Bis(2,6-dimethyl­pyridinium) dichromate

In the title compound, (C₇H₁₀N)₂[Cr₂O₇], the pyridinium cations and dichromate anion are linked by N—H...O hydrogen bonds. π–π stacking and C—H...O inter­actions play subordinate roles in forming the supramolecular structure.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](C-C) = 0.007 Å
• R factor = 0.042
• wR factor = 0.100
• Data-to-parameter ratio = 16.2

checkCIF/PLATON results

No syntax errors found



Alert level B
PLAT111_ALERT_2_B ADDSYM Detects (Pseudo) Centre of Symmetry .....         88 PerFit

Alert level C
PLAT042_ALERT_1_C Calc. and Rep. MoietyFormula Strings Differ ....          ?
PLAT066_ALERT_1_C Predicted and Reported Transmissions Identical .          ?
PLAT199_ALERT_1_C Check the Reported _cell_measurement_temperature        293 K
PLAT200_ALERT_1_C Check the Reported _diffrn_ambient_temperature .        293 K
PLAT341_ALERT_3_C Low Bond Precision on C-C bonds (x 1000) Ang ...          7
PLAT380_ALERT_4_C Check Incorrectly? Oriented X(sp2)-Methyl Moiety         C1
PLAT380_ALERT_4_C Check Incorrectly? Oriented X(sp2)-Methyl Moiety         C7
PLAT790_ALERT_4_C Centre of Gravity not Within Unit Cell: Resd.  #          1
              C7 H10 N
PLAT850_ALERT_2_C Check Flack Parameter Exact Value 0.00 and su ..       0.02

Alert level G
REFLT03_ALERT_4_G Please check that the estimate of the number of Friedel pairs is
            correct. If it is not, please give the correct count in the
            _publ_section_exptl_refinement section of the submitted CIF.
           From the CIF: _diffrn_reflns_theta_max           27.49
           From the CIF: _reflns_number_total               3660
           Count of symmetry unique reflns         2182
           Completeness (_total/calc)            167.74%
           TEST3: Check Friedels for noncentro structure
           Estimate of Friedel pairs measured      1478
           Fraction of Friedel pairs measured     0.677
           Are heavy atom types Z>Si present        yes

   0 ALERT level A = In general: serious problem
   1 ALERT level B = Potentially serious problem
   9 ALERT level C = Check and explain
   1 ALERT level G = General alerts; check

   4 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   2 ALERT type 2 Indicator that the structure model may be wrong or deficient
   1 ALERT type 3 Indicator that the structure quality may be low
   4 ALERT type 4 Improvement, methodology, query or suggestion

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S1. Comment

The oxidation of primary alcohols to aldehydes and secondary alcohols to ketones with pridinium chromate is a well established procedure in organic synthesis, owing mainly to the mild conditions employed (Corey & Schmidt, 1979; Kanemoto et al., 1983; Corey et al., 1985). Nowadays the development of newer chromium(VI) reagents (Sarma & Mahanti, 1991; Li & Li 1998; Yli-Kauhaluoma et al., 1998) for the oxidation of organic substrates continues to be of interest. In our laboratory, the title salt, (I), has been synthesized.

Fig. 1 shows the asymmetric unit of (I), consisting of two 2,6-dimethylpyridinium (DMPM) cations and one dichromate anion. The dichromate has longer bridging Cr—O bonds than terminal Cr—O bonds (Table 1). The shorter distances correspond to double bonds and the longer distances to single bonds. They are in good agreement with those found in bipyridinium dichromates (Martín-Zarza et al., 1995), bis(octyltrimethylammonium) dichromate (Fossé et al., 1998) and tetramethylammonium dichromate (Fossé et al., 2001). The coordination geometry of each Cr atom is distorted tetrahedral.

The C—N—C angle of the pyridinium cation is always widened in comparison with the parent pyridine. For example, the C—N—C angles are 119.0 (3) and 120° for 2,6-dimethylpyridine (Bond et al., 2001) and the 1:1 2,6-dimethylpyridine–urea complex (Lee & Wallwork, 1965), respectively. However, this angle is widened to 123.83 (2), 123.92 (17) and 124.56 (19)° in 2,6-dimethylpyridinium hydrogen phthalate, fumarate and nitrate (Jin et al. 2000, 2003; Pan et al., 2001), respectively. A similar feature is also observed in the title salt (Table 1).

In the crystal structure, there are two pairs of bifurcated hydrogen bonds between the pyridinium and the dichromate ions (Table 2). Furthermore, the pyridinium and dichromate ions are linked by a number of C—H···O hydrogen bonds to form a network structure.

A packing diagram is shown in Fig. 2. The structure is stabilized by π–π interactions between pyridinium cations. The relevant centroid–centroid separations are between DMPM (N1/C2–C6) at (x, y, z) and DMPM (N1/C2–C6) at (x + 3/2, y + 1/2, z + 1) [3.471 (5) Å], and between DMPM (N2/C9–13) at (x, y, z) and DMPM (N2/C9–C13) at (x + 1/2, y + 1/2, z) [3.483 (5) Å].

S2. Experimental

2,6-Dimethylpyridine, CrO₃ and H₂O in a molar ratio of 1:1:8 were mixed together. Crystals of the title salt were formed in the resulting solution by slow evaporation at 293 K for a week.

S3. Refinement

All H atoms were placed in calculated positions and allowed to ride at distances of 0.93 (aromatic), 0.86 (NH) and 0.96 Å (methyl) from their parent atoms, with U_iso(H) values of 1.2–1.5 times U_eq(C,N).

Computing details

Data collection: PROCESS-AUTO (Rigaku, 2001); cell refinement: PROCESS-AUTO; data reduction: CrystalStructure (Rigaku, 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 1998); software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. The structure of the title compound, showing 40% probability displacement ellipsoids and the atom-numbering scheme. Hydrogen bonds are indicated by dashed lines.
	Fig. 2. A packing diagram, viewed along the b axis. Hydrogen bonds are indicated by dashed lines.

Bis(2,6-dimethylpyridinium) dichromate

Crystal data

(C7H10N)2[Cr2O7]	F(000) = 888.0
Mr = 432.32	Dx = 1.615 Mg m^−3
Monoclinic, C2	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: C 2y	Cell parameters from 4141 reflections
a = 13.600 (3) Å	θ = 3.5–27.5°
b = 8.3478 (17) Å	µ = 1.26 mm^−1
c = 17.032 (3) Å	T = 293 K
β = 113.10 (3)°	Prism, yellow
V = 1778.6 (7) Å^3	0.35 × 0.32 × 0.30 mm
Z = 4

Data collection

Rigaku Model? CCD area-detector diffractometer	3660 independent reflections
Radiation source: fine-focus sealed tube	2182 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.030
ϕ and ω scans	θmax = 27.5°, θmin = 3.5°
Absorption correction: multi-scan (ABSCOR; Higashi, 1995)	h = −17→14
Tmin = 0.650, Tmax = 0.685	k = −10→10
9945 measured reflections	l = −22→22

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)] = 0.042	H-atom parameters constrained
wR(F^2) = 0.100	w = 1/[σ^2(Fo^2) + (0.0473P)^2 + 2.3684P] where P = (Fo^2 + 2Fc^2)/3
S = 1.10	(Δ/σ)max < 0.001
3660 reflections	Δρmax = 0.40 e Å^−3
226 parameters	Δρmin = −0.43 e Å^−3
1 restraint	Absolute structure: Flack (1983), 1501 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.00 (2)

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. 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 (Ų)

	x	y	z	Uiso*/Ueq
Cr1	0.23311 (4)	0.64382 (5)	0.28535 (3)	0.02159 (16)
Cr2	0.26458 (4)	0.98596 (6)	0.21240 (3)	0.02090 (15)
O1	0.2878 (2)	0.6855 (3)	0.38559 (17)	0.0360 (7)
O2	0.2625 (2)	0.4639 (3)	0.26863 (18)	0.0378 (6)
O3	0.10372 (19)	0.6611 (3)	0.25219 (16)	0.0313 (6)
O4	0.2828 (2)	0.7736 (3)	0.22656 (17)	0.0317 (6)
O5	0.3814 (2)	1.0705 (4)	0.25065 (18)	0.0361 (6)
O6	0.2065 (3)	1.0189 (3)	0.11124 (17)	0.0394 (8)
O7	0.1915 (2)	1.0473 (4)	0.26094 (18)	0.0365 (6)
N1	0.5518 (2)	1.1575 (4)	0.3997 (2)	0.0234 (6)
H1	0.5093	1.1378	0.3479	0.028*
N2	−0.0484 (3)	0.5429 (4)	0.1064 (2)	0.0244 (7)
H2	−0.0081	0.5698	0.1577	0.029*
C1	0.5613 (4)	0.8684 (5)	0.4154 (3)	0.0340 (9)
H1A	0.5150	0.8768	0.3561	0.051*
H1B	0.5246	0.8142	0.4456	0.051*
H1C	0.6242	0.8091	0.4210	0.051*
C2	0.5926 (3)	1.0318 (4)	0.4517 (3)	0.0270 (9)
C3	0.6586 (3)	1.0620 (5)	0.5351 (3)	0.0303 (9)
H3	0.6888	0.9778	0.5726	0.036*
C4	0.6798 (4)	1.2197 (6)	0.5629 (3)	0.0350 (10)
H4	0.7227	1.2413	0.6197	0.042*
C5	0.6377 (3)	1.3430 (5)	0.5068 (3)	0.0331 (9)
H5	0.6529	1.4483	0.5256	0.040*
C6	0.5731 (3)	1.3126 (5)	0.4230 (3)	0.0273 (8)
C7	0.5239 (3)	1.4367 (5)	0.3570 (3)	0.0385 (10)
H7A	0.4821	1.3860	0.3037	0.058*
H7B	0.5791	1.5001	0.3504	0.058*
H7C	0.4788	1.5044	0.3740	0.058*
C8	−0.0680 (4)	0.8283 (5)	0.0814 (3)	0.0415 (11)
H8A	−0.0212	0.8269	0.1407	0.062*
H8B	−0.1326	0.8842	0.0743	0.062*
H8C	−0.0333	0.8818	0.0493	0.062*
C9	−0.0941 (3)	0.6616 (5)	0.0501 (2)	0.0284 (8)
C10	−0.1590 (3)	0.6175 (6)	−0.0329 (3)	0.0364 (10)
H10	−0.1915	0.6959	−0.0736	0.044*
C11	−0.1753 (3)	0.4590 (7)	−0.0547 (3)	0.0406 (11)
H11	−0.2191	0.4305	−0.1103	0.049*
C12	−0.1273 (3)	0.3401 (6)	0.0052 (3)	0.0365 (10)
H12	−0.1389	0.2326	−0.0099	0.044*
C13	−0.0615 (3)	0.3837 (5)	0.0880 (3)	0.0280 (8)
C14	−0.0044 (4)	0.2694 (5)	0.1585 (3)	0.0397 (10)
H14A	0.0352	0.3282	0.2096	0.060*
H14B	0.0437	0.2051	0.1430	0.060*
H14C	−0.0555	0.2014	0.1681	0.060*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cr1	0.0205 (3)	0.0230 (3)	0.0173 (3)	−0.0014 (2)	0.0031 (2)	0.0028 (2)
Cr2	0.0191 (3)	0.0250 (3)	0.0151 (3)	−0.0033 (2)	0.0029 (2)	0.0021 (2)
O1	0.0426 (16)	0.0357 (17)	0.0181 (12)	−0.0076 (12)	−0.0007 (12)	0.0021 (11)
O2	0.0435 (15)	0.0262 (15)	0.0400 (14)	0.0041 (13)	0.0125 (12)	0.0026 (12)
O3	0.0218 (12)	0.0410 (16)	0.0289 (13)	−0.0048 (12)	0.0076 (10)	0.0019 (12)
O4	0.0375 (15)	0.0255 (13)	0.0391 (15)	0.0013 (11)	0.0225 (13)	0.0079 (11)
O5	0.0244 (13)	0.0408 (16)	0.0359 (15)	−0.0111 (12)	0.0041 (11)	−0.0057 (13)
O6	0.0470 (18)	0.0385 (19)	0.0205 (13)	−0.0025 (13)	0.0002 (12)	0.0073 (11)
O7	0.0386 (16)	0.0380 (15)	0.0373 (15)	0.0062 (12)	0.0195 (13)	0.0047 (13)
N1	0.0189 (13)	0.0268 (16)	0.0232 (14)	−0.0019 (14)	0.0068 (11)	−0.0030 (14)
N2	0.0206 (15)	0.0309 (17)	0.0198 (15)	0.0007 (12)	0.0058 (12)	0.0022 (13)
C1	0.039 (2)	0.023 (2)	0.032 (2)	0.0009 (17)	0.0045 (17)	0.0008 (17)
C2	0.0255 (19)	0.029 (2)	0.0298 (19)	−0.0018 (14)	0.0146 (16)	−0.0042 (15)
C3	0.027 (2)	0.037 (2)	0.0249 (19)	0.0001 (17)	0.0087 (17)	0.0022 (18)
C4	0.033 (2)	0.048 (2)	0.025 (2)	−0.0042 (19)	0.0130 (17)	−0.0114 (18)
C5	0.034 (2)	0.033 (2)	0.038 (2)	−0.0107 (16)	0.0198 (18)	−0.0136 (18)
C6	0.0228 (18)	0.0268 (19)	0.039 (2)	−0.0051 (15)	0.0193 (16)	−0.0057 (17)
C7	0.036 (2)	0.027 (2)	0.052 (3)	0.0002 (16)	0.018 (2)	−0.0019 (18)
C8	0.038 (2)	0.029 (2)	0.051 (3)	0.0033 (18)	0.010 (2)	0.009 (2)
C9	0.0202 (17)	0.038 (2)	0.0266 (18)	0.0053 (16)	0.0083 (15)	0.0084 (17)
C10	0.028 (2)	0.054 (3)	0.026 (2)	0.0005 (19)	0.0097 (17)	0.004 (2)
C11	0.028 (2)	0.070 (3)	0.0233 (18)	−0.010 (2)	0.0104 (16)	−0.006 (2)
C12	0.035 (2)	0.043 (3)	0.037 (2)	−0.0104 (18)	0.0197 (19)	−0.014 (2)
C13	0.0241 (19)	0.033 (2)	0.030 (2)	−0.0022 (17)	0.0137 (16)	−0.0012 (18)
C14	0.047 (3)	0.028 (2)	0.043 (2)	0.0034 (18)	0.017 (2)	0.0008 (18)

Geometric parameters (Å, º)

Cr1—O2	1.608 (3)	C4—H4	0.930
Cr1—O1	1.610 (3)	C5—C6	1.375 (6)
Cr1—O3	1.630 (3)	C5—H5	0.930
Cr1—O4	1.779 (3)	C6—C7	1.481 (6)
Cr2—O7	1.606 (3)	C7—H7A	0.960
Cr2—O6	1.613 (3)	C7—H7B	0.960
Cr2—O5	1.623 (3)	C7—H7C	0.960
Cr2—O4	1.793 (3)	C8—C9	1.482 (6)
N1—C2	1.344 (5)	C8—H8A	0.960
N1—C6	1.352 (5)	C8—H8B	0.960
N1—H1	0.860	C8—H8C	0.960
N2—C9	1.349 (5)	C9—C10	1.390 (6)
N2—C13	1.361 (5)	C10—C11	1.369 (8)
N2—H2	0.860	C10—H10	0.930
C1—C2	1.489 (5)	C11—C12	1.389 (7)
C1—H1A	0.960	C11—H11	0.930
C1—H1B	0.960	C12—C13	1.390 (6)
C1—H1C	0.960	C12—H12	0.930
C2—C3	1.374 (6)	C13—C14	1.490 (6)
C3—C4	1.390 (6)	C14—H14A	0.960
C3—H3	0.930	C14—H14B	0.960
C4—C5	1.368 (7)	C14—H14C	0.960
O2—Cr1—O1	110.25 (15)	N1—C6—C5	117.2 (4)
O2—Cr1—O3	109.34 (15)	N1—C6—C7	117.7 (4)
O1—Cr1—O3	109.57 (16)	C5—C6—C7	125.0 (4)
O2—Cr1—O4	107.06 (15)	C6—C7—H7A	109.5
O1—Cr1—O4	110.23 (14)	C6—C7—H7B	109.5
O3—Cr1—O4	110.36 (13)	H7A—C7—H7B	109.5
O7—Cr2—O6	110.66 (17)	C6—C7—H7C	109.5
O7—Cr2—O5	111.14 (16)	H7A—C7—H7C	109.5
O6—Cr2—O5	110.30 (16)	H7B—C7—H7C	109.5
O7—Cr2—O4	109.07 (14)	C9—C8—H8A	109.5
O6—Cr2—O4	107.24 (14)	C9—C8—H8B	109.5
O5—Cr2—O4	108.31 (15)	H8A—C8—H8B	109.5
Cr1—O4—Cr2	127.95 (16)	C9—C8—H8C	109.5
C2—N1—C6	124.6 (3)	H8A—C8—H8C	109.5
C2—N1—H1	117.7	H8B—C8—H8C	109.5
C6—N1—H1	117.7	N2—C9—C10	117.3 (4)
C9—N2—C13	124.9 (4)	N2—C9—C8	117.1 (4)
C9—N2—H2	117.6	C10—C9—C8	125.5 (4)
C13—N2—H2	117.6	C11—C10—C9	120.2 (4)
C2—C1—H1A	109.5	C11—C10—H10	119.9
C2—C1—H1B	109.5	C9—C10—H10	119.9
H1A—C1—H1B	109.5	C10—C11—C12	120.8 (4)
C2—C1—H1C	109.5	C10—C11—H11	119.6
H1A—C1—H1C	109.5	C12—C11—H11	119.6
H1B—C1—H1C	109.5	C11—C12—C13	119.3 (4)
N1—C2—C3	118.2 (4)	C11—C12—H12	120.4
N1—C2—C1	117.7 (3)	C13—C12—H12	120.4
C3—C2—C1	124.2 (4)	N2—C13—C12	117.5 (4)
C2—C3—C4	119.3 (4)	N2—C13—C14	117.4 (4)
C2—C3—H3	120.3	C12—C13—C14	125.1 (4)
C4—C3—H3	120.3	C13—C14—H14A	109.5
C5—C4—C3	120.1 (4)	C13—C14—H14B	109.5
C5—C4—H4	120.0	H14A—C14—H14B	109.5
C3—C4—H4	120.0	C13—C14—H14C	109.5
C4—C5—C6	120.6 (4)	H14A—C14—H14C	109.5
C4—C5—H5	119.7	H14B—C14—H14C	109.5
C6—C5—H5	119.7
O2—Cr1—O4—Cr2	174.14 (19)	C2—N1—C6—C7	178.3 (4)
O1—Cr1—O4—Cr2	−65.9 (2)	C4—C5—C6—N1	1.3 (6)
O3—Cr1—O4—Cr2	55.2 (2)	C4—C5—C6—C7	−179.6 (4)
O7—Cr2—O4—Cr1	−1.1 (2)	C13—N2—C9—C10	−0.2 (6)
O6—Cr2—O4—Cr1	−121.0 (2)	C13—N2—C9—C8	−178.4 (4)
O5—Cr2—O4—Cr1	120.0 (2)	N2—C9—C10—C11	0.5 (6)
C6—N1—C2—C3	1.6 (6)	C8—C9—C10—C11	178.6 (4)
C6—N1—C2—C1	−179.6 (4)	C9—C10—C11—C12	−0.3 (6)
N1—C2—C3—C4	0.8 (6)	C10—C11—C12—C13	−0.3 (6)
C1—C2—C3—C4	−178.0 (4)	C9—N2—C13—C12	−0.4 (6)
C2—C3—C4—C5	−1.9 (7)	C9—N2—C13—C14	179.8 (4)
C3—C4—C5—C6	0.8 (6)	C11—C12—C13—N2	0.6 (6)
C2—N1—C6—C5	−2.6 (5)	C11—C12—C13—C14	−179.6 (4)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O5	0.86	1.96	2.779 (6)	160
N1—H1···O3^i	0.86	2.45	2.865 (5)	110
N2—H2···O3	0.86	1.89	2.719 (7)	162
N2—H2···O5^ii	0.86	2.58	2.973 (5)	110
C1—H1A···O5	0.96	2.57	3.364 (5)	141
C3—H3···O1^iii	0.93	2.53	3.385 (5)	153
C4—H4···O7^iii	0.93	2.50	3.164 (5)	128
C5—H5···O1^iv	0.93	2.43	3.332 (5)	163
C7—H7B···O7^i	0.96	2.58	3.412 (7)	146
C8—H8A···O3	0.96	2.42	3.246 (5)	143
C10—H10···O4^v	0.93	2.49	3.325 (5)	149
C11—H11···O2^v	0.93	2.54	3.361 (6)	147
C12—H12···O6^vi	0.93	2.40	3.257 (5)	153
C14—H14B···O7^vii	0.96	2.58	3.152 (5)	119

Symmetry codes: (i) x+1/2, y+1/2, z; (ii) x−1/2, y−1/2, z; (iii) −x+1, y, −z+1; (iv) −x+1, y+1, −z+1; (v) −x, y, −z; (vi) −x, y−1, −z; (vii) x, y−1, z.