Crystal structures of two bicyclo[5.1.0]octanes: potassium trans-bicyclo[5.1.0]octane-4-carboxylate monohydrate and cis-bicyclo[5.1.0]octan-4-yl 4-bromobenzenesulfonate

The geometry of potassium trans-bicyclo[5.1.0]octane-4-carboxylate monohydrate, a highly strained trans-fused bicyclic ring system, is compared with that of a less-strained cis-bicyclo[5.1.0]octane derivative.


Chemical context
Extensive studies on the reactivities of the bridge bond in trans-fused bicyclic cyclopropane derivatives (Gassman et al., 1968) led to proposal of the 'twist'-bent bond to describe the bonding in these [5.1.0] bicyclic systems (Gassman, 1967). The [5.1.0]octanes are expected to be more highly strained than the corresponding trans-fused bicyclo[4.2.0]octanes which had previously been prepared (Cava & Moroz, 1962). Our studies were initiated in order to illuminate discussions of bonding by providing accurate geometric parameters for the most strained systems available. Several 4-substituted derivatives of transfused bicyclic [5.1.0]octanes were studied, but in most, disordering of the molecules in the crystal precluded any refined structure that would give useful information. Even the transfused bicyclic [5.1.0]octane 4-carboxylate structure presented here is disordered, but we were able to determine a reasonable geometry for the bicyclic system. The structure of a 4-substituted cis-fused bicyclic [5.1.0]octane was also determined, so that a comparison of the ring geometries could be made. These studies formed part of the MS and PhD theses of one of us (Kershaw, 1972(Kershaw, , 1974, and were presented at the 1973 winter meeting of The American Crystallographic Association. Table 1  show the cis-and trans-fused rings superimposed upon one another. It can be seen that in the cis-fused system (II), chemically equivalent bonds and angles are the same, and ISSN 2056-9890 so are the torsional angles. Thus the cis-fused compound has an excellent, non-crystallographic molecular mirror plane.

Structural commentary
In contrast, while the trans-fused derivative cannot have a molecular mirror plane; the molecule sits astride a crystallographic mirror plane, probably due to the packing requirements of the potassium cation and the carboxylate part of the molecule, and necessarily leading to a disordered structure. Treatment of the disorder is discussed in the Refinement section. One of the assumptions made in the refinements of (I) was that chemically equivalent bonds and angles would be the same, so it was important to verify that this was the case in the cis-fused compound, (II). In both structures, the substituent on C4 is in the exo position. In (I), the plane of the carboxylate substituent on C4 is necessarily at 90 to the molecular plane through C2, C3, C5 and C6, while in (II) the roughly planar set C4, O1, S and C11 is tilted at 71.9 (2) to the molecular plane and at 49.6 (1) to the plane through the phenyl group. In both structures, displacement ellipsoids for the cyclopropane methylene group indicate motion perpendicular to the cyclopropane ring.
The two bicyclic systems are rather similar in the top view given in Fig. 3. trans-Fusion changes the conformation angles around C2-C3 and C5-C6, as seen in Fig. 4 and in Table 1.

Figure 1
The asymmetric unit of compound (I). Displacement ellipsoids are at the 50% probability level. Sizes of the H atoms are arbitrary.

Figure 2
The asymmetric unit of compound (II). Displacement ellipsoids are at the 50% probability level. Sizes of the H atoms are arbitrary.

Figure 3
A superposition of the ring systems found for (I) and (II), viewed normal to the planes through C3, C4 and C5. The trans-fused structure is in black and the cis-fused structure in red. Fig. 3 shows that the trans-fusion is also accommodated by expansion of the angles at C3 and C5 from an average of 112.7 (2) to 117.5 (8) , contraction of the angles at C2 and C6 from an average of 113.1 (3) to 107.1 (4) , an increase in the external angles at C1 and C7 to 130.4 (8) from an average of 121.3 (3) , and a lengthening of bonds C3-C4 and C5-C4 from 1.505 (4) to 1.538 (4) Å . The H1Á Á ÁH7 distance of 2.32 Å in (II) is increased to 2.84 Å in the trans-fused (I) structure. There is a significant shortening of the bridgehead bond C1-C7 in the trans-fused compound, from 1.493 (5) Å in (II) to 1.463 (6) Å in (I), which leads to a distortion of the cyclopropane ring from equilateral triangular geometry, with reduction of the angle at C8 from 60.0 (2) in (II) to 58.4 (3) in (I). Such shortening of the strained twist-bent bond, though counter-intuitive, was expected (Kershaw, 1974, p2), because much of the electron density of the bond would lie outside the internuclear line. We carried out geometry optimization of both trans-and cis-fused C 8 H 14 systems using B3LYP density functional calculations (GAUSSIAN09; Frisch et al., 2013), with results that also showed the trends noted above, including a calculated shortening of the bridgehead C1-C7 bond length by 0.014 Å . A view of the superposition of (I) and (II) at 90 to that in Fig. 3.
bonds (Table 2) to carboxylate oxygen atoms of two separate [5.1.0] octane molecules, linking the anions into chains parallel to the b axis, as can be seen in Fig. 6. The hydrogen-bond lengths are rather short, with O3-HÁ Á ÁO1(x À 1 2 , 1 À y, z) = 2.701 (3) Å and O3-HÁ Á ÁO2(x À 1, y, z) = 2.757 (4) Å . The water O atoms may lie slightly off the mirror plane at y =1/4, as indicated by the displacement ellipsoid values, which would change the hydrogen-bond geometry a little. Strong hydrogen bonds are consistent with retention of the water of hydration even after recrystallization from a non-aqueous solvent, and also with the shifts in O-H stretching frequencies in the IR to 3060 and 3360 cm À1 . The potassium ions lie in between two of the hydrogen-bonded chains, and have four carboxylate and two water oxygen atoms as near neighbors, in a distorted flattened trigonal-prismatic array, with K-O distances ranging from 2.719 (3) to 2.879 (3) Å .

Figure 6
One of the two hydrogen-bonded chains parallel to the b axis in (I).

Figure 5
Projection of (I) down the b axis. Disordered [5.1.0]octane moieties related by the mirror at z = 0.25 are not shown.

Figure 7
Packing diagram for (II), showing halogen bonds in red.
angles are 170.06 (8) and 107.81 (9) , respectively. These parameters are consistent with moderate halogen bonding according to a systematic study of such intermolecular interactions in the CSD (Lommerse et al., 1996). Also, a review of the role of halogen bonding in crystal engineering (Metrangolo et al., 2005), stresses the importance in halogen bonding of the aromatically bound bromine seen in the present compound. There are no other intermolecular contacts of note and the shortest HÁ Á ÁH contact is H3A Á Á Á H8B(x, 1 2 À y, z À 1 2 ), at 2.47 Å .

Database survey
Of 399 hits in the Cambridge Structure Database (CSD, Version 5.35; Groom et al., 2016) for the [5.1.0] ring system, 105 have 3D coordinates available, unsubstituted H atoms at the bridgehead positions, and conventional R factors of 0.05 or less, leading to 244 [5.1.0] geometries. All of the systems are cis-fused; no trans-fused [5.1.0] system was found. The average geometry of the CSD bicyclic ring systems displays the same near-perfect mirror symmetry found in the present cis-fused structure. The geometrical parameters of the cis-fused system described here do not differ significantly from the database geometries. In particular, the average bridgehead C-C bond length in the CSD set does not differ significantly from the other cyclopropane bond lengths, just as in the present cisfused structure, (II), and in contrast to the trans-fused struc-ture, (I), where the bridgehead C-C bond length is shortened. Both the current cis-structure and the ensemble of [5.1.0] structures show the significant lengthening of bonds C2-C3 and C5-C6 relative to other bonds in the ring system noted in Table 1.
Searches for simple bicyclic [6.1.0] systems yielded only 14 hits. Two of these were trans-fused structures, (Szabo et al., 1973;Hayes et al., 2005), with H1Á Á ÁH7 distances of 2.80 and 2.95 Å , respectively. In both structures, the bridgehead C-C bond length was longer by 0.03 Å than the other two cyclopropane C-C bond lengths, in contrast to the shorter bridgehead C-C bond observed in (I).

Synthesis and crystallization
Syntheses of these ring systems are described in Gassman et al. (1971). Samples of trans-fused bicyclo [5.1.0] octane 4-carboxylic acid and crystals of the cis-bicyclo[5.1.0]octan-4-yl 4bromobenzenesulfonate were supplied by Dr Paul G. Gassman. The trans-fused acid was titrated with potassium hydroxide, and crystals of the potassium salt were obtained by evaporation to dryness and recrystallization from a benzenemethanol mixture.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. For the trans-fused structure (I), only one octant of data was collected. Also in (I), reflections with I<2 were not saved when the data were processed. These weak reflections were later patched back into the structure factor file, with intensities set at (I), where (I) was the average value for reflections at a similar value for weak reflections in the data set with 2<I<3. It became apparent, however, that most of the missing reflections were higher order. We chose to use a cut-off value of 0.82 for the resolution of reflections used in final refinements, as about 50% of the intensities at this resolution were above 3, while only 11% of the reflections at resolutions above this value had I>2. After extensive efforts, it was concluded that the nearperfect mirror symmetry in (I) apart from C1 and C7 hampered successful refinement in the non-centrosymmetric space group Pca2 1 . Accordingly, all further refinements were carried out assuming a disordered structure in space group Pbcm. Initially, only atoms C1 and C7 were disordered, but it became apparent that bonded atoms C2 and C6 should be refined individually, and that C8 should also be allowed to move off the mirror plane at z = 0.25. Later, atoms C3 and C5 were also refined individually. It was necessary to impose tight restraints on the geometry to overcome the high correlation between parameters for C2 and C3 and the reflected images of C5 and C6. This was done by tightly restricting differences between chemically equivalent bond lengths and angles on either side of the octane ring.
No special measures were necessary in the refinement of (II).
In both compounds, C-bound H atoms were constrained to idealized positions, with C-H distances of 0.97 Å for CH 2 groups, 0.98 Å for methine CH groups and 0.93 Å for aromatic H atoms, and with U eq values set at 1.3 times the U iso of their bonded atoms for the CH 2 H atoms, and 1.2 times for methine and aromatic H atoms. In (I), H1 and H7 were initially refined independently, in case their positions could throw light on the twist-bent bond, but as they refined into positions indistinguishable from the constrained positions, they were constrained in the final refinements. The water H atoms in (I) were found in a difference-Fourier map, and their positional coordinates were refined whilst their U eq values set at 1.3 times the U iso of the O atom. As a check, the U eq values for these H atoms were allowed to vary, but as there was no appreciable change in these U values, they were constrained in the final refinement.

Computing details
For both structures, data collection: Corfield (1972); cell refinement: Corfield (1972). Data reduction: Data reduction followed procedures in Corfield et al. (1973), with p = 0.05 for (I); Data reduction followed procedures in Corfield et al. (1973), with P = 0.06 for (II). For both structures, program(s) used to solve structure: local superposition program (Corfield, 1972); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: ORTEPIII (Burnett & Johnson, 1996); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 0.15 e Å −3 Δρ min = −0.17 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.