (14)	0.30675 (8)	0.0592 (6)
H3	0.5250	0.1839	0.3409	0.071*
C4	0.5349 (4)	0.28102 (15)	0.29217 (9)	0.0632 (4)
C5	0.5346 (3)	0.30100 (13)	0.23922 (8)	0.0599 (6)
H5	0.5413	0.3556	0.2295	0.072*
C6	0.5247 (3)	0.24286 (13)	0.20346 (7)	0.0534 (5)
C7	0.5090 (4)	0.04775 (15)	0.28765 (9)	0.0757 (8)
H7A	0.5204	0.0428	0.3236	0.114*
H7B	0.6082	0.0188	0.2716	0.114*
H7C	0.3933	0.0249	0.2770	0.114*
C8	0.5235 (4)	0.26134 (16)	0.14825 (8)	0.0748 (8)
H8A	0.5297	0.3195	0.1433	0.112*
H8B	0.4120	0.2406	0.1335	0.112*
H8C	0.6279	0.2358	0.1325	0.112*
C9	0.8037 (3)	0.38771 (15)	0.53736 (9)	0.0618 (6)
C10	0.7467 (3)	0.33882 (14)	0.49346 (9)	0.0594 (6)
C11	0.6900 (3)	0.37668 (13)	0.45153 (8)	0.0545 (6)
H11	0.6567	0.3457	0.4235	0.065*
C12	0.6795 (3)	0.46510 (12)	0.44883 (7)	0.0470 (5)
C13	0.7278 (3)	0.51316 (13)	0.49256 (7)	0.0515 (5)
H13	0.7188	0.5699	0.4910	0.062*
C14	0.7854 (3)	0.47789 (14)	0.53535 (8)	0.0543 (6)
C15	0.7550 (4)	0.24690 (17)	0.49819 (12)	0.0867 (9)
H15A	0.7188	0.2223	0.4668	0.130*
H15B	0.6728	0.2293	0.5245	0.130*
H15C	0.8786	0.2305	0.5063	0.130*
C16	0.8347 (4)	0.52555 (18)	0.58136 (9)	0.0774 (8)
H16A	0.8091	0.5825	0.5758	0.116*
H16B	0.9635	0.5186	0.5885	0.116*
H16C	0.7632	0.5063	0.6095	0.116*
N1	0.5473 (3)	0.33307 (12)	0.33012 (7)	0.0632 (4)
N2	0.6243 (3)	0.49507 (10)	0.40609 (6)	0.0514 (5)
O1	0.4999 (3)	0.10161 (10)	0.18743 (5)	0.0668 (5)
O2	0.5585 (3)	0.41143 (11)	0.31286 (6)	0.0915 (7)
H2	0.5659	0.4428	0.3370	0.110*
O3	0.8661 (3)	0.35446 (13)	0.57542 (7)	0.0924 (7)
O4	0.6124 (3)	0.57921 (9)	0.40651 (5)	0.0673 (5)
H4	0.5773	0.5954	0.3788	0.081*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0641 (14)	0.0485 (13)	0.0428 (11)	−0.0016 (10)	−0.0010 (9)	−0.0049 (9)
C2	0.0683 (14)	0.0504 (12)	0.0427 (10)	−0.0044 (10)	−0.0011 (10)	−0.0016 (10)
C3	0.0748 (15)	0.0604 (14)	0.0425 (11)	−0.0055 (12)	0.0003 (11)	−0.0018 (10)
C4	0.0823 (11)	0.0553 (9)	0.0519 (8)	−0.0053 (8)	0.0000 (8)	−0.0066 (6)
C5	0.0764 (16)	0.0435 (12)	0.0597 (14)	−0.0038 (11)	−0.0001 (11)	−0.0024 (10)
C6	0.0675 (14)	0.0500 (12)	0.0426 (11)	0.0016 (10)	−0.0008 (10)	−0.0012 (10)
C7	0.116 (2)	0.0585 (15)	0.0530 (13)	−0.0067 (15)	−0.0012 (14)	0.0069 (11)
C8	0.113 (2)	0.0618 (15)	0.0497 (13)	−0.0011 (14)	0.0029 (13)	0.0067 (11)
C9	0.0535 (13)	0.0737 (16)	0.0582 (13)	0.0035 (11)	−0.0016 (11)	0.0187 (12)
C10	0.0545 (14)	0.0516 (13)	0.0720 (15)	0.0056 (11)	0.0021 (11)	0.0118 (11)
C11	0.0611 (14)	0.0452 (12)	0.0572 (13)	0.0026 (10)	0.0027 (10)	−0.0029 (10)
C12	0.0538 (12)	0.0453 (11)	0.0420 (10)	0.0031 (9)	0.0012 (9)	0.0008 (9)
C13	0.0583 (14)	0.0486 (12)	0.0477 (12)	−0.0009 (10)	−0.0028 (9)	−0.0006 (9)
C14	0.0523 (13)	0.0631 (14)	0.0474 (11)	−0.0025 (10)	−0.0031 (9)	0.0040 (10)
C15	0.083 (2)	0.0524 (14)	0.124 (2)	0.0067 (14)	−0.0067 (17)	0.0203 (15)
C16	0.0827 (19)	0.097 (2)	0.0530 (13)	−0.0108 (15)	−0.0154 (13)	−0.0033 (13)
N1	0.0823 (11)	0.0553 (9)	0.0519 (8)	−0.0053 (8)	0.0000 (8)	−0.0066 (6)
N2	0.0674 (12)	0.0418 (10)	0.0449 (9)	0.0019 (8)	−0.0013 (8)	−0.0003 (8)
O1	0.1031 (13)	0.0516 (9)	0.0458 (8)	−0.0024 (8)	−0.0074 (8)	−0.0086 (7)
O2	0.156 (2)	0.0573 (11)	0.0616 (11)	−0.0071 (11)	0.0006 (11)	−0.0074 (8)
O3	0.1028 (16)	0.1004 (15)	0.0741 (12)	0.0114 (12)	−0.0219 (11)	0.0325 (11)
O4	0.1092 (14)	0.0428 (9)	0.0500 (8)	0.0030 (8)	−0.0145 (8)	0.0050 (7)

Geometric parameters (Å, º)

C1—O1	1.238 (2)	C9—C14	1.480 (3)
C1—C6	1.466 (3)	C10—C11	1.336 (3)
C1—C2	1.469 (3)	C10—C15	1.508 (3)
C2—C3	1.331 (3)	C11—C12	1.448 (3)
C2—C7	1.500 (3)	C11—H11	0.9300
C3—C4	1.430 (3)	C12—N2	1.297 (2)
C3—H3	0.9300	C12—C13	1.443 (3)
C4—N1	1.320 (3)	C13—C14	1.338 (3)
C4—C5	1.440 (3)	C13—H13	0.9300
C5—C6	1.343 (3)	C14—C16	1.490 (3)
C5—H5	0.9300	C15—H15A	0.9600
C6—C8	1.493 (3)	C15—H15B	0.9600
C7—H7A	0.9600	C15—H15C	0.9600
C7—H7B	0.9600	C16—H16A	0.9600
C7—H7C	0.9600	C16—H16B	0.9600
C8—H8A	0.9600	C16—H16C	0.9600
C8—H8B	0.9600	N1—O2	1.362 (3)
C8—H8C	0.9600	N2—O4	1.377 (2)
C9—O3	1.231 (3)	O2—H2	0.82
C9—C10	1.470 (3)	O4—H4	0.82
O1—C1—C6	120.82 (18)	C10—C9—C14	119.14 (19)
O1—C1—C2	119.64 (19)	C11—C10—C9	119.5 (2)
C6—C1—C2	119.55 (18)	C11—C10—C15	122.8 (2)
C3—C2—C1	118.9 (2)	C9—C10—C15	117.7 (2)
C3—C2—C7	123.1 (2)	C10—C11—C12	121.3 (2)
C1—C2—C7	118.03 (19)	C10—C11—H11	119.4
C2—C3—C4	122.3 (2)	C12—C11—H11	119.4
C2—C3—H3	118.8	N2—C12—C13	124.78 (19)
C4—C3—H3	118.8	N2—C12—C11	115.86 (18)
N1—C4—C3	114.7 (2)	C13—C12—C11	119.36 (18)
N1—C4—C5	126.6 (2)	C14—C13—C12	121.4 (2)
C3—C4—C5	118.7 (2)	C14—C13—H13	119.3
C6—C5—C4	121.8 (2)	C12—C13—H13	119.3
C6—C5—H5	119.1	C13—C14—C9	119.2 (2)
C4—C5—H5	119.1	C13—C14—C16	122.9 (2)
C5—C6—C1	118.72 (18)	C9—C14—C16	118.0 (2)
C5—C6—C8	123.2 (2)	C10—C15—H15A	109.5
C1—C6—C8	118.05 (18)	C10—C15—H15B	109.5
C2—C7—H7A	109.5	H15A—C15—H15B	109.5
C2—C7—H7B	109.5	C10—C15—H15C	109.5
H7A—C7—H7B	109.5	H15A—C15—H15C	109.5
C2—C7—H7C	109.5	H15B—C15—H15C	109.5
H7A—C7—H7C	109.5	C14—C16—H16A	109.5
H7B—C7—H7C	109.5	C14—C16—H16B	109.5
C6—C8—H8A	109.5	H16A—C16—H16B	109.5
C6—C8—H8B	109.5	C14—C16—H16C	109.5
H8A—C8—H8B	109.5	H16A—C16—H16C	109.5
C6—C8—H8C	109.5	H16B—C16—H16C	109.5
H8A—C8—H8C	109.5	C4—N1—O2	110.74 (18)
H8B—C8—H8C	109.5	C12—N2—O4	112.90 (16)
O3—C9—C10	120.7 (2)	N1—O2—H2	109.3
O3—C9—C14	120.1 (2)	N2—O4—H4	109.5
O1—C1—C2—C3	−177.8 (2)	O3—C9—C10—C15	4.2 (4)
C6—C1—C2—C3	1.9 (3)	C14—C9—C10—C15	−175.9 (2)
O1—C1—C2—C7	1.6 (4)	C9—C10—C11—C12	−1.4 (3)
C6—C1—C2—C7	−178.7 (2)	C15—C10—C11—C12	178.4 (2)
C1—C2—C3—C4	−1.4 (4)	C10—C11—C12—N2	179.4 (2)
C7—C2—C3—C4	179.3 (2)	C10—C11—C12—C13	−1.2 (3)
C2—C3—C4—N1	−178.3 (2)	N2—C12—C13—C14	−179.4 (2)
C2—C3—C4—C5	0.3 (4)	C11—C12—C13—C14	1.2 (3)
N1—C4—C5—C6	178.7 (2)	C12—C13—C14—C9	1.4 (3)
C3—C4—C5—C6	0.3 (4)	C12—C13—C14—C16	−179.4 (2)
C4—C5—C6—C1	0.3 (4)	O3—C9—C14—C13	176.0 (2)
C4—C5—C6—C8	179.8 (2)	C10—C9—C14—C13	−4.0 (3)
O1—C1—C6—C5	178.3 (2)	O3—C9—C14—C16	−3.3 (4)
C2—C1—C6—C5	−1.3 (3)	C10—C9—C14—C16	176.8 (2)
O1—C1—C6—C8	−1.2 (3)	C3—C4—N1—O2	179.1 (2)
C2—C1—C6—C8	179.1 (2)	C5—C4—N1—O2	0.6 (4)
O3—C9—C10—C11	−175.9 (2)	C13—C12—N2—O4	−1.1 (3)
C14—C9—C10—C11	4.0 (3)	C11—C12—N2—O4	178.28 (19)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O2—H2···N2	0.82	2.06	2.862 (2)	164
O4—H4···O1^i	0.82	1.84	2.643 (2)	164
C3—H3···O3^ii	0.93	2.58,	3.432 (3)	153
C11—H11···N1	0.93	2.61	3.453 (3)	152

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