(±)-3-Oxocyclo­pentanecarboxylic acid: the smallest keto acid known to aggregate by catemeric hydrogen bonding

The title compound, C₆H₈O₃, is the smallest keto acid yet found to aggregate in the solid as acid-to-ketone hydrogen-bonded catemers. Four translational chains pass through the cell in the a direction [O...O = 2.6915 (14) Å and O—H...O = 166°]. Two inter­molecular C—H...O close contacts exist, involving both carbonyl functions.

S1. Comment

Our study of crystalline ketocarboxylic acids has explored their five known hydrogen-bonding modes. Acid-to-ketone catemers constitute a sizable minority of cases, appearing among a wide variety of keto acids, from polycyclic C₃₀ non-racemates to far smaller racemic compounds. Of interest are the minimum requirements for catemeric aggregation, but the smallest molecules offer experimental problems (low crystallinity, little structural variability), and among C₃–C₆ keto acids, only the crystal structure for pyruvic acid has been reported to date, as a carboxyl dimer (Harata et al., 1977). We now report the structure of the title six-carbon γ-keto acid, (I). To date, this is the smallest crystalline keto acid found to aggregate as a catemer.

Fig. 1 shows the molecule of (I). The ring conformation is not a typical `folded-envelope' cyclopentane. Rather, it is a twist conformation, in which only atom C3 lies in the average ring-plane [deviation for C3 = −0.0083 (9) Å]. The adjoining atoms, C2 and C4, lie nearly equidistant from the average plane on alternate faces [−0.1235 (8) and 0.1381 (9) Å, respectively], as do the remaining two atoms, C1 and C5, but in the opposite sense [0.2147 (9) and −0.2211 (9) Å, respectively]. This arrangement avoids eclipsing among all the ring substituents. The carboxyl group projects outward, with a C6—C1—C2—C3 torsion angle of −153.28 (11)°, and its rotation about C1—C6 places its carbonyl group on the same face of the average plane as atom C5 and nearly eclipsed with it, so that the C5—C1—C6—O2 torsion angle is 6.8 (2)°.

The averaging of C—O bond lengths and C—C—O angles by disorder, often observed in dimeric carboxyls, typically results from tautomerism within the symmetrical eight-membered dimer structure. This is not observed in catemers, whose geometry cannot support this averaging mechanism. Hence in (I), which is catemeric, these values (Table 1) are typical of those in highly ordered dimeric carboxyls (Borthwick, 1980).

Fig. 2 illustrates the packing of (I) (Z = 4), in which the centrosymmetric array about (1/2, 1/2, 1/2), combined with the formation of translational catemers, yields four separate hydrogen-bonded chains, all passing through the cell in the a and −a directions. These chains display the four permutations of molecular chirality plus chain direction; proceeding along c from the origin, these combinations are encountered in the order S + a, S − a, R + a, R − a.

A combination of the C═O···H angle and the C—C═O···H torsion angle characterizes the geometry of hydrogen bonding to carbonyl groups. These describe the approach of the acid H atom to the O receptor in terms of its deviation from, respectively, C═O axiality (ideal = 120°) and planarity with the carbonyl (ideal = 0°). In (I), these two angles are 128 and −21°, respectively. Three close C—H···O non-bonded intermolecular contacts exist (Table 2), with all distances within the 2.7–?.?? Å [Please provide missing upper limit] range we survey as standard for such polar packing interactions (Steiner, 1997).

As previously seen (Thompson et al., 1999), among the minimum molecular requirements for the formation of translational catemers, a `linear-anti' arrangement of ketone and carboxyl, as found in (I), is clearly important. When such arrangements are not forced by inflexible molecular geometry (Lalancette et al., 1997), they may be facilitated when alternative conformations are precluded sterically or discouraged energetically (Lalancette & Thompson, 2003), as also occurs in (I).

S2. Experimental

Compound (I) was prepared by conjugate addition of malonic ester to diethyl itaconate (Hope, 1912; Kay & Perkin, 1906). After hydrolysis and decarboxylation, sublimed material gave crystals suitable for X-ray from EtOAc–hexane (Ratio?) (m.p. 335 K).

Because of the similar but opposite shifts produced by ketone ring-strain and by hydrogen bonding, solid-state versus liquid IR spectra of carboxycyclopentanones are typically ambiguous regarding hydrogen bonding in the crystal. The solid-state (KBr) spectrum of (I) has C═O stretching absorptions at 1734 (acid) and 1713 cm⁻¹ (ketone), consistent with known shifts produced when hydrogen bonding is, respectively, removed from a carboxyl C═O and added to a ketone. In CHCl₃ solution these bands appear, but presumably reversed, at 1743 and 1712 cm⁻¹.

S3. Refinement

All H atoms were located in electron-density difference maps. The O-bound H atom was constrained to its idealized position, with the O—H distance fixed at 0.84 Å and U_iso(H) = 1.5U_eq(O). The methylene and methine H atoms were placed in geometrically idealized positions and constrained to ride on their parent C atoms, with C—H distances of 0.99 and 1.00 Å, respectively, and with U_iso(H) = 1.2U_eq(C).

Computing details

Data collection: SMART (Bruker, 2000); cell refinement: SMART; data reduction: SAINT-Plus (Bruker, 2000); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 2000); software used to prepare material for publication: SHELXTL.

Figures

Fig. 1. A view of the asymmetric unit of (I), with the atom-numbering scheme. Displacement ellipsoids are drawn at the 40% probability level and H atoms are shown as small spheres of arbitrary radii.

Fig. 2. A packing diagram for (I), showing the four catemeric connections created by acid-to-ketone hydrogen bonds (dashed lines) proceeding along chains of molecules translationally related in a. All methylene H atoms have been omitted for clarity. The O atoms involved in the hydrogen bonding are labelled in one of the chains; atom O3AA is at the symmetry position (x − 1, y, z). Displacement ellipsoids are drawn at the 40% probability level.

(±)-3-Oxocyclopentanecarboxylic acid

Crystal data

C6H8O3	F(000) = 272
Mr = 128.12	Dx = 1.443 Mg m^−3
Monoclinic, P21/c	Melting point: 335 K
Hall symbol: -P 2ybc	Cu Kα radiation, λ = 1.54178 Å
a = 7.8564 (8) Å	Cell parameters from 1027 reflections
b = 7.9812 (8) Å	θ = 2.6–67.9°
c = 10.1367 (10) Å	µ = 0.99 mm^−1
β = 111.862 (5)°	T = 100 K
V = 589.90 (10) Å^3	Block, colourless
Z = 4	0.32 × 0.19 × 0.13 mm

Data collection

Bruker SMART APEXII CCD area-detector diffractometer	1027 independent reflections
Radiation source: fine-focus sealed tube	985 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.019
ϕ and ω scans	θmax = 67.9°, θmin = 6.1°
Absorption correction: multi-scan (SADABS; Sheldrick, 2001)	h = −8→9
Tmin = 0.871, Tmax = 0.926	k = −9→9
4895 measured reflections	l = −11→12

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)] = 0.037	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.093	H-atom parameters constrained
S = 1.12	w = 1/[σ^2(Fo^2) + (0.0437P)^2 + 0.3094P] where P = (Fo^2 + 2Fc^2)/3
1027 reflections	(Δ/σ)max < 0.001
83 parameters	Δρmax = 0.23 e Å^−3
0 restraints	Δρmin = −0.22 e Å^−3

Special details

Experimental. 'crystal mounted on cryoloop using Paratone-N'

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
O1	−0.00815 (13)	0.03524 (13)	0.17242 (10)	0.0209 (3)
O2	0.56738 (16)	0.24344 (16)	−0.00345 (11)	0.0337 (3)
O3	0.67891 (14)	0.11760 (15)	0.20710 (11)	0.0284 (3)
H3	0.7699	0.1025	0.1840	0.043*
C1	0.3829 (2)	0.23349 (17)	0.13999 (14)	0.0186 (3)
H1	0.4198	0.3113	0.2233	0.022*
C2	0.29877 (19)	0.07544 (17)	0.17826 (14)	0.0181 (3)
H2A	0.3116	−0.0223	0.1225	0.022*
H2B	0.3583	0.0494	0.2808	0.022*
C3	0.09999 (19)	0.12007 (16)	0.14038 (13)	0.0168 (3)
C4	0.0573 (2)	0.28269 (17)	0.05880 (15)	0.0198 (3)
H4A	0.0420	0.3749	0.1188	0.024*
H4B	−0.0562	0.2727	−0.0270	0.024*
C5	0.2232 (2)	0.31392 (18)	0.01809 (15)	0.0212 (3)
H5A	0.2441	0.4354	0.0113	0.025*
H5B	0.2067	0.2603	−0.0739	0.025*
C6	0.54890 (19)	0.19979 (17)	0.10355 (14)	0.0182 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0167 (6)	0.0237 (5)	0.0239 (5)	−0.0005 (4)	0.0096 (4)	0.0005 (4)
C1	0.0182 (8)	0.0176 (7)	0.0200 (7)	−0.0005 (5)	0.0072 (6)	−0.0013 (5)
O2	0.0264 (7)	0.0535 (8)	0.0252 (6)	0.0121 (5)	0.0142 (5)	0.0111 (5)
C2	0.0169 (8)	0.0178 (7)	0.0206 (7)	0.0013 (5)	0.0082 (5)	0.0014 (5)
O3	0.0186 (6)	0.0392 (7)	0.0311 (6)	0.0084 (4)	0.0137 (4)	0.0119 (5)
C3	0.0174 (8)	0.0182 (7)	0.0148 (6)	−0.0004 (5)	0.0060 (5)	−0.0050 (5)
C4	0.0166 (8)	0.0191 (7)	0.0229 (7)	0.0029 (5)	0.0064 (6)	0.0000 (5)
C5	0.0198 (8)	0.0188 (7)	0.0257 (7)	0.0026 (5)	0.0093 (6)	0.0044 (5)
C6	0.0168 (8)	0.0162 (6)	0.0216 (7)	−0.0018 (5)	0.0071 (6)	−0.0018 (5)

Geometric parameters (Å, º)

O1—C3	1.2212 (17)	O3—C6	1.3330 (17)
C1—C6	1.506 (2)	O3—H3	0.8400
C1—C5	1.5357 (19)	C3—C4	1.5081 (19)
C1—C2	1.5394 (19)	C4—C5	1.527 (2)
C1—H1	1.0000	C4—H4A	0.9900
O2—C6	1.1983 (18)	C4—H4B	0.9900
C2—C3	1.506 (2)	C5—H5A	0.9900
C2—H2A	0.9900	C5—H5B	0.9900
C2—H2B	0.9900
C6—C1—C5	113.57 (11)	C3—C4—C5	104.51 (11)
C6—C1—C2	114.07 (11)	C3—C4—H4A	110.8
C5—C1—C2	104.27 (11)	C5—C4—H4A	110.8
C6—C1—H1	108.2	C3—C4—H4B	110.8
C5—C1—H1	108.2	C5—C4—H4B	110.8
C2—C1—H1	108.2	H4A—C4—H4B	108.9
C3—C2—C1	104.12 (11)	C4—C5—C1	103.78 (11)
C3—C2—H2A	110.9	C4—C5—H5A	111.0
C1—C2—H2A	110.9	C1—C5—H5A	111.0
C3—C2—H2B	110.9	C4—C5—H5B	111.0
C1—C2—H2B	110.9	C1—C5—H5B	111.0
H2A—C2—H2B	109.0	H5A—C5—H5B	109.0
C6—O3—H3	109.5	O2—C6—O3	122.70 (13)
O1—C3—C2	124.01 (12)	O2—C6—C1	125.30 (13)
O1—C3—C4	126.03 (13)	O3—C6—C1	111.98 (11)
C2—C3—C4	109.96 (11)
C6—C1—C2—C3	−153.28 (11)	C6—C1—C5—C4	161.88 (11)
C5—C1—C2—C3	−28.85 (13)	C2—C1—C5—C4	37.13 (13)
C1—C2—C3—O1	−169.93 (12)	C5—C1—C6—O2	6.8 (2)
C1—C2—C3—C4	9.93 (14)	C2—C1—C6—O2	126.07 (15)
O1—C3—C4—C5	−167.17 (13)	C5—C1—C6—O3	−174.83 (12)
C2—C3—C4—C5	12.97 (14)	C2—C1—C6—O3	−55.53 (15)
C3—C4—C5—C1	−30.64 (14)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H3···O1^i	0.84	1.87	2.6915 (14)	166
C4—H4A···O1^ii	0.99	2.57	3.5239 (17)	162
C1—H1···O2^iii	1.00	2.61	3.3611 (17)	132
C2—H2A···O2^iv	0.99	2.52	3.4800 (18)	164

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