Dipyridinium dichromate: an achiral compound forming chiral crystals

The title compound, (C₅H₆N)₂[Cr₂O₇], crystallizes in one of the Sohncke space groups, viz. P2₁2₁2₁. Crystallization of dipyridinium dichromate is thus an example of spontaneous formation of a chiral crystal structure from achiral mol­ecules. The dichromate anion adopts a virtually eclipsed achiral conformation, and the crystal structure is held together primarily by N-H...O and C-H...O inter­actions. The possibility of using dipyridinium dichromate as a reagent in enantio­selective synthesis is discussed.

S1. Comment

Spontaneous resolution is the phenomenon where the two enantiomers in a racemic solution crystallize in separate chiral crystals (Eliel et al., 1994). Racemic solutions of stereochemically inert compounds will yield a racemic conglomerate, which is a 1:1 mixture of the two enantiomorphic crystals. On the other hand, a stereochemically labile substance may undergo total spontaneous resolution, also known as crystallization-induced asymmetric transformation (Jacques et al., 1984). This means that the whole crystal batch may grow from a single nucleus, and the whole batch will be enantiomerically pure. The requirement is, of course, that the two enantiomers will interconvert rapidly in solution, so that the whole amount of solute is transformed into one single enantiomer upon crystallization.

Enantiomerically pure crystals belong to one of the 65 Sohncke space groups (Flack, 2003). What if an achiral substance crystallizes in such a space group? Crystallization of an achiral substance in a Sohncke space group will give chiral crystals, where the molecules or ions will display a chiral packing. Well known examples include α-quartz, magnesium sulfate heptahydrate, strontium formate and sodium chlorate (Groth, 1921). The crystals may appear as two enantiomorphs, and crystallization-induced asymmetric transformation is possible (Kondepudi et al., 1990; McBride & Carter, 1991; Pagni & Compton, 2002). It has been shown that DL-alanine hydrochloride may be enantioselectively adsorbed on quartz crystals (Bonner & Kavasmaneck, 1976), and both quartz (Soai et al., 1999) and sodium chlorate (Sato et al., 2000) induce high enantioselectivity in Soai's asymmetric autocatalytic reaction (Soai et al., 1995).

Crystallization-induced asymmetric transformation may be regarded as a means of achieving absolute asymmetric synthesis, i.e. the creation of optically active substances from achiral or racemic starting materials only (Feringa & van Delden, 1999). Over the years we have studied the total spontaneous resolution of stereochemically labile chemical reagents (Vestergren et al., 2000, 2003; Håkansson et al., 1999) and potential substrates (Johansson & Håkansson, 2005; Lennartson, Salo & Håkansson, 2005; Andersson et al., 1986; Lennartson et al., 2007). A stereochemically labile oxidizing agent would be attractive, since in principle it would be possible to convert, for example, a prochiral sulfide to a chiral sulphoxide, or to partly degrade a chiral (racemic) alcohol to an achiral carbonyl compound leaving an enantiomeric excess in the remaining alcohol. For such a reaction to be successful, the chirality of the crystal structure must induce an enantiomeric excess in the product, and the reaction must be carried out under conditions where the reagent does not dissolve, since dissolution would destroy the chirality of the crystal structure. In the search for such substances, we found that a commercial sample of dipyridinium dichromate, known as PDC to organic chemists, was made up by a phase, (I), crystallizing in the Sohncke space group P2₁2₁2₁.

PDC is a useful oxidizing agent in organic synthesis. A suspension of PDC in methylene chloride is a mild oxidizing agent that converts both saturated and unsaturated alcohols to aldehydes and ketones (Corey & Schmidt, 1979). The reaction is accelerated by the presence of molecular sieves (Herscovici & Antonakis, 1980), and the oxidation of alcohols can also be carried out catalytically using bis(trimethylsilyl)peroxide as bulk-oxidant (Kanemoto et al., 1988). A solution of an aliphatic aldehyde and methanol in dry dimethylformamide gives a methyl ester (O'Connor & Just, 1987), cyclic alkenes give α-iodo-ketones when treated with PDC and iodine in dry methylene chloride (D'Ascoli et al., 1980), and the same group found that trisubstituted alkenes can be transformed into iodohydrines or, with a longer reaction time, to epoxides (Antonioletti et al., 1983). Cyanohydrines give carboxylic acids with PDC in dimethylformamide (Corey & Schmidt, 1980), and Brown et al. (1992) have converted organoboranes to carbonyl compounds using PDC. Today, chemical supplying companies such as Sigma–Aldrich or Acros Organics also offer dipyridinium dichromate on a silica support.

Surprisingly, a search of the Cambridge Structural Database (CSD, Version 5.30, update of November 2008; Allen 2002) indicated that no full crystal structure determination of (I) has been reported to date. Two phases of dipyridiniumdichromate are reported (Gili, 1984), but they do not correspond to compound (I). Both appear to be triclinic but no space group is reported. In addition, only a limited number of dichromates displaying substituted pyridinium counter-ions are found in the CSD. These include bis(5-nitropyridinium) dichromate (Pecaut & Masse, 1993), bis(2,6-dimethylpyridinium) dichromate (Jin et al., 2006), bis(quinolinium) dichromate (Sundar et al., 2003), and bis(2,4'-bipyridinium) dichromate, bis(2,2'-dipyridylaminium) dichromate and bis(4,4'-dipyridinium) dichromate (Martin-Zarza et al., 1995). Among the previously reported substituted pyridinium dichromates, bis(2,6-dimethylpyridinium) dichromate also crystallizes in a Sohncke space group with a low Flack parameter (Flack, 1983; Bernardinelli & Flack, 1985).

The molecular structure of (I) is unremarkable (Fig. 1). The coordination geometries around atoms Cr1 and Cr2 are distorted tetrahedral, and the dichromate ion adopts an eclipsed conformation. This arrangement is similar to that in bis(4,4'-dipyridinium) dichromate, but different from that in bis(2,6-dimethylpyridinium) dichromate, where the dichromate ion adopts a virtually staggered conformation. The assembly of the crystal structure appears to rely largely on N—H···O and C—H···O interactions between anions and cations (Table 1). This was anticipated by Gili (1984) on the basis of ¹H NMR and IR spectroscopy. Each anion forms short C—H···O contacts with surrounding pyridinium cations, and the interaction pattern of (I) is rather complex. Atom O1 forms two contacts: to atoms H1(-x + 3/2, -y + 2, z - 1/2) and H7(-x + 3/2, -y + 2, +z + 1/2). Atom O2 forms two short contacts to atoms H12(-x + 1, y + 1/2, -z + 3/2) and H4(-x + 1, y + 1/2, -z + 3/2), although the C—H···O angles are small and both H atoms are involved in other interactions. Atom O3 forms two short contacts: to H9(-x +1/2, -y +2, z + 1/2) and to H6(-x + 3/2, -y +2, z + 1/2). Atom O4, the O atom bridging atoms Cr1 and Cr2, forms a short contact to atom H8(-x +3/2, -y + 2, z + 1/2) and a short contact with atom H11(-x + 1/2, -y + 2, z - 1/2). Thus, atoms O3 and O4 are interconnected by two adjacent H atoms in one pyridinium cation. Atoms O1 and O3 are interconnected in a similar manner. Atom O5 forms a short contact with atom H5(x + 1, y, z), the same pyridinium cation, forming a short contact with atom O7 through H4(x + 1, y, z). Atom O7 forms an additional contact with atom H10(x + 1, y, z). Atom O6 forms a short contact with H12, the only short contact within the asymmetric unit. All the H atoms appear to be involved in hydrogen bonding except atoms H2 and H3. As a result, (I) forms a layered structure, in which alternating layers of dichromate anions and pyridinium cations are stacked along c (Fig. 2). On inspecting the crystal structure it is not obvious where the chirality originates from, the anion being locked in an achiral conformation; the chirality is therefore to be traced to the orientation of the ions in the unit cell. It therefore appears very unlikely that a reaction between, for example, 1-phenylethanol and (I) would give rise to any measurable enantioselectivity.

It is not trivial to determine if all the crystals in a batch of (I) are of the same enantiomorph or not; such a determination must be carried out in the solid state, and well known methods such as polarimetry are of no use. There are in principle two ways this could be carried out: either by solid-state circular dichroism (CD) spectroscopy or by single-crystal X-ray diffraction on a large number of crystals (distinction between enantiomorphs by observation in polarized light is only possible for crystals of the cubic system). We have developed a method for measuring enantiomeric excess of stereochemically labile molecules by means of solid-state CD spectroscopy (Lennartson , Vestergren & Håkansson, 2005): the value of the solid-state CD of an enantiopure single crystal was found to be proportional to the mass. Enantiomorphic purity can therefore be measured by comparing the CD of a microcrystalline sample with that of a single crystal. In the case of (I), however, we were not able to observe any CD, not even for single crystals. The Flack parameter of the analysed crystal is low, 0.01 (2), and there was no indication of twinning-by-inversion in crystals of (I). It is therefore not unlikely that slow crystallization of (I) would afford exclusive crystallization of one enantiomorph only.

Related literature

For related literature, see: Andersson et al. (1986); Antonioletti et al. (1983); Bernardinelli & Flack (1985); Bonner & Kavasmaneck (1976); Brown et al. (1992); Corey & Schmidt (1979, 1980); D'Ascoli, D'Auria, Nucciarelli, Piancatelli & Scettri (1980); Eliel et al. (1994); Feringa & van Delden (1999); Flack (1983, 2003); Gili (1984); Groth (1921); Håkansson et al. (1999); Herscovici & Antonakis (1980); Jacques et al. (1984); Jin et al. (2006); Johansson & Håkansson (2005); Kanemoto et al. (1988); Kondepudi et al. (1990); Lennartson et al. (2005a, 2005b, 2007); Martin-Zarza, Gili, Rodriguez-Romero, Ruiz-Perez & Solans (1995); McBride & Carter (1991); O'Connor & Just (1987); Pagni & Compton (2002); Pecaut & Masse (1993); Sato et al. (2000); Soai et al. (1995, 1999); Vestergren et al. (2000, 2003).

S2. Experimental

A crystal was carefully selected from a comercial sample of dipyridinium dichromate (Fluka).

S3. Refinement

Atoms H11 and H12 were located in a difference Fourier map and freely refined [N—H = 0.74 (3) and 0.94 (4) Å, respectively]. The C-bound H atoms were included in calculated positions [C—H = 0.93 Å ] and treated as riding [U_iso(H) = 1.2U_iso(C)].

Computing details

Data collection: CrystalClear (Rigaku, 2000); cell refinement: CrystalClear (Rigaku, 2000); data reduction: CrystalClear (Rigaku, 2000); program(s) used to solve structure: SIR92 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997) and PLUTON (Spek, 2009); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. The molecular structure of (I), showing the crystallographic numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are included with an arbitrary radii.
	Fig. 2. A view along the a axis of the crystal structure of (I).

dipyridinium dichromate(VI)

Crystal data

(C5H6N)2[Cr2O7]	F(000) = 760
Mr = 376.22	Dx = 1.762 Mg m^−3
Orthorhombic, P212121	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ac 2ab	Cell parameters from 9462 reflections
a = 8.2797 (17) Å	θ = 2.2–25.5°
b = 12.741 (2) Å	µ = 1.57 mm^−1
c = 13.371 (2) Å	T = 100 K
V = 1410.5 (4) Å^3	Plate, red
Z = 4	0.3 × 0.2 × 0.1 mm

Data collection

Rigaku R-AXIS IIC image-plate system diffractometer	2505 independent reflections
Radiation source: rotating-anode X-ray tube, Rigaku RU-H3R	2441 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.033
Detector resolution: 105 pixels mm^-1	θmax = 25.5°, θmin = 2.2°
ϕ scans	h = −9→9
Absorption correction: multi-scan (CrystalClear; Rigaku, 2000)	k = −15→15
Tmin = 0.647, Tmax = 0.850	l = −16→16
9462 measured 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)] = 0.026	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.063	w = 1/[u^2(Fo^2) + (0.0359P)^2 + 0.8791P] where P = (Fo^2 + 2Fc^2)/3
S = 1.06	(Δ/σ)max = 0.001
2505 reflections	Δρmax = 0.29 e Å^−3
198 parameters	Δρmin = −0.34 e Å^−3
0 restraints	Absolute structure: Flack (1983), 984 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.01 (2)

Special details

Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2u(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
C1	0.1758 (3)	0.9102 (2)	1.12984 (19)	0.0093 (5)
H1	0.2145	0.9299	1.1924	0.011*
C2	0.2816 (3)	0.8755 (2)	1.0569 (2)	0.0119 (6)
H2	0.3920	0.8721	1.0695	0.014*
C3	0.2196 (3)	0.8460 (2)	0.9644 (2)	0.0126 (6)
H3	0.2889	0.8226	0.9144	0.015*
C4	0.0550 (4)	0.8514 (2)	0.9464 (2)	0.0126 (6)
H4	0.0133	0.8314	0.8847	0.015*
C5	−0.0461 (3)	0.8868 (2)	1.0212 (2)	0.0118 (6)
H5	−0.1568	0.8909	1.0103	0.014*
C6	0.3046 (3)	0.8557 (2)	0.5218 (2)	0.0123 (6)
H6	0.4165	0.8503	0.5242	0.015*
C7	0.2266 (4)	0.8797 (3)	0.4331 (2)	0.0181 (7)
H7	0.2854	0.8896	0.3746	0.022*
C8	0.0597 (4)	0.8888 (2)	0.4325 (2)	0.0169 (7)
H8	0.0061	0.9050	0.3734	0.020*
C9	−0.0269 (3)	0.8739 (3)	0.5195 (2)	0.0155 (6)
H9	−0.1386	0.8818	0.5197	0.019*
C10	0.0539 (3)	0.8471 (2)	0.6062 (2)	0.0119 (6)
H10	−0.0031	0.8341	0.6649	0.014*
N1	0.0168 (3)	0.91532 (19)	1.11006 (17)	0.0093 (5)
N2	0.2141 (3)	0.84019 (18)	0.60496 (17)	0.0117 (5)
O1	1.0223 (2)	1.02156 (16)	0.74981 (14)	0.0159 (4)
O2	0.8382 (2)	1.19079 (14)	0.72344 (14)	0.0165 (4)
O3	0.8605 (2)	1.10228 (14)	0.90240 (12)	0.0126 (4)
O4	0.6834 (2)	1.00257 (15)	0.74943 (13)	0.0110 (4)
O5	0.6108 (2)	0.87187 (16)	0.91031 (13)	0.0145 (4)
O6	0.4625 (2)	0.84244 (19)	0.73634 (15)	0.0257 (5)
O7	0.7733 (2)	0.79170 (15)	0.75846 (15)	0.0191 (5)
Cr1	0.85907 (5)	1.08142 (3)	0.78285 (3)	0.00727 (11)
Cr2	0.63172 (5)	0.87140 (3)	0.79057 (3)	0.00832 (11)
H11	−0.037 (4)	0.932 (3)	1.151 (2)	0.015 (9)*
H12	0.274 (4)	0.823 (3)	0.663 (3)	0.033 (10)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C1	0.0094 (15)	0.0101 (12)	0.0082 (12)	−0.0001 (10)	−0.0041 (9)	0.0016 (10)
C2	0.0086 (14)	0.0111 (13)	0.0160 (15)	−0.0005 (11)	−0.0003 (10)	0.0024 (12)
C3	0.0163 (15)	0.0097 (14)	0.0117 (14)	0.0007 (11)	0.0060 (11)	0.0009 (10)
C4	0.0204 (16)	0.0124 (16)	0.0050 (14)	0.0000 (11)	−0.0011 (10)	−0.0013 (11)
C5	0.0127 (15)	0.0106 (14)	0.0120 (14)	−0.0019 (10)	−0.0036 (10)	0.0006 (11)
C6	0.0102 (13)	0.0114 (14)	0.0153 (15)	0.0000 (10)	0.0013 (10)	−0.0015 (11)
C7	0.0264 (18)	0.0189 (15)	0.0089 (14)	−0.0010 (12)	0.0053 (11)	0.0017 (13)
C8	0.0232 (18)	0.0159 (17)	0.0117 (14)	0.0016 (12)	−0.0081 (11)	0.0042 (12)
C9	0.0081 (15)	0.0142 (14)	0.0243 (17)	0.0006 (11)	−0.0062 (11)	−0.0011 (13)
C10	0.0143 (16)	0.0087 (14)	0.0127 (14)	−0.0009 (10)	0.0050 (10)	−0.0043 (11)
N1	0.0123 (12)	0.0109 (12)	0.0048 (11)	0.0000 (10)	0.0033 (9)	0.0004 (9)
N2	0.0183 (14)	0.0099 (12)	0.0070 (11)	0.0018 (9)	−0.0050 (9)	−0.0015 (10)
O1	0.0134 (10)	0.0177 (11)	0.0167 (10)	−0.0005 (8)	0.0043 (7)	−0.0044 (8)
O2	0.0218 (11)	0.0138 (9)	0.0140 (9)	−0.0041 (8)	−0.0033 (8)	0.0045 (8)
O3	0.0126 (10)	0.0197 (10)	0.0056 (9)	−0.0004 (9)	0.0006 (8)	−0.0008 (7)
O4	0.0099 (10)	0.0130 (9)	0.0103 (9)	−0.0019 (7)	−0.0022 (7)	0.0019 (7)
O5	0.0157 (11)	0.0215 (10)	0.0062 (9)	−0.0016 (8)	0.0001 (7)	0.0028 (8)
O6	0.0214 (12)	0.0337 (13)	0.0220 (12)	−0.0122 (9)	−0.0149 (8)	0.0123 (10)
O7	0.0227 (11)	0.0136 (10)	0.0209 (11)	0.0020 (8)	0.0088 (8)	0.0032 (9)
Cr1	0.0082 (2)	0.00878 (19)	0.00481 (19)	−0.00054 (16)	0.00060 (18)	0.00055 (15)
Cr2	0.0074 (2)	0.0111 (2)	0.0064 (2)	−0.00216 (16)	−0.00157 (18)	0.00231 (16)

Geometric parameters (Å, º)

C1—N1	1.345 (4)	C8—C9	1.379 (4)
C1—C2	1.383 (4)	C8—H8	0.9300
C1—H1	0.9300	C9—C10	1.382 (4)
C2—C3	1.392 (4)	C9—H9	0.9300
C2—H2	0.9300	C10—N2	1.330 (3)
C3—C4	1.385 (4)	C10—H10	0.9300
C3—H3	0.9300	N1—H11	0.74 (3)
C4—C5	1.380 (4)	N2—H12	0.94 (4)
C4—H4	0.9300	O1—Cr1	1.614 (2)
C5—N1	1.348 (3)	O2—Cr1	1.6133 (18)
C5—H5	0.9300	O3—Cr1	1.6204 (16)
C6—N2	1.355 (3)	O4—Cr2	1.8106 (19)
C6—C7	1.384 (4)	O4—Cr1	1.8233 (18)
C6—H6	0.9300	O5—Cr2	1.6105 (18)
C7—C8	1.386 (4)	O6—Cr2	1.6204 (19)
C7—H7	0.9300	O7—Cr2	1.609 (2)
N1—C1—C2	119.8 (2)	C8—C9—H9	120.3
N1—C1—H1	120.1	C10—C9—H9	120.3
C2—C1—H1	120.1	N2—C10—C9	119.2 (3)
C1—C2—C3	118.6 (3)	N2—C10—H10	120.4
C1—C2—H2	120.7	C9—C10—H10	120.4
C3—C2—H2	120.7	C1—N1—C5	122.6 (2)
C4—C3—C2	120.3 (3)	C1—N1—H11	117 (2)
C4—C3—H3	119.9	C5—N1—H11	120 (2)
C2—C3—H3	119.9	C10—N2—C6	123.5 (3)
C5—C4—C3	119.2 (3)	C10—N2—H12	122 (2)
C5—C4—H4	120.4	C6—N2—H12	115 (2)
C3—C4—H4	120.4	Cr2—O4—Cr1	128.51 (10)
N1—C5—C4	119.5 (3)	O2—Cr1—O1	111.29 (11)
N1—C5—H5	120.2	O2—Cr1—O3	110.18 (9)
C4—C5—H5	120.2	O1—Cr1—O3	109.96 (10)
N2—C6—C7	118.5 (3)	O2—Cr1—O4	105.65 (9)
N2—C6—H6	120.7	O1—Cr1—O4	109.92 (9)
C7—C6—H6	120.7	O3—Cr1—O4	109.76 (10)
C6—C7—C8	119.2 (3)	O7—Cr2—O5	110.25 (11)
C6—C7—H7	120.4	O7—Cr2—O6	111.51 (12)
C8—C7—H7	120.4	O5—Cr2—O6	110.65 (11)
C9—C8—C7	120.1 (3)	O7—Cr2—O4	109.23 (9)
C9—C8—H8	119.9	O5—Cr2—O4	108.91 (9)
C7—C8—H8	119.9	O6—Cr2—O4	106.18 (10)
C8—C9—C10	119.3 (3)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H11···O4^i	0.74 (3)	1.97 (3)	2.705 (3)	171 (3)
N2—H12···O2^ii	0.94 (4)	2.45 (4)	3.013 (3)	118 (3)
N2—H12···O6	0.94 (4)	1.87 (4)	2.704 (3)	147 (3)
C1—H1···O1^iii	0.93	2.39	3.094 (3)	132
C4—H4···O7^iv	0.93	2.66	3.512 (4)	153
C4—H4···O2^ii	0.93	2.61	3.182 (3)	120
C5—H5···O5^iv	0.93	2.36	3.210 (3)	153
C6—H6···O3^v	0.93	2.53	3.244 (3)	133
C7—H7···O1^v	0.93	2.57	3.451 (4)	159
C8—H8···O4^vi	0.93	2.57	3.459 (4)	160
C9—H9···O3^vi	0.93	2.42	3.190 (3)	140
C10—H10···O7^iv	0.93	2.30	3.169 (3)	156

Symmetry codes: (i) −x+1/2, −y+2, z+1/2; (ii) −x+1, y−1/2, −z+3/2; (iii) −x+3/2, −y+2, z+1/2; (iv) x−1, y, z; (v) −x+3/2, −y+2, z−1/2; (vi) −x+1/2, −y+2, z−1/2.